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PATTERN EVOKED RESPONSES--
SPATIAL AND TEMPORAL FREQUENCY INTERACTIONS
ELMAR THORWALDT SCHMEISSER
A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL
OF THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
Elmar Thorwaldt Schmeisser
It is a pleasure to acknowledge the aid of my
Chairman, Dr. William W. Dawson, not only for providing
the seeds for a sequence of projects which saw publication
and eventually lead to this dissertation, but for
invaluable aid in expanding otherwise too terse statements
as well as deftly managing occasional lapses into
convoluted german grammar. Both the facilities and the
guidance provided are much appreciated. I must also thank
Dr. Jay Enoch for insuring a manageable project and clear
definition of objectives in the planning and execution of
this work. Dr. John Munson, Dr. Arthur Otis and Dr.
Wendell Stainsby all not only shepherded me through the
comprehensive exams, thus building a fund of physiological
training, but also aided in maintaining the relationship
between abstract research results and physiological
reality. Also, I wish to thank Dr. Keith White for ably
stepping in for Dr. Enoch after his departure for
California and providing pointed and interested feedback,
thus leading to a stronger project.
Lastly, I wish to thank my wife who had to put up
with conflicts between courting and lab but came through
with me to the end.
TABLE OF CONTENTS
LIST OF FIGURES .
ABSTRACT . .
REVEIW OF SELECTED LITERATURE
Binocular Data .
Summary . .
EXPERIMENTAL RATIONALE .
METHODS .. . .
. . iii
* . vi
* . viii
. . 1
. . 4
o o 9
S . 15
. . 18
. . 19
. . 21
S . 22
. . 29
. . 30
RESULTS . . 33
Phase Calibration . ... 33
Spectral Power . ... 33
Amplitude Analyses . 54
Phase Analyses . ....... 57
Correlation Analysis . 58
Results Summary. . 60
DISCUSSION. . . 61
CIRCUIT DESCRIPTION AND PARTS LIST ... 66
Voltage Regulators. . 69
Current Regulators . .. 69
External Units . . 70
Switches . . 70
Integrated Circuits . 70
Transformer . ... 70
Capacitors ... . 70
Resistors (Values in Ohms) ... 71
LITERATURE CITED . ..... 72
. . 90
LIST OF FIGURES
1 Schematic illustration of stimulus and
recording arrangement. Al, A2:
preamplifiers; C: computer; F:
band-pass filter; M: first-surface
mirrors; 0: observer; 01: master
display oscillosocpe; 02: slave display
oscilloscope; P: prism array; S:
shield room; SG: signal generator; ST:
stimulus control circuitry; T:
oscilloscope trigger input; Tl:
oscilloscope trigger signals; T2:
computer trigger signal; X: master
oscilloscope horizontal control; Y:
oscilloscope vertical control; Z:
oscilloscope luminance control input;
Z1, Z2: luminance control signals to 01
and 02 respectively . ... 24
2 Stimulus luminance profile metrics for
gratings used . 27
3 Phase run of subject A, session #1. A:
unfiltered data; B: filtered data (5-9
Hz); C: counterphase trace of the
grating luminance contrast; D:
zero-crossing signal. The numbers
between C and D indicate stimulus
events; the numbers over the peaks in A
and B indicate the respective response
positivities . . 35
4 Phase run of subject B, session #3.
Otherwise as in Fig. 3 . 37
5 Grand unweighted, non-normalized digital
sum of all 15 subject-session phase
runs. Otherwise as in Fig. 3. Absolute
amplitude is arbitrary . 39
6 Responses to a monocular presentation of
a complex grating with the components at
a phase angle of 00. A: unfiltered
data; B: filtered data; C: stimulus
traces (sine or square mode
counterphase) as well as time and
amplitude calibration on square wave; D:
power spectra of responses. Note a
factor of two scale shift of the sine
mode versus square mode power spectra 42
7 Responses to a binocular presentation of
a complex grating with the components at
a phase angle of 00. Otherwise as in
Fig. 6 . . 44
8 Responses to a binocular presentation of
a simple (Fl: 2 cycles/degree) grating.
Otherwise as in Fig. 6 . 46
9 Responses to a dichoptically presented
gratings (Fl+F3) with the components at
a phase angle of 1800. Otherwise as in
Fig. 6 .. . . 48
10 Bar graph of response power to component
fundamental (Fl) and second harmonic
(F2) by mode sine (SN) and square (SQ)
mode counterphase and stimulus; means
and standard error of the means (n=15) 51
11 Bar graph of the component power ratio
(fundamental/second harmonic) by mode
and stimulus. -Means and standard error
of the means (n=15) . 53
12 A: means and standard error of the
means of response amplitude, both sine
and square mode, by stimulus:
unfiltered data; B: filtered data; C:
means and standard error of the means of
response phase, unfiltered data; D:
filtered data (n=30) . 56
13 Schematic diagram of the stimulus
circuitry labelled "ST" in Fig. 1 68
Abstract of Dissertation Presented to the Graduate Council
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
PATTERN EVOKED RESPONSES--
SPATIAL AND TEMPORAL FREQUENCY INTERACTIONS
Elmar Thorwaldt Schmeisser
Chairman: William W. Dawson, Ph.D.
Major Department: Physiology
Visual evoked potentials from the scalps of five
human volunteers were recorded in response to 1800 phase
shifts of simple (single spatial frequency) and complex
(sum of two spatial frequencies) sinusoidal gratings of
two and six cycles/degree in a 50 field. Steady-state
responses were obtained by reversing the contrast of the
gratings (counterphasing) at seven shifts/second, either
abruptly (square mode) or smoothly (sine mode). Each eye
always received the same total amount of light during
stimulus presentations, whether monocular, binocular or
dichoptic. Sine mode counterphasing enhances the second
temporal harmonic component of the evoked waveform,
leading to a doubled response peak per stimulus shift.
Square mode counterphasing generates a response with
essentially all the power at the fundamental temporal
frequency. With complex gratings (2 + 6 cycles/degree),
the temporal fundamental to second harmonic ratio is
enhanced when the two grating spatial components are at 00
relative phase angle. Gross amplitude did not differ
between sine and square mode evoked responses, nor between
two and six cycles/degree gratings. Hypothetical
(synthetic) responses to complex gratings were computed by
digitally adding the observed responses to simple gratings
presented alone. These synthetic responses were
significantly greater in amplitude than any observed
response to the presented complex gratings, indicating
that the observed response is not a linear superposition
of the responses to the individual grating components.
Response amplitudes ranked significantly by condition:
binocular>dichoptic>monoptic, indicating that masking of
one grating by another simultaneously presented in the
fellow eye did not occur. Pearson correlation
coefficients calculated between all possible pairs showed
that, in the best possible case, only 61% of the variance
of the observed dichoptic sum of gratings response can be
accounted for by the contribution of the responses to the
simple gratings alone. It is concluded that (a)
sinusoidal counterphasing stimuli lead to two stimulus
events per shift as the grating fades into and out of
visibility (an observed grating onset and offset) which
produces peak doubling in the response waveform, (b)
spatial frequency channels that are harmonically related
in a 1:3 ratio are partially additive in the
suprathreshold steady-state case, and (c) the location of
this interaction is probably cortical rather than
precortical as the monocularly presented complex grating
response does not differ from the dichoptic response.
Since the demise of the homunculus in the pineal body
after Descartes around 1600, modeling of the function of
the visual system has been based on a mechanistic analysis
or decomposition of a visual scene into simpler elements.
