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Temporal studies with flashed gratings: Inferences about human transient and sustained channels
TEMPORAL STUDIES WITH FLASHED GRATINGS: INFERENCES ABOUT HUMAN TRANSIENT AND SUSTAINED CHANNELS’ BRLWO G. BR~TMEYER
Department of Psychology, University of Houston, Houston, TX 77004, U.S.A. and LEO Gtiz Department of Psychology, Stanford University, Stanford, CA 94305, U.S.A. (Received 8 February 1976; in revised form 4 January 1977) Abstract-The temporal response properties of the human visual system to low and high spatial frequency gratings was investigated by two contrast detection threshold techniques. With the tirst technique the contrast threshold for detecting vertical sinusoidal gratings at spatial frequencies of 0.5. 2.8 and 16.0 c/deg was determined at exposure durations ranging from 20 to 4OOmsec. It was found that the critical duration, at and below which reciprocity between contrast and a nonunity power of duration holds, increased from roughly 60 to 2OOmsec as spatial frequency increased from 0.5 to 16.0 c/deg. The second technique involved subthreshold summation of two, 10 msec flashed presentations of either a 1.0 or lO.Oc/deg grating. The stimulus onset asynchrony (SOA) separating the onsets of the two pulses varied from 0 to 210 msec. The results revealed that the subthreshold interaction of the two flashes at high spatial frequencies can be characterized by monophasic sustained excitation and inhibition; at low spatial frequencies, however, this interaction can be characterized by a multiphasic oscillation of excitation and inhibition superimposed on a monophasic excitatory-inhibitory interaction. The findings are related to properties of transient and sustained channels assumed to exist in human vision. _ INTRODUCTION
As a result of recent psychophysical studies, it has been suggested that the human visual system contains at least two classes of channels which can be distinguished on the basis of their temporal response properties and the range of spatial frequencies or stimulus sizes over which each class preferably operates (Breitmeyer and Julesz, 1975; Keesey, 1972; King-Smith and Kulikowski, 1973, 1975: Kulikowski and Tolhurst, 1973; Tolhurst, 1975). Channels of one class, called “transient”, operate at low to moderate spatial frequencies and are characterized by a transient response to the on and offset of a flashed stimulus of prolonged duration (Tolhurst, 1975) and by a relatively high temporal resolution (King-Smith and Kulikowski, 1975; Kulikowski and Tolhurst, 1973); channels of the other class, called “sustained”, operate at moderate to high spatial frequencies and are characterized by a sustained response to a flashed stimulus of prolonged duration and by relatively poor temporal resolution (King-Smith and Kulikowski, 1975; Kulikowski and Tolhurst, 1973; Tolhurst, 1975). The present study further investigates the temporal response properties of these hypothesized transient and sustained channels by measuring the response of the human visual system to flashed gratings ranging from relatively low to high spatial frequencies. Two types of flashed presentation, based on previously
devised methods used to study the temporal response of the visual system to flashed uniform-field stimuli (Bartlett, 1965; Ikeda, 1965; Rashbass, 1970) were employed. With one technique, the threshold contrasts of flashed gratings are measured as a function of their duration of presentation. DilIerences in the temporal integration of contrast at threshold as a function of spatial frequency can then be related to the hypothesized underlying transient and sustained channels. With the other technique, the visual system’s sensitivity to a briefly flashed grating was determined as a function of the temporal interval separating it from a similar flashed presentation of the same grating fixed at a suitably chosen subthreshold contrast value. Again differences in the interaction over time of these two flashed presentations as a function of the spatial frequency of the grating may reveal differences in the temporal response properties of the assumed underlying transient and sustained channels. METHOD Subjects
One subject, CM, was a 20-yr-old, female undergraduate at Stanford University. She had corrected-to-normal vision, was highly practiced as a psychophysical observer, but was naive in regard to the purposes of the experiment. One of the authors, BB, served as the second subject. Stimuli and general procedure
’ This study was supported by the National Eye Institute, National Institute of Health, U. S. Public Health Service. Grant EY01242 to the second author.
The inspection field consisted of a Tektronix 602 oscilloscope (P4 phosphor) viewed at a distance of 110 cm. The oscilloscope screen was masked to produce a rectangular.
