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The analysis of the drift rate of moving sinusoidal gratings
Vision Res. Vol. 13, pp. 2545-2SS5. Pcrplmon
Ress 1973. Printed in Great Britain.
THE ANALYSIS OF THE DRIFT RATE OF MOVING SINUSOIDAL GRATINGS D. J. TOLHURST’,C. R. SHARPE*,and G. HARTZ The Physiological
Laboratory,
Downing Street, Cambridge CB2 3EG England
(Received 15 March 1973)
INTRODUCTION THE HUMAN visual system contains channels which are each sensitive to only a limited number of stimuli, those at particular orientations (CAMPBELLand KULIKOWSKI,1966; SEKIJLER, RUBIN and CUSHMAN,1968) and of particular sizes or spatial frequencies (BLAKEMORE and CAMPBELL,1969; SACHS, NACHMIASand ROBSON, 1971). These psychophysical findings are amply supported by neurophysiological evidence: HUBEL and WIESEL (1962, 1968) have shown that individual neurones in the cat and monkey striate cortex are responsive to only a small number of stimulus orientations, while CAMPBELL, COOPER and ENROTH-CUGELL(1969a) and CAMPBELL,COOPER,ROBSONand SACH~(1969b) have shown that these neurones respond to limited, but different, ranges of spatial frequency of sinusoidal grating. HUBEL and WIE~EL(1965) and PETTIGREW,NIKARA and BISHOP(1968) report that cat cortical neurones have different optimal velocities when tested with single drifting bars. PANTLEand SEKULER(1968) demonstrated the existence of velocity-specific channels in the human visual system by the psychophysical method of spatial adaptation. After adapting to a rectangular-wave grating drifting at a particular velocity, the threshold for detecting that velocity was elevated. If there were adaptable velocity-specific channels, it would be expected that there would be less elevation at velocities differing from that of the adapting grating. To some extent, Pantle and Sekuler found this to be true, but their adaptation experiments suggested that the individual channels showed little or no decline in sensitivity at high velocities. This contradicts the well-known observation that the visual system as a whole displays a very pronounced high-velocity cut (e.g. WATANABE,MORI, NAGATA and HIWATASHI,1968). Because of this important discrepancy, we repeated these experiments and initially could only replicate the results of Pantle and Sekuler, being unable to demonstrate the high-velocity cut in the individual channels. This paper attempts to locate the artifact in the experimental technique which gives rise to the discrepancy. There are two distinct classes of channel in the human visual system: one set analyses the movement of a stimulus, while the second set analyses its spatial structure. The two sets of channels have different spatial and temporal properties, and a drifting grating could be detected by either of these channel systems. Before adaptation, the grating might be detected by one system of channels but, after adaptation, it might well be ’ Correspondence. Recipient of a scholarship from the MRC of Great Britain. ’ Present address: Aviation Medicine Research Unit, Department of Physiology, Montreal, Que., Canada. Recipient of a studentship from the MRC of Canada. ’ Present address: Trinity College, Oxford, England. 2545
McGill University,
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D. J.
TOLHURST, C. R. SHARPE AVDG. HART
detected by the second system (TOLHURST,1973); such an event must confuse the interpretation of experimental results. Thus, we have repeated the adaptation experiments, examining the effects of adaptation on each of the two sets of channels separately. This can be accomplished as the two sets of channels have recognizably different thresholds: at one contrast, the movement of a drifting grating can just be seen (ticker threshold) and, at a second contrast, the spatial structure of the same grating can just be discriminated clearly (pattern discrimination threshold). These two thresholds (for the same stimulus) represent the activity of the movement-analysers and pattern-analysers respectively (KEESEY, 1972; K~LIKOWSKIand TOLHURST,1973). With this precaution, we have been able to show that the velocity-specific channels do, in fact, show a pronounced decline in sensitivity at high velocities. METHODS Sinusoidal gratings were generated on the face of an oscilloscope screen (P31 phosphor). The screen was circular (dia. 13 cm) and, viewed from a distance of 114 cm, subtended 6.5” of visual angle. The contrast of the gratings was defined as: L mm
L,
Ld. + LohI
-
where L is the luminance of a point on the screen. The space-averaged mean luminance was constant at about 100 cd/mz; it was unat%cted by changes in the contrast or other parameters of the grating. The subject set his detection thresholds by adjusting the contrast of the stimulus until it was just visible, using a logarithmic attenuator having steps of 0.025 log units. Five threshold readings were made for each stimulus and the S.E. of each mean was about 6 per cent. The thresholds were expressed as contrast sensiriaity, the reciprocal of the threshold contrast. The gratings were all vertically oriented. They were either stationary on the screen or drifted steadily in a direction perpendicular to their orientation. The direction of drift of the adapting gratings was reversed every l-5 set, so as to prevent the generation of overt apparent-motion aftereffects. The spurrid frequency is defined as the number of cycles of the grating per degree of visual angle (c/deg); the temporal frequency is defined as the number of cycles of the grating which drifted past the fixation point every second (c/s). The authors acted as subjects; GH and DJT were emrnetropic, while CRS was a well-corrected myope. The stimuli were viewed binocularly and with the foveas; natural accommodation was used.
RESULTS
Sensitivity to drifting gratings
Figure 1 shows how the sensitivity to drifting gratings depends on the rate of drift. The results for four spatial frequencies are presented, and the four sets of data have been shifted vertically so that the sensitivity at 6 c/s is the same (indicated by the cross). The sensitivity is expressed relative to that at 6 c/s. It can be seen that one function describes the high frequency region of the four curves (above about 4 c/s); all the curves show a peak at about 6 c/s, which is similar to the best temporal frequency for sinusoidal modulation of the contrast of stationary gratings (ROBSON,1966; KELLY, 1969). If, however, the rate of movement were expressed in deg/sec, the optimal velocity would shift progressively from 12 to 0.4”/sec as the spatial frequency was increased, and there would be little overlap of the four sensitivity curves. This suggests that movement of a periodic stimulus is analysed in terms of temporal modulation (c/s) rather than velocity (deg/sec) (cf. BREITMEYER, 1973). Below 4 c/s, the situation appears to be different: the four sets of data do not overlap and there is a progressive loss of the low temporal frequency cut as the spatial frequency is increased. The high and low frequency portions of the curves are different functions of the spatial frequency. This is illustrated in Fig. 2 where the data of Fig. 1 are replotted as a
2547
Analysis of Drift Rate
lEMF0W. FREQUENCY c/s FIG. 1. The sensitivity to gratings drifting in one direction as a function of the temporal frequency. The results for four spatial frequencies are shown: 05 c/deg (0); 2 c/deg (0); 8 c/deg (A); and 14 c/deg (m). The four sets of data have been normalised on the sensitivity axis so that the sensitivity at 6 c/s is the same (indicated by the cross); the sensitivity is expressed relative to that at 6 c/s. Note that the four curvy coincide at frequencies above about 4 C/S. Subject DJT.