These elements have been conceived in two major groupings:
"features" such as edges, colors, corners etc., and
"components" in a mathematical or "Fourier" sense. The
putative detectors of these elements are neurons at the
various levels in the quasi-hierarchical structure of the
Pattern analysis by apparent feature detection has
been demonstrated in monkey visual cortex (Hubel and
Wiesel, 1977). An order array of cortical cells seems to
respond optimally to differing orientations and locations
of bar or edge stimuli. A visual scene could be conceived
of as an ensemble of such bars or edges. This type of
analysis would, therefore, be a plausible method for
mapping visual space into patterns of neural activity,
especially considering the interconnectivity of those
neurons (Finette, Harth and Csermely, 1978).
A visual scene can be analyzed alternatively, by
analogy, along the lines of the mathematical decomposition
of a complex waveform into its components (Gaskill, 1978).
This has been done optically (Switkes, Mayer and Sloan,
1978) and so raises the question of whether it is also
done neurally (Kelly, 1977; Graham, 1979; Ochs, 1979).
The simplest stimulus in two dimensional visual
space, in a Fourier, or spatial frequency sense, is a
grating pattern whose luminosity varies sinusoidally along
a single dimension (Ramirez, 1974; Kelly and Magnuski,
1975; Weisstein et al., 1977; DeValois, DeValois and Yund,
1979). In this case, there is a single component (the
fundamental) at one orientation, neglecting symmetrical
reflection. Gratings with different luminance
profiles--square, triangular, etc.--will have multiple
Fourier components. The visibility or detectability of
gratings having various luminance profiles can be simply
related to the strengths (power) of their spatial
frequency components (Campbell and Robson, 1968). Such
research, in conjunction with work using compound gratings
synthesized from known spatial frequencies (Graham and
Nachmias, 1971) has led to the concept of independent
information "channels" in the visual system, each
sensitive to a small range of spatial frequencies, and
operating independently (Riggs, 1974; Quick and Reichert,
1975; Abel and Quick, 1978; Watson and Nachmias, 1980) and
has led as well to clinical applications (e.g., Arden,
1979). These channels are generally conceived of as
theoretical constructs, each receiving the same
inputs--the visual scene segment from the fields of its
receptors--but abstracting different information to pass
on to higher processors. Interactions of information
within a channel can be used to model results from spatial
masking experiments. An ensemble of independent channels
can be used to model adaptation effects. Whether there
exists a neural unit identifiable with a channel remains
unclear (see Review below).
Temporal frequency channels are a logical extension
of this concept; i.e., that time-varying stimuli may be
detected as components (Tolhurst, Sharpe and Hart, 1973).
Results from psychophysical masking experiments support a
temporal sustained versus transient dichotomy, paralleling
neural findings (Breitmeyer and Ganz, 1976). Such masking
experiments show interactions between these presumed
channels, and so have questioned the independence of
Fourier channels and the Fourier model in general (e.g.,
Graham and Rogowitz, 1976; Legge, 1976; Bergen, Wilson and
Cowan, 1979; Julesz and Caelli, 1979; Tangney, Weisstein
and Berbaum, 1979). The literature review in this
dissertation will examine the evidence which has been
offered by both psychophysical and electrophysiological
techniques. This will establish a framework for a set of
experiments designed to probe spatiotemporal interactions
as reflected by the pattern visual-evoked potential
recorded from humans.
REVIEW OF SELECTED LITERATURE
Psychophysical exploration of spatial frequency
channels has developed evidence from both detection and
matching paradigms. Detection experiments, such as those
by Campbell and Robson (1968) and Graham and Nachmias
(1971) have shown that the contrast sensitivity function
defines a set of mutually independent thresholds for
different spatial frequencies. By using a compound
grating formed of more than one spatial frequency, a
grating is detected whenever any one component exceeds its
independently determined threshold. Further, gratings
will be differentiated only when a component is above
threshold for one grating, but not for the comparison
grating (Nachmias et al., 1973; Nachmias and Weber, 1975;
Isono, 1979; Ginsburg, Cannon and Nelson, 1980). Graham
and Nachmias' (1971) results also show a phase
independence between spatial frequencies when the
components are separated by more than one octave in the
high frequency ranges. These results have been extended
to circular targets as well (Kelly and Magnuski, 1975).
The hypothetical structure of such channels in terms
of bandwidth and sensitivity has been related to the
psychophysical thresholds for detection (Kelly, 1972;
Abadi and Kulikowski, 1973; Mostafavi and Sakrison, 1976)
as well as to receptive field structure (Legge, 1978b).
The channels also seem to be orientation selective, as
adaptation by a grating at one orientation does not raise
the threshold for detection of a grating at another
(Carlson, Cohen and Gorog, 1977). Such tuning, however,
seems quite broad, being several minutes of arc, compared
to the observed vernier accuracy of the visual system, on
the order of 10 sec of arc (Blakemore and Hague, 1972;
Detectability or threshold level experiments, while
providing much information, do not necessarily predict the
behavior of the visual system in its normal, non-threshold
operating ranges. Suprathreshold experiments in contrast
matching and size perception have to some extent also
supported the multiple parallel narrow band channels model
(Watanabe et al., 1968; Prome et al., 1979; Levinson and
From, 1979; Arend and Lange, 1980), but complications
indicating some interactions between channels have been
found (Stromeyer and Klein, 1975; Tangney, Weissten and
Berbaum, 1979). Composite pattern adaptation effects have
indicated an inhibition between channels (Stecher, Sigel
and Lange, 1973). Nonindependence has also been found
with masking patterns two octaves away (Henning, Hertz and
Broadbent, 1975; Cavanaugh, 1978; Klein and Stromeyer,
1980), with phase perception (Stromeyer, Lange and canz,
1973; Atkinson and Campbell, 1974; Arend and Lange, 1979;
Burr, 1980) and with sensitivity increases by certain
masks (Tolhurst and Barfield, 1978).
Alternative models for pattern perception have
assumed some form of broad band channel or channels in
parallel or replacing the narrow band array (Stromeyer and
Klein, 1974; Kulikowski and King-Smith, 1973; Kulikowski,
Abadi and King-Smith, 1973; Spitzberg and Richards, 1975;
Snyder and Srinivasen, 1979; Arend and Lange, 1979).
Wilson and Bergen (1979) and Bergen, Wilson and Cowan
(1979) reduce the number of mechanisms to four but invoke
large nonlinearities to fit the data. Two-dimensional
periodic patterns seem to require either size-tuned
detectors or extensive interchannel interaction (Burton,
1973, 1976; Carlson, Cohen and Gorog, 1977; Tyler, 1978a).
On the other hand, phase channels as well as spatial
frequency channels have been proposed (Bacon, 1976).
Interactions are also demonstrated with binocular stimuli
(Blake and Levinson, 1977; Legge, 1979; Walker and Powell,
1979; Blake and Rush, 1980; Cormack and Blake, 1980) and
dichoptic masking experiments (Abadi, 1976).
Cortical location of these channel interactions, and
specifically inhibition of neighboring columns as
postulated by Abadi (1976), while often assumed, need not
be the only determinant of spatial frequency selectivity,
although there seems to be a progressive tuning to sharper
selectivity by each stage in the visual pathway
(Armington, Gaarder and Schick, 1967; Armington, Corwin
and Marsetta, 1971; Maffei and Fiorentini, 1973;
Armington, 1977). Kelly (1975), by postulating receptive
field coherence, has presented a retinal model. Retinal
field size for channels has been described by Quick and
Reichert (1975) who maintained that the channels had a
constant retinal area independent of center frequency.
