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Velocity specificity of the flicker to pattern sensitivity ratio in human vision
VELOCITY SPECIFICITY OF THE FLICKER TO PATTERN SENSITIVITY RATIO IN HUMAN VISION M. G. HARRIS Department of Psychology, University of Bristol, S-10, Berkeley Square, Bristol BS8 IHH, Engfand (Received 23 September f979)
Abstract-Flicker and pattern sensitivities were compared for sine-wave gratings drifting at different rates. Relative sensitivity was found to be constant at each velocity, irrespective of spatial and tempcral frequencies, and to vary monotonically with velocity. At velocities below 1 deg/sec, pattern sensitivity exceeded flicker sensitivity. At higher velocities the reverse was true.
INTRODUCTION
Several investigators have reported the existence of separate thresholds for the detection of the spatial
and temporal structure of Bickering or moving stimuli (van Nes et al., 1957; Keesey, 1972; Kulikowski and Tolhurst, 1973; T’olhurst et al., 1973; King-Smith and Kulikowski, 1975). These two types of threshold have been taken as evidence for two distinct mechanisms in the human visual system: one subserving the analysis of spatial “pattern”, the other subserving the analysis of temporal “flicker”. Evidence from sub-threshold summation (King-Smith and Kulikowski, 1975) and adaptation (Blakemore and Campbell, 1969; Tolhurst et al., 1973) experiments suggests that both the pattern and flicker mechanisms consist of a number of “channels”, each tuned to a limited range of spatial or temporal frequencies, respectively. The work described here investigated pattern and flicker thresholds for sine-wave gratings drifting at different velocities. Previous threshold studies have emphasised variations in either spatial or temporal frequency rather than systematically varying both. Any velocity-based relationship between the two thresholds would not have been apparent. Yet flicker and pattern thresholds may well be systematically related by velocity. Consider what role the two mechanisms might ptay in coding stimulus velocity: the velocity of a drifting sine-wave grating is given by the ratio of its temporal to its spatiai frequency, thus its velocity could be signalled by the identities of the maximally responding spatial and temporal channels. Such a coding system would need to be more sophisticated, however, if it were to deaf with aperiodic stimuli such as lines and edges. These stimuli have continuous spatial frequency spectra Since all spatial frequency components of a drifting line must drift at the same velocity, each must have a different temporal frequency. A line of given width drifting at given velocity should consequently excite a characteristic pattern of spatial and temporal channels. An identitybased velocity code would need to operate upon these complex patterns of activity if it were correctly to
associate the corresponding spatial and temporal identities. In fact, several phenomena suggest that velocity coding is more directly related to response ma~itude, or intensity, than to identity. fn particular, perceived velocity decreases during inspection (Wohlgemuth, 1911) and varies with stimulus contrast (Thompson, 1976). Tolhurst et al. (1973) speculated that perceived velocity might depend upon the ratio of the response intensities in the flicker and pattern mechanisms. If this were so, one might expect the relative sensitivities of the two mechanisms to depend upon the velocity of a stimulus, irrespective of its particular spatial and temporal frequencies. The following experiments confirmed this expectation. EXPERIMENT
1: FLICKER AND PA’ITERN
SEN!SITWlT%S AS A FUNCTION OF VELOCITY
Method
Stimuli were generated and the experiment controlled by computer (PDP ll/lO). A horizontallydrifting vertical sinewave grating was displayed upon an oscilloscope (Tektronix 602, P31 phosphor) using a method similar to that described by Watson (1979). The spatial resolution of the display was 100 points/ deg. All stimuli were of the form: f-(x, t) = t[l
+ mW(x) sin(27rfi + 27rgt)]
where the mean Iuminance (Et was 24 cd/m*, f and g were spatial and temporal frequency, respectively, and W was a Gaussian weighting function with a standard deviation of one quarter the display width, such that Michelson contrast (m) was maximal in the centre of the display and zero at its lateral edges. At the viewing distance of 57 cm, the rectangular display subtended lo” horizontally and 8” vertically. Stimuli were presented in a continuous cycle: 1 set on, 1 set off. Each presentation was accompanied by an auditory tone. The subject was instructed to fixate a small dot in the centre of the display and to adjust the contrast of the stimulus until he or she could just detect either
687
41. G. H.ARRIS
4
Sparlal frequency (c /degl
Fig. 1. The relationship between spatial frequency. temporal frequency and velocity. See text explanation. its spatial or its temporal structure. The type of setting required on each trial was cued by the computer. Within each session, the subject set both thresholds for the 16 combinations of 4 spatial and 4 temporal frequencies (1, 2, 5 and 10 c/deg and Hz). Drift direction was reversed after every setting. The order of trials was randomized within sessions and counterbalanced between them. 7 subjects each undertook 4 sessions. Six of the subjects were naive to the detailed purpose of the experiment.
