Title: Developmental differences in the temporal summation of transient and sustained auditory stimuli
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Title: Developmental differences in the temporal summation of transient and sustained auditory stimuli
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Creator: Blumenthal, Terry D., 1954-
Copyright Date: 1985
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DEVELOPMENTAL DIFFERENCES IN THE TEMPORAL SUMMATION
OF TRANS IENT AND SUSTA INED AUD ITORY ST IMUL I






By


TERRY D. BLUMENTHAL


A DI SSERTAT ION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY




UNIVERSITY OF FLORIDA


1985




















Dedicated to my parents,

El la Dora Blumenthal and Otto John Blumenthal,

for giving me the tools I need to get the job done.















ACKNOWLEDGEMENTS


Would like to thank the members of my doctoral committee, Drs.

W. Keith Berg (Chair), Kenneth J. Gerhardt, Peter J. Lang, Patricia H.

Miller, and Donald C. Teas, for their help In bringing this work into

being and for their generous donations of time and ideas. It is always

helpful to have interested and learned experts to provide comments and

critique. I would further like to thank Dr. Berg for his unfaililn9

friendship and guidance throughout my graduate career. His constant and

energetic drive to achieve quality and precision in research and

teaching have been the ideals I compare my own endeavors to, in order

to improve my skills. I would also like to thank Kathy Berg for her

helpful advice, and for writing such an Interesting dissertation.

Would like to thank Margarete Davies, Barry Hurwitz, Kristen

Bomas, and Alonso Avendano for all their help in the gathering and

analysis of the data. I would further like to thank Alonso Avendano for

his patience and hard work beyond the call of duty. If not for his

help, I would probably still be trying to collect these data. I would

also like to thank all of the undergraduates and other adult subjects,

the staff of the Newborn Nursery at Shands Hospital in Gainesville,

Florida, and the parents and their new bundles of joy, for their

patience and cooperation.

Iam grateful to the students, faculty, and staff of the

Department of Psychology at the University of Florida, for making










graduate school such an intense ly enjoyab le exper ience. I am al so

grateful to the facu lty, staff and students of Ham ilton Coll ege for

show ing me that there is life atter graduate school F ina IIy, I wou Id

like to thank Dr. Ber9 again, for setting such a good example.





















TABLE OF CONTENTS



ACKNOWLEDGEMENTS.................................il
ABSTRACT......................................... vt
CHAPTER


1. ITRODUCTION.............................,.... 1


Transient and Sustained Systems................... 1
The Startle Response.............................. 6


I.EXPERIMENT I..................,.................... 15


Method....................................... 15
Results...................................... 18
Discussion.......................,............ 22


Il.EXPERIMENT II..................................... 26


Introduction................................. 26
Method....................................... 26
Results...................................... 26
Discussion. .................................. 30


IV. EXPERIMENT III.................................... 34


Method..... ................................. 34
Results...................................... 36
Discussion... ................................ 40


V. GENERAL DISCUSSION............................... 43


REFERENCES....................................... 52
BIOGRAPHICAL SKETCH..................................... 57















Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the degree of Doctor of Philosophy


DEVELOPMENTAL DIFFERENCES IN THE TEMPORAL SUMMATION
OF TRANSIENT AND SUSTAINED AUDITORY STIMULI

By

Terry D. Blumenthal

August 1985

Chairman: W. Keith Berg
Major Department: Psychology


The present study measured the startle eyeblink electromyographic

(EMG) response to transient and sustained auditory stimuli In human

adults and newborns, to assess Immaturity of temporal summation in

newborns. Stimuli were broadband noise bursts (20Hz-20KHz) with fast

(<0.1 ms) rise times. In Experiments 1 and 2 adults received 95 dB(A)

stimuli, in 13 stimulus conditions. One stimulus was 3 ms long, six

varied in duration from 20 to 100 ms In Experiment 1 and from 30 to 55

ms in Experiment 2, and six consisted of pairs of 3 ms stimuli at

onset-to- onset intervals analogous to the durations used. In

Experiment 3, newborn infants (7-36 hours old) received 100 dB(A)

stimuli analogous to those of Experiments 1 and 2. Startle amplitude

Increased for single and paired stimuli up to 40 ms in adults and 50 ms

in infants, above which single stimuli maintained this response

amplitude while the second pulse in a pair was not effective. Within

these summation windows, single stimuli were not significantly more










effective than analogous paired stimuli. Startle probability increased

for single stimuli up to 20 ms in adults and 35 ms in infants, above

which no significant age difference was found. Single and paired

stimuli were equally effective up to 45 ms for adults and 20 ms for

Infants. For paired stimuli, intervals of 20-45 ms were equally

effective in adults, and intervals of 20-50 ms were equally effective

in infants, with the second pulse at longer intervals having no effect,

but probability for Infants was lower than that for adults at all

intervals tested. Response latency decreased as stimulus duration

increased to 20 ms, for adults but not for infants, and no significant

effect of interpulse interval on latency was found at either age. The

window for transient summation is narrower in adults (40 ms) than in

infants (50 ms), and brief stimuli are more effective in adults than in

infants. These data suggest that the transient system Is immature in

infants and that these developmental differences are illustrated in

different ways by startle amplitude, probability, and latency.















CHAPTER
INTRODUCTION

Transient and Sustained Systems

The processing of sensory input may occur in different ways for

transient and sustained stimuli. Transient stimuli activate onset-

sensitive response systems (detection), while sustained stimuli

activate response systems which are sensitive to more prolonged

stimulus attributes (identification). A stimulus with both transient

and sustained properties may activate both types of response system.

This translent-sustained dichotomy has been found for auditory (Graham,

1979), tactile (Verrillo, 1968), and visual (Schwartz & Loop, 1984)

Input, and underlying neurological mechanisms have been proposed in

each modality (Gersuni, 1971; Gescheider, 1976; Hickey, 1977). Also,

the mechanisms underlying this dichotomy are believed to develop at

different rates, at least in the auditory (Graham, Anthony, & Zelgler,

1983) and visual systems (Hickey, 1977; Norman, Pettigrew, & Daniels,

1977).

One way in which transient and sustained systems differ is in

their temporal summation abilities, with rapid summation occurring in

transient systems, and longer-lasting summation occurring in sustained

systems. In the auditory system, temporal summation has been

Investigated using behavioral scaling (Zwislocki, 1969), single unit

responding (Gersuni, 1971), and brainstem reflex elicitation (Marsh,

Hoffman, & Stitt, 1973). The present study used the acoustic startle










reflex to assess developmental differences in temporal summation in

human transient and sustained systems.

Temporal summation involves the integration of sensory input over

time. This is illustrated by the fact that the effectiveness of a

stimulus can be increased by increasing its duration, to a point. The

duration above which no further summation occurs is called the critical

duration, and is the point at which the summation function (perception

plotted as a function of stimulus duration) reaches asymptote. Also, a

pair of stimuli can be more effective than a single stimulus, and this

increases as interstimulus interval decreases. The interval below which

further reductions of interstimulus interval have no greater effect can

be considered a critical Interval, analogous to the critical duration.

Both the critical duration and the critical interval are the points at

which their respective functions of sensation plotted as a function of

time reach asymptote. Temporal summation has been found in several

sensory modalities, using a variety of experimental techniques.

Psychophysical investigations of visual perception show that

brightness increases as stimulus duration increases to 500 ms, but not

beyond (Stevens, 1966). As stimulus intensity increases, this critical

duration decreases, but the slopes of the summation functions are

parallel across a range of intensities. This suggests that summation

occurs until some specific amount of Input has occurred, and this

amount is reached more rapidly by stimuli of higher intensity. Temporal

summation may asymptote due to sufficient "area under the curve", as

determined by the stimulus envelope.

