After-images observed with intermittent background illumination

VisionRes. Vol. 5, pp. 123-132.

Pergamon Press 1965. Printed in Great Britain.

AFTER-IMAGES

OBSERVED

BACKGROUND

WITH INTERMITTENT

ILLUMINATION1

HALSEY H. MATTESON University of Illinois2 (Received 6 J&y 1964)

INTRODUCTION

AN AFTER-IMAGEis typically referred to as positive in appearance when brightness relations in the after-image are the same as brightness relations in the stimulus configuration. Brightness relations in a negative after-image are opposite to those in the stimulus configuration. After-images are usually negative when viewed on an illuminated background and positive when viewed in the dark (EBBECKE, 1929; ROBERTSONand FRY, 1937). The duration of positive after-images increases with increasing stimulus intensity (FRANZ, 1899; SHUEY, 1924, 1926; BERRY and IMUS, 1935; FEINBLOOM, 1938; PADGHAM, 1953, 1957; ALPERN and BARR, 1962). ROBERTSONand FRY (1937) also found that the duration of a series of after-images observed with intermittent background illumination increased with increasing stimulus intensity. Extremely intense stimuli are necessary to produce longlasting positive after-images, but there are a number of reports that intermittent or continuously changing back~ound illumination can considerably prolong the duration of a series of after-images (MCDOUGALL, 1901; MILES, 1915; TROLAND, 1917; EBBECKE,1929; ROBERTSONand FRY, 1937; BRINDLEY, 1962). A number of studies using relatively low stimulus intensities have shown that sensitivity to positive after-images is considerably higher in the periphery than in the fovea (KARWOSKI and CROOK, 1937; KARWOSKI and WARRENER, 1942; KARWOSKI and PERRY, 1943; KARWOSKIand WAYNER, 1951; TREZONA, 1960), but studies of differences between fovea1 and peripheral positive after-images using relatively intense stimuli have been neither extensive nor unequivocal. Two studies in which the size of centrally fixated stimuli was varied (PADGHAM, 1963; FRANZ, 1899) did not really indicate differences in sensitivity to positive after-images between the fovea and periphery; the results can be accounted for entirely on the basis of area1 summation. Observations made by ROBERTSONand FRY (1937) and also by ALPERN and BARR (1962) suggest that sensitivity to positive after-images is higher peripherally than foveally, but no such difference was obtained by TREZONA (1960). TREZONA’S study was the most extensive of the three studies of peripheral and fovea1 positive after-images using high st~ulus intensities; however, her method, which essentialiy required Ss to sort after-images into two categories, may not have been precise enough to reveal differences. CREEDand GRANIT (1928) found that sensitivity to negative after-images was higher peripherally than foveally. Some observations made by BRINDLEY (1962) 1 This article is based upon a dissertation submitted in partial fulfillment of the requirements for the Ph.D. The author gratefully acknowledges assistance received from Dr. WILLIAM E. KAPPAUF, dissertation advisor, and Dr. HAROLD W. HAKE. degree at the University of Illinois.

2 Now at U.S. Naval Medical Research Laboratory, Groton, Connecticut.

123

HALSEYH. MATTESON

124

suggest that foveally produced series of negative and positive after-images observed with intermittent background illumination fade out more rapidly than peripheral ones. There are no systematic studies on the effects of the luminance of an intermittent background on the duration of a series of positive and negative after-images. ROBERTSON and FRY (1937) did find that the luminance of a continuously varied background required to shift the appearance of an after-image from positive to negative decreased with time after stimulation. -xSK6

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I

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slop

shutter

2

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Eyepiece

1. A schematic diagram of the optical apparatus.

The present study is concerned with the effects of retinal position, of intensity of the original stimulus, and of the luminance of an intermittently illuminated background on the duration of a series of positive and negative after-images. APPARATUS

The optical apparatus, which consisted essentially of two projectors, is diagrammed in Fig. 1. The primary field, a very intense circular field one degree in diameter, was 42.96 in. from S’s eye. It was presented by means of a projector with a 750 W Sylvania model CWA projection bulb operated at 110 V by means of a variable transformer and a voltmeter. The mean of ten measures of the highest primary field intensity level obtained in ten calibration sessions conducted throughout the course of the experiment was 4.2 x lo6 mLamberts (total range of -5-+2 per cent). The median of nine individual readings obtained with a

