Perception, 1992, volume 21, pages 563-568
Sensitivity to relative and absolute motion
Robert J Snowden School of Psychology, University of Wales College of Cardiff, Cardiff, CF1 3YG, UK Received 5 June 1991, in revised form 6 February 1992
Abstract. The threshold of sensitivity to movement could be governed by mechanisms that are sensitive either to change in spatial position, or directly to the movement itself. The use of spatially complex patterns (random-dot patterns) has been suggested to eliminate the former strategy allowing examination of the movement detecting mechanisms in isolation. By means of such a technique, thresholds for directional judgements were determined for patterns which underwent either a simple displacement or a shearing displacement. Thresholds for shearing motion were found to be around one half of those for simple motion, suggesting that relative, rather than absolute, motion governs performance for small displacements. This contrasts with previous experiments which showed that absolute motion governs performance for much larger displacements. 1 Introduction It has been suggested on a number of occasions that the visual processes that, underlie the detection of very slow movements may not be the same as those that detect faster movements (see, eg, Exner 1875; Dimmik and Karl 1930; Leibowitz 1955; Henderson 1971; Cohen and Bonnet 1972; Johnson and Leibowitz 1976; Bonnet 1982; Johnson and Scobey 1982; Snowden and Braddick 1990; Buckingham etal 1991). The general theme behind these authors' ideas is that very slow movements can be 'inferred' from the realisation of position change over time, thus thresholds for detecting movement should be governed by the amplitude of the movement rather than by its velocity. It is thought that, at greater speeds, motion becomes a sensation in its own right; with tremendous foresight, Exner (1875) came to assume that this would involve a neural motion centre—a notion which now appears to have considerable physiological support (Zeki 1980; Salzman etal 1991). Under these conditions it is assumed that it is the velocity of the movement that will govern performance. I shall refer to these two strategies as 'inferred' and 'direct' movement detection, respectively. These two strategies make quite different predictions as the duration of a stimulus is manipulated. The inferred strategy predicts displacement thresholds (the minimum amplitude of movement) to be independent of duration, whereas the direct strategy predicts displacement thresholds to increase with increasing duration. Much evidence has been gathered which appears to support these ideas. It appears that for short durations detection is governed by the displacement of the stimulus, whereas at longer durations it is the velocity that governs thresholds (Johnson and Leibowitz 1976; Bonnet 1982). Nakayama and Tyler (1981) performed a series of elegant experiments which were designed to show how the use of random-dot patterns can overcome the general problem that motion always involves a change in position and can therefore be detected on the basis of either judgement. They argued that the use of random-dot patterns removes 'familiar position cues' and subjects are therefore forced to rely upon their movement-sensitive elements. To support this argument they measured sensitivity to an oscillating movement either of a random-dot pattern or of a single line. It is important to note that the oscillation for both the line and the random dots was along the line of the pattern (ie adjacent sections of the pattern or line were
564
R J Snowden
moving in different directions). When the line stimulus was used, displacement thresholds did not vary across a range of low temporal frequencies of oscillation, suggesting that the amplitude of displacement governed performance. For the randomdot stimulus, displacement thresholds rose as the frequency of oscillation was reduced, so that the threshold was a constant velocity. These results suggest that the random-dot patterns had eliminated the subjects' ability to use the positional system which would have produced lower thresholds. Evidence has accumulated that the inferred strategy acts upon the relative change in position rather than on absolute change. Nakayama and Tyler (1981) once again provided the crucial data. They measured sensitivity to the sinusoidal oscillation of a line stimulus for a series of oscillation spatial frequencies. They found that threshold amplitude varied with spatial frequency of oscillation. If thresholds were governed by absolute position, this would not be the case, as for each spatial frequency