Schwartz and Loop (1984) have shown that visual perception may

involve two partially independent systems, a chromatic and an










achromatic system. The chromatic system is sensitive to stimulus

duration, showing summation functions with critical durations of 500

ms. The achromatic system is believed to be sensitive to transient

stimulation only, showing no effect of duration in the range tested

(43-1000 ms). These systems may be differentiated at the neuronal

level, in that retinal ganglion cells with transient discharge patterns

(Y-cells) are more common in the periphery, where achromatic

stimulation is more pronounced (Hirsch & Levanthal, 1978). Ganglion

cells with sustained discharge patterns (X-cells) are more common in

and around the fovea, where chromatic activation is maximal. This

indicates that the differential temporal summation of chromatic

(sustained) and achromatic (transient) systems may be due to underlying

neurological distinctions.

For intermittent visual stimuli, brightness decreases as

interstimulus interval increases beyond the critical flicker fusion

threshold (Bartley, 1969). Below this threshold, temporal summation

occurs to prevent the brightness from decreasing when the stimulus Is

not on continuously. In the visual system, the critical interval is

illustrated by the critical flicker fusion threshold.

Temporal summation has also been shown for both taste perception

(Bujas & Ostojcic, 1939, as cited in Marks, 1974), and proprioception

(Brown, 1966, 1968). Brown rotated subjects, varying the duration and

acceleration of the rotation. The perceived magnitude of the rotation

increased as the duration of the rotation increased, at a range of

accelerations. As acceleration was increased, the effect of duration on

perceived magnitude of rotation reached asymptote earlier, similar to










the finding on the visual system that critical duration decreases as

stimulus intensity increases.

For tactile stimuli, two patterns of temporal summation exist. If

Pacinlan corpuscles are present, summation occurs with a critical

duration of 500-1000 ms (Gescheider, 1976; Verrillo, 1968). This

critical duration decreases as stimulus intensity increases, just as in

the visual and proprioceptive systems. If Pacinian corpuscles are not

present, summation still occurs, but with a critical duration of less

than 1 ms (Higashlyama & Tashiro,1983). This is analogous to the

rapidly summating achromatic (transient) and slowly summating chromatic

(sustained) systems found in vision. Both visual and tactile systems

are distinguished by functional differences at the periphery which

result in qualitatively different stimulus processing.

In the auditory system, psychophysical methods have been used

extensively to illustrate temporal summation. Loudness matching has

been used to show that increasing stimulus duration results in

decreased threshold, up to a critical duration of 200 ms (Scharf,

1978). Critical duration decreases as stimulus intensity increases

(Richards, 1976), but the slope of the summation function is the same

across a wide range of intensities (Small, Brandt, & Cox, 1962; Stevens

& Hall, 1966), as is the case for tactile, proprioceptive, and visual

stimuli. For pairs of brief pulses, threshold increases as interpulse

interval increases, up to 200 ms (Scharf, 1978; Zwislocki, 1960). Using

psychophysical methods, the critical duration and critical interval for

auditory temporal summation appear to be the same.

While the use of Ioudness matching in assessing temporal summation

has provided much information, this technique is not without problems.










Differences in subject criteria used in the matching tasks can result

in variable findings. For example, when subjects are told to match on

the basis of the loudness at the onset of a stimulus as opposed to the

total loudness of the stimulus, differences in stimulus duration

effects are found (Zwislocki & Sokolich, 1974). These problems could be

avoided by using techniques which are less susceptible to cognitive

Influences, such as psychophysiological or neurophysiological

responses.

The investigation of auditory temporal summation at the

neurophysiological level has led to the identification of two types of

neurons in the auditory pathway which differ in their characteristic

discharge patterns to stimuli (Gersuni, 1971; Radionova, 1971). Short

time constant (STC) units illustrate a transient discharge at stimulus

onset (and at offset, in some cases), while long time constant (LTC)

units discharge in a sustained fashion, throughout the duration of the

stimulus. These units are also sensitive to different stimulus

parameters, with LTC units being sensitive to stimulus duration, but

not to rise time, while the opposite is true of STC units. The two

types of units show different patterns of temporal summation, with STC

units having very brief critical durations(a few milliseconds), and LTC

units having much longer critical durations. These units are found at

all levels of the auditory pathway, and may be organized into

functional systems resulting in differential processing of transient

and sustained stimulus attributes. In this respect, the LTC and STC

systems are analogous to Pacinlan and non-Pacinian systems in tactile

processing, and to chromatic (X-cell) and achromatic (Y-cell) systems

in visual processing. All three modalities illustrate distinctive










transient and sustained systems which possess different temporal

summation properties.

Evidence for auditory temporal summation has also been found at

the level of the receptor (hair cell) in monkeys (Kletsky &

Stengrevics, 1972). Period histograms of receptor activity, which are

correlated with positive excursions of the stimulus waveform, decay

more slowly than does the stimulus, with a lag of .1 to .2 ms. This

implies that some integration may be taking place at the hair cell,

resulting in a prolongation of the half-wave rectified receptor

potential.

Temporal summation appears to be a fundamental property of several

sensory systems. This summation may vary with the stimulus envelope,

until some maximal amount of summation has occurred, since critical

duration decreases as stimulus intensity increases, in several sensory

modalities. Also, in the tactile, visual, and auditory systems,

differential summation of transient and sustained stimuli can be found,

suggesting the possibility of analogous underlying mechanisms in the

three systems. Investigations of such mechanisms would be aided by a

response which is sensitive to small stimulus changes, and which can be

found across a range of ages and species. The startle reflex is such a

response.


The Startle Response

The use of the startle response as a means to assess temporal

summation can be recommended for several reasons. This response is very

reliable, shows considerable plasticity and sensitivity to stimulus

conditions, and is mediated by a relatively simple neural circuit.










Also, the startle response is time-locked to stimulus onset, and can be

measured noninvasively. A wealth of developmental and comparative data

exists, illustrating the generalizability of the startle response

across studies, subjects, and species.

Landis and Hunt (1939) found that the eyeblink is the first

component of the startle response to appear, and that this reflex

eyeblink is very reliable, resisting habituation. The reflex eyeblink

is also a sensitive measure, In that it varies predictably with very

small stimulus changes (Davis & Heninger, 1972).

Lesion and electrical stimulation studies in the rat have shown

that the acoustic startle response is mediated by a relatively simple

neural circuit (Davis, Gendelman, Tischler, & Gendelman, 1982).

Auditory input ascends via the auditory nerve and the ventral cochlear

nucleus to the nuclei of the lateral lemniscus, which connect to the

nucleus reticularis pontis caudalis, and this is the beginning of the

motor output component of the startle pathway. Startle is a brainstem

reflex that can be found in decerebrate animals (Davis & Gendelman,

1977; Fox, 1979), and in anencephalic infants (W. K. Berg, unpublished

data).

Startle has been measured in adult and infant animals and humans,

using a variety of techniques. In animal research, the two most common

measures are electromyographic (EMG) activity of the flexor muscles of

the leg, and the whole-body jerk, as measured by an accelerometer In a

rigidly suspended cage. In humans the eyeblink component is assessed,

either by measuring the movement of the eyelid with a potentiometer, or

by measuring the EMG activity of the orbicularis oculi, the muscle

responsible for the eyeblink. The advantage of using the eyeblink










component of startle in the human is that it has a lower threshold than

either flexor EMG or whole-body jerk, so minimally-startling stimuli

can be used. The EMG measure may be preferred to lid movement, since

EMG activity can be found in the absence of lid movement, whether the

eyes are open or closed, making EMG both more sensitive and more

versatile than lid movement.

Startle responses can be assessed by the application of two

different paradigms. First, the direct effects of stimuli on the

startle response can be studied, by varying the eliciting stimuli and

measuring the resultant changes in the startle response. Second, the

modification of the startle response by a nonstartling stimulus (called

a prepulse), occurring before the eliciting stimulus, can be measured.