This is flatly contradicted by Hoekstra et al. (1974) and
by Howell and Hess (1978), who found a critical response
amplitude dependence on the number of cycles present in
the display to avoid artifactual decreases in recorded
sensitivity, and also in part by a series of papers by
Koenderink et al. (1978) in which target extent in the
visual field had a critical effect on detection. Kelly's
(1976) specific attempt to infer a postretinal location
for Fourier channels showed that for the mechanisms he
tested, the achromatic and opponent-color pathways were
equally affected by pattern adaptation, and so presumably
the effects occurred prior to a neural locus at which
these two pathways specialize, although color
discrimination by spatial frequency channels remains in
debate (Ingling, 1978; King-Smith and Carden, 1978).
Orientation effects are usually assumed to require
cortical processing, however, since it is in the cortical
cells that orientation selectivity of neural elements
seems most strongly expressed, as determined by cat and
monkey studies (Bisti and Maffei, 1974; Boltz, Harwerth
and Smith, 1979; Bauer et al., 1979; Berkeley, Kitterle
and Watkins, 1975; Hubel and Wiesel, 1977; among others),
and also apparent from studies with pattern evoked retinal
responses (Maffei and Campbell, 1970).
Modulating the stimuli in the temporal domain by
flickering or counterphasing the grating leads to changes
in contrast sensitivity (Kelly, 1977; Koenderink and van
Doom, 1979. Such temporal stimuli can also be examined
from a Fourier analysis viewpoint. The interdependence of
temporal and spatial channels can be seen in the effect of
temporal modulation of a grating on the "oblique effect,"
a decrease of response or of sensitivity to stimuli in
non-horizontal or -vertical presentations (May et al.,
1979); the effect is apparent only at low temporal
frequencies (Camisa, Blake and Lema, 1977). Selectively
masking "sustained" channels appears to uncover several
"transient" channels with differing peak sensitivities
(Legge, 1978a). Kelly (1979a) using stabilized-image
stimuli has shown, that without some temporal modulation
such as that provided by the slow drift of a "fixated"
eye, thresholds are elevated by a factor of 20. Using
inverse Fourier transforms, Kelly constructed line spread
functions from the stabilized velocity data that closely
resemble those derived by localized but unstabilized
stimuli and with electrophysiologically determined
receptive field structures (Braddick, Campbell and
Atkinson, 1978; Legge, 1978b; Kelly, 1979b).
With the advent of signal averaging computers,
electrophysiological data from humans could be taken with
high enough signal-to-noise ratios to be generally useful,
both as a research tool (Riggs and Wooten, 1972; MacKay
and Jeffreys, 1973; Riggs, 1974, 1977), and as a clinical
diagnostic technique (Sokol, 1976; Sherman, 1979). Such
data, generally stimulus-evoked potentials, occupy an
intermediate area in stimulus-response analysis, between
cellular or unit recording in neural networks, as in
retina (Dowling, 1970) or cortex (Hubel and Wiesel, 1977),
and whole organism responses, generally explored
psychophysically. Evoked potentials sum the responses of
a population of cells within the electrical range of the
recording electrode, integrating them according to their
strength and distance (Verzeano et al., 1968; Creutzfeld
et al., 1969; Lesevre and Joseph, 1979; Legatt, Arezzo and
Vaughan, Jr., 1980). Sharp depolarizations correlate well
with surface positivity (Creutzfeld and Kuhnt, 1973) and
reflect processes that can be isolated psychophysically
(e.g., cone mechanisms: Gouras and Padmos, 1974; masking:
Samoilovich and Trush, 1979) or electrophysiologically
(e.g., internal/conditioned inhibition: Shul'gina, 1979;
DeToledo-Morrell, Hoeppner and Morrell, 1979; pattern
selective adaptation: Movshon and Lennie, 1979).
Evoked potentials, by their definition, are locked to
a stimulus event, rather than a stimulus condition,
although various conditions [not only experimental (Uren,
Stewart and Crosby, 1979)] can have profound effects on
such potentials, as a vast clinical literature atests
(e.g., Ashworth, Maloney and Townsend, 1978; Ikeda,
Tremain and Sanders, 1978; Kadobayashi et al., 1978;
Noebels, Roth and Kopell, 1978; Rover et al., 1978; Von
Knorring, 1978; Andreassi et al, 1979; Arseni et al.,
1979; Facciolla et al., 1979; Frank and Torres, 1979;
Kataoka et al., 1979; Klassen et al., 1979; Naparstek et
al., 1979; Seppalainen, Raitta and Huuskonen, 1979;
Shagrass et al., 1979; Sokol and Nadler, 1979; Wolfe,
1979; Wright, Craggs and Sergejew, 1979; Lee et al., 1980;
Regan et al., 1980; among others). Visual acuity testing
has used the evoked potential in addition to
psychophysical tests to assess objectively spatial
frequency characteristics of the visual system and to
prescribe corrective refractions (Sokol, 1972; Arden and
Lewis, 1973; Bodis-Wollner and Diamond, 1976; Bostrom,
Keller and Marg, 1978; Hess, Howell and Kitchin, 1978;
Hofmann, Bartl and Gruber, 1978; Enoch, Bedell and
Kaufman, 1979; Enoch, Campos, Greer and Trobe, 1979;
Enoch, Ohzu and Itoi, 1979; Enoch, Yamade and Namba, 1979;
Klingaman and Moskowitz-Cook, 1979; Tyler et al., 1979;
Dobson and Davison, 1980; Julesz, Kropfl and Petrig, 1980;
King-Smith and Kulikowski, 1980).
Evoked potential research to demonstrate spatial
frequency channels has used both masking and adaptation
paradigms, as in similar psychophysical work (Ochs and
Aminoff, 1980). The classic experiment by Campbell and
Maffei (1970) used a grating of a specified spatial
frequency to adapt a channel, and by sampling spatial
frequencies near it with counterphasing gratings, Campbell
and Maffei demonstrated a selective suppression of the
evoked response until the test grating was at least an
octave away from the adapting grating. Similarly,
rotation of a test grating at the same spatial frequency
as the adapting grating showed a 150 specificity for
orientation of a particular channel, paralleling
psychophysical results. Some differences from
psychophysical data have also been demonstrated; recovery
of the depressed evoked potential response is much faster
than the recovery of the contrast detection threshold
(Mecacci and Spinelli, 1976). Other work, however, shows
close correspondences with psychophysics (e.g., contrast
thresholds: Adachi-Usami, 1979; Kojima and Zrenner,
1980). Direct comparison of electrophysiologic and
psychophysiologic measures of apparent contrast shows
close correlations with each other and indicates that the
evoked potential can be an accurate reflection of
perceptual experience (Bodis-Wollner, Hendley and
Kulikowski, 1972; Pranzen and Berkeley, 1975).
Temporal frequency channels, usually conceived as a
dichotomous set of parallel transient and sustained
mechanisms, have been implied by evoked responses to both
slow reversal rate stimuli and by temporal tuning curves
using higher rates of stimulus presentation (Lennie,
1980). Such stimuli are sorted in evoked potential
research into two broad overlapping classes according to
stimulation rate: "steady-state" and "transient"
response recording. Steady-state recording presupposes a
temporally periodic stimulus presented for a sufficiently
long period of time that the effects of the abrupt onset
of stimulation are no longer apparent (Regan, 1977).