362
BRLSO G. BREITMEYERand LEO GASZ
4
x 6 srimulus field. A small fixation point was attached to the screen and centered in the stimulus field. Subjects fixated rhis point throughout an experimental session. The uniform-field luminance of the display was 5.0 ft-L. The stimuli consisted of vertical sinusoidal gratings. They were generated by a sinewave function generator (Wavetek) which modulated the Z-axis input of the oscilloscope. To control the spatial contrast of the gratings, a logarithmic voltage attenuator, variable in 0.05 log unit steps, was placed between the function generator and the Z-axis input of the oscilloscope. The contrast of a grating is given by (I,,,,, - ~,,,)/(~,,, + I,,,,,) where I,, and Imln are the luminance maximum and minimum respectively within one period of the grating. Photometric calibration showed that up to a value of 489, grating contrast varied nearly linearly with voltaee. The subject sat in a chair and his head rested on a chin and head brace while viewing the stimulus field. No optics or artificial pupils were used. Prior to initiating the determination of contrast thresholds for detecting gratings. the subject viewed the uniformly illuminated inspection field-located in an otherwise dark room-for 5 min in order to attain a stable light adaptation level. For any contrast threshold determination the logarithmic voltage attenuator was initially set to produce a clearly subthreshold contrast, and on successive stimulus presentations the voltage was serially incremented in 0.05 log unit steps until the contrast value was sufficiently high so that the subject was able to detect the grating two consecutive times. Repeated threshold observations of this type were obtained and averaged to determine (mean) threshold contrast values. In experiment I threshold observations were made at each of three spatial frequencies of 0.5, 2.8 and 16.0c/deg and at each of I1 stimulus durations ranging from 30-4OOmsec (see Fig. I). Within any given experimental session all spatial frequencies were tested at all durations in a cross randomized manner. Similarly, in experiment 2, two-pulse thresholds (see Fig. 1 and below) were determined at spatial frequencies of 1.0 and lO.Oc/deg and at each of 12 stimulus onset asynchronies (SOAs) ranging from 0 to 2LOmsec. The method for measuring the twopulse threshold was an adaptation of the procedures used by lkeda (1965) and Rashbass (1970). The two pulses each were ot‘ IO msec duration and each contained the same spatial frequency target. The first pulse was always maintained as nearly as possible at 0.15 log unit (3dB) below contrast threshold (based on two threshold observations
made at the start of each experimental session). The conON
EXP I
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20 - 400msecd
OFF
Fig. 1. The stimulus timing of sinusoidal gratings presented in experiments I and 2. In experiment 1 only one presenration was made on a given trial and it cduld range in duration from 20 to 4OOmsec. In experiment 2, two lo-msec grating presentations were made; the onset of the second presentation was separated from the onset of the first by intervals ranging from 0 to 210 msec. The first grating presentation was 0.15 log unit below contrast threshold.
trast of rhe second pulse was adjusted as described above until the subject reported detecting a gratmg. RESULTS Experiment
1
In the first experiment threshold contrasts for detecting a grating at a spatial frequency of 0.5, 2.S and 16.0 c!deg and at durations of 20, 30, 40. 50, 80. 110, 140. 170. 200, 300 and JOOmsec were measured. Assuming that differences in temporal response properties of transient and sustained channels manifest themselves. among other ways, in their respective temporal integration times. one would expect systematic differences in temporal integration of contrast as the spatial frequency of the test gratings varies from low to high. The results confirm this expectation. Figure 2 shows the contrast in units of log per cent plotted against stimulus duration in units of log msec. All data points are based on the mean of 10 threshold measurements. Note that at all three spatial frequencies log threshold contrast tends to decrease linearI> with log duration over a limited and spatial frequency-specific range of durations. The slope of the reciprocity function relating log threshold contrast to log duration is the same at all spatial frequencies and for both subjects. The value of the slope is approx -0.70 (the fit of the straight line segments to the data was determined by eye). That is to say. the contrast-duration reciprocity function can be expressed best by the formula (Contrast) x (Duration)‘.” = Constant or alternately (Contrast)‘.“’ x (Duration) = Constant. Perfect temporal integration of contrast would obtain if the power exponent of both the contrast and the duration terms were 1.0. similar to the results of many studies of temporal integration of luminance which can be expressed in terms of the well known Bloch’s law. Luminance x Duration = Constant (Barlett, 1965; Owens, 1972). The deviation from perfect temporal integration of contrast is discussed later. Despitk the common, non-unity slope of the reciprocity functions. Fig. 2 also shows that the duration at and below which the contrast-duration reciprocity holds and above which it breaks down increases with spatial frequency. As indicated by the vertical dashed lines in Fig. 2, at spatial frequencies of 0.5. 2.5 and 16.0 c/deg the critical durations are 80, 150 and 2OOmsec, respectively, for subject CM and 60. 150 and 200 msec, respectively, for subject BB. Thus, in the human visual system, integration time for contrast increases with the spatial frequency of a grating. Elaboration of this particular finding is left for the Discussion. Experiment 2 In this experiment two-pulse threshold contrasts for detecting a grating at a spatial frequency of 1.0 and lO.Oc/deg were measured. .As stated. the two pulsed presentations of a grating were each of 10msec duration. The first of these grating presentations was 0.15 log unit below threshold contrast whereas the contrast of the second grating could be adjusted until a threshold for the two-pulse presentation was obtained. The SOAs separating the onsets of the two
863
Temporal studies with flashed gratings
STIMULUS
DURATION
(MSECI
Fig. 2. Threshold contrast in per cent at spatial frequencies of 0.5,2.8 and 16.0 cidea as a function of stimulus duration. Both threshold contrasf and stimulus duration are plotted along logarithmic coordinates in order to obtain linear functions indicating the contrast-duration reciprocity at each spatial frequency.