The first technique is aimed at a direct assessment of the startle

system, while the second technique uses the startle stimulus as a probe

to investigate processing of the prepulse.

The degree to which a stimulus will elicit startle is affected by

several factors. Increasing stimulus rise time results in higher

startle thresholds, in both rats (Fleshler, 1965) and human adults

(Berg, 1973). Increasing rise time also results in smaller blinks for

suprathreshold stimuli in human adults (Blumenthal & Berg, 1984) and

infants (Blumenthal & Berg, 1982). Increasing stimulus Intensity yields

larger responses in adults (Blumenthal & Berg, 1984) and in rats

(Marsh, Hoffman, & Stitt, 1973). Increasing stimulus duration up to 8

ms, but not beyond, yields larger responses in rats (Marsh, Hoffman, &

Stitt, 1973), and this critical duration decreases as stimulus

Intensity increases. In human adults, the critical duration is longer

than it is in rats. Berg (1973) increased stimulus duration up to 32










ms, and found a continuing increase in response amplitude. Yamada

(1983) found that response amplitude increased as stimulus duration

increased to 30-50 ms, but not beyond. This illustrates a critical

duration for human adults which is considerably higher than that for

rats. The critical duration for Infants has not previously been

reported.

In rats, increasing the interval between two brief (1 ms) pulses

to 3 ms (onset-to-onset) results in larger responses, with a return to

the level of a single 1 ms pulse when interpulse interval is increased

to 6 ms (Marsh, Hoffman, & Stitt, 1973). The increase in responding as

interpulse interval is increased to 3 ms may be due to a reduction of

neural refractoriness and more complete peripheral recovery. At

interpulse intervals greater than 6 ms, the second pulse appears to

have no effect, illustrating a critical Interval for summation of

transients of 6 ms in rats.

The startle modification technique uses the startle stimulus as a

probe, and can be used to assess stimulus processing In several

modalities. This method Involves the presentation of the stimulus to be

assessed, the prepulse, some brief time before the startle stimulus.

Depending on the stimulus parameters of the prepulse, the startle

response can be Inhibited or facilitated in several ways. The amount of

modification Is wholly dependent upon the parameters of the prepulse,

with the startle-eliciting stimulus having no effect on the

modification (Stitt, Hoffman, & Marsh, 1976).

The time between onset of the prepulse and onset of the startle

pulse is called lead time, and this is an important factor In

determining modification effects. At lead times of 10 ms or less in the










rat, or 30 ms or less in the human, a prepulse above detection

threshold will cause a decrease in the latency of the subsequent

startle response (Hoffman & 1son, 1980). At lead times of 30-240 ms in

both rats and humans, startle amplitude is reduced by the prepulse

(Hoffman & Ison, 1980). The optimal lead time, where the amount of

inhibition of amplitude is maximal, is about 120 ms In adults. If lead

time is equal, prepulses which remain on throughout the lead time are

no more effective than those which have a duration of 20 ms (Graham &

Murray, 1977). However, as the duration of transient prepulses

increases, more amplitude inhibition is found. This may be because the

onset and offset of the transient prepulse become more independent as

they are separated in time, so their combined effect is Increased

(Harbin & Berg, 1984).

In infants, the transient-sensitive system Is believed to be

immature (Graham, Strock, & Zeigler, 1981). For maximal inhibition of

startle amplitude by a transient prepulse in 6-to-9-week-olds, lead

time must be 225 ms, as compared with 120 ms in adults. More intense

prepulses are required with these infants, and the prepulses must be




If a brief, transient stimulus occurs at a very long lead time, it
will have no effect, unless attention Is directed to the prepulse (as
indicated by heart rate deceleration) (Bohlin, Graham, Silverstein, &
Hackley, 1981). If attention Is directed towards the sensory modality
in which the startle stimulus occurs, the startle response will be
facilitated; if attention is directed away from the startled modality,
inhibition occurs. The attentional effect can combine with the effect
of a sustained prepulse, resulting in more facilitation than to either
effect alone (Bohlin, Graham, Silverstein, & Hackley, 1981). The reason
for this attentional effect may be that heart deceleration, or
orienting, may cause an enhancement of stimulus Input so that the
effectiveness of a startle stimulus occurring during orienting to a
prepulse Is increased (Lacey & Lacey, 1974).










slightly longer (25 30 ms in infants, 20 ms in adults) in order to be

effective (Zeigler, Strock, & Graham, 1979). Even at optimal lead

times, Inhibition Is less marked in infants than in adults (Graham,

Strock, & Zeigler, 1981). Since startle inhibition is argued to be a

function of transient system activity, these findings suggest that the

temporal summation of transients is immature in infants.

At very long lead times (2-4 sec) a prepulse which stays on

throughout the lead time (sustained prepulse) can cause an increase in

startle amplitude in adults and infants, but optimal lead time is

longer for infants (4 sec) than for adults (2 sec). However, the degree

of facilitation is similar at the two ages, supporting the belief that

the sustained processing system is relatively mature In infancy

(Graham, Strock, & Zeigler, 1981). In rats, startle Inhibition and

facilitation have been Investigated at the neurochemical level and

appear to be due to the activation of different receptor sites. The

receptors responsible for facilitation of startle appear to mature

earlier than those responsible for inhibition (Gallager, Kehne,

Wakeman, & Davis, 1983). If an analogous arrangement exists in the

human, developmental differences in startle modification may be

expected.

Immaturity of the transient system has been proposed, based on

evidence using the startle modification paradigm (Graham, Strock,

Zelgler, 1981). A more direct assessment of the transient system may be

possible by using the startle elicitation paradigm, investigating

stimulus effects on the startle response directly, rather than its

modification by a prepulse (Dykman & 1son, 1979). Also, if transient

system immaturity is believed to exist, it may be most apparent In









newborns, so this age group, rather than older infants, should be

studied.

In the present study, the temporal summation of transient and

sustained stimuli was assessed In human adults and newborns, using the

startle elicitation technique. The stimuli used were either single

stimuli, varying in duration (3-100 ms), or pairs of brief (3 ms)

stimuli, with onset-to-onset intervals analogous to the duration of the

single stimuli. Since peripheral recovery requires more than 3 ms

(Green, 1973), the offset of very brief pulses may have a minimal

effect, so the paired stimuli were treated as two transients rather

than four. These stimuli were used to assess transient system activity

directly.

The single stimuli longer than 3 ms included transients (at onset

and offset) and a steady-state portion (between onset and offset) so

that responding to these stimuli could be contributed to by both

transient and sustained system activity. The contribution of the

sustained system could be assessed in three ways, all of which involve

the assumption that the onset of single stimuli has an effect

equivalent to that of the first pulse In a pair of brief pulses, that

is, transient system activity. The assessment of sustained system

activity differs in how the stimulus components following onset (i.e.

steady-state portion and offset) affect responding.

Stimulus offset may be assumed to have no effect on the startle

response, if startle is a function of a sufficient Increase In stimulus

Intensity occurring within a sufficiently brief time, as Graham (1975)

states. Therefore, subtracting the response to a 3 ms pulse from that

to a longer pulse (subtraction of onset transient) would yield the










effect of the steady-state portion of the stimulus. However, the

influence of stimulus offset on startle has not been shown to be

Irrelevant, so ignoring this possible effect may result in an

overestimation of sustained system activity (since some of the response

remaining may be due to transient system activity).

A second way to infer sustained system activity is to subtract the

response to a pair of transients from the response to an analogous

single stimulus. This assumes that offset Is as powerful a transient as

onset, and that startle Is determined by sufficient stimulus change in

either direction. With this method, sustained system activity will be

underestimated if offset is not as effective a transient as onset

(since transient system activity will be overestimated).