Transient evoked potentials, by contrast, deliver one
stimulus and wait until the response to that stimulus has
returned to background activity before presenting the next
(Kinney, 1977). In nonlinear signal processing systems,
the steady-state response is not necessarily equivalent to
the superposition of the transient responses at the same
rate. In this regard, the evoked potentials do not behave
as a linear system. Patterns of signal buildup and decay
with various time courses and differing arousal states
have demonstrated this essential nonlinearity in cortex
(Steinberg, 1965; van der Tweel and Verduyn Lunel, 1965).
The analysis of steady-state activity also gives
information on the visual pathways in the form of phase
lag and permits the modeling of the signal processing
system as a resistor-capacitor (R-C) network or frequency
filter (Bode, 1945). Amplitude (or power) alone can only
provide information on the resistive elements of a system;
phase information is required in addition to permit a full
description of a system's behavior. Additionally,
steady-state recording permits the separation of low
versus high temporal frequency "channels," i.e., the
modeling of the visual "processor" as a temporal Fourier
analyzer (Jones and Keck, 1978; Graham, 1979). Such
separation shows a strong interaction with spatial
frequency. By using transient stimuli, Jones and Keck
(1978) were able to show that there were at least two
mechanisms responding to the stimuli. One had a short
latency component and was strongest at 1 or 2
cycles/degree, saturated at low contrast, and appeared
insensitive to precise retinal location. The major
component was tuned to higher spatial frequencies and
showed a linear response with log contrast. This type of
response was also found by Regan (1978) using an online
analog Fourier analyzer and steady-state recording.
Responses to low spatial frequencies seemed enhanced with
fast stimulus presentation rates, and high spatial
frequencies with low reversal rates. These data support a
model of the visual system with many parallel channels
that are specific to spatial frequency, temporal frequency
and orientation, all with relatively narrow bandwidths,
and permit the description of the system response in terms
of a spatiotemporal response surface defined on purely
electrophysiological criteria (Tyler, Apkarian and
Nakayama., 1978). This surface has been shown to be
homologous to the data derived from psychophysical
contrast detection threshold studies (Barris and Dawson,
1980). Relative spatial phase detection, however, seems
to require either channel interaction, separate broadly
tuned channels, or an additional class of mechanisms
specifically sensitive to phase (e.g., Burr, 1980; Ross
and Johnstone, 1980).
Analysis of cat and monkey cortical neurons has
revealed a possible cellular basis for the Fourier model
of visual processing (Maffei and Fiorentini, 1973). Hubel
and Wiesel's (1977) model of monkey cortex as an ensemble
of bar detectors has been recast as an ensemble of spatial
frequency detectors (Noda et al., 1971; Maffei, 1978;
Movshon, Thompson and Tolhurst, 1978a, 1978b, 1978c;
Kulikowski, 1979). This model seems tc. fit the tuning
curves for the neurons better as the cells show a greater
selectivity when tested by gratings rather than bars
(Albrecht, DeValois and Thorell, 1979), although the
concept has been extensively debated (see for example
Glezer, Ivanoff and Tscherbach, 1973; Tyler, 1975; Glezer,
1975; Glezer et al., 1976; Tyler, 1978b; Glezer, 1979).
DeValois, DeValois and Yund (1979) directly tested the
concept of "Fourier" tuning in ccrtical neurons and found
that both simple and complex cells were sensitive to the
orientation of the optical Fourier transform components of
a pattern, rather than the orientation of the edges. This
sensitivity applied also to the spatial frequency axis, as
the response to the higher harmonics of square edged and
missing fundamental patterns was predicted by the Fourier
components (Maffei et al., 1979a). At lower spatial
frequencies, however, some edge detection does occur
(Maffei et al., 1979b; Kulikowski and Bishop, 1980; Ross
and Johnstone, 1980).
The existence of two functional "simultaneously"
seeing eyes in the normal human, and the assumption of
singular independent information channels immediately
raises the problem of binocular integration. Do the two
eyes constitute separate visual channels (Cormack and
Blake, 1980)? By using the performance degrading effects
of stimulus uncertainty on grating detection, Cormack and
Blake (1980) were able to show that eye-of-origin
uncertainty was unable to degrade detection in
stereo-normal subjects, but did lower performance in
stereo-blind subjects, thus indicating that the two eyes,
at threshold, normally behave as a single channel.
However, especially at suprathreshold levels, the
separateness of information from the two eyes is not lost
at higher levels. Binocular rivalry, or alternating
monocular suppression during dichoptic viewing, compounds
the problem of spatial frequency channels and perceptual
fusion (Julesz, 1978). However, even in nondichoptic
situations, i.e., with perceptual fusion, Makous and
Sanders (1978) were able to show that some monocular
The neural location of this suppression remains
unclear as neither evoked potentials need be reduced, nor
McCollough color aftereffects weakened, by a perceptual
invisibility of the inducing stimuli (Cobb, Morton and
Ettlinger, 1967; White et al., 1978). Binocular
suppression, or facilitation, when measured by evoked
potentials, seems critically dependent on the stimulus
parameters, and has generated apparently contradictory
results. If the images from the two eyes are not
perceptually fused, suppression results; if the images are
disparate, but do fuse, enhancement results; if only a
blank field is presented to one eye, that eye is
apparently ignored (Ciganek, 1971; Harter, 1977).
Individual components of the transient evoked potential
behave differently with differing spatial frequencies and
orientations (Harter, Seiple and Salmon, 1973; Harter,
Seiple and Musso, 1974; Harter, Condor and Towle, 1980).
Active suppression of an artificially blurred image in one
eye has been demonstrated with steady-state evoked
potentials (Fiorentini et al., 1978) and has been
correlated with similar results found in human amblyopia
(Srebro, 1978; Wanger and Nilsson, 1978; Levi, Harwerth
and Smith, 1979). Temporally disjoint but spatially
identical and fused stimuli produce a reduction of
steady-state evoked potentials in stereo-normals but a
smaller reduction in humans with impaired binocular vision
(Lennerstrand, 1978a). Inducing greater binocular rivalry
by changing the color and pattern presented to each eye
resulted in a perceptual suppression/alternation, but the
evoked response did not reflect the rivalry (Lennerstrand,
1978b). In fact, the signal depression was greater with
no spatial disparity, supporting the hypothesis that the
depression due to the temporally disjunct stimulation was
producing interference in a single spatial frequency
channel, as a form of stimulus masking. Unit analysis has
shown neurons "tuned" for interocular disparity (Fischer
and Kruger, 1979; Poggio, 1979) but the variability of the
response is so great that a "variability" code has been
proposed (Crawford and Cool, 1970) since occlusions of
response as well as facilitation have been found (Noda,
Creutzfeld and Freeman, 1971). Binocular response of
striate cells seems dependent on the corpus callosum
(Payne et al., 1980) and would imply a separable evoked
response for each eye. In sum, these data would seem to
indicate that at least some aspects of depressed detection
reside in the neural elements of the primary visual
cortex, even if perceptual rivalry may not (Martin, 1970).
To recapitulate the main findings detailed above:
(a) psychophysical data from detection, discrimination and
masking experiments have been modelled as spatial and/or
temporal frequency channels, responsive to Fourier
components of stimuli; (b) electrophysiological recordings
at the single unit level in area 17 are consistent with
activation by spatio-temporal Fourier components; (c)
stimulus evoked potentials, although recording neural
discharges en masse, can be manipulated meaningfully by
appropriate changes in stimulus Fourier components; and
(d) data from psychophysical, evoked potential and
single-unit recording experiments suggest that
eye-of-origin might be an additional characteristic of the
spatiotemporal frequency channels.
The multiple channel model of visual spatial
frequency analysis (Campbell and Robson, 1968) predicts
independence between channels more than an octave apart.