The slope of this plane along lines of equal velocity does not differ significantly from zero (c < 1.96). Thus, irrespective of spatial and temporal frequency, the relative sensitivity of the pattern and flicker systems is constant within each velocity. Moreover, it varies monotonically with velocity. This point is illustrated in Fig. Zb, which shows individual subjects’ data pooled across the appropriate spatial and temporal frequencies and plotted against log velocity. EXPERIMENT
Results
Results were expressed as flicker (Sr) and pattern (S,) sensitivities (the reciprocals of the corresponding thresholds). Each subject’s mean log sensitivity difference (mean log S, - mean log S,) was calculated for each combination of spatial and temporal frequencies. Analysis of variance on these data revealed main effects of spatial and temporal frequencies (F(13,8) = 34.2 and 13.0 respectively, p < 0.001) and no significant interaction between them (F < 1). The relationship between spatial frequency, temporal frequency and velocity is illustrated in Fig. 1. In logarithmic axes of spatial and temporal frequency, stimuli with the same velocity can be joined by a line with a slope of 1. Different velocities are represented by lines with different intercepts, so that velocity varies logarithmically at right angles to this set of “isovelocity” lines. Log sensitivity difference was plotted along the vertical axis of a 3-dimensional graph with horizontal axes of log spatial and log temporal frequency. The resulting surface was presented as though viewed from the azimuth indicated in Fig. 1 and from an altitude of 45”. In the picture plane of this projection, points with equal velocities lie on horizontal lines and the velocity axis can be represented by a vertical line. The data, pooled across subjects, are shown in Fig. 2a The best fitting plane, derived by multiple regression, accounts for 97% of the variance of these data.
for
PERIPHERAL
2: SENSITIVIIY VIEWING
COUNTERPHASE
WlTH
AND TO
GRATISGS
The first part of Experiment 2 extended the findings of Experiment 1 to a greater range of spatial and temporal frequencies for both fovea1 and peripheral viewing. The second part of the experiment used counterphase rather than drifting stimuli. Although counterphase gratings do not usually appear to drift when viewed foveally (but see Georgeson and Harris, 1978) they can be regarded as the sum of two gratings drifting in opposite directions. Since detection of a counterphase grating is best described in terms of the detection of its drifting components (Levinson and Sekuler, 1975) it is meaningful to describe such a stimulus in terms of the velocity of these components. The author, who provided fairly typical data in Experiment 1 (Fig. 2b; inverted triangles), acted as subject in this experiment. Method
Stimuli were generated by conventional analog techniques and contrast was not weighted by the Gaussian function described above. The oscilloscope screen was masked down to a circle subtending 3.75’ at the viewing distance of 115 cm. The remainder of the screen was covered by a translucent mask which reduced the contrast of the stimulus and served to minimize flicker artefacts at the edges of the display.
Velocity specificity in human vision
689
Fig. 2. Data pooled across 7 subjects from Experiment 1. (a) 2.point perspective projection of mean log sensitivity difference (mean log S, - mean log S,) plotted against log spatial and log temporal frequency. Multiple regression: R ’ = 0.970, p < 0.001; F&2,13)= 208.7, p < 0.001. (b) Small open symbols are individual subjects’ data plotted against log velocity. Numbers in parentheses indicate the number of spatio-temporal points over which data at each velocity were pooled. Missing symbols are obscured by large solid circles, which represent the data averaged across subjects. Linear regression (on solid symbols): r2 = 0.988, p < 0.001; F(l.7) = 576.8, p < 0.001.