I
I 03
3
I 10
30
SPATIA FREQUENCY(c/dq) FIG. 2. The sensitivity to gratings drifting in one direction as a function of their spatial frequency. The results for four temporal frequencies are shown: stationary (0); 2.5 C/S(0); 6 c/s (&; and Il.5 c/s (D. The continuous curve has been drawn by eye through the open circles; the dashed line has been drawn by eye through the data for 1l-5 c/s and then shifted up the sensitivity axis to fit the data for 6 c/s. These data include those plotted in Fig. 1, but have not been normalised. Subject DJT.
function of the spatial frequency for four rates of drift. The open symbols show the sensitivity to stationary gratings; the sensitivity to such stimuli is maximal at about 3.5 c/deg. When the gratings drift, the low spatial frequency decline in sensitivity is less severe and the optimal spatial frequency is lowered (2 c/deg). This difference in the spatial frequency sensitivity curves for stationary and drifting gratings suggests the presence of two sets of channels: a high spatial frequency set responding optimally to slowly moving or stationary
2548
D. J.
TOLHURST,
C. R. SHARPEAND G.
HART
patterns (pattern-analysers), and a low spatial frequency set of movement-analysers which respond optimally to stimuli drifting at about 6 c/s (TOLHURST, 1973; SH~ZRPEand TOLHURST, 1973; KULIKOWSKIand TOLHURST, 1973). The data points for a drift rate of 2.5 c/s (filled circles) lie along the curve for stationary gratings at high spatial frequencies but along the curve for drifting gratings at lower frequencies. This is consistent with the appearance of these gratings at threshold: the low spatial frequency gratings appeared to be moving, but the high spatial frequency gratings appeared to be stationary. Velocity-specific channels
PAN~LEand SEKULER(1968) showed the existence of velocity-specific channels by the method of spatial adaptation. Adaptation to a grating drifting at one velocity greatly elevated the detection threshold for that velocity and partially raised the threshold for other velocities: the elevation of threshold was velocity-specific. But the results were not wholly as expected : surprisingly, the channels showed little sign of a high-velocity decline in sensitivity. We have, therefore, re-investigated the problem. Initially we replicated the findings of Pantle and Sekuler. E,rperimenfulprocedure. The subject initially made five threshold settings for sinusoidal gratings drifting at several rates, but all of the same spatial frequency. He then viewed the adapting grating for 3 min, fixating a small spot in the centre of the screen, unless the adapting grating was stationary when he was asked to move his gaze about to prevent the generation of afterimages. The adapting grating had the same spatial frequency as the test gratings, and drifted at one temporal frequency throughout the experiment. Its direction of drift was reversed every I-5 set and its contrast was about 10 times its detection threshold. The subject then made five threshold settings at each of the test temporal frequencies, with 20 set further adaptation between each setting. Threshold elevation is expressed as relative elevation (BLAKEMOREand CAIMPBELL,1969) and is defined as :
rel. elev. =
sensitivity before adapting - 1. sensitivity after adapting
The standard error of each mean of five readings was about 6 per cent and the relative elevation could be considered statistically significant (95 per cent) if it was greater than about O-11.
Figure 3 illustrates the results of adapting to four temporal frequencies of 1 c/deg grating. It can be seen that the shape of the threshold elevation curve depended on the adapting temporal frequency, suggesting that there are channels which each respond to only limited ranges of stimulus temporal frequency. However, the threshold for any one test frequency was affected to much the same extent by adapting to all gratings drifting at higher temporal frequencies than its own, implying that the channels show little or no decline in sensitivity at high temporal frequencies. This is improbable because the overall sensitivity curves (Fig. 1) show a very pronounced decline in sensitivity at high temporal frequencies. Similar results were obtained at S-5 c/deg. The rest of this paper attempts to resolve this discrepancy. Thresholds forjlicker detection andpattern discrimination
Many authors have described two distinct thresholds for detecting drifting gratings: at one contrast, flicker or movement becomes visible, and at a second contrast, the individual bars of the grating can just be clearly seen (e.g. VAN NES, KOENDERINK, NAS and BOUMAN, 1967; WATANABE et al., 1968). These two thresholds are affected differently under stabilized image conditions (RIGGSand WHITTLE,1967; KEESEY,1969) and are independently elevated by adapting tc gratings (TOLHURST,1973 ; TOLHURSTand KULIKOWSKI,1973). KEESEY ( 1972) and KULIKOWSKIand TOLHURS-F (1973) have suggested that the flicker detection threshold
Analysis of Drift Rate
2549
FIG. 3. Relative elevation of the threshold for gratings drifting at various temporal frequencies after adapting to gratings drifting at one temporal frequency. The results for four experiments are illustrated, using four different adapting temporal frequencies: stationary (0); 2.5 c/s (a); 6 c/s (0); and 11.5 c/s (I). The adapting frequencies are indicated by the arrows. The spatial frequency of all the gratings was 1 c/deg, and the contrast of the adapting gratings was 0-l. The adapting gratings drifted backwards and forwards, reversing their direction of drift every l-5 sec. Subject GH.
reflects the activity of movement-analysing channels while the pattern-discrimination threshold reflects the activity of separate pattern-analysing channels. This ambiguity of threshold has so far been neglected in this paper and was presumably neglected also by PANTLEand SEKULER(1968). In the preceding experiments, threshold was taken as the lowest contrast at which it could be seen that the screen was either not spatially uniform or not temporally uniform. Whether the stimulus appeared to be moving or stationary was ignored. If the flicker thresholds do represent the activity of movementanalysers and if the proposed velocity-specific channels are sub-units of the movementanalysing system, it is imperative that flicker thresholds be used in these experiments. This is especially important at low temporal frequencies, where the moving gratings often appeared to be stationary at threshold (the pattern-discrimination threshold would have been used at these frequencies). It is at the low temporal frequencies that the elevation curves plateau. The adaptation experiments were thus repeated and the effects of adaptation were examined for the flicker thresholds and pattern-discrimination thresholds separately. Figure 4 shows the sensitivity for ticker and for pattern discrimination for gratings of 1 c/deg drifting at various temporal frequencies. Before adaptation (filled symbols), the flicker sensitivity shows a peak at about 6 c/s (cf. Fig. 1) while the patter-discrimination sensitivity curve shows little sign of a low temporal frequency cut. These results are similar to those obtained by KEESEY (1972) and KULIKOWSKI and TOLJWIWC (1973) for temporal modulation of the contrast of stationary stimuli. After adapting to a grating which had the same spatial frequency and which drifted at 6 c/s, the sensitivity to flicker was depressed only at temporal frequencies close to that of the
2550
D. J. TOLHURX, C. R. SHARPEASD G. HART
FIG. 4. Two distinct thresholds for drifting gratings: the movement is apparent at one contrast, while the individual bars of the grating become distinct at a second contrast. The sensitivity for detecting flicker or movement as a function of the temporal frequency is indicated by the tilled circles and the sensitivity for discriminating the bars by the filled squares. After adapting to a grating drifting at 6 c/s (arrow), these sensitivities are depressed (open symbols). Ah the gratings had a spatial frequency of 1.0 c/deg; the adapting grating had a contrast of 0*018 and its direction of drift was reversed every 1.5 sec. Subject DJT.
TEMFCRAL
FREQUENCY
c/s
FIG. 5. The elevation of the thresholds for detecting movement (0) and for discriminating the individual bars of the grating(m). The elevation is expressed in log units, and is calculated from the data in Fig. 4. The movement-threshold is elevated over a narrow range of temporal frequency and is most affected at the adapting frequency (arrow). The pattern-discrimination threshold is more or less uniformly elevated over the whole range of frequency tested. grating. The pattern-discrimination sensitivity was depressed more or less uniformly over the whole range of temporal frequency tested. In Fig. 5, the elevations of the fhcker and pattern-discrimination thresholds are plotted. The elevation is expressed in log units and could be considered to be statistically significant (95 per cent) when it was greater than about O-05log units. Figure 6 shows theelevation curves for a similar experiment but with gratings of 4 c/deg. From both figures it can be seen that the flicker threshold was elevated adapting
Analysis of Drift Rate
2551
FIG. 6. The results of an experiment similar to that illustrated in Fig. 5, except that the spatial frequency of all gratings was 4 c/deg. Again, the movement threshold (e) is elevated over a narrow range of temporal frequency and is most elevated at the adapting temporal frequency (6 c/s-arrow). The pattern-discrimination threshold (m) is uniformly elevated. The adapting grating had a contrast of O-01 and its direction of drift was reversed every 1.5 sec. Subject DJT.