If this independence holds in the visual cortex, the
response to the complex gratings will be a linear
superposition of the response to the grating's components
presented separately. Any interaction, constructive or
destructive, will be apparent from the evoked waveform (by
correlation analysis) as well as by amplitude and/or phase
changes. Phase independence of sum of sinusoid gratings,
predicted by the same detection experiments, will also be
If the spatial frequency channels are fully
independent at area 17, i.e., independent cortical columns
(Hubel and Wiesel, 1972), then the mode of stimulus
component addition should not change the response, i.e.,
haploscopic (dichoptic) presentation of a complex grating
should equal monocular presentation. If there is
interaction, then these two modes of stimulation will
indicate whether the interaction is cortical or
precortical (if the response to the complex monocular
stimulus is less than that for the dichoptic input
assuming destructive masking).
Lastly, by adding harmonics to a steady-state
temporal stimulus sinusoidall counterphase vs. square wave
at seven shifts/second), this experiment will show whether
temporal frequency channels also interact to change any of
the above results. If there is no difference, the visual
system is using only the fundamental stimulus frequency.
If enhancement or interference occurs, the model of the
visual system as a strict Fourier analyzer in the temporal
domain cannot be maintained.
In conclusion, these experiments will test the degree
of independence of spatial and temporal frequency channels
in steady-state suprathreshold response of the human
visual system. Should interaction between the channels be
found, these experiments should also indicate a possible
site for them (retina/LGN or primary cortex) i.e., does
the eye-of-origin constitute an appropriate characteristic
of a spatial frequency channel. The results link the
threshold detection experiments in psychophysical research
and suprathreshold visual evoked responses.
Three male and two female volunteers, Caucasian,
between 22 and 30 years old, who gave their informed
consent, served as subjects. The females were between
menses and ovulation in their menstrual cycles. All
subjects had corrected 20/20 Snellen acuities in each eye,
perfect Farnsworth D-15 color discrimination tests in each
eye, Titmus stereoacuity of 50 sec of arc or better, and
normal binocular fusion ranges as tested by prismatic
displacement of a 50 target with a phoropter. Monocular
contrast sensitivity curves were obtained for each eye by
inspection of photographs of Arden type sine wave grating
plates (Arden, 1979) and'were normal in shape and
approximately equal between the two eyes for each subject.
All subjects practiced viewing the stimulus display before
recording sessions were started. No difficulty was
reported in aligning the haploscopically presented
display. Subjects wore their normal corrections, and no
artificial pupils were used. All subjects were tested
three times on separate days, producing 15
subject-sessions of 20 stimuli each (see below).
Two Tektronics RM503 oscilloscopes with blue-green
(P2) phosphors were mounted to face each other (Fig. 1).
Two first surface mirrors were mounted between them to
direct the respective images to a set of prisms mounted on
rotating sleeves, permitting adjustment for individual
interpupillary distances. The circular oscilloscope
screens subtended 50 of arc at the eye. Fixation and
fusion were aided by providing a black fixation spot of
22'48" in the center of each screen (two subjects) or
diagonal crosshairs of 00 black silk of 1'53" diameter
(three subjects). No significant differences were noted
in control runs with one subject in response wave form,
amplitude or phase with the different fixation marks in
distinction to some psychophysical results (Weisstein, et
al. 1977). The horizontal deflection plates of CRO#2
(slave) were connected in parallel to those of CRO#1
(master) to insure perfect synchrony of sweep, triggering
and unblanking as well as phase lock between the two
Vertical sine wave gratings were generated on the
oscilloscope faces by modulating the z (luminance) axes
with the appropriate waveform (Campbell and Robson, 1968).
A mean luminance field was established with a 0.55 MHz
signal from an external function generator fed to the
x-inputs of the oscilloscopes. The field was refreshed at
1.56 KHz and provided approximately 140 traces per degree
Schematic illustration of stimulus and recording
arrangement. Al, A2: preamplifiers;
C: computer; F: band-pass filter; M: first-
surface mirrors; 0: observer; 01: master
display oscillosocpe; 02: slave display
oscilloscope; P: prism array; S: shield room;
SG: signal generator; ST: stimulus control
circuitry; T: oscilloscope trigger input;
Tl: ocilloscope trigger signals; T2: computer
trigger signal; X: master oscilloscope
horizontal control; Y: oscilloscope vertical
control; Z: oscilloscope luminance control
input; Zl, Z2: luminance control signals to 01
and 02 respectively.
T1 Z1 Z2
of visual angle. Due to the line spread of the phosphor,
the individual traces were completely unresolvable. The
brightness was adjusted to 5.16 cd/m2 at the eye although
this level is not critical for contrast evoked potentials
(van der Tweel, Estevez and Cavonius, 1979). The AC
signal from the stimulus circuitry (see Appendix)
modulated the luminance of the screens around this value.
The high refresh rate and the stable triggering of the
oscilloscopes prevented variability of the counterphasing
stimulus, such as found in television systems (van Lith,
van Marle and van Dok-Mak, 1978; Meienberg et al., 1979).
Luminance profiles of four shapes were used (Fig. 2): (1)
a fundamental sine wave, (2) its third harmonic, (3) the
sum of 1 and 2 at a phase angle of 00 (peaks subtract),
and (4) the sum of 1 and 2 at a phase angle of 1800 (peaks
add) (Atkinson and Campbell, 1974). A linearity check of
the screens with a photoresistor showed a distortion of
0.58% (power of the fundamental to the first harmonic) of
voltage versus luminance with a 32 V sine wave grating.
All stimulus waveform luminances were calibrated with a
Pritchard photometer at the plane of the subject's eye and
confirmed the display contrast linearity. A linear
regression calculated between peak to peak voltage of the
stimulus waveforms and per cent contrast
(L -L /2L X100%) yielded a correlation coefficient
max min mean
for the line of 0.9985.
Figure 2. Stimulus luminance profile metrics for gratings
( AKx10 0%)
The fundamental spatial frequency was set at two
cycles per degree, producing 10 iterations of the waveform
on the screen (Hoekstra et al, 1974; Howell and Hess,
1978; McCann, Savoy and Hall, 1978). The stimulus
circuitry permits these waveforms to be presented
monocularly (mean luminance field to the fellow eye),
binocularly, and dichoptically (Fl to one eye, F3 to the
other, either at 00 or 1800 phase angle). Finally, the
circuit permits the waveforms to be counterphased in two
modes: sinusoidal (continuous) and square-wave (abrupt).
The fundamental temporal stimulus frequency was set at
seven shifts (contrast reversals) per second and modulated
both the simple and complex grating phase transitions at
the same rate, as suggested by Pantle (1973). These
spatial and temporal frequency values are near the human
contrast sensitivity peak both psychophysically (Kelly,
1977) and electrophysiologically (Bodis-Wollner, Hendley
and Kulikowski, 1972; Regan, 1978). All 20 stimuli (10
sine mode and 10 square mode) were presented in each
session but in different random orders and to different
eyes. The stimuli were:
FlM, 2 cycles/degree, monocular presentation
FlB, 2 cycles/degree, binocular presentation
F3M, 6 cycles/degree, monocular presentation
F3B, 6 cycles/degree, binocular presentation
SOM, 2+6 cycles/degree at 00 relative phase angle,
SOB, the same as SOM but binocular presentation
SOD, 2 cycles/degree presented to one eye, 6
cycles/degree presented to the fellow eye, the
fused gratings at 00 relative phase angle
SIM, the same as SOM but a relative phase angle of
S1B, the same as SIM but presented binocularly
S1D, the same as SOD but with the fused dichoptic
grating components at a phase angle of 1800.