pulses were 0, 15, 30, 45, 60, 75, 90, 105, 130, 155, 180 and 210msec. By measuring the threshold contrast of the second pulse as a function of its SOA, one can determine how the two responses of the visual system generated by the two-pulse presentation of a grating interact over time. As noted, differences in the form of this interaction over time between the relatively low, l.Oc/deg, spatial frequency and the relatively high, 10.0 c/deg, spatial frequency gratings may indicate differences between the temporal waveform of the responses of transient and sustained channels to flashed stimuli. Figure 3 shows the results of this experiment. The inverse of threshold contrast, i.e. contrast sensitivity, for the second pulse is plotted against SOA. Each data point is based on 15 observations. Since the first pulse presentation of a grating was fixed at 0.15 log unit below detection threshold, the present results give an indication of how the presumably fixed sub threshold activity generated by the first pulse combines over time with the subthreshold activity generated by the second pulse. Thus, the subthreshold summation of the two pulses shows how activity produced by the first pulse temporally modulates the contrast sensitivity of the second pulse. From this one can infer the temporal wave form of the activity generated by the first pulse. Several aspects of these results are noteworthy. Most obvious is the fact that at both spatial frequencies the contrast sensitivity for the second pulse is highest at an SOA of Omsec; i.e. when the second pulse is synchronous with the iirst and consequently when optimal temporal superimposition of the activity generated by the two pulses can occur. A corollary finding is that the contrast sensitivity of the second pulse generally decreases with increases in SOA. The effect of temporal separation of the two pulses is thus an overall decrease in the summation of the activity generated by the two pulses. It should be noted that the decrease in contrast sensitivity with SOA is monotonic for the high spatial frequency grating; however, for the low spatial frequency grating
the contrast sensitivity for the second pulse, although showing an overall decline with SOA, nevertheless reveals a multiphasic or periodic nonmonotonicity. In particular, for both subjects a contrast sensitivity minimum occurs at an SOA of roughly 60 msec, and a secondary maximum occurs at SOAs of lOO-120msec, followed in turn by a decline in contrast sensitivity. The fact that this periodic or multiphasic variation in contrast sensitivity is found at low but not high spatial frequencies signifies clear differences in the temporal response properties of the assumed underlying transient and sustained channels (see Discussion). Another aspect of the present results is also noteworthy. The dashed lines emanating from the SOA =a point indicate the contrast sensitivity (at each spatial frequency and for both subjects) of an isolated 10msec pulse of a test grating. Note that at low spatial frequencies the two-pulse contrast sensitivity is higher than that of a single pulse only at and below SOAs of 20msec. Beyond that SOA value. and despite the above mentioned nonmonotonicity of the contrast sensitivity, the contrast sensitivity for the second pulse always lies below that of the single, isolated pulse. Thus, at the low spatial frequency the multiphasic nature of the contrast sensitivity variations is superimposed what appears to be a monophasic, sustained suppression of the contrast sensitivity. This monophasic suppression of the contrast sensitivity of the second pulse is particularly evident at the high spatial frequency. Here, the two-pulse contrast sensitivity is higher than that of the single isolated pulse only at and below SOAs of 60msec. At higher SOA values the contrast sensitivity of the second pulse is continually diminished relative to that of the single pulse. On the basis of this result. it seems likely that the first pulse can have not only a facilitatory effect on the contrast sensitivity of the second pulse at small SOAs, but also a suppressive effect at larger SOAs. I
I
0
I
I
40
STIMULUS
I
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00 ONSET
I
I
120
I
I
160
ASYNCHRONY
I
I
II
200
I
-I
0)
(MSEC)
Fig. 3. Contrast sensitivity (inverse of threshold contrast) for the second of two IO-msec grating presentations at spatial frequencies of 1.0 and lO.Oc/deg as a function of the stimulus onset asynchrony separating the onset of the second presentation from that of the first. The lirst grating presentation was 0.15 log unit below threshold. Dashed lines indicate the contrast sensitivity of a single lo-msec presentation of the test gratings.
BRLWO G. BRE~EYERand LEOGANZ
864
DISCC’SSION
The two experiments reported here addressed themselves to the study of the human visual system’s temporal response when stimulated with flashed gratings of variable spatial frequency. The first experiment showed, for one thing, that the critical duration, at and below which contrast-duration reciprocity at threshold holds, increases with spatial frequency. At a spatial frequency of 0.5 c/deg the critical duration was roughly 60 msec; at the highest spatial frequency of 16.0 c/deg the critical duration was 200 msec. On the assumption that transient and sustained channels operate at low to moderate and moderate to high spatial frequencies respectively (Tolhurst, 1975), one can infer that transient channels have a substantially briefer integration time than sustained channels. This interpretation is consistent with several other psychophysical findings. For instance, Kulikowski and Tolhurst (1973) found that the fusion frequency for a lOc/deg grating presented in counterphase flicker was lower when subjects were required to detect the spatial periodicity of the grating than when they were required to simply detect flicker. This lower temporal resolution of the spatial pattern detectors most likely is another indicator of the longer integration time of sustained channels as determined in the present study. Moreover, the findings of several recent investigations using flashed square-wave gratings (Nachmias, 1967; Schober and Hilz, 1965) as well as flashed sine-wave gratings (Arend, 1976) indicate that the contrast sensitivity at moderate to high spatial frequencies benefits more from increases in exposure duration than the contrast sensitivity at low to moderate spatial frequencies. This is entirely consistent with the present tindings which show that beyond 60-80 msec increases in stimulus duration benefit the contrast sensitivity only with the higher spatial frequency gratings but not with the low spatial frequency one. Moreover. Kahneman (1964) and Kahneman and Norman (1964) found that whereas the critical duration