After-Images

Observed with Intermittent

Background

Illumination

125

MacBeth illuminometer during each calibration session was used as the measure for each session. A Compur model OX shutter was used to control presentation time of the primary field. A stimulus duration of 6 msec was used in order to minimize the effects of blinking and shifts in fixation position during stimulus presentation. The second projector, a Bell and Howell Robomatic, was used to present the background (a Ganzfeld) which covered the entire visual field with approximately uniform illumination. The 500 W General Electric model CZX projection bulb in the background projector was operated at 106 V by means of a variable transformer and a voltmeter. The mean of twelve measures of the highest background luminance level, made periodically throughout the course of the experiment, was 23-l mLamberts (total range from -3-$-S per cent). Each measure was the median of nine readings taken with a MacBeth illuminometer during each calibration session. An Alphax shutter was used merely to turn the background field on and off. Timing of background presentations was accomplished with two electronic timers. A hemispherical diffuser fashioned from half a table-tennis ball, contoured in such a manner as to fit closely over s’s eye, was placed over the Plexiglas eyepiece of the background assembly (see Fig. 1). A plexiglass diffusing cylinder painted flat white except on the ends and several opal glass diffusers were used to diffuse the light from the background projector thoroughly before entering the eyepiece. Such diffusion was necessary to provide relatively uniform illumination over most of the visual field. The entire eyepiece assembly was movable and remained out of the light beam from the primary source during stimulus presentations. A light-tight enclosure was provided for the subjects. A permanent dental impression from each S was used in order to obtain accurate head ~sitioning. Stimulus con~gurations to guide fixation were placed in the same plane as the field stop nearest s’s eye (see Fig. 1). A fovea1 stimulus position and a peripheral position 5 degrees above the fovea were obtained with two fixation-positions: central and 5 degrees below the center of the primary field. In each case S was to fixate the center of a diamond-shaped configuration of lights. The four lights in each configuration were 1 degree from the center of the configuration, and each light was 18 min in diameter. The lights were illuminated with neon pilot bulbs, one each for two of the lights in the central configuration, and one bulb for all four peripheral lights. Each bulb was regulated with a separate potentiometer. Fixation lights were extinguished by means of a microswitch attached to the shutter on the primary field. SUBJECTS

Three s’s, aged 20,21 and 29, with varying degrees of experience with psychologi~l experimentation, were used. They were essentially emmetropic, and none of them wore corrective lenses while making observations. All S’s were color normal according to American Optical Co. Pseudo-Isochromatic Plates. PROCEDURE

Two retinal positions (central and 5 degrees above the fovea), four levels of stimulus intensity (neutral densities of O-1,0*9, 14 and 2-Oin front of the primary source), and seven background levels (23.1 to 0400231 mLamberts in steps of one log unit plus complete darkness) were used. The s’s were instructed to report as after-images only those visual experiences about the same size and shape as the primary field, and practice trials were conducted until each S feh confident in making the jud~ents. Subjects RB and KM made

126

HALSEY

H. MATTESON

some sixty observations in practice sessions and in a preliminary experiment over a period of one month. Subject DW made some thirty practice observations over a period of one week. Wide spacing of observations was deemed advisable to avoid possible residual effects of previous observations. Each S made only one observation (i.e. the stimulus was presented only once) per session during the main experiment with a minimum of 2 hr between sessions and no more than two observations per day. The main experiment was conducted over a period of 3 months. The trials were divided into two separate replications, and conditions were randomized within replications. In other words, each S made one observation at each combination of levels of the independent variables before making any repeat observations. Subjects RB and DW made two observations each at all forty-two combinations of three primary field intensity levels (0, 1.1 and 1*9), seven background luminance levels, and two stimulus positions. Subject KM made two observations at each of the twenty-eight combinations involving the two highest (I.1 and l-9) primary field intensity levels, The lowest primary field level for her was changed from 0 to O-5 in the second replication, because she often observed no after-images at the 0 (lowest) level in the first replication. Several control conditions will be described later. Each trial was conducted after 10 min of dark adaptation. Viewing was monocular with the right eye. Fifteen set before presentation of the primary field S was asked to bite firmly on a dental impression and to signal preparedness. The experimenter raised the dark flap in the primary field projector and gave S a ready signal 5 set prior to stimulus presentation. The subject indicated the presence of an after-image by pressing a pushbutton switch and keeping the switch depressed until the after-image disappeared. The background eyepiece was moved over in front of S’s right eye approximately 25 set after the last report of an after-image in the initial dark period. The background field was turned on 35 set after the end of the first series of after-images. The series of background presentations was an alternation between 20 set periods of illumination and 20 set periods of darkness, beginning with an illuminated period. The series was continued until 2 min after the disappearance of the last after-image during the background series or for a minimum of 2 min if no afterimage was observed. Measures from each trial were recorded on a Phipps and Bird polygraph with five event markers. Verbal reports of the appearance of the series of after-images were obtained after the conclusion of each trial. RESULTS