The procedure used was similar to that of the previous experiment except that the stimulus was presented for 1.25 set with a delay of 1.25 set between presentations. For the drifting stimulus the subject fixated a small dot in the centre of the display or a small light 5” to the left of this point. For both tixation points he made 10 settings of each threshold for the 100 combinations of 10 spatial and IO temporal frequencies--ranging in half-octave steps from 0.88 to ZOc/deg and Hz. For the counterphase stimulus only fovea1 viewing was used. The subject made 10 settings of each threshold for the 25 combinations of 5 spatial and 5 temporal frequencies--ranging in octave steps from 1.25 to 2Oc/deg and Hz. RESULTS AND DISCUSSION
Although a comparison of flicker and pattern sensitivities in the fovea and periphery is itself interesting, the raw data are not needed for the current discussion and they will be presented in a future paper. The data were treated as in Experiment 1. Results for drifting and counterphase stimuli are shown in Figs 3 and 4, respectively. All the data show trends similar to those revealed by Experiment 1. The log sensitivity difference associated with each velocity is consistently greater in the fovea than in the periphery (Fig. 3~)~ The results of both experiments clearly show that flicker and pattern sensitivities are systematically related by stimulus v&city: the ratio of flicker to pattern sensitivity is constant at each velocity, irres-
pective of spatial and temporal frequencies, and varies
monotonically with velocity. This finding has two possible interpretations. Firstly, it could be argued that moving gratings are detected by a system of velocity-tuned mechanisms and that, at each velocity, flicker and pattern thresholds simply reflect different criteria applied to the response of a unitary mechanism. Although this inte~retation can be reconciled with most previous findings, it remains unlikely for two reasons: (i) It does not explain why the relationship between the two response criteria varies SO systematically with velocity and (ii) there is one report (Tolhurst et al., 1973) that the two thresholds can be independently manipulated by adaptation. Alternatively if the two thresholds do regect the function of two different mechanisms, and if relative sensitivity reflects relative responsiveness above threshold, then for a given retinal region each velocity should be associated with a unique flicker to pattern response magnitude ratio. Moreover, this ratio should be the same for all the combinations of spatial and temporal frequencies which make up each particular velocity. The visual system would thus be ideally organized to encode velocity simply by comparing the average response magnitude in the flicker system with that of the pattern system. It need take no account of the identities of the responding channels. Figure 3c shows that, for each velocity, log sensitivity difference (log 5, - log 5,) is greater in the, fovea than in the periphery. Since apparent velocity is also greater in the fovea than in the periphery (Lich-
690
Xl. G. HARRIS
(a)
Faveol dr:fr Subject MGh
!Si
Peripherel
dr;f f
sobjea
(cl
MGH
Dnft Wlea
MGH
18,I
/il/lllllil/i/
OCWJ
I ! 22
Vekmty
( deg /see
)
Fig. 3. Data for drifting gratings from Experiment 2, displayed as in Fig. 2. (a) Fovea1 viewing. Xlultiple regression: R’ = 0.880, p < 0.001; F(Z. 96) = 352.9, p c 0.001. (b) 5’ peripheral viewing. Data for 20 Hz were incomplete because of contrast limitations of the apparatus and are excluded. Multiple regression: R’ = 0.825, p < 0.001; F(2.86) = 203.3, p c O.OOj. (c) Solid symbols and continuous line are for fovea1 viewing. Linear regression: r2 = 0.956, p c 0.001; F( t. 17) = 367.0, p < 0.001. Open symbols and broken line are for 5” peripheral viewing. Linear regression: rt = 0.922. p c 0.001; F(I. 16) = 186.4. p < 0.001.
tenstein, 1963) this may seem to support the intensitybased model of velocity coding described above. However, stimuli also appear larger (Schneider er a[., 1978) or of lower spatial frequency (Georgeson, 1980) in the fovea than in the periphery. These shifts in apparent size are compatible with the type of simple identity-based velocity code mentioned in the introduction.
Although the data presented here imply that the visual system is ideally equipped to encode velocity on the basis of response intensities, they do not prove that it does so. More direct investigation is obviously needed. It would be particularly interesting to know whether perceived velocity is affected by manipulation of the relative sensitivity of the flicker and pattern systems using, for example, adaptation techniques.
691
Velocity specificity in human vision LoI
Fovea1 counterptwe Sutqect MGH
0
A
r
(b) Fovea1CWWPPQS
/
S&yxtMGH
Fig. 4. Data for foveally-viewed counterphase gratings displayed as in Fig. 2. (a) Data for 20 c/deg were incomplete because of contrast limitations of the apparatus and are excluded. Multiple regression: R* = 0.732, p < 0.001; F(Z,17)= 23.3, p c 0.001. (b) Linear regression: r2 = 0.920, p < 0.001; F(l.6, = 68.1, p < 0.001.