1
30 3 10 TEMPORAL FREQUENCY ck
FIG. 7. The elevation of the movement threshold in log units after adapting to gratings drifting at 10 c/s. Three experiments are illustrated: gratings of 1 c/deg, subject GH (m); 1 c/deg, subject DJT (e); and 5 c/deg, subject DJT (0). The contrast of the adapting gratings was O-018and their direction of drift was reversed every 1.5 sec.
only over a narrow range of temporal frequencies and that the elevation was maximal at the frequency of the adapting grating. The pattern-discrimination thresholds were elevated over the whole range of frequency tested; apart from two points at high frequencies in Fig. 5, the elevation is fairly uniform. The previous figures have shown the results of adapting to gratings drifting at 6 c/s. In Fig. 7, the elevation of the flicker threshold is illustrated after adapting to gratings drifting at 10 c/s; the results for two spatial frequencies and two subjects are shown. Again, the elevation is limited to a narrow range of temporal frequency but the curves no longer peak at 6 c/s. Rather, they peak close to the new adapting temporal frequency, suggesting that
2552
D. J.
TOLHURST,
C. R.
SHARPE AND G. HART
there are families of narrow temporal-frequency different ranges of frequency.
specific channels which each respond to
DISCUSSION-
The existence of temporal-frequency channels
The rate of movement of a drifting spatially-periodic stimulus appears to be analysed in terms of its temporal frequency (c/s) rather than its velocity (deg/sec). The sensitivity to gratings is optimal at about 6 c/s irrespective of the spatial frequency; the optimal velocity in deg/sec is directly proportional to the spatial frequency. BREITMEYER (1973) reached the same conclusion from a very different experiment. Thus, the channels to be discussed should perhaps be regarded as temporal-frequency specific rather than as velocity specific. This distinction is trivial as long as we are concerned with gratings of one spatial frequency, but might be important for understanding how each channel responds to a stimulus which is not its optimal stimulus. Consider a channel which responds optimally to gratings of 5 c/deg drifting at 5 c/s. What will be the optimal temporal frequency when the stimulus is at 4 c/deg? We have no experimental evidence on this point. Neither do we know how the rate of drift of aperiodic stimuli is analysed. Such stimuli as single bars and edges are, perhaps, more common naturally occurring stimuli than periodic gratings. PANTLEand SEKULER(1968) presented initial psychophysical evidence for velocity or temporal-frequency channels. Their results, which we have replicated, suggest that the individual channels can each respond to very high temporal frequencies although the visual system as a whole cannot. This inconsistency suggests an artifact in the method used to demonstrate the channels. The problem can be resolved simply by taking into account the subjective appearance of the test stimuli. Under some circumstances, a moving grating appears to be stationary at threshold and it is possible to distinguish two distinct thresholds for temporally-modulated stimuli: at one contrast, the temporal changes become apparent while, at a second contrast, the spatial structure of the stimulus becomes distinct. It has been proposed by KEESEY(1972), TOLHURS~(1973) and KULIKOWSKIand TOLHURST(1973) that there are two sets of channels in the human visual system. One set analyses the temporal properties of the stimulus and is responsible for the flicker detection threshold. The second set of channels is responsible for the analysis only of the spatial structure of stationary and moving stimuli; the pattern-discrimination threshold represents the activity of this set of channels. It would seem reasonable that only the movement-analysing system would be subdivided into temporal-frequency specific channels. Our results support this suggestion. The elevation of the flicker threshold was limited to a narrow range of temporal frequencies and was maximal at or near the adapting frequency. The threshold elevation curves showed both a high and a low temporal-frequency cut. The elevation of the pattern-discrimination threshold was fairly uniform, suggesting that, at each spatial frequency and orientation, a single channel is responsible for the analysis&f the spatial structure of the stimulus, irrespective of its rate of movement. It should be noted that the temporal-frequency channels, which are sub-units of the movement-analysing system, do not simply generalize for temporal frequency : each movement-sensitive channel is sensitive to stimuli only at certain orientations and spatial frequencies (SHARPE and TOLHURST,1973; TOLHUFGT,1973). SMITH(1971) and PANTLE(1971) examined the effects of adapting to spatially-uniform large fields whose intensity was temporally modulated. The adaptation was temporal-
2553
Analysis of Drift Rate
frequency specific Tbe channels demonstrated with spatially-uniform to those demonstrated here using spatially-structured stimuli
stimuli may be similar
The role of temporal frequency channels
How is the apparent temporal frequency of a drifting grating analysed? It is possible that the apparent rate is determined by which of the several temporal frequency channels is the most excited by the stimulus This would be analogous to the situation in the orientation and spatial frequency domains, where the roles of the channels have been demonstrated by means of figural after-effects (LENME, 1972; BLAKEMORE, NACHMIAS and SUITON, 1970). After adapting to a vertical stimulus, a stimulus at a neighbouring orientation appears to be tilted even further from the vertical. When a group of orientation channels is made less sensitive, the subject’s perception of orientation is distorted. Similar experiments might be possible in the temporal frequency domain. After adapting to a grating drifting at one temporal frequency, the apparent rate of gratings drifting at neighbouring frequencies should be distorted; slower gratings should appear even slower, and faster gratings should appear even faster. However, there is an alternative way in which the apparent drift rate could be encoded. The pattern-analysing channels and movement-analysing channels have very different temporal-frequency sensitivity curves. Perhaps, the apparent rate is encoded in the ratio of the firing rates of these two channel systems. If the pattern-analysers were well stimulated by the drifting grating it might appear to move slowly and steadily; if, however, the patternanalysers were only poorly stimulated, the grating might appear to move fast. Thus, adapting to a stationary grating, which desensitises the pattern-analysers much more than the movement analysers, might cause all drifting gratings to apparently move faster. Adapting to a fast moving grating might affect the movement-analysers more than the patternanalysers and might cause all gratings to apparently move slower. CARLSON(1962) reports the results of an experiment somewhat similar to the one proposed, but he did not use spatially periodic stimuli. He found that, after adapting to one velocity of drifting bar, slower stimuli appeared even slower while the apparent rate of faster stimuli was unaffected. Carlson’s experiments could usefully be repeated with periodic stimuli, and our experiments should be extended to examine the way in which the movement of aperiodic stimuli is analysed. Acknowledgemenfs-We should like to thank DRS. NORMAGRAHAM,R. M. helpful discussions and criticism.