The data are reported for each S separately, since there were scnre rather large differences between S’s Since fovea1 luminance would not be appropriate for the stimulus levels presented at the S-degree peripheral stimulation position, the primary stimulus levels are reported in arbitrary log intensity units relative to the lowest intensity level (absolute value of neutral density in front of the primary source minus two). Total duration Time elapsed between presentation of the primary field and the disappearance of the last after-image during the series of background presentations (“total duration”) is plotted for each S in Fig. 2. A separate sign test (SIEGEL, 1956) was run on the data for each S. Each of the pairs of total durations consisted of the peripheral total duration and the central total duration obtained with the same combination of primary field and background levels in the

After-Images

Observed with Intermittent

Background Ilhnnination

127

same replication. There were thirty-six such pairs for each S (DW and KM had one tie each). For each S, peripheral total durations were significantly longer than the corresponding central total durations (O-05 level, two-tailed). Every one of the peripheral total durations for RB was longer than the central total duration under corresponding primary field and background conditions. The difference between peripheral and central total durations was appreciable for KM only at the highest primary field level. In many fovea1 trials, both RB and KM reported no after-images during the series of background presentations (indicated in the figures by parentheses around points ,

5* peripheral stimulation

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200

100

0 -4.36

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- I.36

0.36

1.36

-4.36

-3.36

Log background luminance,

-2.36

-1.36

0.36

I.36

mlamberts

FIG. 2. Total duration as a function of background intensity at two stimulus positions for three subjects. The primary field intensity levels, expressed in arbitrary units, are: 1*9(O-----_), l~l(o---),0~5(p-~-~-),andO(A~~~~~~ .). Open symbols indicate the first replication, and filled symbols indicate the second replication. Parentheses () around symbols indicate that no after-images were observed during the series of background presentations. I

HALSEY

128

H.

MATTESON

Peripheral total durations for DW were noticeably representing those total durations). longer than central total durations only at the middle (1.1) primary field intensity level, and the difference at this level is considerably smaller than the difference for the other two subjects. The general level of magnitude of total durations was also considerably lower for Variability between replications did not seem to be DW than for the other two subjects. appreciably larger for DW than for the other subjects; however, ail of the total durations obtained from him at the peripheral retinal position were shorter in the second replication than in the first. In general, total duration increased with increasing primary field intensity, although there was sometimes considerable overlap between adjacent primary field intensity levels, particularly for DW. There was a fairly pronounced peak in peripheral total durations for KM at the 0.0231 background luminance level. The curves for DW seemed to be too irregular to permit any inferences about an optimal range of background luminance. The position of maximum total durations for RB seemed to shift toward lower background luminance with decreasing primary field intensity. Other background measures Several other measures of after-images produced with intermittent background illumination were investigated, including duration of after-images during the background series only. The results did not differ appreciably from the results using total duration. In particular, all these measures indicated that more after-images of longer duration were produced peripherally than foveally and that the difference was quite large for RB and KM and quite small for DW. Primary duration The time elapsed from presentation of the primary field to the fading out of the last after-image in the initial dark period before presentation of the background (“primary duration”) generally increased with increasing stimulus intensity. TABLE

1.

