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THESIS

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ABSTRACT

FOVEAL CRITICAL FLICKER FREQUENCY AS A FUNCTION OF
SYNCHKONOUS STIMULATION IN THE PERIPHERY
OF THE CONTRALATERAL EYE

By Terry Lee Hickey

A number of studies have indicated that, under certain
conditions, visual thresholds of one eye are affected by
stimulation of the other eye. Of most relevance to the
present investigation were those studies dealing with
critical flicker frequency. The majority of these studies
report a depressive effect when the stimulation in the two
eyes is out-of-phase and a summative effect when the two
sources of stimulation are in-phase.

The purpose of the present study was to systematically
investigate the effects of in-phase stimulation of various
points along the periphery of one eye on the CFF thresholds
measured at the fovea of the other eye.

The method of serial exploration was used to present
two synchronous, intermittent stimulus targets to three
subjects. Each subject made CFF determinations at the
fovea of the left eye for each of the following points of

stimulation: fovea-left eye only; fovea-left eye, fovea-

Terry Lee Hickey

right eye; fovea-left eye, 10 degrees-right eye; fovea-
left eye, 20 degrees-right eye; fovea-left eye, 30 degrees-
right eye; fovea-left eye, 40 degrees-right eye; fovea-
left eye, 50 degrees-right eye; fovea-left eye, 60 degrees-
right eye; and fovea-left eye, 70 degrees-right eye.

The results of the present study were in agreement
with earlier investigations. The greatest summative
effect was found when both eyes were stimulated foveally;
the greatest depressive effect, at 10 degrees. Beyond 10
degrees, the slight summative effect found was attributed
to differences in the latencies of the rods and cones.

The results of the present study were interpreted as
a further indication of the effects of timing of inputs on
the visual system and, thus, afforded further evidence that

alternation of response can occur in the visual system.

Approved: gain—.43 “@1—
Date: gays. .35?

FOVEAL CRITICAL FLICKER FREQUENCY AS A FUNCTION OF
SYNCHRONOUS STIMULATION IN THE PERIPHERY
OF THE CONTRALATERAL EYE

By
Terry Lee Hickey

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF ARTS

Department of Psychology

1968

$03 5*:
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ACKNOWLEDGEMENTS

I wish to thank Dr. 5. Howard Bartley whose continuous
support and assistance in this endeavor have been invalu-
able. .

I wish to also thank Dr. Charles Hanley and Dr. Mark
Rilling for reading and criticizing the manuscript.

1 wish to thank Mrs. Le Ann Slicer for her invaluable
assistance in typing and preparing the manuscript.

1 am especially grateful to my wife for her encourage-
ment and support and, most of all, for her willingness to
give up so much that I might have the Opportunity to com-

plete this endeavor.

ii

II

III
IV

VI

Introduction .
Method . . . .
Subjects
Apparatus
Procedure
Results . . .
Discussion . .

Summary . . .

Bibliography .

TABLE

OF CONTENTS

iii

13
16
24
29
30

LIST OF FIGURES

FIGURE Page
1. Layout of eXperimental apparatus employed . . . . 10

2. Diagrammatic drawing of stimulus presentation
apparatus 0 O O C O O O O O O O O O O O O O O O O 12

3. Critical flicker frequency determinations for
individual subjects and determinations averaged
over subjects . . . . . . . . . . . . . . . . . . l7

4. Learning or practice effect over days averaged
overBUbjGCtSeeeeeeeeeeeIeeeeeee20

5. CFF determinations for subject I. . . . . . . . . 21
6. OFF determinations for subject 2. . . . . . . . . 22

7. CFF determinations for subject 3. . . . . . . . . 23

iv

LIST OF TABLES

Table Page
1 Summary of Analysis of Variance . . . . . . 18

INTRODUCTION

Since the early work of such people as Sherrington
(1904), Abney and Watson (1916), Piper (1904), and
McDougall (1901), investigators have been concerned with
the possibility that the central convergence of the path-
ways from the two retinae may exert some influence on
binocular'perception.