the maximum oscillation would be the same. It therefore appears that subjects were comparing the position of the line in some parts of the display with its position in other parts—a relative position strategy (see also Tyler 1973). From the notion that small displacements are processed by a position-sensitive mechanism, we might expect that relative motion would have an advantage over absolute motion as the relative motion would cause elements in different parts of the display to move further apart than in the case of absolute motion (for a similar argument with respect to stationary landmarks see Bonnet 1982). Thus this theory predicts that displacement thresholds will be governed by the relative motion of the pattern. If, however, random-dot patterns are employed, then the position-sensitive mechanism should be rendered useless and the motion will be sensed directly. I was therefore interested to see if thresholds would be governed by relative motion under these conditions, or by absolute motion. To test these ideas I measured displacement thresholds using random-dot patterns undergoing either a simple or a shearing motion. 2 Methods The overall procedures and equipment have been reported in previous studies (Snowden and Braddick 1989; 1990) and so only those relevant to the present study will be reported here. 2.1 Stimuli The stimuli were two-frame random-dot kinematograms. Frame 1 consisted of 400 dots (each dot was approximately 0.5 mm in diameter and of high luminance) randomly positioned within a 5 cm x 5 cm square, and frame 2 was produced by shifting all the points in one direction, horizontally, (termed 'simple' motion), or by shifting the dots in the upper half of the display in one direction and the dots in the lower half in the opposite direction (termed 'shearing' motion). Points which now fell outside the 5 cm x 5 cm square were wrapped to the other side of the display, hence the edges of the pattern remained stationary. Each frame was displayed for 100 ms, and there was no interframe interval. Therefore each kinematogram had a duration of 200 ms. 2.2 Procedure and subjects The subjects viewed the screen from 300 cm in a dimly lit room. This enabled them to see clearly the edges of the screen and other laboratory equipment. A fixation point preceded each trial and the subject pressed a button to initiate the stimulus presentation. To prevent any confusion only one type of motion (simple or shearing) was presented with any block of trials, and subjects were informed of the type of motion to expect. Subjects made, a two-alternative forced-choice as to the motion they saw: left versus right for the simple motion, and clockwise versus anticlockwise for the shearing motion.
565
Sensitivity to relative and absolute motion
The method of constant stimuli was used to determine performance. A number of displacement levels was chosen (based on practice trials) for each subject and stimulus, and presented 50 times each, in a random order. As alluded to above, the naive subjects (SS and GJ) were given a brief practice session (of the order of 100 trials) to familiarise themselves with the task. The author also served as a subject and had undergone extensive practice while developing these stimuli. 3 Results The results are displayed in figure 1, in which the percentage of correct judgements made at each displacement size for each of the three subjects is plotted. In the plots on the left displacement is defined in absolute terms, so that a displacement of 10 s arc for the shearing motion means that one half of the pattern moved 10 s arc to the right and the other half 10 s arc to the left (for the simple motion all points moved 10 s arc to the right). The psychometric functions were fitted by Probit analysis (Finney 1971). It is clear that subjects were more sensitive to the shearing than to the
Displacement/s arc
Displacement/s arc
Figure 1. The percentage of correct responses is plotted against displacement size for three subjects. In the figures on the left displacement here is defined in absolute terms (ie a displacement of 10 s arc for the shearing motion means that the upper half was displaced 10 s arc one way and the lower half 10 s arc the opposite way). In the figures on the right displacement is defined in relative terms (ie a displacement of 10 s arc for the shearing motion means that the upper half was displaced 5 s arc one way and the lower half 5 s arc the opposite way). The curves through the data points were fitted by Probit analysis (Finney 1971). The solid circles refer to the shearing displacement and the open circles to the simple displacement.
