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Sustained visual afterimages
LETTER TO THE EDITORS SUSTAINED
VISUAL AFTERIMAGES
(Received 22 October 1973)
If a brief Rash of light is delivered to the eye, an afterimage that mirrors the size, shape and retinal position of the inducing stimulus may be observed. Depending upon changes in the state of the retinal cells themselves (Craik. 1940; Newsome. 1971) an afterimage is a spatially (though not temporally) perfectly stabilized retinal event-an event. moieover, for which there is good reason to believe that is virtually indistinguishable to the more central parts of the visual pathway from a real light image stabilized on the retina by optical devices (Barlow and Sparrock, 1964). Typically during prolonged viewing a pattern viewed as an optically stabilized image or as an afterimage is visible for only a fraction of the total observation period, intermittently fades and regenerates either as a whole or in parts (e.g. Barlow and Sparrock, 1964; Mackinnon, Forde and Piggins, 1969). If, however, during optical image stabilization, the light is modulated in time at suitable frequencies the visibility of the target may be greatly improved (Fiorentini and Ercoles. 1960; West. 1968; Keesy, 1969, 1973). Likewise. it is common knowledge that afterimages may be kept longer “alive” and appear more vivid when viewed under conditions of intermittent retinal stimulation (as produced for example by turning the background illumination on and off, or simply by blinking) than against a steadily illuminated background or in steady darkness. As part of a general study of the prolonged afterimages induced by brief, high-intensity flashes, the effect of intermittent background illumination has been investigated for an extended frequency range. The results are briefly reported here. The afterimage of a small vertical bar, 10’ x 40 visual angle, was generated in the subject’s right-eye fovea by a 0.8 msec stimulus from a Braun F290 electronic flash unit. delivered via a beamsplitter placed in front of the eye. The afterimage was viewed against a 203 circular. luminous background-a diffusing screen shielded off by black cardboard. To aid steady fixation cross hairs were etched in the screen except in the 2” central region. The screen was illuminated from behind by a 250 W tungsten filament source. The lightbeam was chopped by a rotary sector-disc providing approximate square-wave flicker (rise and fall times less than 5 per cent) at the background. The mean luminance during flicker was 2.8 cd/m’ for light-dark ratio 1:l. The subject was comfortably seated with his head supported by a chin-rest and the non-viewing eye 143
occluded. Having adapted to the mean background luminance, the subject fixated the center of the screen and triggered the flash. The presence or absence of the afterimage was reported by pressing or releasing a key, and the responses were continuously recorded on an ink writer. Atria1 was considered terminated when no afterimage had been reported visible for 1 min as approximately timed by the experimenter watching the records. At least 10 mm elapsed between successive trials in order to let any trace of previous afterimages subside. In the experiments reported in this communication the authors acted as subjects. Figure 1 shows the duration and visibility (i.e. the total amount of time for which the afterimage was reported visible) of the afterimage (in seconds) as a function of the frequency of intermittent background illumination, Each point in the graph represents the mean value of at least ten measurements. The two measures are highly correlated in the frequency range investigated: lowering frequency from the CFF (in the neighbourhood of 40 Hz) produces no change before 8 Hz, below which duration and visibility increase steadily to reach their maximum values at the lowest frequency employed (05 Hz). These results agree closely with the results of Fiorentini and Ercoles (1960) and Keesy (1973), employing optical image stabilization. In a subsequent series of observations a delay period was introduced between inducing flash and the onset - 300
Fig. 1. Mean afterimage duration (0) and visibility (0) as a function
of the frequency
of intermittent
background
illu-
mination. The smooth curves are drawn freely through the data points. Results for both subjects,
Letter to the Editors
744
Thus, whilst the final decay of the titerimage under the temporal conditions optimali for sustaining the afterimage may be related to the regeneration uf bleached pigments. is it clear that neural adaptation of border and contrast subserving mechanisms similar to that observed during stabilization of real light images (e.g. Gerritsand Vendrik. 1970) is nnrmally involved in
the fading effect. Similar conclusions have been rtxhed
200
L 0
I 60
I
120
I 180
I 2f.40
Fig. 3. The duration of the afterimage as a function of the temporal separation between inducing flash and on-set of 2 HI background flicker. Each point is the mean value obtained from five measurements, one standard deviation is plotted to each side of the mean. Inset shows the temporal course of visibility when flicker was turned on 3 min after the inducing flash was delivered. as marked by arrow. The records were analyzed for successive 20-set intervals and the percentage of the time for which the afterimage was visible was computed for each interval. Result for subject T.T.