"Since homonymous halves of both retinae

are connected with the same half of the

cerebrum, it is usually assumed that cor-

responding points upon the two retinae out-

side of the fovea are represented in the

visual projection areas of theocortex by

single patterns. It is therefore generally

accepted that the two retinae function inte-

gratively, i.e., that the singleness of

vision with binocular observation is made

possible by the unity of their central con-

nections.” (DeSilva and Bartley, 1930; 241).
While comparing the brightness of an object seen both
monocularly and binocularly, DeSilva and Bartley (1930)
demonstrated that in order for the object seen monocularly
to appear just as brilliant as the object seen binocularly,
it must be 1.27 to 1.44 times as bright. Fry and Bartley
(1933) presented further evidence that binocular vision
results in an increase in brilliance over monocular vision.
They accounted for the fact on the hypothesis "that path-

ways from each pair of corresponding points in the two

2
retinae converge upon a common pathway in the brain and
that summation takes place." (Fry and Bartley, 1933; 693).
Several other investigators (Valerius; Piper; Aubert; and
McDougall) have proposed and shown a facilitative, or
summative, effect upon brightness resulting from the unity
of the central projections.

A summative, and inhibitive, effect has also been
demonstrated with temporally manipulated stimuli. It has
been shown that synchronous flashes delivered to correspond-
ing points on the two retinae yield a higher critical
flicker frequency (OFF) than is obtained with monocular
regard or with binocular regard and alternate flashes.
Sherrington (1904) studying binocular summation by the
flicker method, concluded that: '

"As far as sensual (sic) effect goes, the
light phases at the one eye practically do
not, therefore, interfere or combine at all
with the coincident dark phases at the other;
and conversely. Nor do they, in the alter-
nate left and right arrangement, add them-
selves as a series of additional stimuli to
the like series of stimuli applied at the
other eye." (Sherrington, 1904; 37).
The mechanical device employed by Sherrington lacked pre-
cision and Ireland (1950) partially replicated it utilizing

more modern electronic equipment. Ireland tested twenty-

four subjects under four conditions of stimulation:

3
(a) monocular flicker, dominant eye; (b) monocular flicker,
non-dominant eye; (c) binocular flicker, out-of-phase;
and (d) binocular flicker, in-phase. The results clearly
indicated the CFF for binocular in-phase stimulation was
reliably higher than the monocular. In view of the dif-
ferences between monocular and binocular stimulation,
Ireland postulated some interaction between the two eyes
which appeared possible only by way of some central mech-
anism of the brain.

Thomas (1954) was interested in determining the effect
of interocular differences in intensity on CFF values re-
ported with binocular regard. His measures were made
with both in- and out-of-phase flashes in the two eyes
with the stimuli located centrally and 10 degrees peripher-
ally from the fovea. The results indicated a general
subtractive effect of a less intense stimulus in one eye
on the CFF of a brighter stimulus on the corresponding
area of the other eye. He found the subtractive effect
to be even greater when both light sources were placed
10 degrees peripherally from the fovea.

Thomas (1955) reported comparing CFF measured under
three conditions: uniocular regard; binocular regard,

flashes in-phase; and binocular regard, flashes out-of-phase.

4

He postulated that if the impulses arriving from each eye
were additive, then the synchronous stimulation should
yield a visual effect identical to doubling the flash
luminance of‘a stimulus projected on a given region of
one retina. His results showed that over the range of
luminance studied, binocular CFF with synchronous flashes
was significantly higher than the binocular CFF with out-
of-phase flashes or the uniocular CFF. The difference
was small "being only about one half to one third as much
as would result from doubling the flash luminance of a
stimulus viewed with one eye." (Thomas, 1955; 52-53).
The magnitude of summation was independent of flash lumin-
ance and flash frequency.

In the first of a series of studies dealing with
binocular CFF, Baker (1952) replicated Sherrington's (1904)
study. The ranges between the synchronous and alternate
flicker rates found in the Baker and Sherrington studies
differed in magnitude with Baker's evidence definitely
indicating the presence of some alliance and antagonism
between flicker processes initiated at corresponding retinal
points. Baker's results led him to the following conclu-
sion:

"Binocular fusion involves, in part at least,
some central process which combines and inte-

5
grates the neural processes arising from
stimulation of corresponding retinal areas,
so that the resultant sensation differs from

that arising from either eye alone." (Baker,
1952; 10).