Since the effect of stimulus offset has not been quantified, it

may be useful to consider the situation where the effect of the offset

transient is greater than zero but less than that of the onset (or the

3 ms pulse). If a single stimulus is no more effective than an

analogous pair of stimuli, it is assumed that the steady-state portion

and offset of the single stimulus are no more effective than the second

pulse In a pair. The degree to which single stimuli are more effective

than paired stimuli Indicates sustained system activity, though the

full effect cannot be ascertained unless offset transient effects can

be determined. However, if stimulus duration increases and responding

does not change, sustained system activity may have a minimal

contribution. This does not necessarily mean that sustained activity is

not occurring, it just means that this activity is not contributing

much to the response. When both transient system activity and sustained

system activity are present, their contributions to the response may be










unequal. If the activation of the more efficient system is decreased,

the activity of the other system may become more apparent The only case

in which the contribution of sustained system activity would be

ambiguous is that for paired stimuli which produce larger responses

than single stimuli. The lower responding to single stimuli could be

due to either reduced sustained system activity or to reduced transient

system activity at onset.

To assess the activity of the sustained system more directly,

research using stimuli with minimal transients (long rise and fall

times) is needed. The effects of stimulus onset alone could be examined

with fast-rising, slow-falling stimuli, so that no offset transient

occurs. Stimulus offset effects could be isolated with slow-rising,

fast-falling stimuli, to minimize the onset transient. These avenues of

research are suggested to explore the sustained processing system more

ful ly.















CHAPTER II
EXPERIMENT 1

Method

Subjects

Subjects were 16 adults, 6 males and 10 females, with a mean age

of 20 years (range=18 to 27 years), selected from a university

undergraduate subject pool, who reported no history of hearing loss and

no use of medications on the day of testing.


Stimu li

Broadband noise bursts (20Hz-20kHz), at 95 dB(A), with a rise-fall

time of less than 0.1 ms were used. Seven of the stimuli varied In

duration (3, 20, 35, 50, 65, 80, and 100 ms), and six others consisted

of pairs of 3 ms pulses at intervals analogous to the durations used

(20, 35, 50, 65, 80, and 100 ms, measured from onset to onset).

Intertrial interval ranged from 25-35 sec, averaging 30 sec.


Apparatus

Stimuli were produced by a Grason-Stadler 455C noise generator

gated through an Iconix electronic switch and a Sansul AU517 amplifier,

and presented through a JBL Decade 26 loudspeaker, located

approximately 1.5 m in front of the subjects. Stimulus intensity was

calibrated using a General Radio 1551C sound level meter and monitored

with a Hewlett-Packard 400E AC voltmeter.









Per i-orb ita l e lectromyograph ic ( ENG) responses were colIlected

using miniature Beckman biopotential electrodes (Ag/AgCI) filled with

Synapse conducting paste. The EMG signal was amplified by a Coulbourn

Hi-Gain Bloampliffer/Coupler, with filters passing frequencies of 90-

250 Hz, and a gain of 63,000. The signal was then integrated with a

Coulbourn Contour Following Integrator at a time constant of 80 ms

(integrated EMG). The Integrated EMG was then recorded on a Beckman

R411 polygraph, as well as being digitally sampled (10 bit accuracy) by

a PDP-8 computer every millisecond for 250 ms after stimulus onset.


Procedure

The experimenter explained the procedure and the subject was asked

to read and sign an informed consent statement, and to fill out a

background questionnaire. The experimenter then cleaned the area just

below the left eye with a cotton swab soaked in alcohol, and two

electrodes were attached, one below the center of the eye, and the

other Immediately temporal to the first, as close to the orbital ridge

as possible without impairing eye movement. The subject was then seated

in the testing room and asked to move as little as possible, especially

the eyes and head. Data were used only from trials on which an

experimenter, watching the subject on closed circuit television, judged

the eyes to be open. If a stimulus was presented when the eyes were

closed, or during movement of the head or eyes, the trial was rejected




" The integrator time constant was determined by calibration, not by
the setting on the integrator itself. It was found that the time
constant settings of two integrators of the same model varied widely,
so the only reliable way to determine the actual time constant was to
calibrate each separately.










by an experimenter blind to stimulus conditions. An average of 7.5

percent of the trials were rejected for each subject, and rejection was

not more likely for one stimulus condition than for any other. A 13x13

Latin square was used to determine stimulus order, with the row of

entry into the square being randomly determined. A session was

terminated when six potentially scorable responses In each stimulus

condition were obtained.


his Analys i s

Response amplitude was measured as the difference between response

onset and peak. Response onset was judged only during the window

between 20 and 100 ms after stimulus onset. Initiation of a response

was judged on the basis of a monotonic increase in EMG that continued

for at least 20 ms. Onset latency was then judged as the point where

this increase exceeded the random variability in the 20 ms prior to

this increase. The peak of the response was the first point following

onset where the slope of the Integrated EMG signal equalled or passed

through 0, when this was followed by at least 10 ms with no further

slope reversal. Response latency was measured as the time from stimulus

onset to response onset.

Response probability was measured as the degree to which responses

actually occurred when subject variables were optimal (i.e. eyes open,

no head movement). If the integrated EMG signal during the response

window did not deviate beyond the random noise present prior to the

onset window, a failure to respond was recorded. Due to the

considerable sensitivity of the computer sampling and amplifier, the

degree to which very small responses were not detected was minimal.









The response parameter most often reported by researchers in this

area is average magnitude, which is calculated from trials on which a

response could have been recorded, whether a response actually occurred

or not (e.g. Graham & Murray, 1977). Magnitude changes can be due to

changes in either response probability or amplitude (Prokasy & Ebel,

1967), and these two may be partially Independent measures of startle

responding (Blumenthal & Berg, in press). Therefore, the present study

examined response amplitude and probability separately. For each

subject, response amplitude and latency were averaged across trials for

each condition. ANOVAs Including stimulus type (single or paired) and

time (duration or interval in msec) as within-subject variables were

conducted. To balance the analyses, data from the 3 ms single stimulus

condition were duplicated for use in paired stimulus comparisons, as a

substitute for a pair of pulses at an Interval of 0 ms.



Results


Ampl itude

The effect of increasing stimulus duration or interpulse interval

on response amplitude differed, as illustrated by a significant

stimulus type by time interaction, E (6,90)=8.88, p<.001 (see Figure

1). Increasing stimulus duration to 50 ms, but not beyond, resulted in

larger responses, as shown by significant linear, E (1,15)=18.23,

p<.001, and quadratic, E (1,15)=15.37, p<.001, trends. Responses were

larger at 20 than at 3 ms duration, 1 (15)=4.13, p<.001, and larger at

50 than at 35 ms duration, (15)=2.64, p<.025. No other pairwise

comparisons reached significance, indicating that increasing duration












55



50



45


40



35



30



25



20


'SINGLE



















\ PAIRED


I
100


3 20 35 50 65 80


DURATION / INTERVAL (rns)


Figure 1. Adult response amplitude as a function of time for
single and paired stimuli.







20

above 50 ms had no significant effect. Increasing interpulse Interval

to 35 ms, but not beyond, resulted in larger responses, yielding a

significant quadratic trend, E (1,15)=5.61, p<.05, and a marginally

significant cubic trend, E (1,15)=4.46, p<.052. Responses were larger

for a pair of pulses at 20 ms interval than for a single 3 ms pulse,

(15)=2.20, p<.05. Increasing interval from 3 to 35 ms resulted In

larger responses, (15)=2.85, p<.025, but responses were not

significantly larger at 35 than at 20 ms interval. At Intervals of 50

ms or more, responses were not significantly larger than for a single 3

ms pulse. Up to 35 ms, a single stimulus was not significantly more

effective than a pair of transients, showing that, even with much less

total energy, a second transient was as effective as the sustained

portion and the offset of a single stimulus. This indicates an

asymmetrical Input of transient and sustained stimuli to the startle

system, with transient input being more effective than sustained Input,

up to 35 ms. Above 35 ms, the second transient had little effect,

whereas the sustained portion of the stimulus Increased in

effectiveness, reaching asymptote at 50 ms.