SHAPLEY
and J. G. ROEWN for their
REFERENCES BLAKEMORE,C. B. and CAWBELL, F. W. (1969). On the existence of neurones in the human visual system selectively sensitive to the orientation and size of retinal images. J. Physiof., Land. 203,237-260. BLAKEMORE, C. B., NAC~, J. and SU-ITON,P. (1970). The perceived spatial frequency shift: evidence for frequency-selective neurones in the human brain. J. Physiol., Land. 210,727-750. BREITMEYER, B. G. (1973). A relationship between the detection of size, rate, orientation and direction in the human visual system. Vision Res. 13, 41-58. C~BELL, F. W., COOPER,G. F. and ENROTH-CUGELL,Christina (1969a). The spatial selectivity of visual cortical neurones to moving gratings. J. Physiof.. Lond. 203,223-235. CAMPBELL,F. W., COOPER,G. F., ROBSON,J. G. and SACHS,%I. B. (1969b). The spatial selectivity of visual cells of the cat and the souh-rel monkey. J. Phvsiol.. Lmd. 204. 1X-121P. CAMPBELL,F. W. and KUL%OWSKI, J. J.-(1966): Orientational selectivity of the human visual system. J. Physiol., Land. 187,437-445. CARLSON,V. R. (1962). Adaptation in the perception of visual velocity. J. exp. Psychol. 64, 192-197. Y.R.13/12-z
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HUBEL,D. H. and WI~~SEL. T. N. (1962). Receptive fields, binocular
interaction and functional architecture in the cat’s visual cortex. J. Physioi., Lond. 160,106-l 54. HUBEL, D. H. and WIESEL,T. N. (1965). Receptive fields and functional architecture of two non-striate visual areas (18 and 19) of the cat. J. Neurophysiol. 28,229-289. HUBEL, D. H. and WIESEL.T. N. (1968). Receptive fields and functional architecture of the monkey striate cortex. J. Physiol., Lond. 195, 215-243. KEESEY,ULKER(1969). Visibility of a stabilized target as a function of frequency and amplitude of luminance variation. J. opt. Sot. Am. 59,6&t-610. KEESEY,ULKER (1972). Flicker and pattern detection: a comparison of thresholds. J. opt. Sot. Am. 62, 4464I8. KELLY,D. H. (1969). Flickering patterns and lateral inhibition. J. opt. Sot. Am. 59,1361-1370. KULIKOWSKI,J. J. and TOLHURS~,D. J. (1973). Psvchouhvsical evidence for sustained and transient detectors in human~vision. J. Physiol.,L&d. 23i, 145-162. - _ LENNIE, P. (1972). Mechanisms underlying the perception of orientation. Doctoral thesis, University of Cambridge, England. PANTLE,A. J. (1971). Flicker adaptation-I. Effect on visual sensitivity to temporal fluctuations in light intensity. VisionRes. 11,943-952. PAN-IXE,A. J. and SEKULER,R. W. (1968). VeIocity-sensitive elements in human vision: initial psychophysical evidence. VisionRes. 8,445-450. PI?Y-~IGREW, J. D., NIKARA, T. and BISHOP,P. 0. (1968). Responses to moving slits by single units in the cat striate cortex. Exp. Brain Res. 6,373-390. RIGGS,L. A. and Wnrt-r~e, P. (1967). Human occipital and retinal potentials evoked by subjectively faded visual stimuli. Vision Res. 7, 441451. ROBXIN,J. G. (1966). Spatial and temporal contrast-sensitivity functions of the visual system. J. opt. Sot. Am. 56,1141-1142. SACHS,M. B., NACHMIAS,J. and ROB~ON,J. G. (1971). Spatial-frequency channels in human vision. J. opt. Sot. Am. 61,1176-l 186. SEKTJLER,R. W., RUBIN, E. L. and BUSHMAN,W. H. (1968). Selectivities of human visual mechanisms for direction of movement and contour orientation. J. opt. Sot. Am. 58,1146-l 150. SHARPE,C. R. and TOLHURST,D. J. (1973). The effects of temporal modulation on the orientation channels of the human visual system. ((in preparation.) Sm, R. A. (1971). Studies of temporal frequency adaptation in visual contrast sensitivity. J. PhysioI.,Lund. 216,531-552. TO-T, D. J. (1973). Separate channels for the analysis of the shape and the movement of a moving visual stimulus. J. Physiol., Lond. 231, 385-402. TOLIWFST, D. J. and KULIKOWS~, J. J. (1973). Independent elevation of the threshoids for flicker detection and pattern discrimination. (in preparation). VAN Nss, F. L., KOENDERINIC, J. J., NAS, H. and B~UMAN,M. A. (1967). Spatiotemporal modulation transfer in the human eye. J. opt. Sot. Am. 57,1082-1088. WATANABE,A., Moat, T., NAGATA, S. and HIWATASHI,K. (1968). Spatial sine-wave responses of the human visual system. VisionRes. 8,1245-1263.
Abstract-Evidence for velocity-spechic channels in the human visual system was obtained by adapting to drifting sinusoidal gratings and determining the amount of threshold elevation at other drift rates. The shape of the elevation curve depended on the adapting velocity but, surprisingly, it suggested that the channels had little high-velocity decline in sensitivity. Distinct thresholds for detecting the movement and the spatial structure of the stimulus were distinguished. These two thresholds were elevated independently by adaptation: the movement-threshold was affected only at velocities close to the adapting velocity. but the pattern-discrimination threshold was affected equally at all test velocities. It is suggested that there are two systems of channel: one analyses movement and is composed of velocity-specific units; the other analyses spatial structure and is not sub-divided into velocity-specihc units.
R&sun&-On met en evidence les canaux sp&%ques pour la vitesse dans le systeme visuel humain par adaptation a des r&w sinusoldaux mobiles et determination de l’eltvation du seuil pour d’autres vitesses. La forrne de la courbe d’ilevation depend de la vitesse d’adaptation, mais suggbre, ce qui est surprenant, que les canaux ant peu de d&clin de sensibilite aux grandes vitesses. On distingue des seuils dif&ents pour detecter le mouvement et la structure spatiale du stimulus. Ces deux seuils sUMvent par adaptation d’une fa9on independante: le seuil de mouve-
Analysis of Drift Rate ment n’est affect&qu’a des vitesses voisiies de celle d’adaptation, le seuil de discrimination de structure est affect&egalement pour toutes vitesses du test. On suggere deux systemes de canaux: l’un anaiyse le mouvement et se compose d’unit&s specifiques a la vitesse, l’autre analyse la structure spatiale et n’est pas subdivid en unitis spWiques a la vi&se.
Zusammenfassung-Durch Adaptation auf driftende Sinusgitter und durch Restimmung des Retrages der Schwellenerhijhung bei anderen Driftraten wurde der Beweis fiir die Existenz geschwindigkeits-spezilischer Kan&le im visuellen System des Menschen erbracht. Die Form der Erhiihung hing von der Adaptationsgeschwindigkeit ab, aber iiberraschenderweise ergab sich bei hohen Geschwindigkeiten nur ein geringer Abfall in der Empfmdlichlceit der Ranale. Es wurden deutlich verschiedene Schwellen ftir die Erkennbarkeit der Bewegung und der raumlichen Struktur des Reizes unterschieden. Diese zwei Schwellen wurden unabhtigig voneinander durch Adaptation erhoht: die Rewegungsschwelle wurde nur bei Geschwindigkeiten ganz in der N&he der Adaptationsgeschwindigkeit beeinflusst, die Musterunterscheidungssschwelle dagegen beeiiusst. Es wird daher vermutet, dass es zwei Kanalsysteme gibt: eines analysiert die Bewegung und ist aus geschwindigkeitsspezi6schen Einheiten zusammengesetzt, das andere analysiert die raurnliche Struktur und ist nicht in geschwindigkeitsspezitische Einhalten unterteilt.