566
R J Snowden
simple motion, and thresholds (taken at the 25% error mark) are approximately half of those for the shearing motion. This suggests that the subjects are sensitive to the relative rather than the absolute motion of the patterns. If this is so, the functions should superimpose if re-plotted in terms of the relative motion of the display (thus for the shearing stimulus, if the upper half is displaced 10 s arc to the right and the lower half 10 s arc to the left, this is a relative motion of 20 s arc). This manipulation is shown in the plots on the right. The functions for the shearing and simple motions are now very similar. 4 Discussion The present results clearly suggest that motion sensitivity is governed by the relative motion of the stimulus elements rather than by the absolute movement of any one element. A second important measure of motion detection is that of the greatest displacement which can be seen as directional (Braddick 1974). Baker and Braddick (1982) measured the maximum displacement under conditions where the test displacement of a patch of random dots was defined by the relative or absolute motion with respect to the surrounding random-dot pattern. They found that the maximum displacement was governed by the absolute displacement of either the test pattern or the surrounding pattern, and not by the relative motion between them. Thus it appears that the smallest displacement which can be judged as directional is governed by the relative displacements of elements, whereas the maximum displacement which can be judged is governed by the absolute displacement of the elements. Previous studies have shown that motion sensitivity can be improved by the introduction of stationary landmarks (Aubert 1886; Leibowitz 1955; Mates 1969; Tyler and Torres 1972; Legge and Campbell 1981; Bonnet 1982; Johnson and Scobey 1982), but only under conditions which are thought to favour the inferred motion strategy (Bonnet 1982). Such stationary landmarks can, presumably, serve as a comparison to the target stimulus and hence improve performance. It should be noted that in the present study the subjects could clearly see the edge of the screen and other stationary landmarks for both the shearing stimulus and the simple stimulus. Therefore the present result suggests that comparisons can also be made between movements in opposite directions, as well as those between stationary and moving objects. Despite the use of random-dot patterns, which have been previously suggested to eliminate the use of positional cues (Nakayama and Tyler 1981), it appears that the maximum and minimum displacement limits are still governed by absolute motion and relative motion respectively. This suggests the possibility that the motion system has access to both types of information and can use either to detect movement. Hence at the smallest displacements the relative motion system would be the most sensitive whereas at large displacements the absolute motion system is able to signal the motion. The notion of a system specific for relative motion has considerable support from other lines of evidence. Neurons which are sensitive to relative motion have been described in the pigeon (Frost and Nakayama 1983; Frost 1985; Frost et al 1988); superior colliculus (Mandl 1985) and area 17 of the cat (Burns et al 1972); areaMT of the owl monkey (Allman et al 1985); and the superior colliculus (Bender and Davidson 1986), areaV2 (Orban et al 1987), and areaMT of the macaque (Tanaka et al 1986). It is unclear as yet if the neurons of one particular visual area are responsible for the thresholds measured in this paper, however, Siegel and Andersen (1986) found that discrete lesions to areaMT of the macaque compromise performance on a shear detection task similar to that used in the present study. Recent psychophysical studies have shown that relative motion detectors can be selectively adapted out (Shioiri et al 1991). Interestingly, Richards and Lieberman (1982) report
Sensitivity to relative and absolute motion
567
