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Motion processing in peripheral vision: Reaction time and perceived velocity
Vision Rurmrch Vol. 22. pp Prmkd in Great Britam
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lo 68.
0042.6989/~2/010061-08u)3.00/0
1982
PergamonPress Ltd
MOTION PROCESSING IN PERIPHERAL VISION: REACTION TIME AND PERCEIVED VELOCITY PAUL D. TYNAN and ROBERTSEKULER Departments of Psychology
and Ophthalmology. Northwestern Evanston, IL 60201. U.S.A.
(Receired 8 October 1980; in rrriscdfurm
University.
I June 1981)
Abstract-Reaction times to motion onset were measured as a function of eccentricity of presentation. These were compared with measurements of perceived speed at various eccentricities. For slowly moving targets, both dependent measures changed substantially with eccentricity: RT increased and perceived speed declined. For more rapidly moving targets, both dependent measures were unchanged by eccentricity. These results may be related lo the difference between retinotopic distribution of neural mechanisms responsive to low rates of temporal modulation and the retinotopic distribution of neural mechanisms responsive to higher rates of temporal modulation.
INTRODUCTlON
motion onset, the stimulus variable of interest to us. Other work relevant to this topic was done by Ball and Sekuler (1980). In one of their experiments, they measured reaction time to onset of motion for random dot stimuli presented at eccentricities of 0, 5, 10 and 15 deg. With speed held constant at 4deg/sec. reaction time increased from 236 to 272 msec over the range of eccentricities studied. Unfortunately, they made measurements with only one speed of movement. This makes it impossible to know how speed and eccentricity would interact, a question of importance for us.
At various levels of the mammalian visual system. the receptive fields of cells that respond to moderate and high rates of temporal modulation tend to be more uniformly distributed across the visual field than are those of cells that are less responsive to such temporal modulation (Fukuda and Stone, 1974; Kirk et al., 1976). This difference between retinotopic distributions led us to seek corresponding differences in the retinotopic distribution of psychophysical responses. We wondered whether the distributions of receptive fields of cells responsive to various rates of temporal modulation might affeci psychophysical responses to moving targets at several eccentricities. We examined two dependent variables related to motion perception, reaction time to motion onset and perceived speed of motion. Based on the physiological data above, our hypothesis was that with sufficiently high target speeds (and correspondingly high rates of temporal modulation) psychophysical responses would be invariant with eccentricity. But before describing our own work on motion perception in the periphery and near-periphery, let us briefly consider what is already known about the dependent variables we studied. Reaction
Perceiced
celocity
There is some evidence that peripheral viewing alters the perceived speed of an object. Among the most interesting reports on this topic is Lichtenstein’s (1963) that a moving target appears to slow and finally stop as it is presented further in the periphery. This observation, a confirmation of work more than a century earlier (Czermak, 1854; cited in LeGrand, 1967) has been recently rediscovered yet another time (Campbell and Maffei, 1979). In addition, some of Ball and Sekuler’s (1980) measurements of reaction time to motion onset are consistent with the idea that apparent speed decreases with eccentricity. Diener et ul. (1976) asked subjects to magnitude estimate the speed of periodic, grating targets viewed either centrally or at an eccentricity of 30deg. For velocities higher than about 40 deg/sec, peripheral motion elicited lower magnitude estimates, suggesting that rapid motion in the periphery appears slower than the same motion in the center of vision. Unfortunately, the standard speed against which subjects were to make their comparative magnitude estimates was lOOdeg/sec. Because they differed so much in speed from that of the standard target, slowly moving
time to motion
Surprisingly little consideration has been given to speed of response to movement in peripheral vision; in fact we know of only two relevant studies. Borkenhagen (1974) found that reaction time to a small line accelerating at 1 deg/sec’ was 400 msec slower when the stimulus was located 60 deg in the periphery than when located in the center of vision. When the line’s acceleration was increased to 12deg/sec2, this difference dropped to only 50 msec. However. the use of an accelerating stimulus makes it difficult to relate the work directly to the question of reaction time to 61
PAUL D. TYNAN and ROBERTSEKULER
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targets’ apparent speed probably would not have been gauged properly by the method of Diener c’t trl. (Engen and Levy, 1955). Given the paucity of basic measurements of peripheral motion processing, we set out to determine under what conditions responses to moving targets varied with eccentricity and under what conditions the responses might be invariant with eccentricity. To answer these questions. we measured two different types of responses to motion at various eccentricities: reaction time to motion onset and apparent velocity. In addition. we measured the lower threshold for motion at various eccentricities. This threshold. a measure of the slowest perceptible motion. has been well studied previously (Sekuler and Tynan, 1981).
EXPERIMENT
1. REACTION TIME TO
MOTION
diameter. Note that a 0 deg annulus and a IOdeg patch constitute identical conditions: in both, dots cover the entire area available on the CRT. Reaction times were measured with stimulus velocities of 0.25. I. 4 and I6 deg/sec. Trials were run in blocks of constant velocity but randomly intermixed annulus sizes (in one set of trials) or randomly intermixed patch sizes (in another set). Each session consisted of I25 trials. 25 with each hole or patch size. Velocity was constant throughout a session. The computer discarded any RT less than 100 msec or greater than 5OOmsec: in addition, the highest and lowest RTs in each block of 25 trials were discarded. Over the entire experiment. each condition was presented three times for each observer. Ohserwrs. Both authors and a naive volunteer served as observers. All had visual acuity corrected to at least 20.!20.
ONSET
Stimuli were spatially random, luminous dots generated on the face of a cathode ray tube (CRT) with a P4 phosphor by a small computer. The pattern of dots was isotropic. minimizing the contribution of contour processes to our results. The screen of the CRT was masked by a IO deg dia circular aperture cut in white cardboard. The display was viewed binocularly from a distance of 57cm. with fixation maintained in the center of the display where a small fixation point was situated. In all experiments. the dots moved upward along parallel paths (i.e. the position of the dots relative to one another did not change). Moving dots “wrapped around” the screen so that when a dot moved off the top edge. it immediately reappeared at the bottom edge. The veiling luminance of the CRT was 0.75 cd/m’. Incremental luminance of the dots was adjusted to 50 times detection threshold. The maximum number of dots that could appear (full field condition) was 500. An electronic blanking circuit eliminated dots from either the center of the screen or from its periphery. This circuit produced either a patch of dots in the middle of the screen. or a central area devoid of dots surrounded by an annulus of dots. With either patch or annulus. dots moving into the blanked zone disappeared: dots leaving the zone reappeared. The following examples will clarify the convention we have adopted to describe stimuli: with an annulus of 8 deg i.d.. no dots appeared within 4 deg of fixation: with a central patch of 8 deg dia. no dots appeared outside a circle of radius 4 deg. centered on fixation. On each trial. stationary dots appeared first. Then. following a random foreperiod of 131.7 sec. the dots began to move at the desired. constant velocity. The foreperiod varied randomly from one presentation to the next. preventing the observer from knowing the precise time at which movement would be initiated. Annuli were either 0. 2. 4. 6 or 8 deg in i.d.: central stimulus patches were either 2. 4. 6. 8 or IOdeg in
Figure I shows the main results of Experiment 1. Each point is the mean over all three observers (and so is based on a total of about 200 trials). With annuli (left panel), the lowest velocity, 0.25 deg/sec. yielded reaction times that increased steadily with annulus size; at higher velocities. RT was independent of annulus size. With central patches of moving dots (right panel). patch size influences RT only in going from a 2 deg patch to one of 4 deg at a stimulus speed of 0.25 deg/sec. For all higher velocities, RTs were invariant with patch size. Note. in addition. that in both panels RT declines with increasing stimulus velocity. Standard errors for each condition tended to decrease slightly with increasing target velocity. There was no systematic relationship between the size of the standard errors and either annulus or patch size. The largest standard errors. for responses to 0.25 deg/sec. averaged 15.29 msec. Two analyses of variance were performed. one on the annulus data and the other on the patch data. With annuli. the effect of annulus size. velocity and the interaction of annulus size and velocity were all statistically significant, F(4.8) = 10.80. F(3.6) = 171.01, and F(12.24) = 4.69. all P < 0.01. With patches. the overall effect of patch size was not significant. F(4.8) = 0.07. but the effects of target velocity and the interaction between patch size and target velocity were, F(3.6) = 20.31 and F(12.24) = 4.69. respectively. both P < 0.01. The significant interaction is largely the result of the difference in RTs to the 0.25 degisec movement measured with a patch of 2 deg and with a patch of 4 deg. Similarly. the interaction with annular stimuli is also largely the result of the data with the same slow rate of movement. 0.25 deglsec.