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2555
Ress 1973. Printed in Great Britain.
THE ANALYSIS OF THE DRIFT RATE OF MOVING SINUSOIDAL GRATINGS D. J. TOLHURST’,C. R. SHARPE*,and G. HARTZ The Physiological
Laboratory,
Downing Street, Cambridge CB2 3EG England
(Received 15 March 1973)
INTRODUCTION THE HUMAN visual system contains channels which are each sensitive to only a limited number of stimuli, those at particular orientations (CAMPBELLand KULIKOWSKI,1966; SEKIJLER, RUBIN and CUSHMAN,1968) and of particular sizes or spatial frequencies (BLAKEMORE and CAMPBELL,1969; SACHS, NACHMIASand ROBSON, 1971). These psychophysical findings are amply supported by neurophysiological evidence: HUBEL and WIESEL (1962, 1968) have shown that individual neurones in the cat and monkey striate cortex are responsive to only a small number of stimulus orientations, while CAMPBELL, COOPER and ENROTH-CUGELL(1969a) and CAMPBELL,COOPER,ROBSONand SACH~(1969b) have shown that these neurones respond to limited, but different, ranges of spatial frequency of sinusoidal grating. HUBEL and WIE~EL(1965) and PETTIGREW,NIKARA and BISHOP(1968) report that cat cortical neurones have different optimal velocities when tested with single drifting bars. PANTLEand SEKULER(1968) demonstrated the existence of velocity-specific channels in the human visual system by the psychophysical method of spatial adaptation. After adapting to a rectangular-wave grating drifting at a particular velocity, the threshold for detecting that velocity was elevated. If there were adaptable velocity-specific channels, it would be expected that there would be less elevation at velocities differing from that of the adapting grating. To some extent, Pantle and Sekuler found this to be true, but their adaptation experiments suggested that the individual channels showed little or no decline in sensitivity at high velocities. This contradicts the well-known observation that the visual system as a whole displays a very pronounced high-velocity cut (e.g. WATANABE,MORI, NAGATA and HIWATASHI,1968). Because of this important discrepancy, we repeated these experiments and initially could only replicate the results of Pantle and Sekuler, being unable to demonstrate the high-velocity cut in the individual channels. This paper attempts to locate the artifact in the experimental technique which gives rise to the discrepancy. There are two distinct classes of channel in the human visual system: one set analyses the movement of a stimulus, while the second set analyses its spatial structure. The two sets of channels have different spatial and temporal properties, and a drifting grating could be detected by either of these channel systems. Before adaptation, the grating might be detected by one system of channels but, after adaptation, it might well be ’ Correspondence. Recipient of a scholarship from the MRC of Great Britain. ’ Present address: Aviation Medicine Research Unit, Department of Physiology, Montreal, Que., Canada. Recipient of a studentship from the MRC of Canada. ’ Present address: Trinity College, Oxford, England. 2545
McGill University,
X6
D. J.
TOLHURST, C. R. SHARPE AVDG. HART
detected by the second system (TOLHURST,1973); such an event must confuse the interpretation of experimental results. Thus, we have repeated the adaptation experiments, examining the effects of adaptation on each of the two sets of channels separately. This can be accomplished as the two sets of channels have recognizably different thresholds: at one contrast, the movement of a drifting grating can just be seen (ticker threshold) and, at a second contrast, the spatial structure of the same grating can just be discriminated clearly (pattern discrimination threshold). These two thresholds (for the same stimulus) represent the activity of the movement-analysers and pattern-analysers respectively (KEESEY, 1972; K~LIKOWSKIand TOLHURST,1973). With this precaution, we have been able to show that the velocity-specific channels do, in fact, show a pronounced decline in sensitivity at high velocities. METHODS Sinusoidal gratings were generated on the face of an oscilloscope screen (P31 phosphor). The screen was circular (dia. 13 cm) and, viewed from a distance of 114 cm, subtended 6.5” of visual angle. The contrast of the gratings was defined as: L mm
L,
Ld. + LohI
-
where L is the luminance of a point on the screen. The space-averaged mean luminance was constant at about 100 cd/mz; it was unat%cted by changes in the contrast or other parameters of the grating. The subject set his detection thresholds by adjusting the contrast of the stimulus until it was just visible, using a logarithmic attenuator having steps of 0.025 log units. Five threshold readings were made for each stimulus and the S.E. of each mean was about 6 per cent. The thresholds were expressed as contrast sensiriaity, the reciprocal of the threshold contrast. The gratings were all vertically oriented. They were either stationary on the screen or drifted steadily in a direction perpendicular to their orientation. The direction of drift of the adapting gratings was reversed every l-5 set, so as to prevent the generation of overt apparent-motion aftereffects. The spurrid frequency is defined as the number of cycles of the grating per degree of visual angle (c/deg); the temporal frequency is defined as the number of cycles of the grating which drifted past the fixation point every second (c/s). The authors acted as subjects; GH and DJT were emrnetropic, while CRS was a well-corrected myope. The stimuli were viewed binocularly and with the foveas; natural accommodation was used.
RESULTS
Sensitivity to drifting gratings
Figure 1 shows how the sensitivity to drifting gratings depends on the rate of drift. The results for four spatial frequencies are presented, and the four sets of data have been shifted vertically so that the sensitivity at 6 c/s is the same (indicated by the cross). The sensitivity is expressed relative to that at 6 c/s. It can be seen that one function describes the high frequency region of the four curves (above about 4 c/s); all the curves show a peak at about 6 c/s, which is similar to the best temporal frequency for sinusoidal modulation of the contrast of stationary gratings (ROBSON,1966; KELLY, 1969). If, however, the rate of movement were expressed in deg/sec, the optimal velocity would shift progressively from 12 to 0.4”/sec as the spatial frequency was increased, and there would be little overlap of the four sensitivity curves. This suggests that movement of a periodic stimulus is analysed in terms of temporal modulation (c/s) rather than velocity (deg/sec) (cf. BREITMEYER, 1973). Below 4 c/s, the situation appears to be different: the four sets of data do not overlap and there is a progressive loss of the low temporal frequency cut as the spatial frequency is increased. The high and low frequency portions of the curves are different functions of the spatial frequency. This is illustrated in Fig. 2 where the data of Fig. 1 are replotted as a
2547
Analysis of Drift Rate
lEMF0W. FREQUENCY c/s FIG. 1. The sensitivity to gratings drifting in one direction as a function of the temporal frequency. The results for four spatial frequencies are shown: 05 c/deg (0); 2 c/deg (0); 8 c/deg (A); and 14 c/deg (m). The four sets of data have been normalised on the sensitivity axis so that the sensitivity at 6 c/s is the same (indicated by the cross); the sensitivity is expressed relative to that at 6 c/s. Note that the four curvy coincide at frequencies above about 4 C/S. Subject DJT.