for brightness discrimination in which Bloch’s law holds was on the order of lOOmsec, it was on the order of 2OC-3OOmsec for spatial acuity tasks. Insofar as sustained channels are primarily involved in analysis of pattern detail (Kulikowski and Tolhurst, 1973), the Kahneman (1964) and Kahneman and Norman (1964) results dovetail nicely with the findings of experiment 1. Furthermore. the study of Graham and Margaria (1935) indicates that the integration time for luminance increases as the size of a uniformly illuminated test field decreases (but see O&ens (1972) for a critique). This also is consistent with the present finding of longer integration times at higher spatial frequencies, since an increase in the spatial frequency of a grating is equivalent to a directly proportional decrease in the width of the individual dark and light bars of the grating. Another noteworthy feature of the results of experiment 1 is that the temporal integration of contrast is not perfect. Over the range of duration for which contrast-duration reciprocity holds at each spatial frequency tested. the slope of the function relating log contrast to log duration is -0.7. Perfect temporal integration as expressed by Bloch’s law would require a slope of - 1.0. Results of investigations of visual
spatio-temporal integration reported by Owens (1972) may shed some light on this finding. Owens (1972) found that as the diameter of a uniform test field superimposed on a 9.8 ft-L background increased from 3.6’ to 32’ arc, the slope of the function relating log threshold luminance to log duration changed from -1.0 to -0.85. On the basis of these and similar results, Owens (1972) concluded that Bloch’s law does not apply when stimuli were used whose angular size exceeded a critical value. In Owen’s case, using stimuli presented 10” nasally, the critical diameter was roughly 9’ arc. In the present study a 4” x 6’ foveally centered display was used. Since the spatial frequency ranged from 0.5 to 16.0 cideg. the widths of the alternate black and white bars of the gratings ranged from 2’ arc to I ‘. At or near the fovea, these sizes may already exceed the critical value beyond which Bloch’s law no longer holds. If not the width of the grating bars, then certainly their height of 4.0” would exceed such a critical value, and consequently could give rise to a contrast-reciprocity function with a slope less steep than - 1.0, e.g. -0.7. The second experiment revealed differences in the temporal wave form of the visual system’s response to a flashed, subthreshold grating at each of 1.0 or else 10.0 c,‘deg. The two-pulse subthreshold summation technique used in experiment 2 yielded results which may be interpreted as the autocorrelation function of the temporal impulse response of the visual system (Rashbass, 1970. 1976). Insofar as the low and high spatial frequency gratings yielded different functions, one can infer differences in the impulse responses of transient and sustained channels. For the low, 1.0 c/deg, spatial frequency, the subthreshold summation of two 10msec flashes of the test grating revealed facilitation effects up to an SOA of 20 msec; for the high, lO.Ocjdeg, spatial frequency similar effects were obtained up to an SOA of 60msec. Beyond these two respective SOA values, the twopulse summation effect was of a generally suppressive nature. In particular, at the high spatial frequency there was a continual decline in contrast sensitivity from an SOA of 0 msec to an SOA of 210 msec-with a clear, sustained suppression effect evident over the 60-210 msec SOA interval. This is significant because at the high spatial frequency sustained channels are primarily activated. and from recent electrophysiological studies it is known that sustained channels are characterized not only by sustained excitation but also by sustained inhibition (Ganz and Felder, 1977: Winters and Hamasaki, 1976). The presence of the multiphasic or periodic fluctuaction in facilitation and suppression obtained with the two-pulse subthreshold summation of the low spatial frequency grating suggests that the impulse response of transient channels is characterized by a damped periodic oscillation of excitation alternating with inhibition. This interpretation is consistent with those of Rashbass (1970) who obtained similar results using a uniformly illuminated field rather than a grating stimulus. IMoreover. the present results obtained with the low and high spatial frequency grating are also consistent with the findings of Kelly (1971) who found that the temporal impulse response of uniform-field flicker detectors can be characterized by a damped periodic
Temporal studies with flashed gratings oscillation of excitation alternating with inhibition. However. the temporal impulse response of counterphase-flickered grating detectors was characterized by a nonoscillating. monophasic excitatory-inhibitory interaction. That is to say, on the reasonable assumption that transient channels are responsible for detect-
ing uniform field flicker and sustained channels are responsible for detecting the spatial structure of gratings (Kulikowski and Tolhurst. 1973). the present results are substantially in aseement with Kelly’s (1971) findings. They are also supported by recent electrophysiological results reported by Biittner er 01. (1975). Studying the response of cat retinal ganglion cells to brief light flashes. these investigators distinguished between two types of cells on the basis of their responses. Cells of one type yielded oscillating responses to brief flashes and were thought to be of the transient variety; cells of the other type yielded nonoscillating, monophasic responses to brief flashes and were thought to be of the sustained variety. Thus both electrophysiological as well as psychophysical studies point to fundamental differences between transient and sustained channels in terms of their respective temporal response properties. REFERESCES
Arend L. E. Jr. (1976) Response of the human eye to spatially sinusoidal gratings at various exposure durations. Vision Res. 16, 1311-1315. Bartlett N. R. (1965) Thresholds as dependent on some energy relations and characteristics of the subject. In Vision and Visrml Perceprion (Edited by Graham C. H.). pp. 154-184. Wiley, New York. Breitmeyer B. and Julesz B. (1975) The role of on and off transients in determining the psychophysical spatial frequency response. vision Res. 15, 411-415. Biittner U.. Griisser O.-J. and Schwanz E. (1975) The effect of area and intensity on the response of cat retinal ganglion cells to brief light flashes. Expl Bruin Res. 23. 259-278. Ganz L. and Felder R. C. (1977) Quantitative analysis of motion selectivity in complex neurons of the cat visual cortex: I. Receptive field analysis. (In preparation).