that around 20% of humans do not appear to be sensitive to shearing motion. Such evidence suggests that the perception of shearing motion might be processed independently of simple motion, and could have great significance in depth judgements through motion parallax (Rogers and Graham 1979). Acknowledgements. I thank Gabrielle Jordan and Sophie Snowden for participating in these experiments. Most of this work was completed when I was an SERC post-graduate student at Cambridge University. References AllmanJ, MiezinF, McGuinnes E, 1985 "Stimulus specific responses from beyond the classical receptive field" Annual Review ofNeuroscience 8 407 - 430 Aubert H, 1886 "Die Bewegungsempfindung" Archives of Gestalt Physiology 39 347 - 370 Baker C L, Braddick O J, 1982 "Does segregation of differently moving areas depend upon relative or absolute motion?" Vision Research 22 851-856 Bender D B , Davidson R M , 1986 "Global visual processing in the monkey superior colliculus" Brain Research 3 8 1 3 7 2 - 3 7 5 Bonnet C, 1982 "Thresholds of visual motion" in Tutorials on Motion Perception Eds A H Wertheim, W A Wagenaar, H W Leibowitz (New York: Plenum Press) pp 41 - 78 Braddick O J, 1974 "A short-range process in apparent motion" Vision Research 14 519 - 527 Buckingham T, Watkins R, BinningtonJ, 1991 "The effect of spatial parameters on oscillatory movement displacement thresholds" Vision Research 31 327-331 Burns B D, GassanovV, Webb A C , 1972 "Responses of neurons in the cat cerebral cortex to relative movement patterns" Journal of Physiology (London) 226 133-151 Cohen R L , Bonnet C, 1972 "Movement detection thresholds and stimulus durations" Perception &Psychophysics 12 269-272 DimmickFL, Karl J C, 1930 "The effect of exposure time upon the R. L. of visual motion" Journal of Experimental Psychology 13 365-369 Exner S T, 1875 "Uber das Sehen von Bewegungen und die Theorie des zusammengesetzten Auges" Sitzungsberichte der Akademie der Wissenschaft in Wien 72 156 -190 Finney D J, 1971 Probit Analysis (Cambridge: Cambridge University Press) Frost B J, 1985 "Neural mechanisms for detecting object motion and figure-ground boundaries, contrasted with self-motion detecting systems" in Brain Mechanisms and Spatial Vision Eds D J Ingle, M Jeannerod, D N Lee (Dordrecht: Martinus Nijhoff) pp 415-449 Frost B J, Cavanagh P, Morgan B, 1988 "Deep tectal cells in pigeons respond to kinematograms" Journal of Comparative Physiology A 162 639-641 Frost B J, Nakayama K, 1983 "Single visual neurons code opposing motion independent of direction" Science 220 744-745 Henderson D C , 1971 "The relationship among time, distance and intensity as determinants of motion discrimination" Perception & Psychophysics 1 0 3 1 3 - 3 20 Johnson C A, LeibowitzHW, 1976 "Velocity-time reciprocity in the perception of motion: foveal and peripheral determinations" Vision Research 16 7 7 - 8 0 Johnson C A, ScobeyRP, 1982 "Effects of reference lines on displacement thresholds at various durations of movement" Vision Research 2 2 8 1 9 - 8 2 1 LeggeGE, Campbell F W, 1981 "Displacement detection in human vision" Vision Research 21 205-214 LeibowitzHW, 1955 "Effect of reference lines on the discrimination of movement" Journal of the Optical Society ofAmerica 45 829-830 Mandl G, 1985 "Responses of visual cells in cat superior colliculus to relative pattern movement" Vision Research 25 267-281 Mates B, 1969 "Effects of reference marks and luminance on discrimination of movement" Journal of Psychology 73 209 - 221 Nakayama K, Tyler C W, 1981 "Psychological isolation of movement sensitivity by removal of familiar position cues" Vision Research 21 4 2 7 - 4 3 3 Richards W, LeibermanHR, 1982 "Velocity blindness during shearing motion" Vision Research 22 97-100 Rogers B, Graham M, 1979 "Motion parallax as an independent cue for depth perception" Perception 8 125-134 OrbanGA, Gulyas B, Spileers W, 1987 "A moving noise background modulates responses to moving bars of monkey V2 cells but not monkey VI cells" Investigative Ophthalmology and Visual[ScienceSupplement 28 197
568
R J Snowden
Saltzman C D, Britten K H , Newsome W T, 1991 "Cortical microstimulation influences perceptual judgements of motion detection" Nature (London) 346 174-177 Shioiri S, Ono H, Sato T, 1991 "Adaptation of relative motion detectors" Investigative Ophthalmology and Visual Science Supplement 32 827 SiegelRM, Andersen R A, 1986 "Motion deficits following ibotenic acid lesions of the middle temporal area (MT) in the behaving rhesus monkey" Society for Neuroscience Abstracts 12 1183 SnowdenRJ, Braddick O J, 1989 "The combination of motion signals over time" Vision Research 29 1621 -1630 Snowden R J, Braddick O J, 1990 "Differences in the processing of short-range apparent motion at small and large displacements" Vision Research 30 1211 -1222 TanakaH, Hikosaka K, Saito H-A, YukieM, FukadaY, Iwai E, 1986 "Analysis of local and wide-field movements in the superior temporal visual areas of the macaque monkey" Journal of Neuroscience 6 134 -144 Tyler C W, 1973 "Periodic vernier acuity" Journal of Physiology (London) 288 637-647 Tyler CW, Torres J, 1972 "Frequency response characteristics for sinusoidal movement in the fovea and periphery" Perception &Psychophysics 12 232-236 Zeki S M, 1980 "The response of cells in the middle temporal area (MT) of the owl monkey visual cortex" Proceedings of the Royal Society of London B 277 239 - 248
