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This is to certify that the
thesis entitled

Perceptions of subfusional photic intermittencies
under various intensities and
pulse-to-cycle fractions

presented by

Jane Ellen Ranney

has been accepted towards fulfillment
of the requirements for

Major professor 1

 

0-169

LIBRARY

Michigan State
University

 

 

R003: L336: 033st,

ABSTRACT

PERCEPTIONS OF SUBFUSIONAL PHOTIC
INTERMITTENCIES UNDER VARIOUS INTENSITIES
AND PULSE-TO-CYCLE FRACTIONS

by Jane Ellen Ranney

Previous work has shown that subfusional photic intermittencies
are effective in producing phenomena such as brightness enhancement,
color changes, subjective figures, etc. And many researchers have
reported incidental observations of changes in the appearance of inter-
mittentness. In this research six classes of intermittentness were
defined, flash, coarse flicker, fine flicker, ripple, flutter, and fusion,
and the thresholds determined by the method of limits under four
intensities ranging from 3630 c/ftz to 36. 3 c/ft2 and five pulse-to-cycle
fractions (PCF), 1/8 to 7/8. Three observers determined thresholds
under each combination of PCP and intensity for observations of the
target, and of the surrounding field.

Analysis of variance showed that PCF, intensity, threshold, and
observer were all significant variables. However, shapes of curves
for the three observers were very similar, and when one of the observers
adjusted the setting of another for any given threshold, only one or two
pulses per second difference was found. Results of observations made
on the target or surround differed with intensity. At low intensities
flicker persisted longer in the target, at high intensities, in the field.
This also depended upon the PCF, greater differences being observed
with higher PCFS.

Jane Ellen Ranney

It was shown that the existence of the several thresholds could not
be attributed to the effects of rise time (onset) of the stimulus, since
rise and fall time were too brief to be effective. From the alternation
of response theory as presently stated, one would not predict discrete
stages in the appearance of flicker as pulse rate is increased, but only
a gradual change in appearance. It is only necessary to assume that
all channels in the optic pathway may not be equivalent, in order to
account for the phenomena. Differences in latencies and discharge
rates of on and off responses, and the relations between on and off
responses were suggested as providing a mechanism for the experience
of several thresholds. That is, on or off responses may drop out at
one pulse rate as pulse rate is increased, producing one threshold.

As pulse rate is further increased, the remaining response occurs with
each stimulus up to the point where it no longer responds to every pulse,
producing another threshold. Finally a point is reached where no
response occurs to any pulse, the fusion threshold.

Suggestions were made for further research.

\ I ' .r“ 4\
Approved: Bzydwpvfik) 5 "Thai

 

Date: l‘v-‘l‘d” '7- z ' (r (:3 4

 

PERCEPTIONS OF SUBFUSIONAL PHOTIC
INTERMITTENCIES UNDER VARIOUS INTENSITIES
AND PULSE-TO-CYCLE FRACTIONS

BY

Jane Ellen Ranney

A THESIS

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

DOCTOR OF PHILOSOPHY

Department of Ps ychology

1964

ACKNOWLEDGEMENTS

I am deeply indebted to Dr. S. Howard Bartley for his assist-
ance throughout my graduate program.

Drs. Hanley, Bakan, Collings and Jenkins have been helpful
in planning my program and criticizing this thesis.

To Rosaria Bulgarella and Linda Kerley I extend my sincere
gratitude for their care and patience in making observations.
Kenneth Austin and Leslie Kieth constructed the recording device,
which both increased the accuracy of the data and alleviated some of
the tedious work involved in any psychophysical research.

A National Institutes of Mental Health fellowship made it
possible to devote full time to my study and research during the
final year, and permitted purchase of necessary research equipment.

>:< 3:: >:< >:< 3:: >:< >:< >:< >:< >1: 3:: >:< >:< >:< >',< 3:: >:< >:< >:<

ii

II

III

IV

TABLE OF CONTENTS

INTRODUCTION................
METHOD. .

Apparatus . . . . . . . . . . ......
Observers .

Terminology. . . . . . . . . . . . . . .
Procedure. . . . . . . . . . . . . . . .

RESULTS....................

DISCUSSION . . . . . . . . ..........

SUMMARY .

REFERENCES . . .

APPENDIX............ .......