Prob ab ili ty

For response probability, the effect of increasing stimulus

duration or interpulse Interval also differed, as illustrated by a

s ign if icant st imulIus type by time interaction, E (6,90)=5.34, p<.001

(see Figure 2). A linear effect of duration appeared, E (1,15)=13.29,

p<.01, and this was due mainly to increasing duration from 3 to 20 ms,

1 (15)=2.22, p<.05. In fact, if the 3 ms data were excluded from the

analysis, no significant duration effect was found, E (1,15)=2.12. For












































I


IOO


20 35 50


DU RAT I ON / INTERVAL ( ms )

Figure 2. Adult response probability as a function of time
for single and paired stimuli.





100


SINGLE









PAIRED


90



80



70



60



50


3


1


80









paired stimuli, a quadratic trend appeared, E (1,15)=6.16, p<.025.

Increasing interpulse Interval to 50 ms had no further effect. At

longer Intervals, the contribution of the second pulse disappeared,

with probability at intervals of 65, 80, and 100 ms not significantly

different from that for a single 3 ms pulse. Up to 50 ms, the

difference in probability between single and paired stimuli was not

significant, illustrating a dominant transient contribution. When the

second transient was no longer effective, the sustained portion of the

stimulus maintained a high response probability. However, the

Interpretation of these probability data may be influenced by a ceiling

effect for single stimuli.


Latency

Response latency was affected by increasing stimulus duration or

interpulse interval, E (6,90)=2.22, p<.05, and stimulus type had a

significant effect, E (1,15)=10.40, p<.01, but the stimulus type by

time interaction was not significant (see Figure 3). For single

stimuli, increasing duration had an effect on latency, E (6,90)=4.43,

p<.001, but this was due to the increase from 3 to 20 ms only, since,

if the 3 ms data were not included in the analysis, no duration effect

was found (E (5,75)=0.64). For paired stimuli, increasing interpulse

interval had no effect on response latency.



Discussion

The findings of Experiment 1 show that temporal summation of

transient acoustic input dominates that of sustained input, in adults.

Transients occurring within a window of 35-50 ms summate, and sustained















62



60


\ PAIRED









\SINGLE


58



56



54



52


3


I I
20 35


-L
100


50 65 80


DURATION / INTERVAL (ms)


Figure 3. Adult response latency as a function of time for
single and paired stimuli.







24

summation adds little which cannot be explained by transient summation

in this range. Beyond this window, transient summation does not occur,

and the contribution of sustained summation becomes apparent. Also,

these differential summation effects are reflected by startle response

amplitude, probability, and latency in different ways.

For response amplitude the summation window for transients is 35-

50 ms, and the time between transients has a slight, but not

significant, effect on the response. As interpulse interval within the

window increases, a slight Increase in response amplitude may be

explained by forward masking, which suggests that the first pulse

decreases the effectiveness of the second. As the interpulse interval

Increases the amount of masking decreases (Raab, 1961), so that

responding at the 20 and 35 ms intervals may differ due to a reduction

in responding at the 20 ms interval, but not at the 35 ms interval (or,

at least, not as great a reduction). If forward masking is present, its

influence is not great, possibly due to the brief duration of the first

pulse (Zwislocki, 1978). Also, this marginal masking effect was found

for response amplitude, but not for response probability.

For response probability, the summation window extends to 50 ms,

and the effect of the second transient is not Influenced by where in

this window it occurs, above 20 ms. For response latency, the second

transient had no apparent effect at any of the intervals used. It may

be the case that the temporal summation window for transients is

narrower than 20 ms, so that the second pulse always occurred outside

the window, and this prediction may be supported by future research.

For sustained summation, 20 ms is adequate, but the window may be less

than this.









After the transient summation window has been exceeded, sustained

summation continues to occur, either increasing responding or

maintaining elevated responding after the effect of the second

transient has disappeared. This peaks at 50 ms for response amplitude,

and at 20 ms for response latency. For response probability, the peak

of the sustained summation function is difficult to specify due to the

presence of a ceiling effect at high probabilities, but this peak could

not exceed the peak response latency (approximately 65 ms). Also, the

extent to which the second pulse in a pair elicited a response when the

first pulse failed to do so was minimal (less than 10 instances for all

subjects combined). This means that the second pulse alone is not

effective in eliciting responding.

The temporal summation of transient stimuli peaked at 35 ms for

response amp~ltude and 50 ms for response probability. Within these

ranges, the contribution of the sustained portion of a stimulus was no

greater than that of the second brief transient. Beyond these ranges,

the second stimulus in a pulse pair had little or no effect, but

responding was maintained by the sustained portion of a single

stimulus. A second experiment was conducted, to investigate these

temporal summation properties in greater detail.















CHAPTER III
EXPERIMENT 2

I ntroductfi on

The data of Experiment 1 suggest that the temporal summation

window for transient stimuli Is 35-50 ms for response amplitude and

probability, and less than 20 ms for response latency. To more

accurately assess responding in the critical range of 35-50 ms, a

second experiment was conducted, in which stimulus duration and

interpulse intervals In the 30-55 ms range were varied in 5 ms steps.

Method

Sub jects

Subjects were 12 university students, 6 males and 6 females, with

an average age of 22 years (range=18 to 29), selected in the same way

as in Experiment 1.


Stimuli, Apparatus. Procedure, and Data Analysis

These were identical to those used in Experiment 1, except that

stimulus durations were 3, 30, 35, 40, 45, 50, and 55 ms, with

analogous interpulse intervals for paired (3 ms) stimuli.


Results

AmpItltude

Response amplitude was affected by increasing stimulus duration or

Interstimulus interval, E (6,66)= 3.70, p <.005, and stimulus type had

a significant effect, E (1,11)= 29.71, p <.001, but the two variables









did not interact (see Figure 4). For single stimuli, a significant

linear trend for duration was found, E (1,11)= 9.70, p <.01. A single

30 ms stimulus was more effective than a 3 ms stimulus, It (11)=2.86, p

<.025, but further increases in stimulus duration were not

significantly more effective than a 30 ms stimulus. For paired stimuli,

a significant quadratic time trend was found, E (1,11)= 6.81, p <.025.

Pairs of pulses at Intervals of 35, 40, and 45 ms were slightly, but

not significantly, more effective than a single 3 ms stimulus ( 1 (11)=

2.09, p <.06; 1 (11)= 1.98, p <.08; 1 (11)= 2.02, p <.07, for 35, 40,

and 45 ms, respectively). At 50 and 55 ms intervals, pairs of pulses

were clearly no more effective than single 3 ms stimuli. Single stimuli

were significantly more effective than paired stimuli at 50 ms, 1t (11)=

3.44, p <.01, but not below.


Probability

The effect of increasing stimulus duration or intersimulus

interval differed for response probability, as IIlustrated by a

significant stimulus type by time interaction, E (6,66)= 2.27, p <.05

(see Figure 5). For single stimuli, a linear trend for duration was

found, E 11) 7.53, p <.025. A 30-ms duration stimulus was more

effective than a 3 ms stimulus, (11)= 2.40, p <.05, but no

significant difference was found between a single 3 ms pulse and pairs

of pulses at all other intervals tested. For paired stimuli, a

significant quadratic interval trend was found, E (1,11)= 5.64, p <.05.

A pair of pulses 30 ms apart were more effective than a single 3 ms

stimulus, 1 (11)= 2.52, p <.05, but further increases in interstimulus

interval had no significant effect. A single stimulus was more









































3 30 354 45 50 55

D URAT ION/ IN T ERVA L ( ms)

Figure 4. Adult response amplitude as a function of time for
single and paired stimuli.