© 1992 a Pion publication printed in Great Britain
Sensitivity to relative and absolute motion
Robert J Snowden School of Psychology, University of Wales College of Cardiff, Cardiff, CF1 3YG, UK Received 5 June 1991, in revised form 6 February 1992
Abstract. The threshold of sensitivity to movement could be governed by mechanisms that are sensitive either to change in spatial position, or directly to the movement itself. The use of spatially complex patterns (random-dot patterns) has been suggested to eliminate the former strategy allowing examination of the movement detecting mechanisms in isolation. By means of such a technique, thresholds for directional judgements were determined for patterns which underwent either a simple displacement or a shearing displacement. Thresholds for shearing motion were found to be around one half of those for simple motion, suggesting that relative, rather than absolute, motion governs performance for small displacements. This contrasts with previous experiments which showed that absolute motion governs performance for much larger displacements. 1 Introduction It has been suggested on a number of occasions that the visual processes that, underlie the detection of very slow movements may not be the same as those that detect faster movements (see, eg, Exner 1875; Dimmik and Karl 1930; Leibowitz 1955; Henderson 1971; Cohen and Bonnet 1972; Johnson and Leibowitz 1976; Bonnet 1982; Johnson and Scobey 1982; Snowden and Braddick 1990; Buckingham etal 1991). The general theme behind these authors' ideas is that very slow movements can be 'inferred' from the realisation of position change over time, thus thresholds for detecting movement should be governed by the amplitude of the movement rather than by its velocity. It is thought that, at greater speeds, motion becomes a sensation in its own right; with tremendous foresight, Exner (1875) came to assume that this would involve a neural motion centre—a notion which now appears to have considerable physiological support (Zeki 1980; Salzman etal 1991). Under these conditions it is assumed that it is the velocity of the movement that will govern performance. I shall refer to these two strategies as 'inferred' and 'direct' movement detection, respectively. These two strategies make quite different predictions as the duration of a stimulus is manipulated. The inferred strategy predicts displacement thresholds (the minimum amplitude of movement) to be independent of duration, whereas the direct strategy predicts displacement thresholds to increase with increasing duration. Much evidence has been gathered which appears to support these ideas. It appears that for short durations detection is governed by the displacement of the stimulus, whereas at longer durations it is the velocity that governs thresholds (Johnson and Leibowitz 1976; Bonnet 1982). Nakayama and Tyler (1981) performed a series of elegant experiments which were designed to show how the use of random-dot patterns can overcome the general problem that motion always involves a change in position and can therefore be detected on the basis of either judgement. They argued that the use of random-dot patterns removes 'familiar position cues' and subjects are therefore forced to rely upon their movement-sensitive elements. To support this argument they measured sensitivity to an oscillating movement either of a random-dot pattern or of a single line. It is important to note that the oscillation for both the line and the random dots was along the line of the pattern (ie adjacent sections of the pattern or line were
564
R J Snowden
moving in different directions). When the line stimulus was used, displacement thresholds did not vary across a range of low temporal frequencies of oscillation, suggesting that the amplitude of displacement governed performance. For the randomdot stimulus, displacement thresholds rose as the frequency of oscillation was reduced, so that the threshold was a constant velocity. These results suggest that the random-dot patterns had eliminated the subjects' ability to use the positional system which would have produced lower thresholds. Evidence has accumulated that the inferred strategy acts upon the relative change in position rather than on absolute change. Nakayama and Tyler (1981) once again provided the crucial data. They measured sensitivity to the sinusoidal oscillation of a line stimulus for a series of oscillation spatial frequencies. They found that threshold amplitude varied with spatial frequency of oscillation. If thresholds were governed by absolute position, this