In a sequel to the first paper, Baker (1952a) described

six additional eXperiments concerned with monocular and
binocular CFF. The concensus of the six studies briefly
described by Baker was that binocular fusion involved

some central process which combined and integrated the
neural processes arising from stimulation of correspond-
ing, or non-corresponding, retinal areas of similar size.
The resultant binocular percept is different from that
arising from stimulation of either eye alone. Baker (1952b)
demonstrated a central connection by utilizing "progressive“
and "instantaneous" stimulus occlusion. In the first of

the two eXperiments reported, Baker demonstrated that

higher CFF values could be obtained using an "instantaneous,"
rather than a "progressive," source; "...the receptor and
Optic nerve discharge frequencies for the 'instantaneous'
and 'progressive' methods of stimulus occlusion are dis-
similar at similar stimulus intermittence rates." (Baker,
1952; 126). In the second experiment, Baker used an
"instantaneously" occluded source of stimulation for one

eye and a "progressively" occluded source of stimulation

6

for the other. His hypothesis was that the OFF value.
obtained under this condition should fall somewhere
between that value obtained with ”instantaneous" or "pro-
gressive" occlusion alone. The eXperimental results con-
firmed the hypothesis that central factors are active in
"binocularly fusing neural impulses initiated by stimula-
tion of the respective foveas." (Baker, 1952; 128).

Perrin (1954) found that for a 2 degree visual field
and dark surround, as the field luminance was varied the
increase in binocular in-phase CFF was preportional to
the mean critical frequency for the two eyes. Perrin was
also concerned with the effect of field size on binocular
summation. As would be expected, he found that increasing
the stimulated area - at constant luminance - increased
the critical frequency. He also noted that as the area
of the stimulus target increased from 0 to 12 degrees,
binocular summation increased. "Specifically, the summa-
tion for corresponding areas was about 8 percent, while
for noncorresponding areas it was of the order of 0.4
percent." (Perrin, 1954; 69).

A study by Wolf and Zigler (1958) determined monocular
and binocular thresholds for various points along the

peripheral retina. Their measures extended from 30 degrees

METHOD

Subjects

Three students (1 male and 2 females), including the
author, served as subjects. All subjects had considerable

eXperience in making flicker discriminations.

Apparatus

The apparatus shown in Figure 1 consisted of a metal
rod curved into a half circle with a radius of 16 inches;
a septum which divided the left and right visual fields;
and a chin rest placed just to the left of the center of
the arc. This placement of the chin rest positioned the
entrance pupil of the right eye exactly at the center of
the arc.

Placed on the arc were two clamps each supporting a
rod which held one stimulus target. Each stimulus target
was a circular aperture 1/2 inch in diameter which pro-
jected a visual angle of 1° 47' at 16 inches. A piece of
Opal glass served as the target surface. The two stimulus
sources were equated for brightness by means of a Macbeth
photometer. The luminance was 1.5311 candles/ftz. The

same level of luminance was maintained throughout the

eXperiment. A small red "seed" lamp, positioned in the

9

7
to the right of fixation on a horizontal meridian and on a
parallel line 1.5 degrees below the meridian. They found
that in the binocular curves the thresholds were high at
the center, dropped to low levels between 5 and 20 degrees,
and gradually rose farther in the periphery. Thresholds
were maximal at approximately 17.5 degrees. The monocular
curves were asymmetrical due to the blind spots in the
left and right fields. They also found that the binocular
thresholds were slightly lower than the monocular thresholds,
indicating slight binocular summation.. The Wolf and Zigler
study was not, however, concerned with either CFF or inter-
mittent stimulation.

With the exception of Thomas (1954), Perrin (1954),
and Wolf and Zigler (1958), the studies previously cited
were concerned only with foveal stimulation. After a more
than cursory search of the literature, the present author
has concluded that very little, if any, work has been
done investigating binocular summation by systematically

stimulating points along the peripheral retina.

PURPOSE OF THE STUDY
It is, therefore, the purpose of the present study to
partially duplicate and extend the work of previous inves-

tigators dealing with binocular summation. Using inter-

8
mittent light to stimulate the fovea of one eye and various
points toward the periphery of the other eye it should
be possible to determine the effect of peripheral in-phase

stimulation on foveal CFF with binocular regard.

10

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

4.

 

Fig. 1. Layout of experimental apparatus
employed.

ll
center Of the left stimulus target, served as a fixation
point.

The stimulus target for the left eye remained station-
ary throughout the investigation while the stimulus target
Ifor the right eye was moved along the rod. The right
hand portion of the rod was calibrated in units of 10 de-
grees. 1

Figure 2 represents a diagrammatic drawing of the
stimulus presentation apparatus employed. The intermit-
tent stimulus lights were produced by two Sylvania Rll3lC
glow-modulator tubes activated by square-wave inputs. Two
separate Model 8-4 variable-frequency squaregwave stimu-
lators made by Grass Instrument Company furnished the
square-wave oscillations for the two glowamodulator tubes.
The rate of intermittency of both glow-modulator tubes was
always equal and in-phase. The Operation of the glow-
modulator tubes was completely silent at all times. Adjust-
ment of the frequency of both square-wave stimulators was
always made by the eXperimenter. Flicker frequency was
monitored by a Beckman digital counter.