00000

000mm

10

18

2.4

26

30

LIST OF FIGURES

FIGURE Page
1. Schematic diagrams of the six descriptive classes
of intermittentness. Base line in each diagram
indicates zero brightness. . . . . . . . . . . . . . . 7
2. Graphs of mean pulse rate for each threshold as a
function of PCF when the center was observed . . . ll
3. Graphs of mean pulse rate for each threshold as a
function of PCF when the field was observed. Each
intensity is graphed separately . . . . . . . . . . . 12
4. Mean field threshold minus mean center threshold
as a function of PCF for each intensity of four
thresholds.......................15

iv

INTRODUCTION

Most research on intermittent photic stimulation involves the
effects of variables such as intensity on (1) single pulses, (2) double

pulses, or (3) trains of pulses producing fusion. Relatively little work

 

has been done concerning trains of pulses at subfusional rates. In
spite of this, there have been some important effects observed at sub-
fusional rates of stimulation. For example, brightness enhancement
occurs at subfusional rates (Bartley, 1938, 1939b, 1951; Bartley 81
Wilkinson, 1952; Bartley, Paczewitz 81 Valsi, 1957), and similar rates
affect visual acuity (Senders, 1949, Nachmias, 1958; Bartley, Nelson 81
Soules,(1963), and size (Nelson, Bartley 81 Wise, 1963). Other interest-
ing phenomena may occur within this range. Color changes (Bartley 81
Nelson, 1960; Nelson 81 Bartley, 1961; Bartley, Nelson 81 Mackavey,
1961), rotatory and other movement (Zapparoli 81 Ferradini, 1960)

and forms of subjective figures (Segal, 1939) have all been investigated
under various rates of intermittent photic stimulation. But changes in
the appearance of intermittentness or "flicker, " although frequently
mentioned, have not been systematically described for a wide range of
stimulus conditions. That is the task of this research.

Many investigators have reported changes in the appearance of
intermittencies (flicker) below fusion (Rood, 1863); Schenk, 1897;
O.F.F. Griinbaum, 1897, 1898; A. A. Grunbaum, 1917; Brecher, 1937;
Bartley, 1939a; Duke-Elder, 1942), and in 1927, Haack published a
review of reports to that date.

From observing a rotating black and white disc, Schenk, (1897)

reported the following:

Beobachtet man eine halb schwarze, halb weisse Kreiselscheibe
bei allmahlich zunehmender Drehuns'Geschwindigkeit, so sieht
man zunachst noch das Weisse und das Schwarze getrennt von
einander, wenn auch die Grenzen zwischen ihnen nicht mehr
deutlich zu erkennen sind. Darauf tritt ein Stadium ein, in dem
man die ganze Scheibe schon grau sieht, aber durchaus noch
nicht gleichmassig: Intensitatssschwankungen kommen bald hier
bald dort vor--man hat den Eindruck, als ob Schatten uber die
Scheibe huschten. Schliesslich sieht die Scheibe in allen Theilen
gleichmassig grau aus. (p. 32)
Thus he differentiated between what he called “flackern” (course flicker)
and "flimmern" (fine flicker). Similarly, Brecher (1937) described one
stage in which pulses were seen as clearly separated light flashes, a
second stage in which the pulses no longer were seen as separate, and
the third stage, fusion. The transition point between stages one and
two he called the "flacker-flimmer" boundary, or the boundary between
course and fine flicker. O. F. F. Griinbaum (1897, 1898) described the
intermediate stage between flicker and fusion as "molecular movement, "
"Brownian movement, " or ”glitter. "
Rood (1863) noted that as a black and white sectored disk is
rotated slowly, "the figure remains undistorted. " The perception at
about 10 pulses per second he described as a ”loss of definition, "

and at 30 pulses per second, as "glittering." Glitter he described as

lustrous, with rapid variations in the intensity of the light.

. the strong objective light is seen through the weaker
fading subjective impression, and the latter is of course at
regular intervals perceived distinct by itself, so that the eye
is in effect acted on by two masses of light of unequal intensity,
and is also sensible of their separate presence. (p. 358)

Similarly, Bartley (1939a) described a stage with one phase behind
another. Duke—Elder (1942) also desdribed three stages below fusion:
(1) a stage in which each stimulus is experienced separately, (2) a

coarse, unpleasant stage, and (3) a fine tremulus sensation of flicker.

A. A. Griinbaum (1917) described seven stages: (1) a displace-
ment of dark contours; (2) a rhythmic change, but with motionlessness,
of the field of view; (3) "flackern, " a sudden brightening and darkening,
an impression of irregularity; (4) further unification; (5) "flimmer, "

a restlessness, trembling in the visual field, center darker, periphery
brighter, with a circular movement around the center; (6) center
brighter, edges darker, brightness changes perceived as movement in
various directions; (7) target predominantly bright with no movement.

These observations indicate that there is no regular transition
from flash to fusion. There may be several important distinctions in
the continuum, distinctions which may be relevant to the neurophysiology
of vision. However, most of the foregoing descriptions were based on
incidental observations, rather than on systematic studies.