45


40


35


/SINGLE










/ PAIRED


30


25


f


20


11111


























~ \ PAIRED










3 20 25 303540 45


100


90

80


60

50


D U RAT IO N / IN T ERVA L ( ms )


Figure 5. Adult response probability as a function of time for
single and paired stimuli.









effective than an analogous pair of stimuli at 40, 50, and 55 ms (

(11)= 2.32, 2.28, and 2.54, respectively, p <.05).


Latency

A significant stimulus type by flee interaction was found for

response latency, E (6,66)= 2.54, p <.05 (see Figure 6). For single

stimuli, the overall flee effect was not significant, but latency was

lower for 40 ms stimuli than for 3 ms stimuli, 1 (11)= 3.39, p <.01.

For paired stimuli, a Ilnear effect of interstimulus interval was

found, E (1,11)= 14.65, p <.005. Increasing interval from 30 to 50 ms

resulted in a significant increase in response latency, 1 (11)= 2.75, p

<.025, but no other comparisons of paired stimuli were significant.

Latencies for single stimuli were significantly lower than those for

paired stimuli, at each interval tested.



Discussion

For response amplitude, increasing stimulus duration to 30-55 ms

resulted In larger responses, as Illustrated by a significant linear

trend for duration. Increasing interstimulus interval for paired

stimuli resulted in larger responses up to 40 ms, then a reduction In

response amplitude, as shown by the significant quadratic function for

paired stimuli. Single stimuli were not significantly more effective

than paired stimuli below 50 ms. In Experiment 2, the two functions

diverged between 35 and 50 ms, just as they did in Experiment 1. The

fact that the response amplitude functions are so similar in the two

experiments suggests that these findings are reliable. Up to 35 ms, the

transient aspects of the stimulus seem almost as effective as the



















PAIRED


DURAT IO N / INTERVA L ( ms)


Figure 6. Adult response latency as a function of time for
single and paired stimuli.


,,


68

66

64

62


'
/


60L


58

56

54


SINGLE


IIII
3035 4045


I I
50 55









sustained aspects in determining response amplitude.

In Experiment 1, It was shown that temporal summation for response

probability may peak below 20 ms, and this was supported in Experiment

2. Increasing stimulus duration from 3 to 30 ms resulted in increased

response probability, with further increases to 55 ms having no greater

effect. A pair of stimuli 30 ms apart were more effective than a single

3 ms pulse but further Interval Increases were not effective. Paired

pulses were as effective as single pulses up to 45 ms, above which the

effect of the second pulse in the pair tended to decrease. This

parallels the finding in Experiment 1 that transient summation Is as

effective as sustained summation up to 50 ms.

The response latency data of Experiment 2 are also similar to

those of Experiment 1, but not as clearly as were the amplitude and

probability data. Latency was lower for single than for paired stimuli,

in all cases. However, where the latency effect for single stimuli

peaked at 20 ms in Experiment 1, it peaked at 40 ms in Experiment 2.

Also, where no effect of latency was found for paired stimuli In

Experiment 1, longer latency was found at 50 than at 30 ms intervals in

Experiment 2. The reason for these latency differences In the two

experiments is not clear, but these data suggest that latency may be a

less reliable measure of startle activity than either response

amplitude or probability.

Experiments 1 and 2 show that the temporal summation of acoustic

stimuli occurs In different ways for transient and sustained stimuli,

and that this differential summation is reflected in different ways by

startle amplitude, probability, and latency. For response amplitude,

transient summation dominates for 40 ms, and sustained summation adds










little during this time. Beyond the transient summation window,

sustained summation continues to increase response amp~ltude for a

brief time (<20 ms). For response probability, summation of transients

peaks at less than 20 ms, and continues to 50 ms, with minimal

contribution by sustained summation in this period. Above 50 ms,

susta ined summation maintains elevated probability while the effect of

a second transient disappears. The probability and amplitude functions

are similar in that the temporal summation window for transients is not

more than 50 ms in both cases, but this summation seems to peak earlier

for probability than for amplitude. Response latency appears to reflect

summation properties similar to those shown by probability, but in a

less reliable fashion.

Experiments 1 and 2 illustrate the fact that the startle

elicitation paradigm provides an effective tool for comparing transient

and sustained system activity. Therefore, it would be of interest to

apply this paradigm to infant subjects, since Graham, Anthony, &

Zeigler (1983) hypothesize that the transient system is relatively

immature when compared to the sustained system in infants. Experiment 3

measured startle amplitude, probability, and latency to stimuli

analogous to those used in Experiments 1 and 2. If transient immaturity

exists, these measures should show differences between infants and

adults.














CHAPTER IV
EXPERIMENT 3

Method

Sub jects

All subjects were apparently healthy newborns, with no significant

problems or complications, and were the result of spontaneous vaginal

delivery at Shands Hospital in Gainesville, Florida. Data from 4 of 24

subjects tested were not used, due to a failure to respond to at least

two trials at each stimulus condition. For the 20 remaining Infants,

average age was 23 hours (range=7-36 h), average gestational age by

maturity rating was 39.75 weeks (range=37-42 wk), average 5 minute

Apgar (1953) score was 8.9 (range=8-10), and average birthweight was

3350 gm (range=2640-4220 gm). Testing began an average of 1 hour 10

minutes after the last feeding (range= 30 min- 3 h 30 min). Local

anaesthetic was administered to 11 mothers during delivery, with no

anaesthetic in the remaining 9 cases.


Apparatus

Stimuli were produced by a Grason-Stadler 455C noise generator,

gated through a Coulbourn electronic switch and a Dynaco Stereo 120

amp~lfier, and presented through a JBL Decade 26 Ioudspeaker, located 1

m in front of the subjects. Sound levels were checked before each

session, using a General Radio 1551C sound level meter and a Simpson

260 voltmeter. The EMG signal was amplified and integrated in the same

way as in Experiment 1, except for some variation in Bioampliffer gain









settings. This signal was sampled by an Apple II Plus computer (12 bit

accuracy) every m il Ii second for 250 ms and corrected for gain

variations. EKG was recorded on a Teac A-2300SX tape recorder, and on a

Narco DMP-4B Physiograph. Respiration was recorded using a Parks

Electronics strain gauge and a Parks Electronics Model 270

plethysmograph, and this signal was also displayed on the Narco

Physiograph.


Stimuli

Stimuli were broadband noise bursts (20Hz-20 KHz) at 100 dB(A)

with fast (< 0.1 ms) rise and fall times. Pilot data showed that 95

dB(A) stimuli (the intensity used in Experiments 1 and 2) did not

reliably elicit responding at 3 ms duration. Five of the stimuli varied

In duration (3, 20, 35, 50, and 100 ms), and the other four were pairs

of 3 ms pulses at onset-to-onset intervals of 20, 35, 50, and 100 ms.

Interstimulus interval ranged from 20-60 ms.


Procedure

Each subject was placed supine in a crib in the testing chamber,

and the respiration strain gauge was attached laterally across the

abdomen, just above the navel. EKG electrodes were attached in a

modif led Lead 11 configuration. The area just below the left eye was

then cleaned with a cotton swab soaked in alcohol, and the EMG

electrodes were attached, one just below the center of the eye and the

other immediately temporal to the first, as close to the orbital ridge

as possible. Stimulus presentation was controlled by an experimenter,

sitting beside the loudspeaker, who also recorded the state of the

infant on each trial, using the rating scale of Berg, Berg, and Graham









(1971). These behavioral ratings, along with respiration and EKG

measures, were used to determine the state of the infant, and these

three measures have been shown to be adequate and reliable Indicators

of sleep state in young infants (Anders, Emde, & Parmelee, 1971). Only

trials during which the infant was judged to be in quiet sleep were

Included in the analysis. Stimuli were not presented during movement of

the head, eyes, or arms, or when the subject's eyes were open. A

session was terminated when 9 potentially scorable responses at each

stimulus condition were obtained. If the Infant changed state the

session was halted, and the infant was allowed to return to quiet

sleep. If the subject did not reach quiet sleep within an hour, the

truncated session was terminated. Stimulus order was determined by a

9x9 Latin square, with the row of entry into the square being randomly

determined. Response amplitude, probability, and latency were measured

in the same way as in Experiment 1.