The input channels were controlled by a series of
Hunter Model lOO-C decade relay timers. The timers acti-

vated both glow-modulator tubes simultaneously for three

 

Fig.

12

 

 

 

 

 

 

 

2.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[1'3
0—0
H

Diagrammatic drawing of stimulus presentation
apparatus.

buzzer

six volt transformer
timers

glow modulator

glow modulator
driver

driver

Grass 3-4

Grass 8-4

HID'TJFZUOtrIP

13

seconds, after which both tubes extinguished for seven
seconds. This presentation procedure was continued
throughout the experiment. The timers also controlled a
buzzer which was activated 1 second prior to the activation
of the two glowbmodulators. This warning buzzer signaled
the subject that he should fixate on the red ”seed" lamp

and prepare for the succeeding stimulus presentation.

Procedure

Although all subjects had previous experience in
making flicker discriminations, all subjects were given
several trials prior to actual experhmentation in order
to stabilize their criterion for CFF. During these trials
only the left eye was stimulated. The data from these
trials were not included in the final analysis.

Prior to each eXperimental session, subjects were
dark.adapted for One-half hour. Following dark adaptation
each subject was instructed to place his head as close
to the apparatus as possible with his chin in the chin rest
and to fixate on the red light at the sound Of the tone.
The red "seed" lamp, although not as bright for the right
eye due to the polaroid paper used to block out the left
stimulus light, could be seen by both eyes. Following the

‘warning signal both stimulus lights would come on for three

14

seconds. After the stimulus lights were extinguished,
the subjects were required to report their judgments as to
whether the stimulus target had been steady or flickering.
This judgment was reported verbally.
During the seven seconds the stimulus lights were
off, the eXperimenter recorded the subject's response and
changed the rate setting for the next trial. Between series
of trials the subject was given a brief rest period while
the eXperimenter changed the stimulus setting along the rod.
This procedure was continued throughout the investigation.
The method of serial exploration, with five ascending
and five descending trials, was used for the presentation
of the various rates Of intermittencies at each stimulation
point. The order of presentation Of the various stimulation
points was randomly determined during each eXperimental
session. The following conditions were used for subjects:
(1) fovea-left eye only; (2) fovea-left eye, fovea-right
eye; (3) fovea-left eye, 10 degrees-right eye; (4) fovea-
left eye, 20 degrees-right eye; (5) fovea-left eye, 30
degrees-right eye; (6) fovea-left eye, 40 degrees-right
eye; (7) fovea-left eye, 50 degrees-right eye; (8) fovea- ‘
left eye, 60 degrees-right eye; and (9) fovea-left eye,

70 degrees-right eye.

15
Each experimental session lasted approximately two
hours including the one-half dark adaptation period. A
total of five sessions on consecutive days was conducted

for each_subject.

RESULTS

The flicker contours plotted in cycles per second are
shown in Fig. 3. Open squares represent thresholds for
subject 1; solid squares, subject 2; Open circles, subject
3. Each point for the curves is the mean of 50 CFF deter-
minations. The points denoted by solid circles represent
thresholds averaged over the three subjects and represent
150 CFF determinations each. It is apparent that CFF
thresholds measured with foveal stimulation Of both eyes
are consistently a little higher than the thresholds deter-
mined with either peripheral stimulation Of right eye or
with stimulation of the left eye only: There is also evi-
dent a general depressive effect for all subjects when the
right eye is stimulated between 10 degrees and 20 degrees
in the periphery. Beyond 20 degrees the CFF thresholds
remain fairly constant at approximately the threshold level
for the left,eye alone. In the curves of two subjects
there is a drOp in the thresholds beyond 60 degrees; however,
this tendency is reversed in the curve of the third subject.

The results of a five-way analysis of variance com-
puted are presented in Table I. All of the main effects
‘with the exception of "trials" were significant at the .01
level. One three-way interaction, days 5 trials 5 pre-

16

CFF

40
39
38
37
36
35
34
33
32
31
30
29

28

17

Subject 1:

 

 

a

Subject 2: I

Subject 3: °

‘ Means : e
n
j
a
.4

r I I I I I I I I
BOTH LEFT 10 20 3O 4O 50 60 70

EYES EYES DEGREES

Fig. 3. Critical flicker frequency determin-
ations for individual subjects and
determinations averaged over subjects.