The only attempt to systematically describe the changes in per-
ception of intermittency as pulse rate varies from fusion to zero was
by Bartley (1939a). He listed nine stages for targets viewed with dark
surround as rate is decreased from high frequencies:

(1) a steady field or fusion. . . . (2) slight flicker in relatively

steady fields; (3) a mottling of the test-field, and coarser

flicker; (4) flicker in the field surrounding the test-object when
it subtends a small or medium visual angle, and the beginning

of an appreciable amount of gamma movement; (5) flicker is

seen over the whole visual field outside the test—object, and

vigorous flicker within the test-field when it subtends a large
visual angle, gamma movement becoming vigorous; (6) the

Briicke maximum; (7) the development of definite ocular tension;

(8) the emergence of the dark phase of the cycle; (9) the emerg-

ence of the flashes and dark intervals as entities. (p. 465-466)
He further noted that the appearance in the last stages was affected by
the pulse-to-cycle fraction (PCF), and that with PCF's other than 1/2,

with increase in intensity, the maximum ocular tension increased to a

maximum and then fell. However, he presents no data for direct

comparison of PCF‘s, although he does compare several unspecified
intensities. In addition, similar observations were made for targets
with illuminated surround.

Bartley noted changes in both the te st-object and the surrounding
field. Support for this distinction is provided by Hecht and Schlaer
(1935-36). More importantly, they found that with the 190 central
target they used, the relation between the critical flicker frequency
(CFF) of target and field depends upon the intensity of the target.

That is, they found that the field flickered after the center had fused

at low intensities, and at higher intensities flicker persisted longest

in the center. At still higher intensities, flicker may disappear last
from any part of the field. Support is provided by Crozier and Wolf
(1941) who found that at about the kink in the flash frequency-log intensity
curve (the change from rod to cone function) there is a transition from
a condition where the final flicker occurs in the periphery of the field

to one where it occurs in the center of the target. With a 10 test object,
Bartley (1936) found that CFF for the target was higher than for the
field, and that as the intensity increased to 2500 c/ftz, these thresholds
approached each other.

It seems then that two problems need study. First, natural
distinctions in the perceptions produced by the continuum from slow to
rapid photic pulse frequencies must be delineated. Second, the effects
of several variables on the transition points between these categories
of perceptions must be determined. That is the task of this research.
Once suitable categories of intermittentness were decided upon,
transition points or thresholds between them were measured under
various conditions of intensity and PCF. Also these observations were
made both on the target itself, and on the surrounding field or "halo"
which surrounds a bright target. Some suggestions will be made for

neurophysiological explanations of these phenomena.

METHOD

Apparatus. A 6 volt, 18 amp General Electric ribbon filament

 

lamp mounted in a housing with an opal glass front provided an
intensity of 3630 c/ftz. In addition, Kodak Wratten neutral density
filters 0.2 plus 0. 3, 110, and 2.0 reduced the intensity to 1143, 363
and 36. 3 c/ft‘2 respectively.

A single-open-sector 25.6 cm. plastic episcotister disk (PCF 1/8,
1/4, 1/2, 3/4, or 7/8) was rotated in front of the photic source.
Rotation rate was regulated by the observer with a Variac which con-
trolled the motor driving the shaft and disk. Voltage from a Weston DC
voltage generator driven from the same shaft was adjusted by a
potentiometer calibrated to provide pulse rate directly in pulses per
second. Data were recorded by punched tape output from a digital
voltmeter. A lever depressed by the observer activated the recorder
which recorded the data automatically. This eliminated the need for
an experimenter to record data, and permitted recording to one-tenth
pulse per second. .

Observing position was maintained 76 cm from the target by the
dental impression method. The target was a 2 cm. by 1 cm. trapezoid
whose long nonparallel sides were the radii of a circle, subtended
10 30' by 45' visual angle. A reduction screen with a .8 cm. circular
opening was placed 12 cm. in front of the observer.

Observers. Three women served as observers. One undergraduate

 

and one graduate student served as paid observers in addition to the
author. Each spent about eight hours observing before systematic data

collection began.

Terminology. In our early observations we found we could dis-

 

tinguish six stages in the continuum from flash to fusion for most
conditions. These stages are described below and depicted in Fig. l.
(l) _F_‘_1_a_sli_. This is a stage with definite dark and light periods.
There are definite on and off portions, and each may have finite
duration. For conditions below PCF 1/2 it is a flash of light. With
PCF 1/2, light and dark periods appear nearly equal in duration.
With PCF‘s above 1/2, the effect is that of a "dark flash. “ The light
appears interrupted by dark rather than vice versa as in the lower PCFs.

(2) Coarse flicker. The light and dark ”blend in. " There is no

 

clear boundary between light and dark. The target darkens and lightens,
or dims and brightens.

(3) Fine flicker. There is less difference between the dim and

 

bright stages. The change is much faster, although speed was not to
be used as a criterion. As one observer described it, "The alternation
becomes too rapid to have dark and light phases. ”

(4) Ripple. The light is constant with a little ripple or undulation
overlaying it. The variation is regular in contrast to that of flutter.

(5) Flutter. The target is almost completely steady; it quivers
only occasionally. The movement is irregular and may appear in
different portions of the target. The target is basically steady, but
every now and then a little movement appears somewhere in the target.

(6) Fusion. The target is steady.