In Experiment 1, changing the diameter of annulus or patch also changed the total area of retina stimu-
Motion
, 0
processing
in peripheral
vision
63
4
2
4
6
1
e
Annulus Inner Diameter
2
4
(degrees)
Central
6
Patch
6
Diameter
IO
(degrees)
Fig. 1. Reaction time to sudden onset of movement presented either within a central patch of the cathode ray display (right panel) or within an annular region of the display (left panel). The parameter of the family of curves is target velocity in deg/sec. The abscissa in the left panel shows the id. of the annular stimulus; o.d. was constant, IOdeg. The abscissa in the right panel shows the diameter of the circular patch within movement was presented. The average SE for each point shown is approx. 7.5 msec.
lated as well as the number of dots in the stimulus. If the total area stimulated were held constant. we wondered whether differences between annulus and patch conditions would remain. To check on this point, we replotted the data. Figure 2 shows the data as a function of total area stimulated (lower abscissa) and
NUMBER
OF DOTS
IN
TARGET
572
490
409
327
245
163
I
I
I
I
I
1
350
_-
,4-_______t_-------
:
,/’
z -30
number of dots presented (upper abscissa). Reaction times to the slowest rate of movement, 0.25 deg/sec, are plotted as diamonds; reaction times to increasingly faster rates of movement are plotted as pentagons, octagons and triangles. In each case, data points from annular configurations are connected by dashed
CT”
1
0
I
-4” ANNULUS PATCH
. .
.
61
: i= z 0 i=
250
2 k!2
-! 70
60
50
TARGET
40
AREA
30
20
IO
0
(in cm2 1
Fig. 2. Reaction time as a function of the area covered by the moving array of dots (lower abscissa) or as a function of the number of dots visible in the array (upper-abscissa). These data have been replotted from Fig. 1. Solid curves connect data points produced by moving dots confined to a central, circular patch: dashed lines connect data points produced by moving dots confined to an annular region. Filled circles: dots moving at 0.25 deg/sec; filled pentagons: dots moving at 1 degjsec; filled hexagons: dots moving at 4 deg/sec: filled triangles: dots moving at 16 deg/sec.
PAUL D. TYNAN and ROBERT SEKLLER
64
lines; data points from patch configurations are connected by solid lines. For the three fastest speeds (lower three pairs of curves) it does not seem to matter whether the stimuli were presented in the form of annuli or of patches; RT for the both conligurations are the same. However. for the slowest speed (upper two curves) equivalent total stimulus areas produce shorter reaction times when central patches of moving dots are presented than when annuli of moving dots are presented. For the slowest speed only then, does it matter whether the dots appear as annuli or central patches. Looking at Figs I and 2. one is struck by how the reaction time curves flatten at higher velocities. For example. in Fig. 2. at the highest speeds RT is vsirtually unchanged as the number of dots varies by more than 20 times. One possibility is that the region of the held we tested has uniform sensitivity to onset of motion at moderate speeds. Alternatively. the curves might have flattened because our dependent measure had reached an irreducible minimum. But as our own data and those of others show (Ball and Sekuler. 1980). RTs for speeds less than 16degsec in our experiment are definitely not asymptotically fast. In other words. our RTs are not invariant with eccentricity simply because the dependent measure is pinned at some minimum level. Visual functions that depend upon spatial resolution -vacuity and. very likely, RT to very slow movement -change significantly over the portion of the held studied in this experiment (LeGrand. 1967). We believe that such visual functions depend upon physiological mechanisms that respond preferentially to lower temporal frequencies. The rapid decline in psychophysical spatial resolution is consistent with the hypothesis that cells responsive to lower temporal frequencies are more likely to have receptive fields in the center of vision. RT to moderate speeds of motion shows no decline over this same range of eccentricities. Very likely. cells with appreciable sensitivity to higher rates of temporal modulation participate in the detection of such motion. The invariance in RT with eccentricity is consistent with the hypothesis that cells responsive to higher temporal rates have more uniformly distributed receptive fields. EXPERIMENT
2. PERCEIVED
SPEED
OF MOTION
This experiment measured the perceived velocity of dots moving at different velocities and presented at various eccentricities.
Three hundred twenty moving dots were presented within one of five vertical strips on the display. Each strip was 28 deg high by 4.7 deg wide and could be presented at eccentricities as great as 30deg in the temporal held. Viewing was monocular, through a 2.5 mm artificial pupil in front of the right eye. The
observer’s eye was positioned 20.6cm from the display and accommodation facilitated with a f4 D lens. The mean luminance used in the display was 29.5 cd/m’. Note that this is higher than that used in Experiment 1: but, as before. the incremental luminance of the dots was approximately 50 times contrast threshold. Since the screen’s face was flat. all points on its surface could not be equally distant from the observer’s eye. We dealt with this by positioning the screen so that both edges were equidistant from the observer. making the screen‘s center one centimeter closer to the observer than were its edges. This produced an error in target angular velocity and size of 5”,, from center of screen to its ends: we ignored this source of variation. A dark spot (20 min in diameter) at the right hand side of the display afforded a tixation point. The region within which dots could be presented was either immediately to the left of the fixation point. or at one of four distances from it: 7.5. IS. 22.5 and 30 deg. We were concerned that our measurements might be affected by the reference mark provided by the hxation point. This would be particularly bad since the influence of the reference mark would likely fall otf as the distance between fixation point and moving dots increased. This diminished reference effect would be confounded with the effect of eccentricity in which we were interested. To keep the potential reference effect constant at all eccentricities. we added several other dark spots (20 min dia) to the face of the display. evenly spaced along an imaginary horizontal line half way up the display. These spots were positioned so that. no matter within which of the five vertical strips moving dots appeared. the moving dots would always have one dark spot on either side. The duration of any movement varied randomly between 1.5 and 2.5 sec. This made it difficult to judge velocity simply from the distance traveled by any particular element in the pattern. In addition. we used another technique to keep observers from judging speed on the basis of local movements: the initial position of the pattern along the vertical axis was randomized from trial to trial. Each trial consisted of a pair of moving targets that repeated cyclically. In each cycle. the first strip appeared immediately to the temporal side of the lixation point. One-half second after the offset of this central pattern. another pattern appeared at any one of five locations: coincident with the first pattern (Odeg eccentricity) or at either 7.5, 15. 22.5 or 30deg of eccentricity in the temporal field. One second after the offset of the second pattern. the first pattern was again presented. and the sequence was repeated. The velocity of the first. centrally located pattern was controlled by a knob available to the observer. His instructions were to adjust the velocity of dots in the central strip to match that of the dots in the peripheral strip. When the observer was satisfied with the match. he pushed a switch. terminating the trial. The
65
Motion processing in peripheral vision 1.0
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4 dag/nc
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1
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7.5
Eccentricity
:
;\
15
22.5
of Standard
30
(degrees)
Fig. 3. Ratio of central. matching speed to eccentrically presented, standard speed. Horizontal dashed line indicates locus of veridical percepts. Ordinate values less than unity imply the standard motion would appear slower than a central motion of the same actual speed.