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Graham C. H. and Margaria R. (1935) Area and intensitytime relation in the peripheral retina. .&I. J. Physiol. 113. 299-305. Ikeda H. (1965) Temporal summation of positive and neeative flashes in the visual system. J. opt. Sot. .+n. b. 1527-1534. Kahneman D. (1964) Temporal summation in an acuity task at different energy levels-a study of the determinants of summation. Vision Res. 4. 357-566. Kahneman D. and Norman J. (1964) The time-intensity relation in visual perception as a function of observer’s task. J. esp. Psychol. 68, X5-220. Keesey U. (1972) Flicker and pattern detection: a comparison of thresholds. J. opt. Sot. Am. 62. u6-439. Kelly D. H. (1971) Theory of flicker and transient responses-11: counterphase gratings. J. opt. Sot. .-lm. 61. 632-640. King-Smith P. E. and Kulikowski J. J. (1973) Spatial arrangement of the flicker and pattern detectors for a fine line. J. Ph,vsioL, Lond. 234, 3335P. King-Smith P. E. and Kulikowski J. J. (1975) Pattern and flicker detection analyzed by subthreshold summation. .I. Physiol., Lond. 249, 519-548. Kulikowski J. J. and Tolhurst D. J. (1973) Psychophysical evidence for sustained and transient detectors in human vision. J. Physiol., Lond. 232, 149-162. Nachmias J. (1967) Effect of exposure duration on visual contrast sensitivity with square-wave gratings. J. opt. Sot. Am. 51, 421-427. Owens W. G. (1972) Spatio-temporal integration in the human peripheral retina. Vision Res. 12, 1011-1026. Rashbass C. (1970) The visibility of transient changes of luminance. J. Physiol.. Lond. 210, 165-186. Rashbass C. (1976) Unification of two contrasting models of the visual increment threshold. Vision Rrs. 16, 1281-1283. Schober H. A. W. and Hilz R. (1965) Contrast sensitivity of the human eye for square-wave gratings. J. opr. Sot. Am. 5.5. 10861091. Tolhurst D. J. (1975) Sustained and transient channels in human vision. Vision Res. 15, 1151-1155. Winters R. W. and Hamasaki D. I. (1976) Temporal characteristics of peripheral inhibition of sustained and transient ganglion cells in cat retina. Vision Res. 16, 37-45.
Department of Psychology, University of Houston, Houston, TX 77004, U.S.A. and LEO Gtiz Department of Psychology, Stanford University, Stanford, CA 94305, U.S.A. (Received 8 February 1976; in revised form 4 January 1977) Abstract-The temporal response properties of the human visual system to low and high spatial frequency gratings was investigated by two contrast detection threshold techniques. With the tirst technique the contrast threshold for detecting vertical sinusoidal gratings at spatial frequencies of 0.5. 2.8 and 16.0 c/deg was determined at exposure durations ranging from 20 to 4OOmsec. It was found that the critical duration, at and below which reciprocity between contrast and a nonunity power of duration holds, increased from roughly 60 to 2OOmsec as spatial frequency increased from 0.5 to 16.0 c/deg. The second technique involved subthreshold summation of two, 10 msec flashed presentations of either a 1.0 or lO.Oc/deg grating. The stimulus onset asynchrony (SOA) separating the onsets of the two pulses varied from 0 to 210 msec. The results revealed that the subthreshold interaction of the two flashes at high spatial frequencies can be characterized by monophasic sustained excitation and inhibition; at low spatial frequencies, however, this interaction can be characterized by a multiphasic oscillation of excitation and inhibition superimposed on a monophasic excitatory-inhibitory interaction. The findings are related to properties of transient and sustained channels assumed to exist in human vision. _ INTRODUCTION
As a result of recent psychophysical studies, it has been suggested that the human visual system contains at least two classes of channels which can be distinguished on the basis of their temporal response properties and the range of spatial frequencies or stimulus sizes over which each class preferably operates (Breitmeyer and Julesz, 1975; Keesey, 1972; King-Smith and Kulikowski, 1973, 1975: Kulikowski and Tolhurst, 1973; Tolhurst, 1975). Channels of one class, called “transient”, operate at low to moderate spatial frequencies and are characterized by a transient response to the on and offset of a flashed stimulus of prolonged duration (Tolhurst, 1975) and by a relatively high temporal resolution (King-Smith and Kulikowski, 1975; Kulikowski and Tolhurst, 1973); channels of the other class, called “sustained”, operate at moderate to high spatial frequencies and are characterized by a sustained response to a flashed stimulus of prolonged duration and by relatively poor temporal resolution (King-Smith and Kulikowski, 1975; Kulikowski and Tolhurst, 1973; Tolhurst, 1975). The present study further investigates the temporal response properties of these hypothesized transient and sustained channels by measuring the response of the human visual system to flashed gratings ranging from relatively low to high spatial frequencies. Two types of flashed presentation, based on previously
devised methods used to study the temporal response of the visual system to flashed uniform-field stimuli (Bartlett, 1965; Ikeda, 1965; Rashbass, 1970) were employed. With one technique, the threshold contrasts of flashed gratings are measured as a function of their duration of presentation. DilIerences in the temporal integration of contrast at threshold as a function of spatial frequency can then be related to the hypothesized underlying transient and sustained channels. With the other technique, the visual system’s sensitivity to a briefly flashed grating was determined as a function of the temporal interval separating it from a similar flashed presentation of the same grating fixed at a suitably chosen subthreshold contrast value. Again differences in the interaction over time of these two flashed presentations as a function of the spatial frequency of the grating may reveal differences in the temporal response properties of the assumed underlying transient and sustained channels. METHOD Subjects
One subject, CM, was a 20-yr-old, female undergraduate at Stanford University. She had corrected-to-normal vision, was highly practiced as a psychophysical observer, but was naive in regard to the purposes of the experiment. One of the authors, BB, served as the second subject. Stimuli and general procedure
’ This study was supported by the National Eye Institute, National Institute of Health, U. S. Public Health Service. Grant EY01242 to the second author.