Basically the same descriptions apply to observations of the field
surrounding the target. However, at least for the author, the field
possessed a spatial aspect as well as temporal. Gamma movement
played a large role. Thus in flash, the light in the field “moved back
into the target, " as when an incandescent light is turned off. With
coarse flicker this movement was incomplete, and in fine flicker,

restricted to the edges of the field. In ripple, this movement was

fusion

 

 

 

————————_—_—_.————_—

—__——_~-—_——~——-——

 

 

 

flash

 

 

 

 

 

 

 

 

Figure 1. Schematic diagrams of the six descriptive classes of
intermittentness. Base line in each diagram indicates
zero brightness.

completely gone. In its place were rapidly fluctuating rays, like spokes
of light. The lower portions of this stage may be what Griinbaum has
called glitter.

Once these stages and their descriptions were decided upon,
systematic observations could begin. The observations were made on
the transition points between stages ("thresholds"). Preliminary exami-
nation showed that the observers were able to repeat observations quite
reliably. In addition, if one observer adjusted the rate for a particular
"threshold, " when the others examined the target, adjustments of at
most only a few pulses per second were necessary to satisfy them.

Procedure. Each of the five pulse-to-cycle fractions (PCF),

 

1/8, 1/4, 1/2, 3/4, and 7/8, was combined with each of the four in-
tensities (I), 3630, 1143, 363, and 36.3 c/ftz, making a total of 20
conditions. These will be referred to here as PCF-I conditions to
distinguish them from conditions of observing, _i._e_., field or central.
Since all PCF-I conditions were observed under two observing conditions,
there were 40 conditions in all.

All observations were monocular in a darkened room. The ob-
server increased and then decreased pulse rate to observe the changes
and become oriented to each condition before recording any data for
that condition. Thus the observer was light adapted. Then the observer
increased the pulse rate to the first threshold, depressed the lever
activating the recorder which automatically recorded the pulse rate on
punched tape. Then the observer continued to increase pulse rate
toward the one producing the next threshold, and so on. The ascending
readings for all thresholds under one PCF-I condition were obtained,
followed by all descending readings for that condition. Ascending and
descending observations were made first on the target, and second, on

the surrounding field for each PCF-I condition.

A total of ten readings, five ascending, five descending, were
obtained for each threshold under each condition for one observer;
and four readings, two ascending and two descending for the two other
observers. That is, one observer made five replications of the series
of conditions and two observers made two replications. PCF-I con-
ditions were randomized within each replication. From two to eight

PCF-I conditions were observed at a sitting.

RESULTS

Means and standard deviations for each observer were computed
by the CDC 3600 computer for each condition. See Appendix. Group
means are plotted in Figures 2 and 3 where the lines in each case indi-
cate the transition points or thresholds between the classes of inter-
mittentness. That is, the region of ripple, for example, is defined by
the flutter-ripple and fine flicker-ripple thresholds. The observer
means were then used in computing an analysis of variance. Since the
coarse flicker-fine flicker threshold was not observable under some
conditions, this threshold was omitted from the analysis. The results
of this analysis are presented in Table 1. Main effects and interactions
not involving observers were tested using mean squares for the next
higher order interaction involving observers. Since Bartlett's test
showed the variances of three- and four-way interactions involving
observers not homogeneous (corrected x2 = 33.05, df = 5, p < .01;
corrected 1 z = 16.71, df = 3, p < . 01), and no logical basis for using
any other mean square could be determined, interactions involving
observers were not tested. The observers main effect was tested with
each of the four-way interactions involving observers and fOund to be
significant in each case (p < . 001).

Several factors may have contributed to the differences between
observers: (a) Differences in neurophysiologicalfunctioning;

(b) Factors such as brightness out-of-doors may have contributed more
to the results of the two observers who made only two pairs of observ-
ations on each condition. (c) Slight differences in the criteria, both
between observers, and from day to day. ((1) One observer (RB) wore

contact lenses. However, Boynton (1963) tested the effect of contact

10

fl Gil-A

vi

v

v

Pulse Rate (pulses per second)

Pulse Rate (pulses per second)

11

 

fusion 3630 c/ft‘Z

flutter

N U)
C O
1 m l 1
1 m T r

ripple

fusion

1143 c/it2

x

 

 

 

 

    
 
  

ripple

fine flicker

10 J3— \/\

coarse flicker

 

 

 

ripple

fine flicker 1*- fine flicker
10 - ..
coarse flicker coarse flicker
..,_ ¥ _‘\
flash flash
3 3 '3 3 3 ._ 3 3 3 3 3
fusion 363 c/ftz fusion 36.3 c/ftz
flutter m
“r

fine flicker

A

coarse flicker

 

Pulse -to - Cycle Fraction

Pulse-to-Cycle Fraction

\ /—/—-\\

\— ,_
flash flash

3 3 3 3 n 3 3 3 3 3

1/8 1/4 1/2 3/4 7/8 1/8 1/4 1/2 3/4 7/8

Figure 2. Graphs of mean pulse rate for each threshold as a function
of PCF when the center was observed.