~cknowfe~~e~e~rs-I thank Mark Georgeson for supervising the work and for his comments on an earlier version of this report. I thank Lindsay Evett for technical assistance. Supported by an SRC Research Studentship.
REFERENCES
Blakemore C. and Campbell F. W. (1969) On the existence of neurones in the human visual system selectively sensitive to the orientation and size of retinal images. J. Fhysiol. 2tJ3, 237-260. Georgeson M. A. (1980) Spatial frequency analysis in early visual processing. Phil. Trans. R. Sot. Lord B in press. Georgeson M. A. and Harris M. G. (1978) Apparent foveofusal drift of counterphase gratings. Perception 7, 527-536. Keesey U. T. (1972) Ricker and pattern detection: a comparison of thresholds. J. opt. Sot. Am. 62,446-448. King-Smith P. E. and Kulikowski J. J. (1975) Pattern and flicker detection analysed by subthreshold summation. J. Physiol. 249, 519-549.
Kulikowski J. J. and Tolhunt D. J. (1973) Psychophysical evidence for sustained and transient detectors in human vision. J. Physiof. 232, 149-162. Levinson E. and Sekuler R. (1975) The independence of channels in human vision selective for direction of movement. J. Phy~of. Z!%, 347-366. Lichtenstein M. (1963) Spatio-temporal factors in cessation of smooth apparent movement. J. opt. .Soc. Am. 53, 302-306. Nes F. L. van. Koenderink J. J., Nas H. and Bouman M. A. (2967) Spat&temporal modulation transfer in the human eye, J. opt. Sot. Am. 3, 1082-1088. Schneider B., EhrlicdD. J., Stein R., Flaum M. and Manse1 S. (1978) Changes in the apparent length of lines as a function of degree of retinal eccentricity. Perception 7, 215-223. Thompson P. G. (1976) Velocity aftereffects and the perception of movement. Unuublished Ph.D. thesis. University of Cambridge. _ Tothurst D. J,, Sharpe C. R. and Hart G. (1973) Analysis of the drift rate of moving sinusoidal gratings. Vision Res. 13.254%.2555.
Watson A. B. (1979) Probability
summation
over time.
Vision Res. 19, 515-522.
Wohlgemuth A. (1911) On the aftereffect of seen movement. Br. J. Psychol. Mono@. Suppl. I, l-117.
Abstract-Flicker and pattern sensitivities were compared for sine-wave gratings drifting at different rates. Relative sensitivity was found to be constant at each velocity, irrespective of spatial and tempcral frequencies, and to vary monotonically with velocity. At velocities below 1 deg/sec, pattern sensitivity exceeded flicker sensitivity. At higher velocities the reverse was true.
INTRODUCTION
Several investigators have reported the existence of separate thresholds for the detection of the spatial
and temporal structure of Bickering or moving stimuli (van Nes et al., 1957; Keesey, 1972; Kulikowski and Tolhurst, 1973; T’olhurst et al., 1973; King-Smith and Kulikowski, 1975). These two types of threshold have been taken as evidence for two distinct mechanisms in the human visual system: one subserving the analysis of spatial “pattern”, the other subserving the analysis of temporal “flicker”. Evidence from sub-threshold summation (King-Smith and Kulikowski, 1975) and adaptation (Blakemore and Campbell, 1969; Tolhurst et al., 1973) experiments suggests that both the pattern and flicker mechanisms consist of a number of “channels”, each tuned to a limited range of spatial or temporal frequencies, respectively. The work described here investigated pattern and flicker thresholds for sine-wave gratings drifting at different velocities. Previous threshold studies have emphasised variations in either spatial or temporal frequency rather than systematically varying both. Any velocity-based relationship between the two thresholds would not have been apparent. Yet flicker and pattern thresholds may well be systematically related by velocity. Consider what role the two mechanisms might ptay in coding stimulus velocity: the velocity of a drifting sine-wave grating is given by the ratio of its temporal to its spatiai frequency, thus its velocity could be signalled by the identities of the maximally responding spatial and temporal channels. Such a coding system would need to be more sophisticated, however, if it were to deaf with aperiodic stimuli such as lines and edges. These stimuli have continuous spatial frequency spectra Since all spatial frequency components of a drifting line must drift at the same velocity, each must have a different temporal frequency. A line of given width drifting at given velocity should consequently excite a characteristic pattern of spatial and temporal channels. An identitybased velocity code would need to operate upon these complex patterns of activity if it were correctly to