would not be the case, as for each spatial frequency the maximum oscillation would be the same. It therefore appears that subjects were comparing the position of the line in some parts of the display with its position in other parts—a relative position strategy (see also Tyler 1973). From the notion that small displacements are processed by a position-sensitive mechanism, we might expect that relative motion would have an advantage over absolute motion as the relative motion would cause elements in different parts of the display to move further apart than in the case of absolute motion (for a similar argument with respect to stationary landmarks see Bonnet 1982). Thus this theory predicts that displacement thresholds will be governed by the relative motion of the pattern. If, however, random-dot patterns are employed, then the position-sensitive mechanism should be rendered useless and the motion will be sensed directly. I was therefore interested to see if thresholds would be governed by relative motion under these conditions, or by absolute motion. To test these ideas I measured displacement thresholds using random-dot patterns undergoing either a simple or a shearing motion. 2 Methods The overall procedures and equipment have been reported in previous studies (Snowden and Braddick 1989; 1990) and so only those relevant to the present study will be reported here. 2.1 Stimuli The stimuli were two-frame random-dot kinematograms. Frame 1 consisted of 400 dots (each dot was approximately 0.5 mm in diameter and of high luminance) randomly positioned within a 5 cm x 5 cm square, and frame 2 was produced by shifting all the points in one direction, horizontally, (termed 'simple' motion), or by shifting the dots in the upper half of the display in one direction and the dots in the lower half in the opposite direction (termed 'shearing' motion). Points which now fell outside the 5 cm x 5 cm square were wrapped to the other side of the display, hence the edges of the pattern remained stationary. Each frame was displayed for 100 ms, and there was no interframe interval. Therefore each kinematogram had a duration of 200 ms. 2.2 Procedure and subjects The subjects viewed the screen from 300 cm in a dimly lit room. This enabled them to see clearly the edges of the screen and other laboratory equipment. A fixation point preceded each trial and the subject pressed a button to initiate the stimulus presentation. To prevent any confusion only one type of motion (simple or shearing) was presented with any block of trials, and subjects were informed of the type of motion to expect. Subjects made, a two-alternative forced-choice as to the motion they saw: left versus right for the simple motion, and clockwise versus anticlockwise for the shearing motion.
565
Sensitivity to relative and absolute motion
The method of constant stimuli was used to determine performance. A number of displacement levels was chosen (based on practice trials) for each subject and stimulus, and presented 50 times each, in a random order. As alluded to above, the naive subjects (SS and GJ) were given a brief practice session (of the order of 100 trials) to familiarise themselves with the task. The author also served as a subject and had undergone extensive practice while developing these stimuli. 3 Results The results are displayed in figure 1, in which the percentage of correct judgements made at each displacement size for each of the three subjects is plotted. In the plots on the left displacement is defined in absolute terms, so that a displacement of 10 s arc for the shearing motion means that one half of the pattern moved 10 s arc to the right and the other half 10 s arc to the left (for the simple motion all points moved 10 s arc to the right). The psychometric functions were fitted by Probit analysis (Finney 1971). It is clear that subjects were more sensitive to the shearing than to the
Displacement/s arc
Displacement/s arc
Figure 1. The percentage of correct responses is plotted against displacement size for three subjects. In the figures on the left displacement here is defined in absolute terms (ie a displacement of 10 s arc for the shearing motion means that the upper half was displaced 10 s arc one way and the lower half 10 s arc the opposite way). In the figures on the right displacement is defined in relative terms (ie a displacement of 10 s arc for the shearing motion means that the upper half was displaced 5 s arc one way and the lower half 5 s arc the opposite way). The curves through the data points were fitted by Probit analysis (Finney 1971). The solid circles refer to the shearing displacement and the open circles to the simple displacement.