 

Aflrd

Pulse Rate (pulses per second)

Pulse Rate (pulses per second)

12

 

 

 

40 “F fusion 3630 c/ft2 .3. fusion 1143 c/ft2 1‘
/f1utterj J
c-U- -+. 1
30 /\ \ flutter
ripple L
ripple
20 'IU“ qr— up

fine flicker
fine flicker

L
r
i
l
L
1

 

 

 

10 3» coarse fileker «— coarse flicker \ 3
M f
«3- «~ \
flash flash _.
L 1 1 1 1 1 1 1 l 1
—3— I T T l 1 '31- I l T 1 T a?
fusion 363 c/ftz 36. 3 c/ftz
" lb fusion J
30 -.. ’\ ~1. fir
flutter flutter
l- - _.
l

ri le
20 pp 4 ripple

L 1
if T

l

l T

l l

T r

fine flicke r

fine flicker
10 it ‘” ‘”
coarse flicker .

\__._____F coarse flicker
d— \ —0-— k fir.

 

 

 

 

 

 

flash flash x
3 3 ‘ 33 3 3 3 3 3 3
1/8 1/4 172 3/4 7/8 1/8 1/4 1/2 3/4 7/8
Pulse-to-cycle fraction Pulse-to-cycle fraction

Figure 3. Graphs of mean pulse rate for each threshold as a function
of PCF when the field was observed. Each intensity is
graphed separately.

13'

 

 

 

 

Table 1. Summary of analysis of variance based on mean for each
observer under each condition.
Source df MS F
Pulse-to-Cycle Fraction (PCF) 4 358.8522 24.683033<
Intensity (I) 3 339.9093 9.16*
Threshold (Th) 3 18416.1418 69.80*>3<>:<
Field-Center (F-C) 1 158.0108 2.95
Observer (O) 2 76. 3864 ****<
PCF x1 12 7.7676 .93
PCF x Th 12 25.7926 17.353330:<
PCF x F-C 4 38.8638 3.38
Ix Th 9 22,1329 4.91**
I x F-C 3 391.9156 29.69***
ThxF-C 3 63.9650 2.13
PCF x O 8 14.5385
Ix O 6 37.1267
Th x O 6 263.8585
F-C x O 2 53. 5022
PCF x I x Th 36 1.1652 .63
PCFxIxF-C 12 18.5410 5.65***
PCFxThxF-C 12 5.1954 2.86*
IxThxF-C 9 37.6980 5.65**
PCFxIxO 24 8.3131
PCFx Tth 24 1.4863
PCFxF-CxO 8 11.5142
IxTth 18 4.5037
IxF-CxO 6 13.1999
Thx F-C xO 6 30.0278
PCFx IxThxF-C 36 2.7624 2.10***
PCFxIxTth 72 1.8433
PCFxIxF-CxO 24 3.2798
PCFxThxF-CxO 24 1.8190
Ix ThxF-CxO 18 6.6750
PCFxIxThxF-CxO 72 1.3174
Total 479,
* P < .05
** P < .005
*** P < .001

**** Each four-way interaction involving Observers was used as the
error term, with p < . 001 in each case.

See text.

l4

lenses in his research on stray light and reported that

The glare effect, independent of wavelength, was greater by

about 0. 35 log unit with the lens in than with the lens out. .

There was no indication, however, that scatter contributed

either by the lens or its effects was of appreciable magnitude

compared to stray light resulting from reflection and scatter-

ing in the normal media of the eye. (p. 446)

(e) One observer (JR) had had far more previous experience as an
observer in flicker experiments, and consequently her variance was
less than that of the less trained observers (Nelson, Bartley , 81
DeHardt, 1960).

Examination of Figures 2 and 3 along with the analysis of variance
table reveals several things. First of all, PCF is a significant
variable (p < . 001). In addition, PCF by intensity by Field-Center is
significant at the . 001 level. Another finding is that PCF is less
effective for lower thresholds than higher. Some support for this con-
clusion is provided by the significant PCF by Threshold interaction
(p < . 001).

The most obvious result is, of course, that it is possible to
distinguish the six stages described on page 6. While only four of the
thresholds between these categories were included in the analysis of
variance, as explained above, the effect of thresholds was significant
at the . 001 level.