Subjects were seated in a darkened radio-frequency
shielded room and viewed the display through the opened
door. The laboratory was dimmed to reduce distraction.
VERs were recorded from a point on the scalp over the
visual cortex, 2.5 cm above the inion on the mid-saggital
plane with a Beckman Agc-AgCl disk electrode attached with
electroconductive paste and referenced to yoked Ag-AgCl
earclip electrodes; ground was established with another
Ag-AgCl disk electrode on the forehead. Response signals
were preamplified (0.8-50 Hz, -3 dB points) and fed to the
input of a Nicolet MED-80 computer. The signals were also
narrow band filtered (Krohn-hite. band pass filter: 5-9
Hz) and fed simultaneously to the computer. Both raw and
filtered data as well as the stimulus trigger were
recorded during acquisition on FM tape (Tandberg
Instrumentation Recorder) for later off-line analysis.
The bandpass filter was calibrated for gain (0.9) and
phase lag (1060 at 7 Hz) to correct the filtered data
amplitude and latency. Two stimulus shifts were recorded
per averaging epoch; the time occupied by a third shift
was used by the computer for artifact reject, baseline
correction and variance calculations on the just completed
sweep. Thus, both "left" and "right" shifts were averaged
together in the final record. Two hundred sweeps were
taken per stimulus and averaged with a time resolution of
280 lUsec per acquired point and a vertical resolution of
Both filtered and unfiltered averaged signals were
evaluated for amplitude between the most positive point
following a stimulus shift (Pl) and the most negative
point preceding that Pl (Nl). This gave two values per
record for both the filtered and unfiltered signals.
Latencies were calculated from the Pl and referenced to
the zero crossing of the stimulus, thus giving a phase lag
figure in degrees. The absolute phase lag for the visual
system was calculated from the following test run, also
performed at each subject-session. A sinusoidal grating
at F1 was presented binocularly, and by gating the
temporal waveform generator, sinusoidally counterphased
six times, followed by an equivalent quiet period of time
with the grating stationary. Two hundred sweeps were
taken of this 1.4 second epoch. The average phase lag of
the resulting six Pl peaks was taken to represent the
response time of the visual system to a 7 Hz shifts/sec
phase translation. The phase lag figures of the
continuous stimulation runs were referenced to this lag.
DC "noise" runs showed signal to noise ratios of 2-1 or
better in all subjects.
Pearson correlation coefficients were calculated
between the 512 point records of all possible stimulus
pairs, both filtered and unfiltered, and averaged across
all subject-sessions for each stimulus. Under the
assumption of linear superposition, expected response
waveforms, which will be referred to as "synthetic"
responses, were created by digital addition of the
responses to the Fl and F3 stimulation (FSM=F1M+F3M;
FSB=FlB+F3B). These synthetic responses were also
compared to the observed sum of sines responses.
Unfiltered data were reacquired from tape on a long
time base (1.44 sec=10 stimulus shifts per computer sweep)
to avoid low frequency window effects (leakage) in the
subsequent Fourier analysis. The Nyquist sample frequency
cutoff was 358 Hz, which is well above the 50 Hz upper
limit of the recorded data, thus preventing aliasing
(Ramirez, 1974). Power spectra were calculated for each
stimulus condition, and the three bins surrounding the
fundamental (bin width=0.7 Hz) were summed to give the
power of the VER at the stimulus frequency. The second
and third harmonics were similarly quantified.
Numerical amplitude and phase data as well as the
power figures and Pearson r correlation matrix were
evaluated by Repeated Measures Analysis of Variance and
Duncan's Multiple Range Test by the Statistical Analysis
System of the Northeast Regional Data Center.
Figures 3 and 4 show phase runs of two of the five
subjects. Each zero-crossing of the stimulus produces a
response peak that lags by at least 3600 at this stimulus
frequency, depending on the individual. A double response
can clearly he seen in the unfiltered traces, indicating a
strong second harmonic component in the response. The
overall stationarity and reproducibility of the response
can be seen from the grand unweighted non-normalized
digital sum of all 15 subject-session phase runs in the
study (Fig. 5). This method of gated stimuli permits,
therefore, an absolute calibration of the phase lag in the
human visual system to (quasi-) steady state stimuli, in
agreement with Diamond's (1977) rejection of Regan but
without his complex methodology. The values thus obtained
provide the baseline latencies against which the
steady-state responses are measured.
The "doubling" of the response peaks to sinusoidally
as opposed to abruptly (square mode) counterphasing
Phase run of subject A, session #1.
A: unfiltered data; B: filtered data (5-9
Hz); C: counterphase trace of the grating
luminance contrast; D: zero-crossing signal.
The numbers between C and D indicate stimulus
events; the numbers over the peaks in A and B
indicate the respective response positivities.
1 2 3 4 5 6
Figure 4. Phase run of subject B, session #3, Otherwise
as in Fig. 3.
v V V \l
O U U L o
Figure 5. Grand unweighted, non-normalized digital sum of
all 15 subject-session phase runs. Otherwise
as in Fig. 3. Absolute amplitude is arbitrary.
1 2 3 4 5 6
" Ur UL -
stimuli can be seen to occur in monocular, binocular and
dichoptic presentations, whether of simple or complex
gratings (Figs. 6, 7, 8 and 9). The enhancement of the
second harmonic by the sinusoidal mode can be clearly seen
in the power spectra of each response.
Means of the fundamental and second harmonic power
are graphed in Pig. 10; their ratios in Fig. 11.
Statistical analysis showed that for square mode
counterphasing stimuli, the fundamental frequency of the
response is greater than the second harmonic (p=0.0016)
for all stimuli, but that the fundamental and second
harmonic are not significantly different in the sinusoidal
case (p>0.05 by Duncan's). The third harmonic showed
essentially no mode or stimulus differences. The analysis
of the frequency component power ratios confirmed this,
but the large variability of the non-normalized data
allowed only four of the 10 stimuli (FlM, SlB, SOB and
SOM) to show a significant mode difference (p<0.05, SQ>SN)
although all the others also tended in the same direction.
The response power of all stimuli showed overlapping
ranges by Duncan's test in the sine mode, but SOM proved a
significantly higher fundamental to second harmonic power
ratio in the square mode presentation than all other
stimuli (see below).
Analysis of the monocular vs binocular subsets of the
power data for "simple" gratings (FlM, FlB, F3M, F3B)
showed no significant differences in the fundamental,
Responses to a monocular presentation of a
complex grating with the components at a phase
angle of 00o. A: unfiltered data; B: filtered
data; C: stimulus traces (sine or square mode
counterphase) as well as time and amplitude
calibration on square wave; D: power spectra
of responses. Note a factor of two scale shift
of the sine mode versus square mode power
7 14 21
7 14 21
143.92 Ms 2.44 uV
Figure 7. Responses to a binocular presentation of a
complex grating with the components at a phase
angle of 00. Otherwise as in Fig. 6,
7 14 21
7 14 21
Figure 8. Responses to a binocular presentation of a
simple (Fl: 2 cycles/degree) grating.
Otherwise as in Fig. 6.