Table I.

14.

15.

Source
Subjects
Point
Day
Trial
Presentation
Point X Day
Point X Trial
Point X Presentation
Day X Trial
Day X Presentation
Trial X Presentation
Point X Day X Trial

Point X Day X Pre-
sentation

Day X Trial X Pre-
sentation

Point X Day X Trial
X Presentation
* Significant at .01

** Significant at .05

18

SS
9824.4
309.9
1196.2
3.2
109.9
198.3
8.7
1.9
3.0
13.3
3.7
36.7

7.7

4.7

17.3

Level

Level

Summary Of Analysis of Variance

'0
.p a~ co to m

32
32

16

128

32

16

128

M§_
4912.2
38.7
299.0
.8
109.9

§
354.9*
4.6*
9.3*
.8
422.6*

l9
sentations, was significant at the .05 level. A test of
individual comparisons was not made due to the small num-
ber of subjects.

Figure 4 shows the "learning" or "practice".effect
over days. Each point is the mean of 270 CFF determinations.
Note the curve becomes asymptotic at about day four. The
significant "days" main effect was attributed to the rise
in CFF thresholds over days. This shift in the CFF curve
agrees with data presented by Ireland (1950).

Figures 5, 6, and 7 show CFF curves for subjects 1, 2,
and 3, respectively. Open circles represent an average of
the CFF determinations during the first two days of the
experiment; solid circles represent averages for the last
two days. Note that although the curves for the last two
days are typically displaced toward the higher frequencies,
the overall shapes of the curves are generally similar.
These curves tend to indicate that rather than differences
in the general shape of the curve occurring over days,
the subjects, as a whole, become more sensitive, as evi-

denced by the higher CFF determinations.

CFF

41

40

39

38

37

36

35

34

33

32
31

”30

29

20

 

 

DAY

Fig. 4. Learning or practice effect over days
averaged over subjects.

CFF

4O

39

38

37

36
35

34

33

32

31

30

29 .

28

21

Mean-days l and 2: O
Mean-days 4 and S: O

 

 

. ° '-v,
.1 e

'IIIIIjII
BOTH LEFT 10 20 30 4O 50 6O 70
EYES.EYES ‘ DEGREES.

Fig. 5. CFF determinations for subject 1.

40

39

38

37

36 -

35

34

33

32

31

30

29

28

22

Mean-days 1 and 2:0
Mean-days 4 and 5:.

 

 

I l I I I I 1 I I

BOTH LEFT 10 20 3O 4O 50 60 7O
EYES EYES DEGREES

Fig. 6. CFF determinations for subject 2.

23

Mean-days 1 and 2:0
Mean-days 2 and 4:.

40 -
39 «
38 q
37 d
36.
35.

334

314
30-
29.

28*

 

 

BOTH LEFT 1b 20 so '40 330 ‘60 '70
EYES EYES DEGREES

Fig. 7. CFF determinations for subject 3.

DISCUSSION

The finding that synchronous stimulation Of the foveas
of both eyes yields a significantly elevated CFF agrees
with the results Obtained by several authors (Ireland,
1950; Thomas, 1955a, 1955b; and Baker, 1952a, 1952b, 1952c).
These results, along with the results Of the present study,
indicate a certain degree of binocular summation.

Thomas (1955) reported average differences of 1.37
and 0.89 flashes per second between binocular and uniocular
CFF thresholds. In the present study, the value Obtained
for the same comparison - 1.78 flashes per second - closely
resembles those reported by Thomas.

The depressive effect found at 10 degrees in the pre-
sent study at first appears to coincide with the depressive
effect reported by Thomas (1954). However, in comparing
the results from the two studies, certain procedural dif-
ferences must be kept in mind. Thomas was concerned with
CFF thresholds as a function of stimulus intensity and the
depressive effect he reported was due to differences in
intensity of the stimulus targets. Also in the Thomas
study, both stimulus patches were displaced 10 degrees,
whereas, in the present study only the right target was
displaced and the CFF determinations were always made at

24

25

the fovea Of the left eye. The fact, however, that Thomas
did find a greater depressive effect at 10 degrees than
at the fovea appears to coincide with the results of the
present study.