Figure 4, which presents the differences between mean thresholds
for target and surrounding field for four thresholds, and the significant
intensity by Field-Center interaction (p < . 001) indicate that the relation
between target and surrounding field is not as simple as Bartley (1936)
indicated. In Figure 4, the coarse flicker-fine flicker threshold was
again omitted because of the number of missing observations. Bartley's
curve is only for PCF 1/2, and shows CFF for test object and field

becoming nearly identical at about 2500 c/ftz. In‘Figure 4 all positive

Field threshold minus center threshold (pulses per second)

 

 

 

 

 

 

 

 

15

 

 

 

 

 

 

 

 

3630 c/ftz
6 t 3630 c/ftz
4 ~3- 8 .1.-
2 - 6 ..
/\ 1143 1143
o / 4 «L
_2 fl // 363 Z r
'4 " / 0 363
-6 .. -2 ~~ I
-3 .. -4 «3
36. 3
-10 .. -6 ..
flutter-fusion (CFF)
3 3 i i i -8 4-
1/81/4 1/2 3/4 7/8
PCF -10 ..
3630 o/ftflz ,_ 36.3
6 0 ripple -f1utter
4 .. 1/8 1/4 1/2 3/4 7/8
. PCF
2 l.
0 / 363
2.. 3630 o/itZ
'2 ‘3 @4363
0 A VA1143
-4 .. \1143
36. 3 '2" flash-coarse flicker
-6 3- f’ {1' k - ' 1 3 3 3 3 3
1:16 ,1“ er :le e , , 1/8 1/4 1/2 3/4 7/8
1/8 1/4 1/2 3/4 7/8 PCF
PCF
Figure 4. Mean field threshold minus mean center threshold as a

function of PCF for each intensity of four thresholds.

16

points indicate that the threshold for the field was greater than that of
the target. It is apparent from the figure that not only do the CFFs
become equal, but the field CFF exceeds that of the target for the
highest intensity tested here (3630 c/ftz) at PCF 1/2 and higher, and
for intensity 1143 c/ftz, PCF 3/4 and 7/8. One-sided _t_ tests showed
the difference significant, p < .05 for PCF 1/2 (_t_ = 1.85, df = 17),
p < .001 for PCF 3/4 (t_ = 4.16, df = 25 by Welch‘s formula, Walker 81
Lev, 1953), and p < .001 for PCF 7/8 (E. = 4.92, df = 17) at 3630 c/ftz.
At 1143 c/ftz, differences were not significant. Comparison of Figures
2 and 3 shows that intensity is more effective in observations of the
field than of the center. The lack of a significant Field-Center main
effect is attributable to the high Intensity by Field-Center interaction.

These results combined with those of Hecht and Schlaer (1935-36)
and Crozier and Wolf (1941) seem to indicate that at low intensities
(scotopic level) flicker persists longer (CFF is higher) in the periphery
than in the target. When intensity is above the inflection point between
rod and cone function CFF is higher in the target. But as intensity
increases further, a point may be reached when CFF is again higher in
the periphery or surrounding field. One might suppose that the rods
are again being stimulated. Bartley (1936) found that flicker in the
field was produced by entoptic stray light, which would stimulate rods.
Indeed, Boynton (1953) has found both image and stray light components
in the electroretinogram (ERG), and compared the effects of stray and
direct illumination on the ERG. He found that to elicit a given b wave
in the ERG a small stimulus required four times as much luminous
flux as a large stimulus.

The differences between field and target for the ripple-flutter and
fine flicker-ripple thresholds show similar relationships to that of CFF.

However, the lower thresholds, coarse-fine flicker, although not

17

graphed here, and flash-coarse flicker, are irregular. This difference
is brought out by the significant Intensity by Threshold by Field-Center
interaction (p < . 005). This may be complicated by the fact that data for
the flash-coarse flicker threshold may be somewhat skewed. These
observations were difficult to make because the low speeds required
could not be readily maintained. This difference then provides further
evidence for differences between rods and cones, i_._e_2., a difference in

the response (perception) to low and high pulse rates.

DISCUSSION

The existence of the several thresholds might be attributable
to the effects of less-than-square pulses produced by the episco-
tister at low pulse rates. Indeed a great deal of work has been done
on the role of temporal gradients of intensity in retinal response.
In the light-adapted eye, with intensity of 100 lux, both size and shape
of the ERG changed with rise times greater than 100 msec. whereas
no remarkable changes were produced when the rise time was varied
between 1 and 70 msec. (Ronchi 81 Grazi, 1956). Ronchi 81 Moreland
(1957) report no change in the ERG with change in rise time, with
constant exposure time and peak intensity. The results of a study
comparing effects of stimuli with rise times of 50-75 msec. or 2 sec.
recorded from ganglion cells in the cat (Enroth-Cugell 81 Jones, 1961)
provided support for this position. However, Bornschein (1961) ,
varying rise time from 7 to 180 msec. , found the amplitude of the ERG
decrease nearly linear to about 50 per cent in the cone retina of the
European Ziesel (Suslik, Cason or earless marmot). With human sub-
jects using red light, the x-wave of the ERG was influenced by a
stimulus gradient of between 8 and 30 msec. (Bornschein, 1962b).
In another study (Bornschein, 1962a) the reciprocity of the antagonistic
on-center and off-center neurons of the cat was shown to partially
disappear with increase in the rise or fall time of the stimulus. In a
complementary psychophysical experiment, humans distinguished
stimuli with rise times of 7 and 100 msec. (intensity 830 c/mz). Fall
time was significantly more often identified correctly than rise time

for this intensity and for a log reduction of 1. 5 (Bornschein, 1962c).