computer recorded the velocity of the central strip selected as a match and then repeated the procedure with new parameters. The velocity of the central strip was offset randomly before each trial so that the observer was forced to make each adjustment independently of the preceding one. Eccentricity and velocity were randomized. Velocities of 0.25, 1, 4 and 16 deg/sec were factorially combined with the five eccentricities, 0, 7.5, 15, 22.5 and 30 deg. Over the course of four sessions, each observer made a total of eight matches for each combination. Ohsercers. One of the authors (P.T.) and the naive volunteer from the first experiment were joined by another naive volunteer who had not previously served. Results
md
discussion
Figure 3 shows the mean of the velocity matches for dots moving at 0.25, 1,4 and 16 deg/sec. Note that no data are given for the 0.25 deg/sec stimulus at eccentricities beyond 7.5 deg. These data have been omitted because on more than half the trials the 0.25 deg/sec stimulus appeared stationary. Values plotted against the ordinate have been normalized by dividing each by the actual speed of the test pattern. Plotted in this way, ordinate values less than unity indicate the target dots appeared to move more slowly than did the central dots of the same velocity. A score of unity indicates that the dot velocity had been perceived veridically. Our results are simple: eccentrically-viewed dots appear to move more slowly than centrally-viewed ones. This slowing effect increases with eccentricity and decreases with increasing pattern velocity. This
decrease in apparent speed reached quite substantial proportions, as three examples indicate. First, as Fig. 3 shows, at 30deg eccentricity the 4deg/sec stimulus appeared to be moving fully 25”,: more slowly than it really was. Second, at the same eccentricity, the 1 deg/ set stimulus appeared to have slowed by more than 50”,,. Finally, even at eccentricities as small as 15 deg, the 0.25 deg/sec stimulus seemed to have stopped altogether. Our description of the data was verified by an analysis of variance. The analysis revealed two significant sources of variance: the overall effect of eccentricity and the interaction of velocity and eccentricity. F(4,8) = 20.69 and F(8.16) = 2.98. respectively, both P < 0.01. Because the analysis used proportional changes in perceived velocity rather than raw, matching velocities, the main effect of velocity is difficult to interpret, F(2.4) = 6.50, P > 0.06. It is well known that various aspects of motion perception can be altered by whatever tracking eye movements the observer may make (LeGrand, 1967). We were concerned that our observers might have been tracking the stimuli they were being asked to match and that our results might have been influenced by such eye movements. Our concern was not entirely eliminated by the fact that observers were instructed to be very careful about maintaining fixation. As a check on their ability to avoid significant tracking movements, two of these subjects were retested under the conditions of this experiment using the afterimage technique of Verheijen (1961). Both were able to main fixation within 20min arc over a period of several seconds. As a result, we do not think that tracking eye movements played any appreciable role in determining our results (but see Kowler and Steinman, 1981). EXPERIMENT
3. LOWER THRESHOLD
FOR MOTION
The lower threshold for motion is among the most common measures in the literature on motion perception (Sekuler and Tynan, 1981). This threshold is the slowest rate of movement that yields a percept of a moving stimulus. Experiment 2 revealed that the slowest stimulus we used, 0.25 deg/sec, may have been close to or below this threshold when it was presented at eccentricities 15 deg or greater. The rationale for this belief is the observation that the 0.25 deg/sec stimulus appeared stationary at those eccentricities on more than half the trials. We had two reasons for determining how the lower threshold varied with eccentricity. First. we wished to establish the relationship between the velocities used in Experiment 2 and the lower threshold for motion. Second, Experiments 1 and 2 showed that, with slowly moving targets, two different measures of motion response changed rapidly with eccentricity: RT went up and perceived speed dropped. The second of these related findings leads to a clear prediction: if,
66
PAUL D. TYNAN and ROBERTSEKULI:W
with a slowly moving array of random dots. perceived speed drops with eccentricity, then in order to keep perceived speed constant. as one went further from fixation one would have to increase the actual speed. Since the lower threshold for motion is one such possible constant perceived speed (i.e. 0). the lower threshold should rise with increasing eccentricity. The third experiment examined this possibility.
The apparatus and observers were the same as in Experiment 2. Upward moving dots were presented in one of five vertical rectangular regions. 0, 7.5. 15, 22.5 or 30deg in the temporal field. The observer used a potentiometer to adjust the velocity of the dots so that they just appeared stationary. On each trial, moving dots appeared within one of the vertical strips for durations varying randomly between 1.5 and 2.5 sec. One half second later, dots were presented again in the same vertical strip. Periods of random duration. during which dots moved. alternated with 0.5 set long intervals during which the display was blank. In the 0.5 set blank interval. the observer used the potentiometer to adjust the dots’ velocity prior to their next appearance. When satisfied that the threshold had been reached. the observer pressed a button. The computer recorded the velocity the observer had selected and then chose another verticalstrip at which to test during the next trial. At the start of each trial the computer randomly selected some low velocity with which to initiate testing. The order in which the live positions was tested was randomized under the constraint that no position be tested II + I times until all positions had been tested II times. Over several sessions. measurements were made six times with each observer at each test position.
The main results of Experiment 3 are shown in Fig. 4. where the lower threshold for seeing motion is plotted against eccentricity. As expected. the lower threshold for seeing motion rises sharply as testing proceeds further into the periphery of the field. An analysis of variance on the data from Experiment 3 confirmed that the effect of eccentricity was statistically significant. F(4.8) = 11.38. P < 0.01. We were surprised by one finding: that at 15 and 22.5 deg. the lower threshold was less than 0.25 deg, sec. Recall that at these eccentricities in Experiment 2 the same observers could not see movement of 0.25 deg!sec but could see movement of 1 deg/‘sec. As a result. we expected the corresponding lower thresholds here to be between 0.25 deg/sec and 1 deg. sec. There is a likely explanation for the inconsistency between Experiments 2 and 3: observers. assigned quite different psychophysical tasks in the two experiments, might well have adopted different criteria in the two. This possibility is strengthened by the fact
0
75
Eccentrlclty Fig. 4. Threshold for seeing eccentricity of presentation.
15
225
30
( degrees) movement
as a function
01”
Each point is the mean of 18 observations.