566
R J Snowden
simple motion, and thresholds (taken at the 25% error mark) are approximately half of those for the shearing motion. This suggests that the subjects are sensitive to the relative rather than the absolute motion of the patterns. If this is so, the functions should superimpose if re-plotted in terms of the relative motion of the display (thus for the shearing stimulus, if the upper half is displaced 10 s arc to the right and the lower half 10 s arc to the left, this is a relative motion of 20 s arc). This manipulation is shown in the plots on the right. The functions for the shearing and simple motions are now very similar. 4 Discussion The present results clearly suggest that motion sensitivity is governed by the relative motion of the stimulus elements rather than by the absolute movement of any one element. A second important measure of motion detection is that of the greatest displacement which can be seen as directional (Braddick 1974). Baker and Braddick (1982) measured the maximum displacement under conditions where the test displacement of a patch of random dots was defined by the relative or absolute motion with respect to the surrounding random-dot pattern. They found that the maximum displacement was governed by the absolute displacement of either the test pattern or the surrounding pattern, and not by the relative motion between them. Thus it appears that the smallest displacement which can be judged as directional is governed by the relative displacements of elements, whereas the maximum displacement which can be judged is governed by the absolute displacement of the elements. Previous studies have shown that motion sensitivity can be improved by the introduction of stationary landmarks (Aubert 1886; Leibowitz 1955; Mates 1969; Tyler and Torres 1972; Legge and Campbell 1981; Bonnet 1982; Johnson and Scobey 1982), but only under conditions which are thought to favour the inferred motion strategy (Bonnet 1982). Such stationary landmarks can, presumably, serve as a comparison to the target stimulus and hence improve performance. It should be noted that in the present study the subjects could clearly see the edge of the screen and other stationary landmarks for both the shearing stimulus and the simple stimulus. Therefore the present result suggests that comparisons can also be made between movements in opposite directions, as well as those between stationary and moving objects. Despite the use of random-dot patterns, which have been previously suggested to eliminate the use of positional cues (Nakayama and Tyler 1981), it appears that the maximum and minimum displacement limits are still governed by absolute motion and relative motion respectively. This suggests the possibility that the motion system has access to both types of information and can use either to detect movement. Hence at the smallest displacements the relative motion system would be the most sensitive whereas at large displacements the absolute motion system is able to signal the motion. The notion of a system specific for relative motion has considerable support from other lines of evidence. Neurons which are sensitive to relative motion have been described in the pigeon (Frost and Nakayama 1983; Frost 1985; Frost et al 1988); superior colliculus (Mandl 1985) and area 17 of the cat (Burns et al 1972); areaMT of the owl monkey (Allman et al 1985); and the superior colliculus (Bender and Davidson 1986), areaV2 (Orban et al 1987), and areaMT of the macaque (Tanaka et al 1986). It is unclear as yet if the neurons of one particular visual area are responsible for the thresholds measured in this paper, however, Siegel and Andersen (1986) found that discrete lesions to areaMT of the macaque compromise performance on a shear detection task similar to that used in the present study. Recent psychophysical studies have shown that relative motion detectors can be selectively adapted out (Shioiri et al 1991). Interestingly, Richards and Lieberman (1982) report
Sensitivity to relative and absolute motion
567