18

19

However, when the pulses used in this study were monitored with
a Tektronix oscilloscope model 502A with a solenium photoemissive
type photocell at the observer‘s normal eye position, it was found that
the rise time at 3 pulses per second was 4 msec. , and decreased
rapidly to 1. 2 msec. at 10 pps. ~ Since in all cases these are briefer
rise times than those which have been shown to produce changes in ERG,
or responses of ganglion cells, or to be perceptually distinguishable,
it is safe to conclude that the existence of the several thresholds dis-
tinguished in this study cannot be explained by effects of gradients of
intensity produced by the episcotister.

The alternation of response theory (Bartley 81 Nelson, 1963)
postulates a number of channels in the optic pathway (individual fibers
in the Optic nerve) which may be activated in groups. At very slow pulse
rates, with sufficient recovery time between pulses, and with sufficient
intensity, all channels will be activated simultaneously. When pulse
rate or PCF is such that there is not sufficient recovery time for all
channels, only those channels which have recovered will respond.

Thus after a number of pulses, channels will have distributed themselves
to an alternating pattern. ~ This continues as pulse rate is increased,
distributing activity so that fewer and fewer channels respond to any
one pulse in a train. This theory accounts very nicely for the changes
in brightness as pulse rate is increased to fusion where the Talbot-
Plateau law is effective. However, from this theory one would predict
a very gradual change in the appearance of flicker. Yet many earlier
observations have indicated relatively discrete changes in appearance.
This research distinguished five classes of intermittentness over all
the conditions investigated. These results are not explained by the
alternation of response of the channels in the optic pathway. Some

additional mechanism must be responsible for the effects.

20

One possible explanation of the existence of the several thresholds
is derived from Enroth‘s data (1952) from dark adapted eyes, supported
by data of Ogawa, Levick 81 Bishop (1962), and Dodt and Enroth's data
(1953) from light adapted eyes, in which they recorded from the
individual ganglion cells in the cat retina. Several stages can be seen
in their records, particularly those of elements responding to both on
and off of the stimulus. In this case, generally one or the other is the
dominant, _i_._§. , the discharge rate is higher and the latency shorter.

As pulse rate is increased, at some point the on and off responses
"clash, " and the weaker response disappears, leaving only the dominant,
which may be either the on or off, depending on the particular unit being
measured. The dominant response continues as pulse rate is increased,
and then becomes arrhythmic, responding only to some pulses, and

then finally ceases to respond at all. Thus there are four stages in a
fiber responding to both onset and offset of pulses: (1) both on and off
responses occurring, (2) only the dominant occurring, (3) an arrhythmic
response of the dominant, and (4) no response. The similarity of

these last two stages to those of the subjective stages is obvious.

Enroth (1952) has distinguished "fixed" and "labile" off responses,
the difference being that the "fixed" type of discharge responds com-
pletely to the first pulse, whereas the "labile" off discharge requires
a number of pulses before full response, for example, seven pulses.
Unless the differences in this respect between cat and human are much
greater than might be anticipated, the difference in these two types most
likely does not contribute to the thresholds for two reasons: (1) a number
of pulses generally had been observed in a given pulse train before a
threshold was reached, (2) if the lower threshold(s) were attributable
in some way to this difference, then the threshold(s) should not be ob-

servable on descending trials. . Of course this was not the case.

21

Similarly, the non-sustained type of on response would be eliminated
within the first few pulses, and since it depends on the total time

of light exposure as well as the pulse frequency, would not contribute
in any regular manner to any threshold.

The fact that in flutter, movement seemed to occur now here,
now there in the target suggests that this arrhythmic firing of elements
may be responsible for the sensation.

The implication of this for alternation of response theory is that,

because of the differences in on and off responses, the channels in the
pathway are not necessarily equivalent. - This has never been explicitly
stated by Bartley, even though he has used the inhibition of the off
response as an explanation for the occurrence of fusion at higher PCFs
(1939b). This explanation suggests that fusion results when the null
periods between pulses are so brief that the succeeding pulse inhibits
the off response from the preceding, leaving only the on response.
But if the Enroth, and Dodt and Enroth results are correct, that an
_ofi response may persist beyond the elimination of the on response,
the question becomes one of the subjective meaning of a train of off
responses.

It may be that the meaning of a response depends upon other
activity in the system. At a high pulse rate where either on or off
responses have dropped out, the remaining response may be interpreted
as either. It may simply indicate change.