that the stimulus ensembles to which observers were exposed in Experiments 2 and 3 were quite different. In Experiment 2, near-threshold velocities only appeared on a small fraction of the trials; in Experiment 3. virtually all of the moving arrays that observers saw were near-threshold. This difference in stimulus ensemble might well have encouraged observers to adopt different criteria for “movement” in the two experiments. But at least qualitatively the results of Experiment 3 are consistent with our expectations from the preceding experiments: the lower threshold for seeing movement rises steeply with increasing eccentricity. This change with eccentricity differs from the result with rapidly moving targets (see Figs l-3); but it ,does resemble the strong influence of eccentricity we had previously found with slowly moving targets. The variation in lower threshold with eccentricity was reflected in some comments otl’ered by one of our observers. He noted that occasionally, when the speed had been adjusted appropriately, he could see motion in the center of the vertical strip although the dots further out in the strip appeared to be static. We take this to mean that the dots were moving at a speed above threshold for the center of vision but below threshold for more peripheral regions. Our lower thesholds for seeing motion are considerably higher than many reported in the classic literature on this problem (LeGrand. 1967). This may well be the result of the large of the stimulus strips used in our experiment or the fact that more than one moving element was present (Brown. 1931; Mates and Graham. 1970). GENERAL DISCUSSION
As noted before, there is consistency among the outcomes of our experiments. In all cases. psychophysical responses to slowly moving targets change
Motion
processing
rapidly as a function of eccentricity of presentation. Also, in all cases, psychophysical responses to rapidly moving targets are nearly invariant with eccentricity of presentation. Although there is a monotonic relation between reaction time and the speed of target motion (Ball and Sekuler, 1980) that relation is sufficiently complex and non-linear to make precise quantitative comparisons among the experiments difficult. There is another consideration that weighs in the interpretation of our results. In all the experiments we report here, eccentricity was varied along the horizontal half-meridian in the temporal field. It has been shown (McColgin, 1960) that contours connecting points in the visual field at which the lower threshold for motion is constant define an ellipse, with major axis horizontal. Extrapolating from McColgin’s study, we might suppose that for equal distances of stimulus from fixation, eccentricity is likely to have a smaller effect if the stimulus’ distances are along a horizontal axis through fixation than if the stimulus’ distances are along a vertical axis through fixation. Additionally, our stimulus was a moving pattern or texture. The results might well have differed if the stimulus had been a single object or textured stimulus of spatial frequency content different from our own. We need to consider two variables that might have mediated our results: change in apparent contrast with eccentricity and change in spatial frequency sensitivity with eccentricity. Suppose that increasing eccentricity leads to a decrease in the apparent contrast of moving dots. Could this decrease have played a role in our results‘! For two reasons, we think not. First, at any single eccentricity, perceived speed of movement might increase, not decrease, with declining contrast (Thompson, 1976). As a result, variation in perceived contrast would lead us to expect results opposite those we actually obtained: perceived speed should have increased with eccentricity and reaction time should have decreased with eccentricity. Second, our results show that variation in response to movement as a function of eccentricity depends critically upon the speed of movement. At slow speeds, response declines with eccentricity; at more rapid speeds, response is invariant with eccentricity. This kind of interaction between eccentricity and speed is incompatible with an explanation of our results that focuses exclusively upon variation in apparent contrast with eccentricity. Consider the role that might have been played by the spatial frequency characteristics of our stimuli. Our dot arrays have complex spectra, covering a substantial range of spatial frequencies. We measured their spectrum using a high contrast photographic positive of our display and a laser interferometer. With 500 dots on the display, the spectrum peaks below 1 c/deg and is non-negligible out to 556 c/deg. It is known that the visual system has diminished sensitivity to high spatial frequencies as eccentricity increases (Wilson, 1978). This means that with peripheral viewing, the high frequency content of our dot
in peripheral vision
61
arrays would be progressively attenuated. But it is difficult to know what effect this would have on our results because prior reports on the effect of spatial frequency upon perceived speed are contradictory. Diener et (I/. (1976) reported that perceived speed increases with spatial frequency; Campbell and Maffei (1979) reported the opposite. As indicated before, we have some questions about the methodology used by Diener er crl., particularly as it relates to speeds within the range used in our work. If Campbell and Maffei (1979) were correct, we should expect the attenuation of high frequencies with eccentricity to increase perceived speed, not decrease it. Another of our results, that RT decreases with target speed, requires some additional comment. One obvious interpretation suggests that motion is detected when the target has moved through some critical displacement (Graham, 1965; Kinchla, 1976). Since spatial displacement per unit of time is directly related to target velocity, this critical displacement would be reached sooner with high velocities than with low ones. Qualitatively, the outcome would resemble our own result: shorter RTs to more rapidly moving targets. But, when quantitative predictions are tested, the “critical displacement” model, fares rather badly. The following will give some idea of the model’s success. To generate quantitative predictions, the “critical displacement” model assumes any RT consists of two parts. One component of the RT varies inversely with stimulus speed: the interval between onset of motion and the time at which the critical distance has been travelled. The second component of any RT is independent of stimulus speed. This component is Movement Time (MT), the time it takes to execute the response. Moving at a speed of 4 degjsec, our dots would require four times longer to travel the critical distance than if they were moving at 16deg/sec. The righthand panel of Fig. I shows that with a central patch of lOdeg, mean RTs were 203.6, 210.7, 241.1, and 312.5msec, for stimulus speeds of 16. 4. 1. and 0.25 deg/sec, respectively. According the critical distance model, the difference between the mean RT to 16 deg/sec, 203.6 msec, and the mean RT to 4deg/sec, 210.7, is the difference between the times required for the critical displacement. Let x be the time required to traverse the critical distance when the stimulus moves at 16deg/sec; 4x is time taken to traverse the same distance when the stimulus moves at only 4deg/sec. We can then write RT to 4 deg/sec = x + MT RT to 16deg/sec
= 4x + MT
Subtracting one equation from the other shows that the difference between the two RTs equals 3x, thrice the time required to travel the critical at 16deg/sec. Substituting the obtained RTs (203.6 and 210.7 msec). gives 2.37 msec as the time needed to travel the criti-
68
PAUL
D. TYNAN and ROBERT SEKULER
cal distance at 16 deg/sec. During this time the stimulus moves 2.28 min arc, the putative critical distance. Substituting .Y = 2.37 msec, gives a value of 201.2 msec for the Movement Time. According to model, the RT when dots move at 1 deg,sec should be this same Movement Time, 201.2. plus 16 x 2.37. This predicted RT. 239.1 msec. is within 2 msec of the obtained mean, 241.1 msec. But using the same approach, the model predicts a mean RT to 0.25 deg/sec of 352.9 msec. nearly 40msec longer than the mean RT actually obtained. 312.5 msec. Predictions for mean RT to 0.25 deg/sec are even further off (approx. 75 msec) when applied to the smallest stimulus patch we used. Generally, the claim that RT depends upon the traversal of some critical distance does not provide a satisfactory fit to our results. Returning to our initial comments. our goal was to test an hypothesis about psychophysical parallels to the retinopic distributions of neural cells whose temporal responses differ from one another. Some effects we obtained do parallel the retinopic distributions of neural cells that respond best to low rates of temporal modulation and of neural cells that respond best to higher rates of modulation (Fukuda and Stone. 1976; Kirk PI ul., 1978). Obviously, our results need to be followed up with measurements of response to other kinds of temporally modulated stimuli at various eccentricities. Presumably such stimuli should include spatially localized targets whose eccentricity is easier to specify unequivocally. A~knorc,/~tl~rn~rnrs--Research supported by U.S. Army contract MDA903-79-M-3971. We thank Drs Gordon Shulman and Randolph Blake for most helpful comments on earlier versions of this paper. REFERENCES
Ball K. and Sekuler R. (1980) Models of stimulus uncertainty in motion perception. Psycho/. Rtr. 87, 4355469. Ball K. and Sekuler R. (1980) Human vision favours centrifugal motion. Pr,c,rprion 9, 317-325. Borkenhagen J. M. (1974) Rotary acceleration of a subject inhibits choice reaction time to motion in peripheral vision. J. e.up. Ps~~hol. 102, 484487. Breitmeyer B. G. and Ganz L. (1976) Implications of sustained and transient channels for theories of visual pattern masking. saccadic suppression. and information processing. Ps~~c~/w/. Rrr. 83, I-36.