The inspection field consisted of a Tektronix 602 oscilloscope (P4 phosphor) viewed at a distance of 110 cm. The oscilloscope screen was masked to produce a rectangular.
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BRLSO G. BREITMEYERand LEO GASZ
4
x 6 srimulus field. A small fixation point was attached to the screen and centered in the stimulus field. Subjects fixated rhis point throughout an experimental session. The uniform-field luminance of the display was 5.0 ft-L. The stimuli consisted of vertical sinusoidal gratings. They were generated by a sinewave function generator (Wavetek) which modulated the Z-axis input of the oscilloscope. To control the spatial contrast of the gratings, a logarithmic voltage attenuator, variable in 0.05 log unit steps, was placed between the function generator and the Z-axis input of the oscilloscope. The contrast of a grating is given by (I,,,,, - ~,,,)/(~,,, + I,,,,,) where I,, and Imln are the luminance maximum and minimum respectively within one period of the grating. Photometric calibration showed that up to a value of 489, grating contrast varied nearly linearly with voltaee. The subject sat in a chair and his head rested on a chin and head brace while viewing the stimulus field. No optics or artificial pupils were used. Prior to initiating the determination of contrast thresholds for detecting gratings. the subject viewed the uniformly illuminated inspection field-located in an otherwise dark room-for 5 min in order to attain a stable light adaptation level. For any contrast threshold determination the logarithmic voltage attenuator was initially set to produce a clearly subthreshold contrast, and on successive stimulus presentations the voltage was serially incremented in 0.05 log unit steps until the contrast value was sufficiently high so that the subject was able to detect the grating two consecutive times. Repeated threshold observations of this type were obtained and averaged to determine (mean) threshold contrast values. In experiment I threshold observations were made at each of three spatial frequencies of 0.5, 2.8 and 16.0c/deg and at each of I1 stimulus durations ranging from 30-4OOmsec (see Fig. I). Within any given experimental session all spatial frequencies were tested at all durations in a cross randomized manner. Similarly, in experiment 2, two-pulse thresholds (see Fig. 1 and below) were determined at spatial frequencies of 1.0 and lO.Oc/deg and at each of 12 stimulus onset asynchronies (SOAs) ranging from 0 to 2LOmsec. The method for measuring the twopulse threshold was an adaptation of the procedures used by lkeda (1965) and Rashbass (1970). The two pulses each were ot‘ IO msec duration and each contained the same spatial frequency target. The first pulse was always maintained as nearly as possible at 0.15 log unit (3dB) below contrast threshold (based on two threshold observations
made at the start of each experimental session). The conON
EXP I
+---
20 - 400msecd
OFF
Fig. 1. The stimulus timing of sinusoidal gratings presented in experiments I and 2. In experiment 1 only one presenration was made on a given trial and it cduld range in duration from 20 to 4OOmsec. In experiment 2, two lo-msec grating presentations were made; the onset of the second presentation was separated from the onset of the first by intervals ranging from 0 to 210 msec. The first grating presentation was 0.15 log unit below contrast threshold.
trast of rhe second pulse was adjusted as described above until the subject reported detecting a gratmg. RESULTS Experiment
1
In the first experiment threshold contrasts for detecting a grating at a spatial frequency of 0.5, 2.S and 16.0 c!deg and at durations of 20, 30, 40. 50, 80. 110, 140. 170. 200, 300 and JOOmsec were measured. Assuming that differences in temporal response properties of transient and sustained channels manifest themselves. among other ways, in their respective temporal integration times. one would expect systematic differences in temporal integration of contrast as the spatial frequency of the test gratings varies from low to high. The results confirm this expectation. Figure 2 shows the contrast in units of log per cent plotted against stimulus duration in units of log msec. All data points are based on the mean of 10 threshold measurements. Note that at all three spatial frequencies log threshold contrast tends to decrease linearI> with log duration over a limited and spatial frequency-specific range of durations. The slope of the reciprocity function relating log threshold contrast to log duration is the same at all spatial frequencies and for both subjects. The value of the slope is approx -0.70 (the fit of the straight line segments to the data was determined by eye). That is to say. the contrast-duration reciprocity function can be expressed best by the formula (Contrast) x (Duration)‘.” = Constant or alternately (Contrast)‘.“’ x (Duration) = Constant. Perfect temporal integration of contrast would obtain if the power exponent of both the contrast and the duration terms were 1.0. similar to the results of many studies of temporal integration of luminance which can be expressed in terms of the well known Bloch’s law. Luminance x Duration = Constant (Barlett, 1965; Owens, 1972). The deviation from perfect temporal integration of contrast is discussed later. Despitk the common, non-unity slope of the reciprocity functions. Fig. 2 also shows that the duration at and below which the contrast-duration reciprocity holds and above which it breaks down increases with spatial frequency. As indicated by the vertical dashed lines in Fig. 2, at spatial frequencies of 0.5. 2.5 and 16.0 c/deg the critical durations are 80, 150 and 2OOmsec, respectively, for subject CM and 60. 150 and 200 msec, respectively, for subject BB. Thus, in the human visual system, integration time for contrast increases with the spatial frequency of a grating. Elaboration of this particular finding is left for the Discussion. Experiment 2 In this experiment two-pulse threshold contrasts for detecting a grating at a spatial frequency of 1.0 and lO.Oc/deg were measured. .As stated. the two pulsed presentations of a grating were each of 10msec duration. The first of these grating presentations was 0.15 log unit below threshold contrast whereas the contrast of the second grating could be adjusted until a threshold for the two-pulse presentation was obtained. The SOAs separating the onsets of the two
863
Temporal studies with flashed gratings
STIMULUS
DURATION
(MSECI
Fig. 2. Threshold contrast in per cent at spatial frequencies of 0.5,2.8 and 16.0 cidea as a function of stimulus duration. Both threshold contrasf and stimulus duration are plotted along logarithmic coordinates in order to obtain linear functions indicating the contrast-duration reciprocity at each spatial frequency.