It was originally thought that the curves for the various thresholds
might be of different shapes, and therefore offer some explanation for
the diverse results that various authors have obtained. That is, some
of the differences in the curves of critical flicker frequency could be
attributed to differences in the criteria of fusion. Schenk (1897) sug-
gested this possibility when he discussed his replication of Sherrington's

work. He noted that what he called ”flackern" Sherrington called

22

”flicker, " and likewise what he called "flimmern, " Sherrington denoted
as "steady sensation. " The upper two thresholds are most likely to

be defined as CFF. From Figure 2 it appears that these thresholds do
not produce different shaped curves. The thresholds are nearly parallel.
In spite of this, the PCF by Threshold interaction was significant at the

. 001 level. However, this difference may be contributed largely by the
difference between the upper and lower thresholds, particularly flash-
coarse flicker.

The finding of differences in field and target observing at high
intensities does have one implication for methods used in CFF studies.
That is, at high intensities care must be taken that the observers are
instructed to observe the target, and report fusion, or flicker, in the
target rather than surround. At high intensities the flicker in the sur-
round is overwhelming. However if results are to be comparable to
those for lower intensities, this distinction must be observed. Other-
wise the problem becomes a different one, as fusion is defined as the
disappearance of any variation any\_avhere in the observed area, and a
different portion of the retina is involved.

With well trained observers, this sort of study should be repeated
using the method of single stimuli. Such a study may show that dis-
tinctions between coarse and fine flicker should be dropped. For some
of the conditions in this study these were not distinguishable. In using
the method of limits, what was responded to was a change in appearance,
and the preceding perception affected the determination of the threshold.
The method of single stimuli would eliminate this problem. For a
trained observer, though I believe five of the categories, omitting fine
flicker, could be distinguished using single stimuli.

Another possibility is that what has been designated here as the
flash-coarse flicker threshold, at the higher PCFs, might more properly

be the coarse-fine flicker threshold. The curves in Figure 2 would

23

indicate that this may be the case, since if a line is drawn from the
last observed point for coarse-fine flicker to the value for flash-coarse
flicker for the next higher PCF, the curves are roughly parallel to
those of the higher thresholds. The distinction between dark flash
observed at these PCFs, and coarse flicker is indeed a fine one. The
method of single stimuli provides a test of this suggestion.

One might think the categories derived here to be arbitrary dis-
tinctions. Perhaps they are. But the evidence does not support such
a position. Observers were able to repeat observations quite reliably.
And observers agreed at about what point a given threshold existed
when they made direct comparisons with each others' observations.
Finally, similar distinctions under a variety of labels, have been re-
ported in the literature far too frequently not to be real.

It may be possible to pinpoint the thresholds in another way by
using a modulated intensity. Pulse rate could be maintained at a slow
rate and intensity varied between some high value and a lower intensity.
Because the sensation at higher pulse rates is one of dimming and
brightening, it may be possible to match an intermittent stimulus with
a null period and variable rate to an intermittent stimulus alternating
between two intensities, with constant pulse rate. In this way the
experience of coarse flicker, for example, may be found to be produced

by a modulation of, say, 85 per cent.

SUMMARY

Previous work has shown that subfusional photic intermittencies
are effective in producing phenomena such as brightness enhancement,
color changes, subjective figures, etc. And many researchers have
reported incidental observations of changes in the appearance of inter-
mittentness. In this research six classes of intermittentness were
defined, flash, coarse flicker, fine flicker, ripple, flutter, and fusion,
and the thresholds determined by the method of limits under four
intensities ranging from 3630 c/ftZ to 36. 3 c/ftz and five pulse-to-cycle
fractions (PCF), 1/8 to 7/8. Three observers determined thresholds
under each combination of PCF and intensity for observations of the
target, and of the surrounding field.

Analysis of variance showed that PCF, intensity, threshold, and
observer were all significant variables. However, shapes of curves
for the three observers were very similar, and when one of the ob-
servers adjusted the setting of another for any given threshold, only
one or two pulses per second difference was found. Results of observ-
ations made on the target or surround differed with intensity. At low
intensities flicker persisted longer in the target, at high intensities,
in the field. This also depended upon the PCF, greater differences
being observed with higher PCFs.

It was shown that the existence of the several thresholds could
not be attributed to the effects of rise time (onset) of the stimulus,
since rise and fall time were too brief to be effective. From the
alternation of response theory as presently stated, one would not

predict discrete stages in the appearance of flicker as pulse rate is

24

25

increased, but only a gradual change in appearance. It is only necessary
to assume that all channels in the optic pathway may not be equivalent,
in order to account for the phenomena. Differences in latencies and
discharge rates of on and off responses, and the relations between on
and off responses were suggested as providing a mechanism for the
experience of several thresholds. That is, on or off responses may
drop out at one pulse rate as pulse rate is increased, producing one
threshold. As pulse rate is further increased, the remaining response
occurs with each stimulus up to the point where it no longer responds to
every pulse, producing another threshold. Finally a point is reached
where no response occurs to any pulse, the fusion threshold.

Suggestions were made for further research.

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28

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29

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APPENDIX

30

 

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