J. F. (1931) The visual perception of velocity. PsvForsch. 14, 199-232. Campbell F. W. and Maffei L. (1979) Stopped visual motion. Ntrrurr 278, 192. Diener H. C.. Wist E. R.. Dichgans J. and Brandt T. (1976) The spatial frequency effect of perceived velocity. Vision Rex 16. 169-176. Engen T. and Levy N. (1955) The Influence of standards on psychophysical judgments. Pcwepr. mol. Ski/. 5, 193 197. Fankhauser R. and Schmidt T. (1960) Die optimalen Bedingungen fur die Untersuchung der raumlichen Summation mit stehender Reizmarke nach der Methode der quantitativen Lichtsehnperimetrie. Ophthtrlmoloyicrr 139, 409 -423. Fukuda Y. and Stone J. (1974) Retinal distribution and central projections of Y-. X- and W-cells of the cat’s retina. J. Neurophwiol. 37, 749-772. Graham C. H. (19651 Perception of movement. In Vision cd C’iwul Purcrprion (Edited by Graham C. H. cr ul.). pp. 575-588. Wiley. New York. Harweth R. S. and Levi D. M. (1978) Reaction time as a measure of suprathreshold grating detection. Vision Rex 18, 1579-1586. Kinchla R. A. (1976) A psychophysical model of visual movement-perception. In Err M0wmenf.s ctntl P.s~c/Iohqicctl Procrssr.s (Edited by Monty R. A. and Senders J. W.). pp. 2633276. Wiley. New York. Kirk D. L.. Levick W. R. and Cleland B. G. (1976) Crossed and uncrossed representation of the visual field by brisksustained and brisk-transient cat retinal ganglion cells. Vision Rr.s. 16, 2255232. Kowler E. and Steinman R. M. (1981) The effect of expectations on slow oculomotor control-Ill. Guessing unpredictable target displacements. Vi.Gon Rex 21, Brown dud.
19lL204.
LeGrand Y. (1967) Form trntl Spwe Vi.sion. Indiana University Press. Lichtenstein M. (1963) Spatio-temporal factors in cessation of smooth apparent motion. J. opf. Sot. Ant. 53, 3022306. Mates B. and Graham C. H. (1970) Effect of rectangle length on velocity thresholds for real movement. Proc. w/n. Actrtl. %i. L’.S.A. 65, 5 I6 520. McGolgm F. H. (1960) Movement thresholds in peripheral vision. J. opr. Sot. Am. SO, 774.-779. Sekuler R. and Tynan P. (1981) S~rrre~~ of femporol ,jkror.\ tmfecriny riwtrf tlispltrl~ cuptrhilirirs. Report for U.S. Army. Thompson P. G. (1976) C>/ocir\. &rc/jkr.s untl r/w perwprkm c~J’mo~~n~rnt Ph.D. Dissertation. Cambridge University. Verheijen F. J. (1961) A simple after-image method demonstrating the invsoluntary. multi-directional eye movements during fixation. Oprktrl Acttr 8, 309-31 I. Wtlson H. R. (1978) Quantitative characterization of two types of spread function near the fovea. Vision Rrs. 18, 971-982.
61
lo 68.
0042.6989/~2/010061-08u)3.00/0
1982
PergamonPress Ltd
MOTION PROCESSING IN PERIPHERAL VISION: REACTION TIME AND PERCEIVED VELOCITY PAUL D. TYNAN and ROBERTSEKULER Departments of Psychology
and Ophthalmology. Northwestern Evanston, IL 60201. U.S.A.
(Receired 8 October 1980; in rrriscdfurm
University.
I June 1981)
Abstract-Reaction times to motion onset were measured as a function of eccentricity of presentation. These were compared with measurements of perceived speed at various eccentricities. For slowly moving targets, both dependent measures changed substantially with eccentricity: RT increased and perceived speed declined. For more rapidly moving targets, both dependent measures were unchanged by eccentricity. These results may be related lo the difference between retinotopic distribution of neural mechanisms responsive to low rates of temporal modulation and the retinotopic distribution of neural mechanisms responsive to higher rates of temporal modulation.
INTRODUCTlON
motion onset, the stimulus variable of interest to us. Other work relevant to this topic was done by Ball and Sekuler (1980). In one of their experiments, they measured reaction time to onset of motion for random dot stimuli presented at eccentricities of 0, 5, 10 and 15 deg. With speed held constant at 4deg/sec. reaction time increased from 236 to 272 msec over the range of eccentricities studied. Unfortunately, they made measurements with only one speed of movement. This makes it impossible to know how speed and eccentricity would interact, a question of importance for us.
At various levels of the mammalian visual system. the receptive fields of cells that respond to moderate and high rates of temporal modulation tend to be more uniformly distributed across the visual field than are those of cells that are less responsive to such temporal modulation (Fukuda and Stone, 1974; Kirk et al., 1976). This difference between retinotopic distributions led us to seek corresponding differences in the retinotopic distribution of psychophysical responses. We wondered whether the distributions of receptive fields of cells responsive to various rates of temporal modulation might affeci psychophysical responses to moving targets at several eccentricities. We examined two dependent variables related to motion perception, reaction time to motion onset and perceived speed of motion. Based on the physiological data above, our hypothesis was that with sufficiently high target speeds (and correspondingly high rates of temporal modulation) psychophysical responses would be invariant with eccentricity. But before describing our own work on motion perception in the periphery and near-periphery, let us briefly consider what is already known about the dependent variables we studied. Reaction
Perceiced
celocity
There is some evidence that peripheral viewing alters the perceived speed of an object. Among the most interesting reports on this topic is Lichtenstein’s (1963) that a moving target appears to slow and finally stop as it is presented further in the periphery. This observation, a confirmation of work more than a century earlier (Czermak, 1854; cited in LeGrand, 1967) has been recently rediscovered yet another time (Campbell and Maffei, 1979). In addition, some of Ball and Sekuler’s (1980) measurements of reaction time to motion onset are consistent with the idea that apparent speed decreases with eccentricity. Diener et ul. (1976) asked subjects to magnitude estimate the speed of periodic, grating targets viewed either centrally or at an eccentricity of 30deg. For velocities higher than about 40 deg/sec, peripheral motion elicited lower magnitude estimates, suggesting that rapid motion in the periphery appears slower than the same motion in the center of vision. Unfortunately, the standard speed against which subjects were to make their comparative magnitude estimates was lOOdeg/sec. Because they differed so much in speed from that of the standard target, slowly moving
time to motion
Surprisingly little consideration has been given to speed of response to movement in peripheral vision; in fact we know of only two relevant studies. Borkenhagen (1974) found that reaction time to a small line accelerating at 1 deg/sec’ was 400 msec slower when the stimulus was located 60 deg in the periphery than when located in the center of vision. When the line’s acceleration was increased to 12deg/sec2, this difference dropped to only 50 msec. However. the use of an accelerating stimulus makes it difficult to relate the work directly to the question of reaction time to 61
PAUL D. TYNAN and ROBERTSEKULER
62
targets’ apparent speed probably would not have been gauged properly by the method of Diener c’t trl. (Engen and Levy, 1955). Given the paucity of basic measurements of peripheral motion processing, we set out to determine under what conditions responses to moving targets varied with eccentricity and under what conditions the responses might be invariant with eccentricity. To answer these questions. we measured two different types of responses to motion at various eccentricities: reaction time to motion onset and apparent velocity. In addition. we measured the lower threshold for motion at various eccentricities. This threshold. a measure of the slowest perceptible motion. has been well studied previously (Sekuler and Tynan, 1981).
EXPERIMENT
1. REACTION TIME TO
MOTION
diameter. Note that a 0 deg annulus and a IOdeg patch constitute identical conditions: in both, dots cover the entire area available on the CRT. Reaction times were measured with stimulus velocities of 0.25. I. 4 and I6 deg/sec. Trials were run in blocks of constant velocity but randomly intermixed annulus sizes (in one set of trials) or randomly intermixed patch sizes (in another set). Each session consisted of I25 trials. 25 with each hole or patch size. Velocity was constant throughout a session. The computer discarded any RT less than 100 msec or greater than 5OOmsec: in addition, the highest and lowest RTs in each block of 25 trials were discarded. Over the entire experiment. each condition was presented three times for each observer. Ohserwrs. Both authors and a naive volunteer served as observers. All had visual acuity corrected to at least 20.!20.