of flicker, during
which the afterimage was viewed against a 56cd/m2 steady field. Figure 2 shows the duration of the afterimage as a function of the temporal separation between inducing Rash and onset of 2 Hz ba~k~uund flicker. It is clear that the period during which the afterimage is capabk of being reviled is the same whether flicker is continuously present or turned on in later stages. Inset, Fig. 2 depicts the tem-
poral course of visibility when flicker was turned on 3 min after the inducing f&h was delivered. During the steady-light delay period the afterimage completely faded after about 2 min; however, when flicker was turned on after three minutes the afterimage immediately returned and was maintained for another I.5 min before slowly fading away.
by others.
rlcknowle~getnents--We thank A. Glad and D. J. Piggins ior comments on an earlier draft. This study was supported by a grant from the Norwizgian Research Council for Science and the Humanities to S.M. Institute of Psychology, Uniwrsity of Oslo, ~ii~er~ Oslo 3, Norway
SVEIN MAGNG~EN TORE
TORRJSSEN
REFERENCES
Barlow H. B. and Sparrock J. M. B. (1964) The role of afterimages indark%daption. Science, N.l! 144,13@+-1314. Craik K. J. W. (1940)Origin of visual tifter-images. Mature, Land. 145,512. Fiorentini A. and Ercoles A. M. (1960) Vision with stabil@d images and intermittent illumination. Atti. Fond. G. Ronchi 15,618-633. Gerritz H. J. M. and Vendrik A. J. H. (1970) Simultaneous contrast, filling-in prooesp and information processing in man’s v&al system. Exp. Bruin Res. 11,41 f-430. Keesy U. T. (1969j Vis~biIity of a stabilized target as function of frequency and amp&t&e of lumiti#nce variation. J. opt. Sot. Am. S9,604--6ICJ. Keesy U. T. (1973) Stabilized target visibility as a function of contrast and flicker frequency. Vision Res. 13, l3671373. MacKinnon G. E., Forde J. and Piggips D. J, (19691 Stabtlized images, steadily fixated figures, ~nd.prolonged aft&images. Can. 6. Psyckol. 23,184-195 Yewsome D.A. (1971) Afterimage and pupiilary activity following strong light exposure. viLs@ Res.-lE, 275-288; West D. C. (1968) Fliekr- and the stabilized retinal imqe. Vision Res. 8, 7i9-745.
VISUAL AFTERIMAGES
(Received 22 October 1973)
If a brief Rash of light is delivered to the eye, an afterimage that mirrors the size, shape and retinal position of the inducing stimulus may be observed. Depending upon changes in the state of the retinal cells themselves (Craik. 1940; Newsome. 1971) an afterimage is a spatially (though not temporally) perfectly stabilized retinal event-an event. moieover, for which there is good reason to believe that is virtually indistinguishable to the more central parts of the visual pathway from a real light image stabilized on the retina by optical devices (Barlow and Sparrock, 1964). Typically during prolonged viewing a pattern viewed as an optically stabilized image or as an afterimage is visible for only a fraction of the total observation period, intermittently fades and regenerates either as a whole or in parts (e.g. Barlow and Sparrock, 1964; Mackinnon, Forde and Piggins, 1969). If, however, during optical image stabilization, the light is modulated in time at suitable frequencies the visibility of the target may be greatly improved (Fiorentini and Ercoles. 1960; West. 1968; Keesy, 1969, 1973). Likewise. it is common knowledge that afterimages may be kept longer “alive” and appear more vivid when viewed under conditions of intermittent retinal stimulation (as produced for example by turning the background illumination on and off, or simply by blinking) than against a steadily illuminated background or in steady darkness. As part of a general study of the prolonged afterimages induced by brief, high-intensity flashes, the effect of intermittent background illumination has been investigated for an extended frequency range. The results are briefly reported here. The afterimage of a small vertical bar, 10’ x 40 visual angle, was generated in the subject’s right-eye fovea by a 0.8 msec stimulus from a Braun F290 electronic flash unit. delivered via a beamsplitter placed in front of the eye. The afterimage was viewed against a 203 circular. luminous background-a diffusing screen shielded off by black cardboard. To aid steady fixation cross hairs were etched in the screen except in the 2” central region. The screen was illuminated from behind by a 250 W tungsten filament source. The lightbeam was chopped by a rotary sector-disc providing approximate square-wave flicker (rise and fall times less than 5 per cent) at the background. The mean luminance during flicker was 2.8 cd/m’ for light-dark ratio 1:l. The subject was comfortably seated with his head supported by a chin-rest and the non-viewing eye 143