MEDIANS

AND

RANGES

(HIGH

DURATIONS

s RB RB KM KM DW DW

AND

LOW

SECONDS

MEASURES

FOR

ALL

GIVEN

THREE

IN

PARENTHESES)

OF

PRIMARY

SUBJECTS

Primary field intensity level

Stimulation position C3 P” C P C P

IN

0 4 24 0 20 8 29

(3-9) (8-58) (O-23) (O-43) (O-41) (I 5-59)

1.1

0.5

0 (O-25) 14 (O-18)

10 44 40 18 38 52

(629) (12-83) (35-53) (13-28) (22-67) (19-78)

1.9

8 1 (22-106) 95 (54-139) 100 (77-132) 29 (26-37) 101 (72-132) 59 (23-98)

3 Central (foveal) stimulation. 4 5-degree peripheral stimulation.

Primary duration did not follow the same course with increasing stimulus two retinal positions for all S’s (see Table 1). Median primary durations for peripherally than foveally over the entire range of primary field intensity. On median primary durations for KM and DW were longer peripherally at the

intensity at the RB were longer the other hand, lowest primary

After-Images

Observed with Intermittent

Background Illumination

129

field intensity level but longer centrally than peripherally at the highest primary field intensity level. There were no appreciably large differences in primary durations between replications. Controls

In the first control condition, the dark slide control, S viewed a completely dark background in place of the usual alternation between 20 set periods of light and darkness. Two Ss observed no after-images at all during the control periods, and a third (RB) observed a single very short (2-3 set), indistinct after-image in only two of the twelve dark slide control trials. Thus, it seems that the 25 set waiting period before presentation of the background series was sufficiently long to allow after-images to fade out completely in the dark. Another control condition, introduced during the second replication, consisted of a 2 min, 20 set period of continuous illumination. Background luminance levels were selected from those levels which resulted in the longest total durations in the first replication. The data from continuous illumination control trials are presented in Table 2. Only in the case of DW at the highest intensity level were durations of after-images appreciably longer than 20 set or did total durations from continuous illumination control trials approach the order of magnitude of total durations obtained with intermittent background illumination. In general, the results of the control trials indicate that the considerable prolongation of after-images obtained in this study can be attributed to the effects of intermittent background illumination rather than to spontaneous reappearance of after-images with steady background conditions.

TABLE 2. CONTINUOUSILLUMINATION

S

RB

KM

DW

Primary field level

Background intensity

1.9 1.9 1.1 1.1 0 0

2

1.9 1.9 1.1 1.1 0.5

2 2 3

1.9 1.9 1.1 1.1 0

3 3

f f 2

:

: 3

Stim. pos. CT

F

P C P C P C P P

C : P P

CONTROLCONDITION

Duration on background

DATA

Number of phases

0 29

Total duration 90

142 169 104 47 54

2 13 I 8 0 9 0

14 15

1 1

111 75 37 12 66

48 51 0 16 0

3 1 0 2 0

217 152 37 93 0

Background duration (time elapsed between background presentation to disappearance of the last afterimage on the background), number of distinct after-image phases reported, and total duration at two stimulation positions and three primary field intensity levels are reported for three subjects. 5 Central stimulation position. 6 5-degree peripheral stimulation position.

130

HALSEYH. MATTESON

General appearance

Typically, after-images observed on the illuminated background were described as negative in appearance (a dark circle on the background). RB and DW observed positive after-images on the illuminated background only at the lowest background luminance level, which was very dim. RB was the only S who reported an appreciable number of after-images during the dark intervals in the background alternation. Those after-images which were observed during dark intervals in the background alternation were invariably described as positive in appearance (a bright spot). The actual elapsed time during which an after-image was reported was typically quite short in comparison with primary duration or total duration. The after-images appeared as a series of individual images frequently separated by long blank intervals rather than a continuous image. In all peripheral trials and in all central trials at the lowest stimulus intensity level, all three S’s reported that the after-images in the initial dark period were colorless and positive Color sequences in central trials with higher stimulus intensities were not in appearance. consistent between S’s; however, all S’s reported that the last after-image observed in the initial dark period appeared as a dark circle with a bright corona. DISCUSSION