ONSET
Stimuli were spatially random, luminous dots generated on the face of a cathode ray tube (CRT) with a P4 phosphor by a small computer. The pattern of dots was isotropic. minimizing the contribution of contour processes to our results. The screen of the CRT was masked by a IO deg dia circular aperture cut in white cardboard. The display was viewed binocularly from a distance of 57cm. with fixation maintained in the center of the display where a small fixation point was situated. In all experiments. the dots moved upward along parallel paths (i.e. the position of the dots relative to one another did not change). Moving dots “wrapped around” the screen so that when a dot moved off the top edge. it immediately reappeared at the bottom edge. The veiling luminance of the CRT was 0.75 cd/m’. Incremental luminance of the dots was adjusted to 50 times detection threshold. The maximum number of dots that could appear (full field condition) was 500. An electronic blanking circuit eliminated dots from either the center of the screen or from its periphery. This circuit produced either a patch of dots in the middle of the screen. or a central area devoid of dots surrounded by an annulus of dots. With either patch or annulus. dots moving into the blanked zone disappeared: dots leaving the zone reappeared. The following examples will clarify the convention we have adopted to describe stimuli: with an annulus of 8 deg i.d.. no dots appeared within 4 deg of fixation: with a central patch of 8 deg dia. no dots appeared outside a circle of radius 4 deg. centered on fixation. On each trial. stationary dots appeared first. Then. following a random foreperiod of 131.7 sec. the dots began to move at the desired. constant velocity. The foreperiod varied randomly from one presentation to the next. preventing the observer from knowing the precise time at which movement would be initiated. Annuli were either 0. 2. 4. 6 or 8 deg in i.d.: central stimulus patches were either 2. 4. 6. 8 or IOdeg in
Figure I shows the main results of Experiment 1. Each point is the mean over all three observers (and so is based on a total of about 200 trials). With annuli (left panel), the lowest velocity, 0.25 deg/sec. yielded reaction times that increased steadily with annulus size; at higher velocities. RT was independent of annulus size. With central patches of moving dots (right panel). patch size influences RT only in going from a 2 deg patch to one of 4 deg at a stimulus speed of 0.25 deg/sec. For all higher velocities, RTs were invariant with patch size. Note. in addition. that in both panels RT declines with increasing stimulus velocity. Standard errors for each condition tended to decrease slightly with increasing target velocity. There was no systematic relationship between the size of the standard errors and either annulus or patch size. The largest standard errors. for responses to 0.25 deg/sec. averaged 15.29 msec. Two analyses of variance were performed. one on the annulus data and the other on the patch data. With annuli. the effect of annulus size. velocity and the interaction of annulus size and velocity were all statistically significant, F(4.8) = 10.80. F(3.6) = 171.01, and F(12.24) = 4.69. all P < 0.01. With patches. the overall effect of patch size was not significant. F(4.8) = 0.07. but the effects of target velocity and the interaction between patch size and target velocity were, F(3.6) = 20.31 and F(12.24) = 4.69. respectively. both P < 0.01. The significant interaction is largely the result of the difference in RTs to the 0.25 degisec movement measured with a patch of 2 deg and with a patch of 4 deg. Similarly. the interaction with annular stimuli is also largely the result of the data with the same slow rate of movement. 0.25 deglsec.
In Experiment 1, changing the diameter of annulus or patch also changed the total area of retina stimu-
Motion
, 0
processing
in peripheral
vision
63
4
2
4
6
1
e
Annulus Inner Diameter
2
4
(degrees)
Central
6
Patch
6
Diameter
IO
(degrees)
Fig. 1. Reaction time to sudden onset of movement presented either within a central patch of the cathode ray display (right panel) or within an annular region of the display (left panel). The parameter of the family of curves is target velocity in deg/sec. The abscissa in the left panel shows the id. of the annular stimulus; o.d. was constant, IOdeg. The abscissa in the right panel shows the diameter of the circular patch within movement was presented. The average SE for each point shown is approx. 7.5 msec.
lated as well as the number of dots in the stimulus. If the total area stimulated were held constant. we wondered whether differences between annulus and patch conditions would remain. To check on this point, we replotted the data. Figure 2 shows the data as a function of total area stimulated (lower abscissa) and
NUMBER
OF DOTS
IN
TARGET
572
490
409
327
245
163
I
I
I
I
I
1
350
_-
,4-_______t_-------
:
,/’
z -30
number of dots presented (upper abscissa). Reaction times to the slowest rate of movement, 0.25 deg/sec, are plotted as diamonds; reaction times to increasingly faster rates of movement are plotted as pentagons, octagons and triangles. In each case, data points from annular configurations are connected by dashed
CT”
1
0
I
-4” ANNULUS PATCH
. .
.
61
: i= z 0 i=
250
2 k!2
-! 70
60
50
TARGET
40
AREA
30
20
IO
0
(in cm2 1
Fig. 2. Reaction time as a function of the area covered by the moving array of dots (lower abscissa) or as a function of the number of dots visible in the array (upper-abscissa). These data have been replotted from Fig. 1. Solid curves connect data points produced by moving dots confined to a central, circular patch: dashed lines connect data points produced by moving dots confined to an annular region. Filled circles: dots moving at 0.25 deg/sec; filled pentagons: dots moving at 1 degjsec; filled hexagons: dots moving at 4 deg/sec: filled triangles: dots moving at 16 deg/sec.
PAUL D. TYNAN and ROBERT SEKLLER
64
lines; data points from patch configurations are connected by solid lines. For the three fastest speeds (lower three pairs of curves) it does not seem to matter whether the stimuli were presented in the form of annuli or of patches; RT for the both conligurations are the same. However. for the slowest speed (upper two curves) equivalent total stimulus areas produce shorter reaction times when central patches of moving dots are presented than when annuli of moving dots are presented. For the slowest speed only then, does it matter whether the dots appear as annuli or central patches. Looking at Figs I and 2. one is struck by how the reaction time curves flatten at higher velocities. For example. in Fig. 2. at the highest speeds RT is vsirtually unchanged as the number of dots varies by more than 20 times. One possibility is that the region of the held we tested has uniform sensitivity to onset of motion at moderate speeds. Alternatively. the curves might have flattened because our dependent measure had reached an irreducible minimum. But as our own data and those of others show (Ball and Sekuler. 1980). RTs for speeds less than 16degsec in our experiment are definitely not asymptotically fast. In other words. our RTs are not invariant with eccentricity simply because the dependent measure is pinned at some minimum level. Visual functions that depend upon spatial resolution -vacuity and. very likely, RT to very slow movement -change significantly over the portion of the held studied in this experiment (LeGrand. 1967). We believe that such visual functions depend upon physiological mechanisms that respond preferentially to lower temporal frequencies. The rapid decline in psychophysical spatial resolution is consistent with the hypothesis that cells responsive to lower temporal frequencies are more likely to have receptive fields in the center of vision. RT to moderate speeds of motion shows no decline over this same range of eccentricities. Very likely. cells with appreciable sensitivity to higher rates of temporal modulation participate in the detection of such motion. The invariance in RT with eccentricity is consistent with the hypothesis that cells responsive to higher temporal rates have more uniformly distributed receptive fields. EXPERIMENT
2. PERCEIVED
SPEED
OF MOTION
This experiment measured the perceived velocity of dots moving at different velocities and presented at various eccentricities.