pulses were 0, 15, 30, 45, 60, 75, 90, 105, 130, 155, 180 and 210msec. By measuring the threshold contrast of the second pulse as a function of its SOA, one can determine how the two responses of the visual system generated by the two-pulse presentation of a grating interact over time. As noted, differences in the form of this interaction over time between the relatively low, l.Oc/deg, spatial frequency and the relatively high, 10.0 c/deg, spatial frequency gratings may indicate differences between the temporal waveform of the responses of transient and sustained channels to flashed stimuli. Figure 3 shows the results of this experiment. The inverse of threshold contrast, i.e. contrast sensitivity, for the second pulse is plotted against SOA. Each data point is based on 15 observations. Since the first pulse presentation of a grating was fixed at 0.15 log unit below detection threshold, the present results give an indication of how the presumably fixed sub threshold activity generated by the first pulse combines over time with the subthreshold activity generated by the second pulse. Thus, the subthreshold summation of the two pulses shows how activity produced by the first pulse temporally modulates the contrast sensitivity of the second pulse. From this one can infer the temporal wave form of the activity generated by the first pulse. Several aspects of these results are noteworthy. Most obvious is the fact that at both spatial frequencies the contrast sensitivity for the second pulse is highest at an SOA of Omsec; i.e. when the second pulse is synchronous with the iirst and consequently when optimal temporal superimposition of the activity generated by the two pulses can occur. A corollary finding is that the contrast sensitivity of the second pulse generally decreases with increases in SOA. The effect of temporal separation of the two pulses is thus an overall decrease in the summation of the activity generated by the two pulses. It should be noted that the decrease in contrast sensitivity with SOA is monotonic for the high spatial frequency grating; however, for the low spatial frequency grating
the contrast sensitivity for the second pulse, although showing an overall decline with SOA, nevertheless reveals a multiphasic or periodic nonmonotonicity. In particular, for both subjects a contrast sensitivity minimum occurs at an SOA of roughly 60 msec, and a secondary maximum occurs at SOAs of lOO-120msec, followed in turn by a decline in contrast sensitivity. The fact that this periodic or multiphasic variation in contrast sensitivity is found at low but not high spatial frequencies signifies clear differences in the temporal response properties of the assumed underlying transient and sustained channels (see Discussion). Another aspect of the present results is also noteworthy. The dashed lines emanating from the SOA =a point indicate the contrast sensitivity (at each spatial frequency and for both subjects) of an isolated 10msec pulse of a test grating. Note that at low spatial frequencies the two-pulse contrast sensitivity is higher than that of a single pulse only at and below SOAs of 20msec. Beyond that SOA value. and despite the above mentioned nonmonotonicity of the contrast sensitivity, the contrast sensitivity for the second pulse always lies below that of the single, isolated pulse. Thus, at the low spatial frequency the multiphasic nature of the contrast sensitivity variations is superimposed what appears to be a monophasic, sustained suppression of the contrast sensitivity. This monophasic suppression of the contrast sensitivity of the second pulse is particularly evident at the high spatial frequency. Here, the two-pulse contrast sensitivity is higher than that of the single isolated pulse only at and below SOAs of 60msec. At higher SOA values the contrast sensitivity of the second pulse is continually diminished relative to that of the single pulse. On the basis of this result. it seems likely that the first pulse can have not only a facilitatory effect on the contrast sensitivity of the second pulse at small SOAs, but also a suppressive effect at larger SOAs. I
I
0
I
I
40
STIMULUS
I
4
00 ONSET
I
I
120
I
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160
ASYNCHRONY
I
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II
200
I
-I
0)
(MSEC)
Fig. 3. Contrast sensitivity (inverse of threshold contrast) for the second of two IO-msec grating presentations at spatial frequencies of 1.0 and lO.Oc/deg as a function of the stimulus onset asynchrony separating the onset of the second presentation from that of the first. The lirst grating presentation was 0.15 log unit below threshold. Dashed lines indicate the contrast sensitivity of a single lo-msec presentation of the test gratings.