7 14 21
MS 12.44 uV
7 14 21
Figure 9. Responses to a dichoptically presented gratings
(F1+F3) with the components at a phase angle of
1800. Otherwise as in Fig. 6.
7 14 21
7 14 21
second harmonic or their ratio in either sine or square
modes. For the complex stimuli (SOM, SOB, SOD, SlM, SlB,
SlD), the fundamentals were not significantly different
(p>0.05) but the second harmonic showed a significant
difference due to condition (p=0.0074); i.e., binocular
and dichoptic values are both significantly above
monocular values. The frequency component ratio showed
complex interactional effects (phase by condition by mode:
p=0.0002); however, the pattern of significance in the
breakdown showed merely that monocular ratios tended to be
greater than binocular and dichoptic ratios, and that
square mode ratios tended to be greater than sine mode
ratios, echoing the earlier analysis. Also, a significant
difference due to component phase appeared in the square
mode monocular power ratio (p=0.0049) indicating that
complex stimuli with a phase angle of 00 between the first
and third harmonic components (SOM) show a higher temporal
response component power ratio (fundamental/second
harmonic) than with a phase angle of 1800 (SIM), i.e., an
enhancement of the fundamental compared to the second
harmonic when the components are in a square wave phase
relationship; but as neither component alone is
significantly affected, the significance of this result
0 0 C
*5 Ce s~I,-~r~;.ri.3C**.eegS
a.... &aS S B SL aJ
.**; : ..~i^ ^^ ^ 0
--- -- ..e.e .. L .J-.e.----------..
- CU NM
UL IL Uh. UL
- p z<:;- T;; >::T.II'r.:.:'.'.'.j
(SIlVM gljPI) 3UMOd
Figure 11. Bar graph of the component power ratio
(fundamental/second harmonic) by mode and
stimulus. Means and standard error of the
F3B SOM SOB S
The mean unfiltered and filtered response amplitude
values are graphed in Figs. 12 A and B for all stimuli as
well as for the synthetic responses. Immediately apparent
is that the synthetic binocular sum (FSB) is significantly
higher than all other stimuli in both sine and square
modes, unfiltered, and in the filtered square mode (all
p's=0.0001); the filtered sine mode values showed no
significant ranking of stimuli (p=0.5142). Actual
responses to complex (sum of sinusoid) gratings tended to
cluster in the center of the ranking and indicate that the
response to input on "separate" spatial frequency channels
is in general not a linear addition of the responses to
the components presented separately. Relative stimulus
component phase angle had no significant effect on the
With one exception .(F3M), square and sine mode
unfiltered amplitudes do not differ significantly; square
mode responses tend to be larger than sine mode responses
in the filtered case but not strongly so (p<0.05 in F3B,
F3M and SlM; other stimuli rank square>sine but have
p<0.05). Also as expected, unfiltered data tend to be
larger than filtered data for all modes and stimuli
Rearranging the amplitude data for simple gratings
into subsets, the responses to Fl and F3 grating stimuli
(2 and 6 cycles/degree) do not differ significantly;
.( ** Cd
04 4 0
p r-I d
H 0 00
(d m 4
' 4 4J
ra to co
* 0 IfjiB l u
* .B 4mB8BBsl~B q u
ID U) nj-- v)
A35gB 5MSMA .5A 5
5.v 55.ma 55
S1 OAM 3Q0flaIAdHV 8
jOCa3iOg 0 E
*5 s 55
I I -~
however, they are both less than the synthetic responses
(FSM and FSB: p=0.0001 for monocular, p=0.0003 for
binocular conditions). For all stimulus frequencies (Fl,
F3 and FS) and both modes (sine and square), binocular
responses were higher than monocular but not significantly
so (0.18>p>0.05). With complex stimuli (SO and Sl), the
sine mode unfiltered responses did show a significant
ranking (p=0.0148: binocular>dichoptic>monocular, with
overlapping Duncan's ranges), which was weakly confirmed
with the square mode responses (p=0.0709). This indicates
that the monocular response is not depressed by "masking"
in the fellow eye when the "mask" is of a (square wave)
harmonically related frequency but rather the response is
enhanced to fall between monocular and binocular levels.
Figures 12 C and D show the phase values of the
filtered and unfiltered responses respectively. In
general, the square mode phase is unaffected by filtering
(p=0.2220), whereas the sine mode is, although the
difference reaches significant levels for only two stimuli
(FlM and SiB, p<0.05: unfiltered>filtered); the other
stimuli follow the same pattern. Also, in general, the
trend is for unfiltered sine mode responses to have
larger,- i.e., longer phase lags than square mode
(p=0.0772) whereas the opposite holds true for filtered
responses (mode by stimulus: p=0.0228; SlD and SlM:
p<0.05, square>sine; other stimuli nonsignificant but in
the same pattern). The differences are due to the
separate phase of the second harmonic in the sine case.
The stimuli did not rank in any significant order as the
Duncan's ranges overlapped extensively. Grouping the data
by condition (monocular, binocular and dichoptic) also
revealed no significant relationships.
Correlation Analysi s
Pearson correlation coefficients were calculated
between all possible response pairs for each subject
session, both filtered and unfiltered, to define a data
base for comparing individual mean correlations to the
general mean. A selection of mean correlations across
subject-session comparing the synthetic sum of spatial
frequency responses to the actual responses is given in
Table 1. Of the entire matrix of 276 mean correlations of
unfiltered responses, only the correlation of FSB-SQ vs
SlD-SO (unfiltered) fell more than one standard deviation
above the mean (neglecting the trivial correlations of
*e,.g., FlM with FSM, as FlM+F3M=FSM). None of the 276
filtered data correlations fell above one standard
deviation over the mean. The analysis of variance on the
matrices indicated that the ranking of correlations was
significant (p=0.0001 for both filtered and unfiltered
cases), but all the Duncan's ranges overlapped
extensively. However, even in this one case, only 61% of
Selected Pearson Correlation Coefficients
comparing Synthetic sum of component grating
responses (FSM and FSB) to actual responses
(SOM, SOBSOD, SIM SLB, SID); U: unfiltered
F: filtered data; SN: sine mode; : square
mode. sne ode; SQ: square
SN SQ SN FSB SQ
.5102 .1620 .5032 .1969 SN SM
.5853 .2273 .4608 .2633 F
.4827 .7231 .4324 .7833 U SQ
.4928 .9191 .3951 .9336 F
Mean S.D. ean D Mean S
+ -S.D* Mean S.D ---
.7429 ~ -.1756
the variance in the observed dichoptic sum of spatial
frequencies response is accounted for by the individual
responses to the single spatial frequencies. This
indicates a definite nonlinearity, 'i.e., interaction, at
suprathreshold levels, between these spatial frequency
In short, therefore, the power spectral analysis has
shown a significant enhancement of the second temporal
harmonic component of the evoked waveform to the
sinusoidal counterphase mode of stimulation. The
amplitude analysis has refuted the concept of a linear
superposition of spatial frequency channels greater than
one octave apart. The phase analysis shows complex
effects that are not easily relatable to either
stimulation mode or spatial stimulus waveform. Lastly,
the correlation analysis has shown a partial prediction of
the response waveforms to complex grating stimulation by
the summed responses to their simpler components, that
does not depend on eye-of-origin information.
Sinusoidally counterphased gratings physically
present stimulus power at only one temporal frequency.
Abrupt counterphasing presents energy at all the odd
harmonics of the fundamental as well. Thus, square wave
counterphased gratings, having only one stimulus event per
shift, will necessarily show power in the fundamental
frequency in the response waveform, and may show power in
the odd harmonics as well. Regrettably, whether the third
harmonic of the stimulus is also generating a response
waveform, the non-normalized data are too variable to
show. The unexpected finding of peak splitting of the
response waveform to sinusoidal stimulation, however,
indicates that the visual system is not responding in a
linear fashion in the temporal domain. An explanation can
be proposed based on the contrast sensitivity curve,
namely that the grating is not being perceived as
counterphasing, but rather that two separate gratings are
alternately appearing and vanishing, with each onset into
visibility and offset into the period of perceived
"non-grating" mean luminance acting as a stimulus event
and generating its own peak (Spekreijse, van der Tweel and
Zuidema, 1973). This would explain the appearance of a
"second harmonic" to the presumed stimulation frequency.