I
I 03
3
I 10
30
SPATIA FREQUENCY(c/dq) FIG. 2. The sensitivity to gratings drifting in one direction as a function of their spatial frequency. The results for four temporal frequencies are shown: stationary (0); 2.5 C/S(0); 6 c/s (&; and Il.5 c/s (D. The continuous curve has been drawn by eye through the open circles; the dashed line has been drawn by eye through the data for 1l-5 c/s and then shifted up the sensitivity axis to fit the data for 6 c/s. These data include those plotted in Fig. 1, but have not been normalised. Subject DJT.
function of the spatial frequency for four rates of drift. The open symbols show the sensitivity to stationary gratings; the sensitivity to such stimuli is maximal at about 3.5 c/deg. When the gratings drift, the low spatial frequency decline in sensitivity is less severe and the optimal spatial frequency is lowered (2 c/deg). This difference in the spatial frequency sensitivity curves for stationary and drifting gratings suggests the presence of two sets of channels: a high spatial frequency set responding optimally to slowly moving or stationary
2548
D. J.
TOLHURST,
C. R. SHARPEAND G.
HART
patterns (pattern-analysers), and a low spatial frequency set of movement-analysers which respond optimally to stimuli drifting at about 6 c/s (TOLHURST, 1973; SH~ZRPEand TOLHURST, 1973; KULIKOWSKIand TOLHURST, 1973). The data points for a drift rate of 2.5 c/s (filled circles) lie along the curve for stationary gratings at high spatial frequencies but along the curve for drifting gratings at lower frequencies. This is consistent with the appearance of these gratings at threshold: the low spatial frequency gratings appeared to be moving, but the high spatial frequency gratings appeared to be stationary. Velocity-specific channels
PAN~LEand SEKULER(1968) showed the existence of velocity-specific channels by the method of spatial adaptation. Adaptation to a grating drifting at one velocity greatly elevated the detection threshold for that velocity and partially raised the threshold for other velocities: the elevation of threshold was velocity-specific. But the results were not wholly as expected : surprisingly, the channels showed little sign of a high-velocity decline in sensitivity. We have, therefore, re-investigated the problem. Initially we replicated the findings of Pantle and Sekuler. E,rperimenfulprocedure. The subject initially made five threshold settings for sinusoidal gratings drifting at several rates, but all of the same spatial frequency. He then viewed the adapting grating for 3 min, fixating a small spot in the centre of the screen, unless the adapting grating was stationary when he was asked to move his gaze about to prevent the generation of afterimages. The adapting grating had the same spatial frequency as the test gratings, and drifted at one temporal frequency throughout the experiment. Its direction of drift was reversed every I-5 set and its contrast was about 10 times its detection threshold. The subject then made five threshold settings at each of the test temporal frequencies, with 20 set further adaptation between each setting. Threshold elevation is expressed as relative elevation (BLAKEMOREand CAIMPBELL,1969) and is defined as :
rel. elev. =
sensitivity before adapting - 1. sensitivity after adapting
The standard error of each mean of five readings was about 6 per cent and the relative elevation could be considered statistically significant (95 per cent) if it was greater than about O-11.
Figure 3 illustrates the results of adapting to four temporal frequencies of 1 c/deg grating. It can be seen that the shape of the threshold elevation curve depended on the adapting temporal frequency, suggesting that there are channels which each respond to only limited ranges of stimulus temporal frequency. However, the threshold for any one test frequency was affected to much the same extent by adapting to all gratings drifting at higher temporal frequencies than its own, implying that the channels show little or no decline in sensitivity at high temporal frequencies. This is improbable because the overall sensitivity curves (Fig. 1) show a very pronounced decline in sensitivity at high temporal frequencies. Similar results were obtained at S-5 c/deg. The rest of this paper attempts to resolve this discrepancy. Thresholds forjlicker detection andpattern discrimination
Many authors have described two distinct thresholds for detecting drifting gratings: at one contrast, flicker or movement becomes visible, and at a second contrast, the individual bars of the grating can just be clearly seen (e.g. VAN NES, KOENDERINK, NAS and BOUMAN, 1967; WATANABE et al., 1968). These two thresholds are affected differently under stabilized image conditions (RIGGSand WHITTLE,1967; KEESEY,1969) and are independently elevated by adapting tc gratings (TOLHURST,1973 ; TOLHURSTand KULIKOWSKI,1973). KEESEY ( 1972) and KULIKOWSKIand TOLHURS-F (1973) have suggested that the flicker detection threshold
Analysis of Drift Rate
2549
FIG. 3. Relative elevation of the threshold for gratings drifting at various temporal frequencies after adapting to gratings drifting at one temporal frequency. The results for four experiments are illustrated, using four different adapting temporal frequencies: stationary (0); 2.5 c/s (a); 6 c/s (0); and 11.5 c/s (I). The adapting frequencies are indicated by the arrows. The spatial frequency of all the gratings was 1 c/deg, and the contrast of the adapting gratings was 0-l. The adapting gratings drifted backwards and forwards, reversing their direction of drift every l-5 sec. Subject GH.
reflects the activity of movement-analysing channels while the pattern-discrimination threshold reflects the activity of separate pattern-analysing channels. This ambiguity of threshold has so far been neglected in this paper and was presumably neglected also by PANTLEand SEKULER(1968). In the preceding experiments, threshold was taken as the lowest contrast at which it could be seen that the screen was either not spatially uniform or not temporally uniform. Whether the stimulus appeared to be moving or stationary was ignored. If the flicker thresholds do represent the activity of movementanalysers and if the proposed velocity-specific channels are sub-units of the movementanalysing system, it is imperative that flicker thresholds be used in these experiments. This is especially important at low temporal frequencies, where the moving gratings often appeared to be stationary at threshold (the pattern-discrimination threshold would have been used at these frequencies). It is at the low temporal frequencies that the elevation curves plateau. The adaptation experiments were thus repeated and the effects of adaptation were examined for the flicker thresholds and pattern-discrimination thresholds separately. Figure 4 shows the sensitivity for ticker and for pattern discrimination for gratings of 1 c/deg drifting at various temporal frequencies. Before adaptation (filled symbols), the flicker sensitivity shows a peak at about 6 c/s (cf. Fig. 1) while the patter-discrimination sensitivity curve shows little sign of a low temporal frequency cut. These results are similar to those obtained by KEESEY (1972) and KULIKOWSKI and TOLJWIWC (1973) for temporal modulation of the contrast of stationary stimuli. After adapting to a grating which had the same spatial frequency and which drifted at 6 c/s, the sensitivity to flicker was depressed only at temporal frequencies close to that of the
2550
D. J. TOLHURX, C. R. SHARPEASD G. HART
FIG. 4. Two distinct thresholds for drifting gratings: the movement is apparent at one contrast, while the individual bars of the grating become distinct at a second contrast. The sensitivity for detecting flicker or movement as a function of the temporal frequency is indicated by the tilled circles and the sensitivity for discriminating the bars by the filled squares. After adapting to a grating drifting at 6 c/s (arrow), these sensitivities are depressed (open symbols). Ah the gratings had a spatial frequency of 1.0 c/deg; the adapting grating had a contrast of 0*018 and its direction of drift was reversed every 1.5 sec. Subject DJT.
TEMFCRAL
FREQUENCY
c/s
FIG. 5. The elevation of the thresholds for detecting movement (0) and for discriminating the individual bars of the grating(m). The elevation is expressed in log units, and is calculated from the data in Fig. 4. The movement-threshold is elevated over a narrow range of temporal frequency and is most affected at the adapting frequency (arrow). The pattern-discrimination threshold is more or less uniformly elevated over the whole range of frequency tested. grating. The pattern-discrimination sensitivity was depressed more or less uniformly over the whole range of temporal frequency tested. In Fig. 5, the elevations of the fhcker and pattern-discrimination thresholds are plotted. The elevation is expressed in log units and could be considered to be statistically significant (95 per cent) when it was greater than about O-05log units. Figure 6 shows theelevation curves for a similar experiment but with gratings of 4 c/deg. From both figures it can be seen that the flicker threshold was elevated adapting
Analysis of Drift Rate
2551
FIG. 6. The results of an experiment similar to that illustrated in Fig. 5, except that the spatial frequency of all gratings was 4 c/deg. Again, the movement threshold (e) is elevated over a narrow range of temporal frequency and is most elevated at the adapting temporal frequency (6 c/s-arrow). The pattern-discrimination threshold (m) is uniformly elevated. The adapting grating had a contrast of O-01 and its direction of drift was reversed every 1.5 sec. Subject DJT.