Resu lts

Ampl itude

A significant effect of stimulus type was found, E (1,19)=5.13,

p<.05, with single stimuli eliciting larger responses than paired

stimuli (see Figure 7). A significant time effect was also found, E

(4,76)=6.46, p<.001, but stimulus type and time did not interact. For

single stimuli, increasing duration from 3 to 20 ms yielded larger

responses, 1 (19)= 2.09, p<.05, and a further increase from 20 to 35 ms

resulted in still larger responses, (19)=4.07, p<.001. Increasing

stimulus duration above 35 ms had no further effect. For paired

stimuli, increasing interpulse interval from 20 to 50 ms resulted in

larger responses, 1 (19)=2.12, p<.05. Responding to a pair of pulses at









100


90





80


'SING LE






PA IRED


60





50


/


20


35 50


100


DURAT ION /INTERVAL (ms)


Figure 7. Infant response amplitude as a function of time for
single and paired stimuli.









an interval of 100 ms was not significantly different from that to a

single pulse at 3 ms duration, suggesting that the second pulse in the

pair did not contribute to the response.

A single stimulus 20 ms long was not significantly more effective

than a pair of pulses 20 ms apart. At 35 ms, single stimuli were more

effective than pairs of pulses, f (19)=2.47, p<.025, but this advantage

was not found at 50 or 100 ms. These findings suggest that a pair of

pulses can be nearly as effective as an analogous single stimulus, up

to 50 ms. The transient contribution seemed to be dominant in

determining responding, with the sustained portion of the stimulus

adding Ilttle.


Probab Iiity

For response probability, an Interaction between stimulus type and

time was found, E (4,76)=20.67, p<.001 (see Figure 8). For single

stimuli, a quadratic duration trend appeared, E (1,19)=24.28, p<.001.

Response probability increased as stimulus duration was increased from

3 to 20 ms, 1 (19)=2.76, p<.025, and from 20 to 35 ms, (19)=4.41,

p<.001, with duration increases above 35 ms having little effect. For

paired stimuli, a quadratic interval effect appeared, E_ (1,19)= 19.79,

p<.001. Probability was not affected by Increasing interpulse interval

to 50 ms. At 100 ms interval, probability returned to the level found

with a single 3 ms pulse, suggesting that the second pulse at 100 ms

did not contribute to responding.

A pair of pulses at 20 ms interval was more effective than a

single 3 ms pulse, 1 (19)=2.82, p<.01, and no less effective than a 20

ms long stimulus. Single stimuli were more effective than paired pulses










































_ ~1


100


---- SINGLE
















\ PAIRED


90



80



70



606



50


III


100


3 20 35 50


D URAT IO N /INTERVA L ( ms)


Figure 8. Infant response probability as a function of time for
single and paired stimuli.









at 35, 50, and 100 ms, indicating a significant contribution of

sustained stimulus attributes above 20 ms.


Latency

A significant effect of stimulus type on response latency was

found, E (1,19)=4.53, p<.05, with latency being slightly longer for

paired than for single stimuli (except at 100 ms) (see Figure 9). No

significant time effect or stimulus type by time interaction was found.

Pairwise comparisons, looking at time comparisons for each stimulus

type, and stimulus type comparisons for each time, showed that only the

50 ms single and paired stimuli resulted in different response latency,

S(19)=2.42, p<.05. These data are difficult to interpret, due to the

high latency variability, an effect also reported by Zeigler (1978) in

6-to-9-week-old infants.


D iscu ssilon

The present study suggests that the temporal summation of auditory

stimuli in human newborns follows a different time course for transient

and sustained stimuli. Also, startle amplitude, probability, and

latency do not reflect this differential summation in the same way.

Response amplitude increases if a second transient is added up to

50 ms after the f first. In general, the sustained portion of the

stimulus does not seem to contribute much that cannot be accounted for

by the transient components of the single stimuli. This suggests that

the window for temporal summation of transients is 50 ms for response

amplitude, and that transient summation dominates sustained summation

in this range.
















60


58 .






~""~c~\ PAIIRED

mc 52 \
o \

I* 50 SINGLE





3 20 35 50 100
D URAT IO N / INTERVA L ( ms)

Figure 9. Infant response latency as a function of time for
single and paired stimuli.







42

The fact that response amplitude increases as interpulse interval

increases to 50 ms suggests that forward masking may be present, with

the first pulse in the pair decreasing the response to the second

pulse. As interpulse interval increases, masking decreases (Raab,

1961), allowing the second pulse to contribute more to the response.

The findings for response probability are quite different. Adding

a second transient 20-50 ms after the first results in Increased

likelihood of responding, and the effect of the second transient is not

influenced by where It occurs within this window. Sustained summation

becomes important above 20 ms, resulting in an increase in probability

which asymptotes at 55 ms. This suggests that the window for temporal

summation of transients is 20-50 ms, and that sustained summation above

20 ms can add to this, with this sustained contribution reaching Its

maximum within 35-50 ms of stimulus onset. The effect of the second

transient is less pronounced for probability than for response

amplitude, so transient and sustained system activity coexist, and both

contribute significantly to responding in infants.















CHAPTER V
GENERAL DISCUSSION


The startle response is influenced by activity in both the

transient and sustained systems, a finding contrary to the hypothesis

of Dykman and Ison (1979). These two systems are at least partially

independent, with the startle response being the net result of activity

in both systems. These experiments show that manipulation of the

startle-eliciting stimuli can demonstrate Independent Influences of

transient and sustained system activity. However, the relative

contribution to the startle response of these two systems may not be

equal, since these results suggest that the transient system is more

efficient than the sustained system. As one system becomes dominant

over the other, their relative contributions to the response will

change.

To find the relative contribution of transient and sustained

activity, it is first necessary to calculate the effect of a second 3

ms pulse in a pair if no transient activity were to occur for that

pulse. This is accomplished by calculating the slope of the single

stimulus function (in units per ms), and adding 3 times this value to

the response to a single 3 ms stimulus. If responding to two pulses is

simply additive, then 6 ms of stimulation should be equivalent to two 3

ms stimuli. The extent to which the second pulse in a pair affects

responding above this predicted level indicates the influence of

transient system activity.









The ratio of actual responding to predicted responding (based on

linear energy integration) to the second pulse IIlustrates the relative

dominance of transient or sustained system activity. If this ratio is

greater than 1.0, transient activity is dominant, since the actual data

exceed the predicted values. Ratios less than 1.0 illustrate sustained

system dominance.

For response amplitude, the ratios of actual to predicted (linear)

response changes due to the presence of a second pulse In a pair are

7.28, 6.13, and 8.72, for Experiments 1, 2, and 3, respectively. For

response probability, these ratios are 5.33, 5.87, and 5.23, for

Experiments 1, 2, and 3, respectively. Clearly, the effect of the

second pulse in a pair is much greater than would be expected if only

linear energy integration were occurring. The transient properties of

the second pulse enhance responding, and transient system activity

dominates sustained system activity when the two occur together.

The response amplitude data suggest that the result of temporal

summation of transients is more efficient than that of summation of

sustained stimulus attributes, for both neonates and adults. However,

the window of temporal summation is narrower in adults (40 ms) than in

infants (50 ms). Beyond this window, sustained summation becomes

evident. Research using longer durations and intervals is necessary to

more accurately specify the transient summation window for neonates.

The infant and adult response probability data suggest that

transient summation is maximal within 20 ms, and extends to at least 50

ms, with infant probability being about 20 percent lower than adult

probability (see Figure 10), despite the fact that the stimulus was

more intense for infants than for adults. The temporal summation of










































I 1


100


90



80



70



60



50


III


3 20 35 50 65 80


100


INTERVAL (ms)


Figure 10. Adult and infant response probability as a function of
time for paired stimuli.