Three hundred twenty moving dots were presented within one of five vertical strips on the display. Each strip was 28 deg high by 4.7 deg wide and could be presented at eccentricities as great as 30deg in the temporal held. Viewing was monocular, through a 2.5 mm artificial pupil in front of the right eye. The
observer’s eye was positioned 20.6cm from the display and accommodation facilitated with a f4 D lens. The mean luminance used in the display was 29.5 cd/m’. Note that this is higher than that used in Experiment 1: but, as before. the incremental luminance of the dots was approximately 50 times contrast threshold. Since the screen’s face was flat. all points on its surface could not be equally distant from the observer’s eye. We dealt with this by positioning the screen so that both edges were equidistant from the observer. making the screen‘s center one centimeter closer to the observer than were its edges. This produced an error in target angular velocity and size of 5”,, from center of screen to its ends: we ignored this source of variation. A dark spot (20 min in diameter) at the right hand side of the display afforded a tixation point. The region within which dots could be presented was either immediately to the left of the fixation point. or at one of four distances from it: 7.5. IS. 22.5 and 30 deg. We were concerned that our measurements might be affected by the reference mark provided by the hxation point. This would be particularly bad since the influence of the reference mark would likely fall otf as the distance between fixation point and moving dots increased. This diminished reference effect would be confounded with the effect of eccentricity in which we were interested. To keep the potential reference effect constant at all eccentricities. we added several other dark spots (20 min dia) to the face of the display. evenly spaced along an imaginary horizontal line half way up the display. These spots were positioned so that. no matter within which of the five vertical strips moving dots appeared. the moving dots would always have one dark spot on either side. The duration of any movement varied randomly between 1.5 and 2.5 sec. This made it difficult to judge velocity simply from the distance traveled by any particular element in the pattern. In addition. we used another technique to keep observers from judging speed on the basis of local movements: the initial position of the pattern along the vertical axis was randomized from trial to trial. Each trial consisted of a pair of moving targets that repeated cyclically. In each cycle. the first strip appeared immediately to the temporal side of the lixation point. One-half second after the offset of this central pattern. another pattern appeared at any one of five locations: coincident with the first pattern (Odeg eccentricity) or at either 7.5, 15. 22.5 or 30deg of eccentricity in the temporal field. One second after the offset of the second pattern. the first pattern was again presented. and the sequence was repeated. The velocity of the first. centrally located pattern was controlled by a knob available to the observer. His instructions were to adjust the velocity of dots in the central strip to match that of the dots in the peripheral strip. When the observer was satisfied with the match. he pushed a switch. terminating the trial. The
65
Motion processing in peripheral vision 1.0
__
r~,.______
\w\
‘li\
Y
-
4 dag/nc
\ t I drg/arc
1
.4
0
7.5
Eccentricity
:
;\
15
22.5
of Standard
30
(degrees)
Fig. 3. Ratio of central. matching speed to eccentrically presented, standard speed. Horizontal dashed line indicates locus of veridical percepts. Ordinate values less than unity imply the standard motion would appear slower than a central motion of the same actual speed.
computer recorded the velocity of the central strip selected as a match and then repeated the procedure with new parameters. The velocity of the central strip was offset randomly before each trial so that the observer was forced to make each adjustment independently of the preceding one. Eccentricity and velocity were randomized. Velocities of 0.25, 1, 4 and 16 deg/sec were factorially combined with the five eccentricities, 0, 7.5, 15, 22.5 and 30 deg. Over the course of four sessions, each observer made a total of eight matches for each combination. Ohsercers. One of the authors (P.T.) and the naive volunteer from the first experiment were joined by another naive volunteer who had not previously served. Results
md
discussion
Figure 3 shows the mean of the velocity matches for dots moving at 0.25, 1,4 and 16 deg/sec. Note that no data are given for the 0.25 deg/sec stimulus at eccentricities beyond 7.5 deg. These data have been omitted because on more than half the trials the 0.25 deg/sec stimulus appeared stationary. Values plotted against the ordinate have been normalized by dividing each by the actual speed of the test pattern. Plotted in this way, ordinate values less than unity indicate the target dots appeared to move more slowly than did the central dots of the same velocity. A score of unity indicates that the dot velocity had been perceived veridically. Our results are simple: eccentrically-viewed dots appear to move more slowly than centrally-viewed ones. This slowing effect increases with eccentricity and decreases with increasing pattern velocity. This
decrease in apparent speed reached quite substantial proportions, as three examples indicate. First, as Fig. 3 shows, at 30deg eccentricity the 4deg/sec stimulus appeared to be moving fully 25”,: more slowly than it really was. Second, at the same eccentricity, the 1 deg/ set stimulus appeared to have slowed by more than 50”,,. Finally, even at eccentricities as small as 15 deg, the 0.25 deg/sec stimulus seemed to have stopped altogether. Our description of the data was verified by an analysis of variance. The analysis revealed two significant sources of variance: the overall effect of eccentricity and the interaction of velocity and eccentricity. F(4,8) = 20.69 and F(8.16) = 2.98. respectively, both P < 0.01. Because the analysis used proportional changes in perceived velocity rather than raw, matching velocities, the main effect of velocity is difficult to interpret, F(2.4) = 6.50, P > 0.06. It is well known that various aspects of motion perception can be altered by whatever tracking eye movements the observer may make (LeGrand, 1967). We were concerned that our observers might have been tracking the stimuli they were being asked to match and that our results might have been influenced by such eye movements. Our concern was not entirely eliminated by the fact that observers were instructed to be very careful about maintaining fixation. As a check on their ability to avoid significant tracking movements, two of these subjects were retested under the conditions of this experiment using the afterimage technique of Verheijen (1961). Both were able to main fixation within 20min arc over a period of several seconds. As a result, we do not think that tracking eye movements played any appreciable role in determining our results (but see Kowler and Steinman, 1981). EXPERIMENT
3. LOWER THRESHOLD
FOR MOTION
The lower threshold for motion is among the most common measures in the literature on motion perception (Sekuler and Tynan, 1981). This threshold is the slowest rate of movement that yields a percept of a moving stimulus. Experiment 2 revealed that the slowest stimulus we used, 0.25 deg/sec, may have been close to or below this threshold when it was presented at eccentricities 15 deg or greater. The rationale for this belief is the observation that the 0.25 deg/sec stimulus appeared stationary at those eccentricities on more than half the trials. We had two reasons for determining how the lower threshold varied with eccentricity. First. we wished to establish the relationship between the velocities used in Experiment 2 and the lower threshold for motion. Second, Experiments 1 and 2 showed that, with slowly moving targets, two different measures of motion response changed rapidly with eccentricity: RT went up and perceived speed dropped. The second of these related findings leads to a clear prediction: if,
66
PAUL D. TYNAN and ROBERTSEKULI:W
with a slowly moving array of random dots. perceived speed drops with eccentricity, then in order to keep perceived speed constant. as one went further from fixation one would have to increase the actual speed. Since the lower threshold for motion is one such possible constant perceived speed (i.e. 0). the lower threshold should rise with increasing eccentricity. The third experiment examined this possibility.
The apparatus and observers were the same as in Experiment 2. Upward moving dots were presented in one of five vertical rectangular regions. 0, 7.5. 15, 22.5 or 30deg in the temporal field. The observer used a potentiometer to adjust the velocity of the dots so that they just appeared stationary. On each trial, moving dots appeared within one of the vertical strips for durations varying randomly between 1.5 and 2.5 sec. One half second later, dots were presented again in the same vertical strip. Periods of random duration. during which dots moved. alternated with 0.5 set long intervals during which the display was blank. In the 0.5 set blank interval. the observer used the potentiometer to adjust the dots’ velocity prior to their next appearance. When satisfied that the threshold had been reached. the observer pressed a button. The computer recorded the velocity the observer had selected and then chose another verticalstrip at which to test during the next trial. At the start of each trial the computer randomly selected some low velocity with which to initiate testing. The order in which the live positions was tested was randomized under the constraint that no position be tested II + I times until all positions had been tested II times. Over several sessions. measurements were made six times with each observer at each test position.