1
30 3 10 TEMPORAL FREQUENCY ck
FIG. 7. The elevation of the movement threshold in log units after adapting to gratings drifting at 10 c/s. Three experiments are illustrated: gratings of 1 c/deg, subject GH (m); 1 c/deg, subject DJT (e); and 5 c/deg, subject DJT (0). The contrast of the adapting gratings was O-018and their direction of drift was reversed every 1.5 sec.
only over a narrow range of temporal frequencies and that the elevation was maximal at the frequency of the adapting grating. The pattern-discrimination thresholds were elevated over the whole range of frequency tested; apart from two points at high frequencies in Fig. 5, the elevation is fairly uniform. The previous figures have shown the results of adapting to gratings drifting at 6 c/s. In Fig. 7, the elevation of the flicker threshold is illustrated after adapting to gratings drifting at 10 c/s; the results for two spatial frequencies and two subjects are shown. Again, the elevation is limited to a narrow range of temporal frequency but the curves no longer peak at 6 c/s. Rather, they peak close to the new adapting temporal frequency, suggesting that
2552
D. J.
TOLHURST,
C. R.
SHARPE AND G. HART
there are families of narrow temporal-frequency different ranges of frequency.
specific channels which each respond to
DISCUSSION-
The existence of temporal-frequency channels
The rate of movement of a drifting spatially-periodic stimulus appears to be analysed in terms of its temporal frequency (c/s) rather than its velocity (deg/sec). The sensitivity to gratings is optimal at about 6 c/s irrespective of the spatial frequency; the optimal velocity in deg/sec is directly proportional to the spatial frequency. BREITMEYER (1973) reached the same conclusion from a very different experiment. Thus, the channels to be discussed should perhaps be regarded as temporal-frequency specific rather than as velocity specific. This distinction is trivial as long as we are concerned with gratings of one spatial frequency, but might be important for understanding how each channel responds to a stimulus which is not its optimal stimulus. Consider a channel which responds optimally to gratings of 5 c/deg drifting at 5 c/s. What will be the optimal temporal frequency when the stimulus is at 4 c/deg? We have no experimental evidence on this point. Neither do we know how the rate of drift of aperiodic stimuli is analysed. Such stimuli as single bars and edges are, perhaps, more common naturally occurring stimuli than periodic gratings. PANTLEand SEKULER(1968) presented initial psychophysical evidence for velocity or temporal-frequency channels. Their results, which we have replicated, suggest that the individual channels can each respond to very high temporal frequencies although the visual system as a whole cannot. This inconsistency suggests an artifact in the method used to demonstrate the channels. The problem can be resolved simply by taking into account the subjective appearance of the test stimuli. Under some circumstances, a moving grating appears to be stationary at threshold and it is possible to distinguish two distinct thresholds for temporally-modulated stimuli: at one contrast, the temporal changes become apparent while, at a second contrast, the spatial structure of the stimulus becomes distinct. It has been proposed by KEESEY(1972), TOLHURS~(1973) and KULIKOWSKIand TOLHURST(1973) that there are two sets of channels in the human visual system. One set analyses the temporal properties of the stimulus and is responsible for the flicker detection threshold. The second set of channels is responsible for the analysis only of the spatial structure of stationary and moving stimuli; the pattern-discrimination threshold represents the activity of this set of channels. It would seem reasonable that only the movement-analysing system would be subdivided into temporal-frequency specific channels. Our results support this suggestion. The elevation of the flicker threshold was limited to a narrow range of temporal frequencies and was maximal at or near the adapting frequency. The threshold elevation curves showed both a high and a low temporal-frequency cut. The elevation of the pattern-discrimination threshold was fairly uniform, suggesting that, at each spatial frequency and orientation, a single channel is responsible for the analysis&f the spatial structure of the stimulus, irrespective of its rate of movement. It should be noted that the temporal-frequency channels, which are sub-units of the movement-analysing system, do not simply generalize for temporal frequency : each movement-sensitive channel is sensitive to stimuli only at certain orientations and spatial frequencies (SHARPE and TOLHURST,1973; TOLHUFGT,1973). SMITH(1971) and PANTLE(1971) examined the effects of adapting to spatially-uniform large fields whose intensity was temporally modulated. The adaptation was temporal-
2553
Analysis of Drift Rate
frequency specific Tbe channels demonstrated with spatially-uniform to those demonstrated here using spatially-structured stimuli
stimuli may be similar
The role of temporal frequency channels
How is the apparent temporal frequency of a drifting grating analysed? It is possible that the apparent rate is determined by which of the several temporal frequency channels is the most excited by the stimulus This would be analogous to the situation in the orientation and spatial frequency domains, where the roles of the channels have been demonstrated by means of figural after-effects (LENME, 1972; BLAKEMORE, NACHMIAS and SUITON, 1970). After adapting to a vertical stimulus, a stimulus at a neighbouring orientation appears to be tilted even further from the vertical. When a group of orientation channels is made less sensitive, the subject’s perception of orientation is distorted. Similar experiments might be possible in the temporal frequency domain. After adapting to a grating drifting at one temporal frequency, the apparent rate of gratings drifting at neighbouring frequencies should be distorted; slower gratings should appear even slower, and faster gratings should appear even faster. However, there is an alternative way in which the apparent drift rate could be encoded. The pattern-analysing channels and movement-analysing channels have very different temporal-frequency sensitivity curves. Perhaps, the apparent rate is encoded in the ratio of the firing rates of these two channel systems. If the pattern-analysers were well stimulated by the drifting grating it might appear to move slowly and steadily; if, however, the patternanalysers were only poorly stimulated, the grating might appear to move fast. Thus, adapting to a stationary grating, which desensitises the pattern-analysers much more than the movement analysers, might cause all drifting gratings to apparently move faster. Adapting to a fast moving grating might affect the movement-analysers more than the patternanalysers and might cause all gratings to apparently move slower. CARLSON(1962) reports the results of an experiment somewhat similar to the one proposed, but he did not use spatially periodic stimuli. He found that, after adapting to one velocity of drifting bar, slower stimuli appeared even slower while the apparent rate of faster stimuli was unaffected. Carlson’s experiments could usefully be repeated with periodic stimuli, and our experiments should be extended to examine the way in which the movement of aperiodic stimuli is analysed. Acknowledgemenfs-We should like to thank DRS. NORMAGRAHAM,R. M. helpful discussions and criticism.