ADU LT









I NFA NT









transients follows the same time course in adults and neonates, for

response probability. For adults, sustained summation contributed

little more than could be accounted for by transient summation, and

this may have been due to a ceiling effect. In Infants, where maximal

temporal summation of transients still yielded probabilities below 70

percent, a clear additive effect of sustained summation was found above

20 ms. It may be the case that sustained summation would be more

apparent in adults if stimuli which avoid the response probability

ceiling were used. When comparing infant and adult probability for

single stimuli, the most striking finding is the much lower response

probability for very brief stimuli in infants (see Figure 11). Adults

require much less stimulation than do infants to achieve equivalent

response probabilities. However, at longer durations single stimuli are

quite effective in infants, supporting the belief that the sustained

system is relatively mature In infants (Graham, Strock, & Zelgler,

1981).

Graham, Strock, and Zeigler (1981) suggest that transient

immaturity may be due to both motor system and sensory system

immaturity. The response of the orbicularis oculi, the muscle

responsible for the eyeblink, to a startle stimulus consists of two

components, R1 and R2, which are neurologically dissimilar (Sanes,

Foss, & Ison, 1982; Shahani & Young, 1973). The R1 response is a short

latency (5-15 ms) response of motoneurons found in the palpebral

segment of orbicularis oculi. This R1 component occurs Ipsilaterally to

electrical and tactile, but not auditory, stimuli. The R2 component has

a longer latency (20-100 ms), and occurs in the orbital segment of

orbicularis ocull bilaterally. The neural circuit for R2 is more
















100


--ADULTS


- INFANTS



















I
100


9 O



80



70



06



50 /




3


1111
20 35 50 65 80


(ms)


DUR AT ION


Figure 11. Adult and infant response probability as a function of
time for single stimuli.









complicated than that for R1, Involving brain stem nuclei at the level

of the inferior colliculus. This R2 component is what Is measured as

the startle eyeblink by a periorbital electrode placement. For this

reason, the R1 component may be irrelevant to the present study. This

means that the distinction between transient and sustained motoneurons

may not be a relevant factor in defining the immaturity of the

transient system, contrary to the suggestion of Graham, Strock, &

Zelgler (1981). On the other hand, the suggestion by Graham, Strock, &

Zeigler (1981) of a neurological dichotomy of transient and sustained

sensory neurons is supported by the present research. The fact that

pairs of stimuli can be as effective as analogous single stimuli, which

contain much more energy, suggests that the input to the startle center

may be equivalent for the two types of stimuli. This means that

temporal summation occurs either before or at the startle center, due

to differential activation of transient and sustained sensory systems.

In a previous report (Blumenthal & Berg, In press), It was

suggested that startle amplitude and probability may be determined by

partially independent underlying mechanisms, a startle trigger and a

startle amplifier. This is analogous to the distinction made in

cognitive perceptual research between stimulus detection and stimulus

identification (Posner, 1978).

The extent to which the two stimuli in a pair of transients have

common neural elements may have an effect on their summation. If the

units have not fully recovered from the effects of the first pulse,

then the contribution of the second pulse may be decreased. This is a

type of forward masking, and the recovery of these units may be

immature in neonates. This would explain the finding that response









amplitude Increases as interstimulus interval increases above 20 ms for

infants, but not for adults. This neural recovery may be more extended

for the startle amplifier than for the trigger, since no evidence of

increase in forward masking was found over the 20-50 ms range of

intervals tested, for either infant or adult response probability.

The exact location and nature of the trigger and amplifier

mechanisms is unknown, but they are most likely to be found either in

the auditory pathway below the Infer for colliculus, or in the startle

center itself. Also, the two mechanisms may have different locations,

since they appear to be partially independent in their responding. If

either the trigger or the amplifier is located in the auditory pathway,

similar mechanisms may be found in other sensory pathways, and further

studies investigating startle trigger and amplifier mechanisms in

visual and tactile modalities are suggested. The fact that analogous

temporal summation abilities are found across modalities suggests that

this summation may be a very useful means of investigating startle

trigger and amplifier mechanisms cross-modally. It would also be useful

to conduct comparative studies using animals, to more accurately locate

the mechanisms underlying these differential findings. Such studies

might show that the trigger and amplifier mechanisms are due to

specific startle center involvement, or to basic organizing principles

found in all sensory systems.

In adults, sensitivity to transients may be greater for the

trigger than for the amplifier, since response probability peaks at 20

ms for both single and paired stimuli, while the amplifier continues to

summate until 40 ms. This implies that the amount of energy required to

reach maximal activation is lower for the trigger than for the









amplifier. In fact, this critical level may be below 20 ms, and further

research at durations and intervals of less than 20 ms Is needed to

measure the transient sensitivity of the startle trigger.

In infants, the immature transient sensitivity of the startle

trigger is illustrated by the low response probabilities for brief

duration stimuli (see Figure 11). The immaturity of the startle trigger

is further illustrated by the fact that pairs of pulses are much less

effective in Infants than in adults (see Figure 10). However, the

infant and adult curves are parallel, so it may be the case that

interstimulus interval is not the crucial parameter. The probability

data suggest that both the single and paired stimulus functions show

age differences which are due to immature responding to very brief

stimuli, regardless of the interstimulus interval. Making individual

pulses in each pair longer, so that the critical energy could be

reached at the shortest interval, Is necessary to separate the effects

of increasing stimulus duration and interstimulus interval. Future

research using pairs of longer stimuli is suggested, to separate these

duration and interstimulus interval effects more fully.

The present data support Graham and coworkers (1981) in their

belief that the transient system is immature in young infants. The

infant response is immature in three ways. First, the window of

temporal summation of transients, as illustrated by response amplitude,

is wider in infants than in adults. Second, the degree to which brief

stimuli elicit the startle response, as illustrated by response

probability, is more pronounced in adults than in infants. Third, pairs

of brief stimuli are less effective in eliciting transient system

activation at all intervals in infants than in adults, allowing






51


transient and sustained summation to occur together in infants, for

response probabiIlity.















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BIOGRAPHICAL SKETCH

Terry D. Blumenthal was born in Edmonton, Alberta, Canada, on July

17, 1954. He grew up in and around Rolly View, Alberta, and eventually

obtained a Bachelor of Science degree, with a specialization In

psychology, at the University of Alberta in Edmonton. With this feat

behind him, he decided to move from the top of a large pile to the

bottom of a smaller pile, and he applied for graduate school. The

University of Florida retained his services for five years, during

which time he enjoyed Ilfe to the fullest, with considerable help from

his friends and fellow graduate students in a variety of departments.

While at the University of Florida, he worked under the supervision of

Dr. W. Keith Berg, who had a profound influence on how Mr. Blumenthal

viewed the world. Five years after his arrival in Gainesville, Mr.

Blumenthal moved on to a teaching position at Hamilton College In

upstate New York, where he taught courses, wrote his doctoral

dissertation, and had a very productive year on all fronts. At this

writing, matrimony looms on the horizon. Mr. Blumenthal has been

consistently reminded of the considerable rewards which follow hard

work, personal application, and a well-told joke.










I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.




W. Keith Berg, Chair
Associate Professor of Psychology



I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.



Kenneth J \Gerhardt
Associate professorr of Speech



I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.




Pqfter J.vLang
Gra ~ate Researc /ro essor
of Clinical Psy biologyy



I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.



Patricia H. Miller
Associate Professor of Psychology









I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.



Donald C. Teas
Professor of Psychology



This dissertation was submitted to the Graduate Faculty of the Department
of Psychology in the College of Liberal Arts and Sciences and to the
Graduate School, and was accepted as partial fulfillment of the
requirements for the degree of Doctor of Philosophy.

August 1985


Dean, Graduate School




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