associate the corresponding spatial and temporal identities. In fact, several phenomena suggest that velocity coding is more directly related to response ma~itude, or intensity, than to identity. fn particular, perceived velocity decreases during inspection (Wohlgemuth, 1911) and varies with stimulus contrast (Thompson, 1976). Tolhurst et al. (1973) speculated that perceived velocity might depend upon the ratio of the response intensities in the flicker and pattern mechanisms. If this were so, one might expect the relative sensitivities of the two mechanisms to depend upon the velocity of a stimulus, irrespective of its particular spatial and temporal frequencies. The following experiments confirmed this expectation. EXPERIMENT
1: FLICKER AND PA’ITERN
SEN!SITWlT%S AS A FUNCTION OF VELOCITY
Method
Stimuli were generated and the experiment controlled by computer (PDP ll/lO). A horizontallydrifting vertical sinewave grating was displayed upon an oscilloscope (Tektronix 602, P31 phosphor) using a method similar to that described by Watson (1979). The spatial resolution of the display was 100 points/ deg. All stimuli were of the form: f-(x, t) = t[l
+ mW(x) sin(27rfi + 27rgt)]
where the mean Iuminance (Et was 24 cd/m*, f and g were spatial and temporal frequency, respectively, and W was a Gaussian weighting function with a standard deviation of one quarter the display width, such that Michelson contrast (m) was maximal in the centre of the display and zero at its lateral edges. At the viewing distance of 57 cm, the rectangular display subtended lo” horizontally and 8” vertically. Stimuli were presented in a continuous cycle: 1 set on, 1 set off. Each presentation was accompanied by an auditory tone. The subject was instructed to fixate a small dot in the centre of the display and to adjust the contrast of the stimulus until he or she could just detect either
687
41. G. H.ARRIS
4
Sparlal frequency (c /degl
Fig. 1. The relationship between spatial frequency. temporal frequency and velocity. See text explanation. its spatial or its temporal structure. The type of setting required on each trial was cued by the computer. Within each session, the subject set both thresholds for the 16 combinations of 4 spatial and 4 temporal frequencies (1, 2, 5 and 10 c/deg and Hz). Drift direction was reversed after every setting. The order of trials was randomized within sessions and counterbalanced between them. 7 subjects each undertook 4 sessions. Six of the subjects were naive to the detailed purpose of the experiment.
The slope of this plane along lines of equal velocity does not differ significantly from zero (c < 1.96). Thus, irrespective of spatial and temporal frequency, the relative sensitivity of the pattern and flicker systems is constant within each velocity. Moreover, it varies monotonically with velocity. This point is illustrated in Fig. Zb, which shows individual subjects’ data pooled across the appropriate spatial and temporal frequencies and plotted against log velocity. EXPERIMENT
Results
Results were expressed as flicker (Sr) and pattern (S,) sensitivities (the reciprocals of the corresponding thresholds). Each subject’s mean log sensitivity difference (mean log S, - mean log S,) was calculated for each combination of spatial and temporal frequencies. Analysis of variance on these data revealed main effects of spatial and temporal frequencies (F(13,8) = 34.2 and 13.0 respectively, p < 0.001) and no significant interaction between them (F < 1). The relationship between spatial frequency, temporal frequency and velocity is illustrated in Fig. 1. In logarithmic axes of spatial and temporal frequency, stimuli with the same velocity can be joined by a line with a slope of 1. Different velocities are represented by lines with different intercepts, so that velocity varies logarithmically at right angles to this set of “isovelocity” lines. Log sensitivity difference was plotted along the vertical axis of a 3-dimensional graph with horizontal axes of log spatial and log temporal frequency. The resulting surface was presented as though viewed from the azimuth indicated in Fig. 1 and from an altitude of 45”. In the picture plane of this projection, points with equal velocities lie on horizontal lines and the velocity axis can be represented by a vertical line. The data, pooled across subjects, are shown in Fig. 2a The best fitting plane, derived by multiple regression, accounts for 97% of the variance of these data.