BRLWO G. BRE~EYERand LEOGANZ
864
DISCC’SSION
The two experiments reported here addressed themselves to the study of the human visual system’s temporal response when stimulated with flashed gratings of variable spatial frequency. The first experiment showed, for one thing, that the critical duration, at and below which contrast-duration reciprocity at threshold holds, increases with spatial frequency. At a spatial frequency of 0.5 c/deg the critical duration was roughly 60 msec; at the highest spatial frequency of 16.0 c/deg the critical duration was 200 msec. On the assumption that transient and sustained channels operate at low to moderate and moderate to high spatial frequencies respectively (Tolhurst, 1975), one can infer that transient channels have a substantially briefer integration time than sustained channels. This interpretation is consistent with several other psychophysical findings. For instance, Kulikowski and Tolhurst (1973) found that the fusion frequency for a lOc/deg grating presented in counterphase flicker was lower when subjects were required to detect the spatial periodicity of the grating than when they were required to simply detect flicker. This lower temporal resolution of the spatial pattern detectors most likely is another indicator of the longer integration time of sustained channels as determined in the present study. Moreover, the findings of several recent investigations using flashed square-wave gratings (Nachmias, 1967; Schober and Hilz, 1965) as well as flashed sine-wave gratings (Arend, 1976) indicate that the contrast sensitivity at moderate to high spatial frequencies benefits more from increases in exposure duration than the contrast sensitivity at low to moderate spatial frequencies. This is entirely consistent with the present tindings which show that beyond 60-80 msec increases in stimulus duration benefit the contrast sensitivity only with the higher spatial frequency gratings but not with the low spatial frequency one. Moreover. Kahneman (1964) and Kahneman and Norman (1964) found that whereas the critical duration for brightness discrimination in which Bloch’s law holds was on the order of lOOmsec, it was on the order of 2OC-3OOmsec for spatial acuity tasks. Insofar as sustained channels are primarily involved in analysis of pattern detail (Kulikowski and Tolhurst, 1973), the Kahneman (1964) and Kahneman and Norman (1964) results dovetail nicely with the findings of experiment 1. Furthermore. the study of Graham and Margaria (1935) indicates that the integration time for luminance increases as the size of a uniformly illuminated test field decreases (but see O&ens (1972) for a critique). This also is consistent with the present finding of longer integration times at higher spatial frequencies, since an increase in the spatial frequency of a grating is equivalent to a directly proportional decrease in the width of the individual dark and light bars of the grating. Another noteworthy feature of the results of experiment 1 is that the temporal integration of contrast is not perfect. Over the range of duration for which contrast-duration reciprocity holds at each spatial frequency tested. the slope of the function relating log contrast to log duration is -0.7. Perfect temporal integration as expressed by Bloch’s law would require a slope of - 1.0. Results of investigations of visual
spatio-temporal integration reported by Owens (1972) may shed some light on this finding. Owens (1972) found that as the diameter of a uniform test field superimposed on a 9.8 ft-L background increased from 3.6’ to 32’ arc, the slope of the function relating log threshold luminance to log duration changed from -1.0 to -0.85. On the basis of these and similar results, Owens (1972) concluded that Bloch’s law does not apply when stimuli were used whose angular size exceeded a critical value. In Owen’s case, using stimuli presented 10” nasally, the critical diameter was roughly 9’ arc. In the present study a 4” x 6’ foveally centered display was used. Since the spatial frequency ranged from 0.5 to 16.0 cideg. the widths of the alternate black and white bars of the gratings ranged from 2’ arc to I ‘. At or near the fovea, these sizes may already exceed the critical value beyond which Bloch’s law no longer holds. If not the width of the grating bars, then certainly their height of 4.0” would exceed such a critical value, and consequently could give rise to a contrast-reciprocity function with a slope less steep than - 1.0, e.g. -0.7. The second experiment revealed differences in the temporal wave form of the visual system’s response to a flashed, subthreshold grating at each of 1.0 or else 10.0 c,‘deg. The two-pulse subthreshold summation technique used in experiment 2 yielded results which may be interpreted as the autocorrelation function of the temporal impulse response of the visual system (Rashbass, 1970. 1976). Insofar as the low and high spatial frequency gratings yielded different functions, one can infer differences in the impulse responses of transient and sustained channels. For the low, 1.0 c/deg, spatial frequency, the subthreshold summation of two 10msec flashes of the test grating revealed facilitation effects up to an SOA of 20 msec; for the high, lO.Ocjdeg, spatial frequency similar effects were obtained up to an SOA of 60msec. Beyond these two respective SOA values, the twopulse summation effect was of a generally suppressive nature. In particular, at the high spatial frequency there was a continual decline in contrast sensitivity from an SOA of 0 msec to an SOA of 210 msec-with a clear, sustained suppression effect evident over the 60-210 msec SOA interval. This is significant because at the high spatial frequency sustained channels are primarily activated. and from recent electrophysiological studies it is known that sustained channels are characterized not only by sustained excitation but also by sustained inhibition (Ganz and Felder, 1977: Winters and Hamasaki, 1976). The presence of the multiphasic or periodic fluctuaction in facilitation and suppression obtained with the two-pulse subthreshold summation of the low spatial frequency grating suggests that the impulse response of transient channels is characterized by a damped periodic oscillation of excitation alternating with inhibition. This interpretation is consistent with those of Rashbass (1970) who obtained similar results using a uniformly illuminated field rather than a grating stimulus. IMoreover. the present results obtained with the low and high spatial frequency grating are also consistent with the findings of Kelly (1971) who found that the temporal impulse response of uniform-field flicker detectors can be characterized by a damped periodic
Temporal studies with flashed gratings oscillation of excitation alternating with inhibition. However. the temporal impulse response of counterphase-flickered grating detectors was characterized by a nonoscillating. monophasic excitatory-inhibitory interaction. That is to say, on the reasonable assumption that transient channels are responsible for detect-
ing uniform field flicker and sustained channels are responsible for detecting the spatial structure of gratings (Kulikowski and Tolhurst. 1973). the present results are substantially in aseement with Kelly’s (1971) findings. They are also supported by recent electrophysiological results reported by Biittner er 01. (1975). Studying the response of cat retinal ganglion cells to brief light flashes. these investigators distinguished between two types of cells on the basis of their responses. Cells of one type yielded oscillating responses to brief flashes and were thought to be of the transient variety; cells of the other type yielded nonoscillating, monophasic responses to brief flashes and were thought to be of the sustained variety. Thus both electrophysiological as well as psychophysical studies point to fundamental differences between transient and sustained channels in terms of their respective temporal response properties. REFERESCES
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