SHAPLEY
and J. G. ROEWN for their
REFERENCES BLAKEMORE,C. B. and CAWBELL, F. W. (1969). On the existence of neurones in the human visual system selectively sensitive to the orientation and size of retinal images. J. Physiof., Land. 203,237-260. BLAKEMORE, C. B., NAC~, J. and SU-ITON,P. (1970). The perceived spatial frequency shift: evidence for frequency-selective neurones in the human brain. J. Physiol., Land. 210,727-750. BREITMEYER, B. G. (1973). A relationship between the detection of size, rate, orientation and direction in the human visual system. Vision Res. 13, 41-58. C~BELL, F. W., COOPER,G. F. and ENROTH-CUGELL,Christina (1969a). The spatial selectivity of visual cortical neurones to moving gratings. J. Physiof.. Lond. 203,223-235. CAMPBELL,F. W., COOPER,G. F., ROBSON,J. G. and SACHS,%I. B. (1969b). The spatial selectivity of visual cells of the cat and the souh-rel monkey. J. Phvsiol.. Lmd. 204. 1X-121P. CAMPBELL,F. W. and KUL%OWSKI, J. J.-(1966): Orientational selectivity of the human visual system. J. Physiol., Land. 187,437-445. CARLSON,V. R. (1962). Adaptation in the perception of visual velocity. J. exp. Psychol. 64, 192-197. Y.R.13/12-z
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HUBEL,D. H. and WI~~SEL. T. N. (1962). Receptive fields, binocular
interaction and functional architecture in the cat’s visual cortex. J. Physioi., Lond. 160,106-l 54. HUBEL, D. H. and WIESEL,T. N. (1965). Receptive fields and functional architecture of two non-striate visual areas (18 and 19) of the cat. J. Neurophysiol. 28,229-289. HUBEL, D. H. and WIESEL.T. N. (1968). Receptive fields and functional architecture of the monkey striate cortex. J. Physiol., Lond. 195, 215-243. KEESEY,ULKER(1969). Visibility of a stabilized target as a function of frequency and amplitude of luminance variation. J. opt. Sot. Am. 59,6&t-610. KEESEY,ULKER (1972). Flicker and pattern detection: a comparison of thresholds. J. opt. Sot. Am. 62, 4464I8. KELLY,D. H. (1969). Flickering patterns and lateral inhibition. J. opt. Sot. Am. 59,1361-1370. KULIKOWSKI,J. J. and TOLHURS~,D. J. (1973). Psvchouhvsical evidence for sustained and transient detectors in human~vision. J. Physiol.,L&d. 23i, 145-162. - _ LENNIE, P. (1972). Mechanisms underlying the perception of orientation. Doctoral thesis, University of Cambridge, England. PANTLE,A. J. (1971). Flicker adaptation-I. Effect on visual sensitivity to temporal fluctuations in light intensity. VisionRes. 11,943-952. PAN-IXE,A. J. and SEKULER,R. W. (1968). VeIocity-sensitive elements in human vision: initial psychophysical evidence. VisionRes. 8,445-450. PI?Y-~IGREW, J. D., NIKARA, T. and BISHOP,P. 0. (1968). Responses to moving slits by single units in the cat striate cortex. Exp. Brain Res. 6,373-390. RIGGS,L. A. and Wnrt-r~e, P. (1967). Human occipital and retinal potentials evoked by subjectively faded visual stimuli. Vision Res. 7, 441451. ROBXIN,J. G. (1966). Spatial and temporal contrast-sensitivity functions of the visual system. J. opt. Sot. Am. 56,1141-1142. SACHS,M. B., NACHMIAS,J. and ROB~ON,J. G. (1971). Spatial-frequency channels in human vision. J. opt. Sot. Am. 61,1176-l 186. SEKTJLER,R. W., RUBIN, E. L. and BUSHMAN,W. H. (1968). Selectivities of human visual mechanisms for direction of movement and contour orientation. J. opt. Sot. Am. 58,1146-l 150. SHARPE,C. R. and TOLHURST,D. J. (1973). The effects of temporal modulation on the orientation channels of the human visual system. ((in preparation.) Sm, R. A. (1971). Studies of temporal frequency adaptation in visual contrast sensitivity. J. PhysioI.,Lund. 216,531-552. TO-T, D. J. (1973). Separate channels for the analysis of the shape and the movement of a moving visual stimulus. J. Physiol., Lond. 231, 385-402. TOLIWFST, D. J. and KULIKOWS~, J. J. (1973). Independent elevation of the threshoids for flicker detection and pattern discrimination. (in preparation). VAN Nss, F. L., KOENDERINIC, J. J., NAS, H. and B~UMAN,M. A. (1967). Spatiotemporal modulation transfer in the human eye. J. opt. Sot. Am. 57,1082-1088. WATANABE,A., Moat, T., NAGATA, S. and HIWATASHI,K. (1968). Spatial sine-wave responses of the human visual system. VisionRes. 8,1245-1263.
Abstract-Evidence for velocity-spechic channels in the human visual system was obtained by adapting to drifting sinusoidal gratings and determining the amount of threshold elevation at other drift rates. The shape of the elevation curve depended on the adapting velocity but, surprisingly, it suggested that the channels had little high-velocity decline in sensitivity. Distinct thresholds for detecting the movement and the spatial structure of the stimulus were distinguished. These two thresholds were elevated independently by adaptation: the movement-threshold was affected only at velocities close to the adapting velocity. but the pattern-discrimination threshold was affected equally at all test velocities. It is suggested that there are two systems of channel: one analyses movement and is composed of velocity-specific units; the other analyses spatial structure and is not sub-divided into velocity-specihc units.
R&sun&-On met en evidence les canaux sp&%ques pour la vitesse dans le systeme visuel humain par adaptation a des r&w sinusoldaux mobiles et determination de l’eltvation du seuil pour d’autres vitesses. La forrne de la courbe d’ilevation depend de la vitesse d’adaptation, mais suggbre, ce qui est surprenant, que les canaux ant peu de d&clin de sensibilite aux grandes vitesses. On distingue des seuils dif&ents pour detecter le mouvement et la structure spatiale du stimulus. Ces deux seuils sUMvent par adaptation d’une fa9on independante: le seuil de mouve-
Analysis of Drift Rate ment n’est affect&qu’a des vitesses voisiies de celle d’adaptation, le seuil de discrimination de structure est affect&egalement pour toutes vitesses du test. On suggere deux systemes de canaux: l’un anaiyse le mouvement et se compose d’unit&s specifiques a la vitesse, l’autre analyse la structure spatiale et n’est pas subdivid en unitis spWiques a la vi&se.
Zusammenfassung-Durch Adaptation auf driftende Sinusgitter und durch Restimmung des Retrages der Schwellenerhijhung bei anderen Driftraten wurde der Beweis fiir die Existenz geschwindigkeits-spezilischer Kan&le im visuellen System des Menschen erbracht. Die Form der Erhiihung hing von der Adaptationsgeschwindigkeit ab, aber iiberraschenderweise ergab sich bei hohen Geschwindigkeiten nur ein geringer Abfall in der Empfmdlichlceit der Ranale. Es wurden deutlich verschiedene Schwellen ftir die Erkennbarkeit der Bewegung und der raumlichen Struktur des Reizes unterschieden. Diese zwei Schwellen wurden unabhtigig voneinander durch Adaptation erhoht: die Rewegungsschwelle wurde nur bei Geschwindigkeiten ganz in der N&he der Adaptationsgeschwindigkeit beeinflusst, die Musterunterscheidungssschwelle dagegen beeiiusst. Es wird daher vermutet, dass es zwei Kanalsysteme gibt: eines analysiert die Bewegung und ist aus geschwindigkeitsspezi6schen Einheiten zusammengesetzt, das andere analysiert die raurnliche Struktur und ist nicht in geschwindigkeitsspezitische Einhalten unterteilt.
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