that around 20% of humans do not appear to be sensitive to shearing motion. Such evidence suggests that the perception of shearing motion might be processed independently of simple motion, and could have great significance in depth judgements through motion parallax (Rogers and Graham 1979). Acknowledgements. I thank Gabrielle Jordan and Sophie Snowden for participating in these experiments. Most of this work was completed when I was an SERC post-graduate student at Cambridge University. References AllmanJ, MiezinF, McGuinnes E, 1985 "Stimulus specific responses from beyond the classical receptive field" Annual Review ofNeuroscience 8 407 - 430 Aubert H, 1886 "Die Bewegungsempfindung" Archives of Gestalt Physiology 39 347 - 370 Baker C L, Braddick O J, 1982 "Does segregation of differently moving areas depend upon relative or absolute motion?" Vision Research 22 851-856 Bender D B , Davidson R M , 1986 "Global visual processing in the monkey superior colliculus" Brain Research 3 8 1 3 7 2 - 3 7 5 Bonnet C, 1982 "Thresholds of visual motion" in Tutorials on Motion Perception Eds A H Wertheim, W A Wagenaar, H W Leibowitz (New York: Plenum Press) pp 41 - 78 Braddick O J, 1974 "A short-range process in apparent motion" Vision Research 14 519 - 527 Buckingham T, Watkins R, BinningtonJ, 1991 "The effect of spatial parameters on oscillatory movement displacement thresholds" Vision Research 31 327-331 Burns B D, GassanovV, Webb A C , 1972 "Responses of neurons in the cat cerebral cortex to relative movement patterns" Journal of Physiology (London) 226 133-151 Cohen R L , Bonnet C, 1972 "Movement detection thresholds and stimulus durations" Perception &Psychophysics 12 269-272 DimmickFL, Karl J C, 1930 "The effect of exposure time upon the R. L. of visual motion" Journal of Experimental Psychology 13 365-369 Exner S T, 1875 "Uber das Sehen von Bewegungen und die Theorie des zusammengesetzten Auges" Sitzungsberichte der Akademie der Wissenschaft in Wien 72 156 -190 Finney D J, 1971 Probit Analysis (Cambridge: Cambridge University Press) Frost B J, 1985 "Neural mechanisms for detecting object motion and figure-ground boundaries, contrasted with self-motion detecting systems" in Brain Mechanisms and Spatial Vision Eds D J Ingle, M Jeannerod, D N Lee (Dordrecht: Martinus Nijhoff) pp 415-449 Frost B J, Cavanagh P, Morgan B, 1988 "Deep tectal cells in pigeons respond to kinematograms" Journal of Comparative Physiology A 162 639-641 Frost B J, Nakayama K, 1983 "Single visual neurons code opposing motion independent of direction" Science 220 744-745 Henderson D C , 1971 "The relationship among time, distance and intensity as determinants of motion discrimination" Perception & Psychophysics 1 0 3 1 3 - 3 20 Johnson C A, LeibowitzHW, 1976 "Velocity-time reciprocity in the perception of motion: foveal and peripheral determinations" Vision Research 16 7 7 - 8 0 Johnson C A, ScobeyRP, 1982 "Effects of reference lines on displacement thresholds at various durations of movement" Vision Research 2 2 8 1 9 - 8 2 1 LeggeGE, Campbell F W, 1981 "Displacement detection in human vision" Vision Research 21 205-214 LeibowitzHW, 1955 "Effect of reference lines on the discrimination of movement" Journal of the Optical Society ofAmerica 45 829-830 Mandl G, 1985 "Responses of visual cells in cat superior colliculus to relative pattern movement" Vision Research 25 267-281 Mates B, 1969 "Effects of reference marks and luminance on discrimination of movement" Journal of Psychology 73 209 - 221 Nakayama K, Tyler C W, 1981 "Psychological isolation of movement sensitivity by removal of familiar position cues" Vision Research 21 4 2 7 - 4 3 3 Richards W, LeibermanHR, 1982 "Velocity blindness during shearing motion" Vision Research 22 97-100 Rogers B, Graham M, 1979 "Motion parallax as an independent cue for depth perception" Perception 8 125-134 OrbanGA, Gulyas B, Spileers W, 1987 "A moving noise background modulates responses to moving bars of monkey V2 cells but not monkey VI cells" Investigative Ophthalmology and Visual[ScienceSupplement 28 197
568
R J Snowden
Saltzman C D, Britten K H , Newsome W T, 1991 "Cortical microstimulation influences perceptual judgements of motion detection" Nature (London) 346 174-177 Shioiri S, Ono H, Sato T, 1991 "Adaptation of relative motion detectors" Investigative Ophthalmology and Visual Science Supplement 32 827 SiegelRM, Andersen R A, 1986 "Motion deficits following ibotenic acid lesions of the middle temporal area (MT) in the behaving rhesus monkey" Society for Neuroscience Abstracts 12 1183 SnowdenRJ, Braddick O J, 1989 "The combination of motion signals over time" Vision Research 29 1621 -1630 Snowden R J, Braddick O J, 1990 "Differences in the processing of short-range apparent motion at small and large displacements" Vision Research 30 1211 -1222 TanakaH, Hikosaka K, Saito H-A, YukieM, FukadaY, Iwai E, 1986 "Analysis of local and wide-field movements in the superior temporal visual areas of the macaque monkey" Journal of Neuroscience 6 134 -144 Tyler C W, 1973 "Periodic vernier acuity" Journal of Physiology (London) 288 637-647 Tyler CW, Torres J, 1972 "Frequency response characteristics for sinusoidal movement in the fovea and periphery" Perception &Psychophysics 12 232-236 Zeki S M, 1980 "The response of cells in the middle temporal area (MT) of the owl monkey visual cortex" Proceedings of the Royal Society of London B 277 239 - 248
© 1992 a Pion publication printed in Great Britain