for
PERIPHERAL
2: SENSITIVIIY VIEWING
COUNTERPHASE
WlTH
AND TO
GRATISGS
The first part of Experiment 2 extended the findings of Experiment 1 to a greater range of spatial and temporal frequencies for both fovea1 and peripheral viewing. The second part of the experiment used counterphase rather than drifting stimuli. Although counterphase gratings do not usually appear to drift when viewed foveally (but see Georgeson and Harris, 1978) they can be regarded as the sum of two gratings drifting in opposite directions. Since detection of a counterphase grating is best described in terms of the detection of its drifting components (Levinson and Sekuler, 1975) it is meaningful to describe such a stimulus in terms of the velocity of these components. The author, who provided fairly typical data in Experiment 1 (Fig. 2b; inverted triangles), acted as subject in this experiment. Method
Stimuli were generated by conventional analog techniques and contrast was not weighted by the Gaussian function described above. The oscilloscope screen was masked down to a circle subtending 3.75’ at the viewing distance of 115 cm. The remainder of the screen was covered by a translucent mask which reduced the contrast of the stimulus and served to minimize flicker artefacts at the edges of the display.
Velocity specificity in human vision
689
Fig. 2. Data pooled across 7 subjects from Experiment 1. (a) 2.point perspective projection of mean log sensitivity difference (mean log S, - mean log S,) plotted against log spatial and log temporal frequency. Multiple regression: R ’ = 0.970, p < 0.001; F&2,13)= 208.7, p < 0.001. (b) Small open symbols are individual subjects’ data plotted against log velocity. Numbers in parentheses indicate the number of spatio-temporal points over which data at each velocity were pooled. Missing symbols are obscured by large solid circles, which represent the data averaged across subjects. Linear regression (on solid symbols): r2 = 0.988, p < 0.001; F(l.7) = 576.8, p < 0.001.
The procedure used was similar to that of the previous experiment except that the stimulus was presented for 1.25 set with a delay of 1.25 set between presentations. For the drifting stimulus the subject fixated a small dot in the centre of the display or a small light 5” to the left of this point. For both tixation points he made 10 settings of each threshold for the 100 combinations of 10 spatial and IO temporal frequencies--ranging in half-octave steps from 0.88 to ZOc/deg and Hz. For the counterphase stimulus only fovea1 viewing was used. The subject made 10 settings of each threshold for the 25 combinations of 5 spatial and 5 temporal frequencies--ranging in octave steps from 1.25 to 2Oc/deg and Hz. RESULTS AND DISCUSSION
Although a comparison of flicker and pattern sensitivities in the fovea and periphery is itself interesting, the raw data are not needed for the current discussion and they will be presented in a future paper. The data were treated as in Experiment 1. Results for drifting and counterphase stimuli are shown in Figs 3 and 4, respectively. All the data show trends similar to those revealed by Experiment 1. The log sensitivity difference associated with each velocity is consistently greater in the fovea than in the periphery (Fig. 3~)~ The results of both experiments clearly show that flicker and pattern sensitivities are systematically related by stimulus v&city: the ratio of flicker to pattern sensitivity is constant at each velocity, irres-
pective of spatial and temporal frequencies, and varies
monotonically with velocity. This finding has two possible interpretations. Firstly, it could be argued that moving gratings are detected by a system of velocity-tuned mechanisms and that, at each velocity, flicker and pattern thresholds simply reflect different criteria applied to the response of a unitary mechanism. Although this inte~retation can be reconciled with most previous findings, it remains unlikely for two reasons: (i) It does not explain why the relationship between the two response criteria varies SO systematically with velocity and (ii) there is one report (Tolhurst et al., 1973) that the two thresholds can be independently manipulated by adaptation. Alternatively if the two thresholds do regect the function of two different mechanisms, and if relative sensitivity reflects relative responsiveness above threshold, then for a given retinal region each velocity should be associated with a unique flicker to pattern response magnitude ratio. Moreover, this ratio should be the same for all the combinations of spatial and temporal frequencies which make up each particular velocity. The visual system would thus be ideally organized to encode velocity simply by comparing the average response magnitude in the flicker system with that of the pattern system. It need take no account of the identities of the responding channels. Figure 3c shows that, for each velocity, log sensitivity difference (log 5, - log 5,) is greater in the, fovea than in the periphery. Since apparent velocity is also greater in the fovea than in the periphery (Lich-
690
Xl. G. HARRIS
(a)
Faveol dr:fr Subject MGh
!Si
Peripherel
dr;f f
sobjea
(cl
MGH
Dnft Wlea
MGH
18,I
/il/lllllil/i/
OCWJ
I ! 22
Vekmty
( deg /see
)
Fig. 3. Data for drifting gratings from Experiment 2, displayed as in Fig. 2. (a) Fovea1 viewing. Xlultiple regression: R’ = 0.880, p < 0.001; F(Z. 96) = 352.9, p c 0.001. (b) 5’ peripheral viewing. Data for 20 Hz were incomplete because of contrast limitations of the apparatus and are excluded. Multiple regression: R’ = 0.825, p < 0.001; F(2.86) = 203.3, p c O.OOj. (c) Solid symbols and continuous line are for fovea1 viewing. Linear regression: r2 = 0.956, p c 0.001; F( t. 17) = 367.0, p < 0.001. Open symbols and broken line are for 5” peripheral viewing. Linear regression: rt = 0.922. p c 0.001; F(I. 16) = 186.4. p < 0.001.