Ross (1936) determined CFF thresholds at the fovea
and at 5, 10, 20, 30, and 45 degrees in the retinal periphery
using a single stimulus target. He found that fusion fre-
quency values decreased as the stimulus target was moved
toward the periphery. His curves showed that at 45 degrees
the CFF values were about 50% as large as they had been
at the fovea, with the greatest drop occurring between 0
and 10 degrees. Beyond 10 degrees the curve continued to
drOp but more gradually. In contrast to the curves pre-
sented for binocular thresholds in the present study, the
curves presented by Ross indicate no increase in CFF
thresholds beyond 10 degrees. The increase in threshold
values beyond 10 degrees found in the present study would,
thus, appear to indicate some degree of binocular summa-
tion occurring during peripheral stimulation.

Since the proportion Of rods to cones increases toward
the periphery of the eye, the points Of stimulation beyond
10 degrees may be viewed as stimulation to two different

populations of receptors. Although the stimulus inputs to

26

the two populations of receptors were synchronous in the
present study, it is unlikely that their inputs to any
central processes mediated by the receptors would be syn-
chronous. Bartley (1942), working with single flashes of
light, found that when the area of the stimulus target

was increased, the subject reported seeing two flashes.
Bartley deduced from his finding that the two flashes were
obtained ”when both the rod and the cone pOpulations are
activated, and that the one pOpulation reacts more quickly
than the other population." (Bartley, 1958; 127). Gouras
(1966) presented electroretinogram records showing rod and
cone threshold response latencies in the dark adapted
retina of the Rhesus monkey. The records indicated a con-
siderable (100 msec.) time delay between the cone and rod
responses with the cone response being the faster. There-
fore, if there is a difference in the latencies of the
rods and cones, as there well appears to be, then the points
beyond 10 degrees on the curve in Fig. 3 may actually repre-
sent CFF determinations under cortical out-of-phase stimu-
lation rather than under synchronous stimulation as was
originally assumed. Such an interpretation would seem
plausible in light of the following data. Perrin (1953)

found that with a 2 degree field, subjects' monocular CFF

27

threshold values were approximately midway between the
binocular thresholds with alternate flashes and those
with synchronous flashes. Thomas (1954) also reported
that binocular thresholds with out-Of-phase flashes tended
to be the same as the uniocular thresholds, and in some
cases lower.

The results of the present study indicate that the
timing of stimulus inputs, and thus the receptor outputs,
has a considerable influence on processes within the visual
system. Baker (1952c) interpreted his results as offering
additional evidence that alternation of response can occur
in the visual system. Baker states that:

"...the central factors active in effecting
binocular vision can perform their task when
non-corresponding retinal points are stimu-
lated, for, within an fovea, the receptors
(correSponding points alternate in respond-
ing to intermittent stimulation." (Baker,
1952c; 129).
An interpretation of the results of the present study along
the lines of the interpretation given by Baker appears
reasonable. I

The present study is taken to be another example of

the fact that certain gross temporal features of input to

the cortex exercise a demonstrable effect on sensory out-

come a

28
It is, therefore, another example of the temporal

factor that has received so much study in this laboratory.

29
SUMMARY

The method of serial exploration was used to deter-
mine CFF thresholds for three subjects. With stimulation
being supplied synchronously by two Sylvania glow-modulator
tubes, the subjects made CFF determinations at the fovea
of the left eye under the following nine conditions:
fovea-left eye; fovea-left eye, fovea-right eye; fovea-
left eye, 10 degrees-right eye; fovea-left eye, 20 de-
grees-right eye; fovea-left eye, 30 degrees-right eye;
fovea-left eye, 40 degrees-right eye; fovea-left eye, 50
degrees-right eye; fovea-left eye, 60 degrees-right eye;
and fovea-left eye, 70 degrees-right eye.

A five-way analysis of variance was computed and the
results presented in tables and graphs. The basic find-
ings were in accord with previous studies dealing with

binocular summation and inhibition.

BIBLIOGRAPHY

Abney, W. and Watson, W. Phil, 15323. A, CCXVI, 91,

1916. In Parsons, J. H. Ag Introduction £9,5hg
I Sggnggf Colour Vision. Cambridge: Univ. Press,‘
1924.

Baker, C. H. The dependence of binocular fusion on timing
of peripheral stimuli and on central process, 1.
Symmetrical flicker. ggggg. g, ggzgh., 1952a, 6:
1-10.

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