An alternative explanation could be proposed from the
observation that a sinusoidally counterphasing grating can
be decomposed into two gratings drifting across one
another (Sekuler, Pantle and Levinson, 1978). This would
require the hypothesis that the mechanism responding to
e.g., the rightward drifting grating respond with a
different latency to that responding to the leftward
drifting grating; however, such differential directional
responses have not been noted in the evoked potential
literature, unlike spatial frequency differences which do
correlate with latency changes (Vassilev and Strashimirov,
Channel independence in the spatial frequency domain
assumes independently responding mechanisms, and thus,
presumably, independent evoked responses. Psychophysical
masking experiments have postulated a destructive
interaction between channels that suppresses the response
below monocular levels in a dichoptic paradigm; however,
the data reported here support neither of these two
positions. If the two and six cycles/degree channels were
independent, the digital sum of the monocular responses
would be of the same amplitude and phase as well as highly
correlated with the observed dichoptic sum of gratings
response. The dichoptic response not only is lower than
the synthetic response whether monocular or binocular,
indicating some occlusion, but the correlation
coefficients are also low (see Table 1), for example,
FSM/SOD, square mode, unfiltered, shows only 43% of the
variance accounted for. The filtered cases, of course,
have higher values, but even with the data reduced to the
response fundamental, only 79% of the variance is
accounted for. Neither of these values falls more than
one standard deviation above the mean correlation of the
entire data matrix, and therefore is of doubtful
significance. But it is also clear that a suppression of
the evoked potential by the dichoptic condition did not
occur either. Separate channels are still, however, a
valid model of some of the data, since the relative phase
angle of the stimulus components does not affect the
response fundamental or amplitude, despite a 10%
difference in overall contrast. What remains, therefore,
is that in the steady-state, suprathreshold case, gratings
that are square wave related,' i.e., in a 1:3 harmonic
relationship, and phase locked, do interact and enhance
the evoked potential approximately 15% above the monoptic
case but not to the level of the pure binocular
stimulation response (approximately 24%).
Comparing the responses to two-spatial-frequency-
channel-stimulation by condition of stimulus (monoptic vs.
dichoptic) leads to evidence suggesting the site of
interaction of these channels. The data for simple
gratings showed a small, if non-significant, binocular
versus monocular enhancement (approximately 17%),
presumably explained by the fact that the monocular case
in this study was quasi-binocular. There was always the
same total luminance field presented to each eye at all
times, rather than a dark field to one eye as in classical
binocular facilitation studies which demonstrate 1.4 fold
enhancement. Thus, one can assume that at least some
aspects of the binocular mechanisms were also activated in
the "monocular" case, i.e.,* the various conditions of
stimulus presentation did not radically alter the
responding population of cortical neurons giving rise to
the evoked signal. If the interaction between the two and
six cycle/degree channels is cortical, the cortical
population of neurons can "see" a difference between the
monoptic and dichoptic sum of gratings only with a
monocular suppression (or by integrating the information
from purely monocular cells), not otherwise (Braccini,
Gambardella and Suetta, 1980). This implies that SOM=SOD
and S1M=S1D in the fused case. On the other hand, if the
interaction is precortical, i.e., before integration of
information of the two eyes, not only would FSM=SOD, but
SOMySOD. Since the Duncan's ranges showed a monocular and
dichoptic overlap in agreement with Sturr and Teller's
(1973) dichoptic disk/anulus sensitization experiments (as
well as a binocular-dichoptic overlap), none of the
conditions for a precortical locus are satisfied, whereas
those for a cortical one are. Additionally, eye-of-origin
information does not appear to be an important
characteristic of spatial frequency channels when examined
with an equal-luminance paradigm.
CIRCUIT DESCRIPTION AND PARTS LIST
The circuit diagrammed in Fig. 13 consists of four
1. a fundamental plus third harmonic spatial
2. a logic switch pair to abruptly replace this
waveform with its inverse
3. a temporal waveform synchronizer, and
4. a programmable variable gain amplifier for
The first section consists of FG1, IC1 and IC2 with
their associated discrete components. The F3 frequency is
set by C4, and FG1 is tuned to 1/3 this value. The SYNC
pulse from FG1 locks the peaks of the two waveforms
together. Phase selectivity (00 or 1800) is determined by
which SYNC pulse (peak or valley) is seen by IC1.
Switches Sl and S2 determine whether a composite waveform
(Fl+F3=Z) is seen by the remainder of the circuit, or only
the simple waveforms.
The second section (ICs 3-7) creates the inverted
waveforms, and in response to IC8's output, switches
between them and the noninverted signals. Final gain
adjustment for the square wave mode counterphase output is
accomplished by ICs 6 and 7.
LI -- 2 r
a a .
-m --- H -ID'ce *-A -
0 cc... a
^^g. 1I .i ..
> t I ,
Q ra Q
uj__ h CC r^
I, r^ n- 1 r+
U, C C ,
I"~ ~ ~ t -- J L? a ^
c3 oT'1^'ti 1'?A
2_Ja -- ?o
The third section has two parts: a zero-crossing
detector (IC8) and an ideal rectifier (IC9). The input
waveform comes from FG2 and determines the counterphase
rate as well as the programming waveform for the last
stage. FG2 also produces a square wave, synched to its
sine wave output, to generate a computer trigger pulse
through Tl, CR1 and CR2.
The final stage (ICs 10-13) takes the rectified
temporal signal to control the gain of the square wave
mode counterphasing signal of section two and sinusoidally
varies the amplitude. As the TTL switches are
counterphasing at the zero-crossing of the temporal
waveform from FG2, the output will change phase by 1800 at
that point at which the instantaneous output voltage from
ICs 10 and 11 are zero. This produces the sine-mode
Square- and sine-mode outputs are selected by S3 and
fed to the Z-axes of the two oscilloscopes comprising the
21 1N761A 4.9 V zeener diode
Z2 1N754Z 6.8 V zeener diode
CR1 SA 1181/631-1 Diode
CR2 SA 1181/631-1 Diode
CR3 1N4067 Diode
CR4 1N4067 Diode
CR5 1N4146 Diode
CR6 1N4148 Diode
External Unit s
WAVETEC model 110
EXACT model 505
(+15 volts/-15 volts)
TTL DPDT switch
TTL DPDT switch
Resistors (Values in Ohms)
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The author was born on October 3, 1950 in Stuttgart,
West Germany to Gerhardt Heinz and Evelyn Martha (Poenitz)
Schmeisser. His parents now live in San Mateo,
California, and are naturalized U.S. citizens.
After emmigrating first to Canada in 1951 and then to
California in 1958, the author attended Menlo School in
Menlo Park, California, graduating from the high school in
1968. Thereafter, he attended the University of
California at San Diego, receiving the Bachelor of Arts
degree in 1972 with a major in biology. In 1974, he
received the Master of Science degree from the University
of Massachusetts at Amherst from the Department of
Zoology. After working for two.years at the Stanford
University Medical Center as a research technician and,
later, research assistant, he entered the University of
Florida in 1976 for further graduate training in the
Department of Physiology.
The author married the former Jean Marie Wollenberg
in September 1980.
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