tenstein, 1963) this may seem to support the intensitybased model of velocity coding described above. However, stimuli also appear larger (Schneider er a[., 1978) or of lower spatial frequency (Georgeson, 1980) in the fovea than in the periphery. These shifts in apparent size are compatible with the type of simple identity-based velocity code mentioned in the introduction.
Although the data presented here imply that the visual system is ideally equipped to encode velocity on the basis of response intensities, they do not prove that it does so. More direct investigation is obviously needed. It would be particularly interesting to know whether perceived velocity is affected by manipulation of the relative sensitivity of the flicker and pattern systems using, for example, adaptation techniques.
691
Velocity specificity in human vision LoI
Fovea1 counterptwe Sutqect MGH
0
A
r
(b) Fovea1CWWPPQS
/
S&yxtMGH
Fig. 4. Data for foveally-viewed counterphase gratings displayed as in Fig. 2. (a) Data for 20 c/deg were incomplete because of contrast limitations of the apparatus and are excluded. Multiple regression: R* = 0.732, p < 0.001; F(Z,17)= 23.3, p c 0.001. (b) Linear regression: r2 = 0.920, p < 0.001; F(l.6, = 68.1, p < 0.001.
~cknowfe~~e~e~rs-I thank Mark Georgeson for supervising the work and for his comments on an earlier version of this report. I thank Lindsay Evett for technical assistance. Supported by an SRC Research Studentship.
REFERENCES
Blakemore C. and Campbell F. W. (1969) On the existence of neurones in the human visual system selectively sensitive to the orientation and size of retinal images. J. Fhysiol. 2tJ3, 237-260. Georgeson M. A. (1980) Spatial frequency analysis in early visual processing. Phil. Trans. R. Sot. Lord B in press. Georgeson M. A. and Harris M. G. (1978) Apparent foveofusal drift of counterphase gratings. Perception 7, 527-536. Keesey U. T. (1972) Ricker and pattern detection: a comparison of thresholds. J. opt. Sot. Am. 62,446-448. King-Smith P. E. and Kulikowski J. J. (1975) Pattern and flicker detection analysed by subthreshold summation. J. Physiol. 249, 519-549.
Kulikowski J. J. and Tolhunt D. J. (1973) Psychophysical evidence for sustained and transient detectors in human vision. J. Physiof. 232, 149-162. Levinson E. and Sekuler R. (1975) The independence of channels in human vision selective for direction of movement. J. Phy~of. Z!%, 347-366. Lichtenstein M. (1963) Spatio-temporal factors in cessation of smooth apparent movement. J. opt. .Soc. Am. 53, 302-306. Nes F. L. van. Koenderink J. J., Nas H. and Bouman M. A. (2967) Spat&temporal modulation transfer in the human eye, J. opt. Sot. Am. 3, 1082-1088. Schneider B., EhrlicdD. J., Stein R., Flaum M. and Manse1 S. (1978) Changes in the apparent length of lines as a function of degree of retinal eccentricity. Perception 7, 215-223. Thompson P. G. (1976) Velocity aftereffects and the perception of movement. Unuublished Ph.D. thesis. University of Cambridge. _ Tothurst D. J,, Sharpe C. R. and Hart G. (1973) Analysis of the drift rate of moving sinusoidal gratings. Vision Res. 13.254%.2555.
Watson A. B. (1979) Probability
summation
over time.
Vision Res. 19, 515-522.
Wohlgemuth A. (1911) On the aftereffect of seen movement. Br. J. Psychol. Mono@. Suppl. I, l-117.