As has been found in many previous studies (FRANZ, 1899; SHUEY, 1924, 1926; BERRY and IMUS, 1935; FEINBLOOM, 1938; PADGHAM, 1953, 1957; ALPERN and BARR, 1962), primary duration increased with increasing stimulus intensity. The finding that central primary durations were shorter than peripheral durations at the lowest stimulus intensity level is in agreement with results of previous studies of after-images observed in the dark using relatively low stimulus intensities (KARWOSKI et al., 1937, 1942, 1943, 195 I : TREZONA, 1960). Conflicting results from previous studies of after-images observed in the dark using high stimulus intensities (ROBERTSON and FRY, 1937; TREZONA, 1960; ALPERN and BARR, 1962) might have resulted from the sort of individual differences obtained in the present study. Differences between fovea1 and peripheral after-images observed in the dark obtained in the present study were not the same for all Ss at the two highest stimulus intensity levels. Total duration of after-images observed with intermittent background illumination also increased with increasing stimulus intensity. ROBERTSON and FRY (1937) obtained similar results with continuously changing background illumination. It has been demonstrated in the present study that intermittent background illumination can prolong after-images considerably, and that after-images observed with intermittent background illumination are of longer duration in the periphery. Only a few primary durations were longer than 2 min, and none were as long as 3 min. Total durations were as long as 11, 9 and 6 min for RB, KM and DW, respectively. For RB and KM, this considerable prolongation of after-images was possible only with peripheral stimulation. Peripheral durations were generally longer for DW, but not a great deal longer. The results of this study, as well as results obtained by BRINDLEY (1962), tend to indicate that afterimages observed with intermittent background illumination last longer in the periphery than in the fovea. BRINDLEY found that with a 1.5 degree stimulus fovea1 after-images did fade out more rapidly; however, durations of his fovea1 after-images were also quite long. Anatomical studies have indicated that the rod-free area of the fovea is 45 min (~STERBERG, 1935) to 1 degree, 40 min (POLYAK, 1941) in diameter. The stimulus fields used in the present study and in BRINDLEY’S study would seem to be small enough to confine most of

After-Images Observed with Intermittent Background Illumination

131

the stimulation to the rod-free area; however, retinal stray light could be considerable with the extremely high stimulus intensities used. Positive after-images can reasonably be attributed to some sort of persistent after-effect of stimulation. Negative after-images can reasonably be attributed to some sort of local adaptation (i.e. the previously stimulated area would be less sensitive to subsequent background illumination than surrounding retinal areas). It is interesting to note that the total adaptation effect produced with stabilized images can be almost entirely eliminated with intermittent illumination (DITCHBURN and FENDER, 1955). Perhaps some sort of adaptation, which might account for the disappearance of images under steady conditions of illumination, is interfered with by transient effects of changes in illumination both in the case of after-images and in the case of stabilized images. REFERENCES ALP~RN M. and BARR,L. (1962). Durations of the after-images of brief light flashes and the theory of the Broca and Sulzer phenomenon. J. opt. Sot. Amer. E&219-221. BERRY,W. and IMUS,H. (1935). Quantitative aspects of the flight of colors. Amer. J. Psycho/. 47,449-457. BRINDLEY,G. S. (1962). Two new properties of fovea1 after-images and a photochemical hypothesis to explain them. J. Pbysiol. 164, 168-179. CREED,R. S. and GRANIT, R. (1928). On the latency of negative after-images following stimulation of different areas of the retina. J. Physiol. 66, 281-298. DITCHBURN,R. W. and FENDER,D. H. (1955). The stabilized retinal image. Opticu Acta 2, 128-132. EBBECKE,U. (1929). Uber positive tmd negative Nachbilder, ihre gegenseitige Beziehung, und den Einfluss der lokalen Adaptation. PpUg. Arch. ges. Physioi. 221, 160-188. FE~NBLOOM, W. (1938). A quantitative study of the visuai after-image. Arch. Psychoi. N. Y. 33, No. 233. FRANZ, S. I. (1899). After-images. Psychof. Rev. monogr. suppl. 3, no. 2 (whole no. 12). KARWOSKI,‘I. F. and CROOK,M. N. (1937). Studies in the peripheral retina: I. The Purkinje after-image. J. gen. Psychof. 16,323-356.