occluded. Having adapted to the mean background luminance, the subject fixated the center of the screen and triggered the flash. The presence or absence of the afterimage was reported by pressing or releasing a key, and the responses were continuously recorded on an ink writer. Atria1 was considered terminated when no afterimage had been reported visible for 1 min as approximately timed by the experimenter watching the records. At least 10 mm elapsed between successive trials in order to let any trace of previous afterimages subside. In the experiments reported in this communication the authors acted as subjects. Figure 1 shows the duration and visibility (i.e. the total amount of time for which the afterimage was reported visible) of the afterimage (in seconds) as a function of the frequency of intermittent background illumination, Each point in the graph represents the mean value of at least ten measurements. The two measures are highly correlated in the frequency range investigated: lowering frequency from the CFF (in the neighbourhood of 40 Hz) produces no change before 8 Hz, below which duration and visibility increase steadily to reach their maximum values at the lowest frequency employed (05 Hz). These results agree closely with the results of Fiorentini and Ercoles (1960) and Keesy (1973), employing optical image stabilization. In a subsequent series of observations a delay period was introduced between inducing flash and the onset - 300
Fig. 1. Mean afterimage duration (0) and visibility (0) as a function
of the frequency
of intermittent
background
illu-
mination. The smooth curves are drawn freely through the data points. Results for both subjects,
Letter to the Editors
744
Thus, whilst the final decay of the titerimage under the temporal conditions optimali for sustaining the afterimage may be related to the regeneration uf bleached pigments. is it clear that neural adaptation of border and contrast subserving mechanisms similar to that observed during stabilization of real light images (e.g. Gerritsand Vendrik. 1970) is nnrmally involved in
the fading effect. Similar conclusions have been rtxhed
200
L 0
I 60
I
120
I 180
I 2f.40
Fig. 3. The duration of the afterimage as a function of the temporal separation between inducing flash and on-set of 2 HI background flicker. Each point is the mean value obtained from five measurements, one standard deviation is plotted to each side of the mean. Inset shows the temporal course of visibility when flicker was turned on 3 min after the inducing flash was delivered. as marked by arrow. The records were analyzed for successive 20-set intervals and the percentage of the time for which the afterimage was visible was computed for each interval. Result for subject T.T.
of flicker, during
which the afterimage was viewed against a 56cd/m2 steady field. Figure 2 shows the duration of the afterimage as a function of the temporal separation between inducing Rash and onset of 2 Hz ba~k~uund flicker. It is clear that the period during which the afterimage is capabk of being reviled is the same whether flicker is continuously present or turned on in later stages. Inset, Fig. 2 depicts the tem-
poral course of visibility when flicker was turned on 3 min after the inducing f&h was delivered. During the steady-light delay period the afterimage completely faded after about 2 min; however, when flicker was turned on after three minutes the afterimage immediately returned and was maintained for another I.5 min before slowly fading away.
by others.
rlcknowle~getnents--We thank A. Glad and D. J. Piggins ior comments on an earlier draft. This study was supported by a grant from the Norwizgian Research Council for Science and the Humanities to S.M. Institute of Psychology, Uniwrsity of Oslo, ~ii~er~ Oslo 3, Norway
SVEIN MAGNG~EN TORE
TORRJSSEN
REFERENCES
Barlow H. B. and Sparrock J. M. B. (1964) The role of afterimages indark%daption. Science, N.l! 144,13@+-1314. Craik K. J. W. (1940)Origin of visual tifter-images. Mature, Land. 145,512. Fiorentini A. and Ercoles A. M. (1960) Vision with stabil@d images and intermittent illumination. Atti. Fond. G. Ronchi 15,618-633. Gerritz H. J. M. and Vendrik A. J. H. (1970) Simultaneous contrast, filling-in prooesp and information processing in man’s v&al system. Exp. Bruin Res. 11,41 f-430. Keesy U. T. (1969j Vis~biIity of a stabilized target as function of frequency and amp&t&e of lumiti#nce variation. J. opt. Sot. Am. S9,604--6ICJ. Keesy U. T. (1973) Stabilized target visibility as a function of contrast and flicker frequency. Vision Res. 13, l3671373. MacKinnon G. E., Forde J. and Piggips D. J, (19691 Stabtlized images, steadily fixated figures, ~nd.prolonged aft&images. Can. 6. Psyckol. 23,184-195 Yewsome D.A. (1971) Afterimage and pupiilary activity following strong light exposure. viLs@ Res.-lE, 275-288; West D. C. (1968) Fliekr- and the stabilized retinal imqe. Vision Res. 8, 7i9-745.