KARWOSKI,T. F. and PERRY,W. B. (1943). Studies in the peripherai retina: III. The Purkinje after-image bulge. J. gen. Psychol. 29,63-85. KARWOSKI,T. F. and WARRENER,H. (1942). Studies in the peripheraf retina: II. The Purkinje after-image on the fovea1 area of the retina. J. gen. Psychol. 29, 129-l 51. KARWOSKI,T. F. and WAYNER,M. (19.51). Studies in the peripheral retina: IV. The interactions of rods and cones in after-sensations. J. gen. Psychol. 44, 215-233. MCDOUGALL,W. (1901). Some new observations in support of Thomas Young’s theory of light- and colour-vision (I). Mind 10 (new series), 52-97. MILES,G. H. (1915). The formation of projected visual images by intermittent retinal stimulation. &it. J. Psychol. 7,42U434.

~STERBERG,G. (1935). Topography of the layer of rods and cones in the human retina. Acrn oprid., Kbh. 164, Suppl. v. PADGHAM,C. A. (1953). Quantitative study of visual after-images. &if. J. Oohrhul. 37. 165-170. PADGHAM,C. A. (1957). Further studies of the positive after-image. Optica ,kfa 4, 102-107. POLYAK,S. L. (1941). The Retina. Chicago: Univ. of Chicano Press. ROBERTSON, V.‘M. and FRY, G. A. (1937): After-images observed in complete darkness. Amer. J. Psychoi. 49,265-276.

SHUEY,A. M. (1924). The flight of colors. Amer. J. Psychol. 35, 559-582. SHUEY,A. M. (1926). The effects of varying periods of dark adaptation on the flight of colors. Amer. J. Psychol. 37, 528-537.

SIEGEL,S. (1956). Nonparametric Statisfics. McGraw-Hill, New York. TREZONA,P. W. (1960). The after-effects of a white light stimulus. J. Physiol. MO, 67-78. TROLAND,L. T. (1917). Preliminary note: the influence of changes of illumination upon after-images. Amer. J. Psychol. 28,497-503.

Abstract-It has been reported that an intermittent background can considerably prolong the duration of after-images. The luminances of both an inte~ittently illuminated background and a one degree stimulus were varied as three subjects made observations foveally and five degrees in the periphery.

132

HALSEY

The results mittent

confirmed

background

that

longer

compared

H.

MATTESON

durations

to the durations

of

after-images

of those

were

observed

with

an

inter-

showed lasted longer than fovea1 ones. but the effects of background

observed

in the dark,

that with the intermittent background peripheral after-images After-image duration also increased with stimulus intensity, luminance varied with subjects.

and further

Resum&-On a constati qu’un fond intermittent peut prolonger considerablement la duree des images consecutives. On presente a trois sujets, en vision foveale et a cinq degres periphtriquement, un stimulus de un degre, dont la luminance ainsi que celle du fond intermittent Ctaient variables. Les resultats confirment que les images consecutives durent plus longtemps avec un fond intermittent que dans l’obscurite, et en outre que sur fond intermittent les images periphtriques durent plus longtemps que les fovtales. La duree de l’image consecutive augmente aussi avec I’intensitt du stimulus, mais l’effet de la luminance du fond varie avec les sujets. Zusammenfassung-Es wurde berichtet, dass ein flimmerndes Umfeld die Dauer der Nachbilder bedeutend verllngern kann. Bei drei Versuchspersonen werden sowohl bei Leuchtdichte des flimmemden Umfeldes als such eines Testzeichens von 1” variiert, das fovea1 und unter 5” peripher dargeboten wurde. Die Ergebnisse bestltigen, dass bei einem flimmemden Umfeld lingerdauemde Nachbilder beobachtet werden als ohne Umfeld. Weiterhin zeigte sich, dass bei flimmemdem Umfeld periphere Nachbilder linger dauem als foveale. Die Nachbilddauer nimmt such mit der Reizintensitat zu, der Einfluss der Umfeldleuchtdichte war jedoch bei den einzelnen Versuchspersonen verschieden. Pe3mMe-Coo6manoCb, yBeAU’IUTb MblMU

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