The main results of Experiment 3 are shown in Fig. 4. where the lower threshold for seeing motion is plotted against eccentricity. As expected. the lower threshold for seeing motion rises sharply as testing proceeds further into the periphery of the field. An analysis of variance on the data from Experiment 3 confirmed that the effect of eccentricity was statistically significant. F(4.8) = 11.38. P < 0.01. We were surprised by one finding: that at 15 and 22.5 deg. the lower threshold was less than 0.25 deg, sec. Recall that at these eccentricities in Experiment 2 the same observers could not see movement of 0.25 deg!sec but could see movement of 1 deg/‘sec. As a result. we expected the corresponding lower thresholds here to be between 0.25 deg/sec and 1 deg. sec. There is a likely explanation for the inconsistency between Experiments 2 and 3: observers. assigned quite different psychophysical tasks in the two experiments, might well have adopted different criteria in the two. This possibility is strengthened by the fact
0
75
Eccentrlclty Fig. 4. Threshold for seeing eccentricity of presentation.
15
225
30
( degrees) movement
as a function
01”
Each point is the mean of 18 observations.
that the stimulus ensembles to which observers were exposed in Experiments 2 and 3 were quite different. In Experiment 2, near-threshold velocities only appeared on a small fraction of the trials; in Experiment 3. virtually all of the moving arrays that observers saw were near-threshold. This difference in stimulus ensemble might well have encouraged observers to adopt different criteria for “movement” in the two experiments. But at least qualitatively the results of Experiment 3 are consistent with our expectations from the preceding experiments: the lower threshold for seeing movement rises steeply with increasing eccentricity. This change with eccentricity differs from the result with rapidly moving targets (see Figs l-3); but it ,does resemble the strong influence of eccentricity we had previously found with slowly moving targets. The variation in lower threshold with eccentricity was reflected in some comments otl’ered by one of our observers. He noted that occasionally, when the speed had been adjusted appropriately, he could see motion in the center of the vertical strip although the dots further out in the strip appeared to be static. We take this to mean that the dots were moving at a speed above threshold for the center of vision but below threshold for more peripheral regions. Our lower thesholds for seeing motion are considerably higher than many reported in the classic literature on this problem (LeGrand. 1967). This may well be the result of the large of the stimulus strips used in our experiment or the fact that more than one moving element was present (Brown. 1931; Mates and Graham. 1970). GENERAL DISCUSSION
As noted before, there is consistency among the outcomes of our experiments. In all cases. psychophysical responses to slowly moving targets change
Motion
processing
rapidly as a function of eccentricity of presentation. Also, in all cases, psychophysical responses to rapidly moving targets are nearly invariant with eccentricity of presentation. Although there is a monotonic relation between reaction time and the speed of target motion (Ball and Sekuler, 1980) that relation is sufficiently complex and non-linear to make precise quantitative comparisons among the experiments difficult. There is another consideration that weighs in the interpretation of our results. In all the experiments we report here, eccentricity was varied along the horizontal half-meridian in the temporal field. It has been shown (McColgin, 1960) that contours connecting points in the visual field at which the lower threshold for motion is constant define an ellipse, with major axis horizontal. Extrapolating from McColgin’s study, we might suppose that for equal distances of stimulus from fixation, eccentricity is likely to have a smaller effect if the stimulus’ distances are along a horizontal axis through fixation than if the stimulus’ distances are along a vertical axis through fixation. Additionally, our stimulus was a moving pattern or texture. The results might well have differed if the stimulus had been a single object or textured stimulus of spatial frequency content different from our own. We need to consider two variables that might have mediated our results: change in apparent contrast with eccentricity and change in spatial frequency sensitivity with eccentricity. Suppose that increasing eccentricity leads to a decrease in the apparent contrast of moving dots. Could this decrease have played a role in our results‘! For two reasons, we think not. First, at any single eccentricity, perceived speed of movement might increase, not decrease, with declining contrast (Thompson, 1976). As a result, variation in perceived contrast would lead us to expect results opposite those we actually obtained: perceived speed should have increased with eccentricity and reaction time should have decreased with eccentricity. Second, our results show that variation in response to movement as a function of eccentricity depends critically upon the speed of movement. At slow speeds, response declines with eccentricity; at more rapid speeds, response is invariant with eccentricity. This kind of interaction between eccentricity and speed is incompatible with an explanation of our results that focuses exclusively upon variation in apparent contrast with eccentricity. Consider the role that might have been played by the spatial frequency characteristics of our stimuli. Our dot arrays have complex spectra, covering a substantial range of spatial frequencies. We measured their spectrum using a high contrast photographic positive of our display and a laser interferometer. With 500 dots on the display, the spectrum peaks below 1 c/deg and is non-negligible out to 556 c/deg. It is known that the visual system has diminished sensitivity to high spatial frequencies as eccentricity increases (Wilson, 1978). This means that with peripheral viewing, the high frequency content of our dot
in peripheral vision
61
arrays would be progressively attenuated. But it is difficult to know what effect this would have on our results because prior reports on the effect of spatial frequency upon perceived speed are contradictory. Diener et (I/. (1976) reported that perceived speed increases with spatial frequency; Campbell and Maffei (1979) reported the opposite. As indicated before, we have some questions about the methodology used by Diener er crl., particularly as it relates to speeds within the range used in our work. If Campbell and Maffei (1979) were correct, we should expect the attenuation of high frequencies with eccentricity to increase perceived speed, not decrease it. Another of our results, that RT decreases with target speed, requires some additional comment. One obvious interpretation suggests that motion is detected when the target has moved through some critical displacement (Graham, 1965; Kinchla, 1976). Since spatial displacement per unit of time is directly related to target velocity, this critical displacement would be reached sooner with high velocities than with low ones. Qualitatively, the outcome would resemble our own result: shorter RTs to more rapidly moving targets. But, when quantitative predictions are tested, the “critical displacement” model, fares rather badly. The following will give some idea of the model’s success. To generate quantitative predictions, the “critical displacement” model assumes any RT consists of two parts. One component of the RT varies inversely with stimulus speed: the interval between onset of motion and the time at which the critical distance has been travelled. The second component of any RT is independent of stimulus speed. This component is Movement Time (MT), the time it takes to execute the response. Moving at a speed of 4 degjsec, our dots would require four times longer to travel the critical distance than if they were moving at 16deg/sec. The righthand panel of Fig. I shows that with a central patch of lOdeg, mean RTs were 203.6, 210.7, 241.1, and 312.5msec, for stimulus speeds of 16. 4. 1. and 0.25 deg/sec, respectively. According the critical distance model, the difference between the mean RT to 16 deg/sec, 203.6 msec, and the mean RT to 4deg/sec, 210.7, is the difference between the times required for the critical displacement. Let x be the time required to traverse the critical distance when the stimulus moves at 16deg/sec; 4x is time taken to traverse the same distance when the stimulus moves at only 4deg/sec. We can then write RT to 4 deg/sec = x + MT RT to 16deg/sec
= 4x + MT
Subtracting one equation from the other shows that the difference between the two RTs equals 3x, thrice the time required to travel the critical at 16deg/sec. Substituting the obtained RTs (203.6 and 210.7 msec). gives 2.37 msec as the time needed to travel the criti-
68
PAUL
D. TYNAN and ROBERT SEKULER
cal distance at 16 deg/sec. During this time the stimulus moves 2.28 min arc, the putative critical distance. Substituting .Y = 2.37 msec, gives a value of 201.2 msec for the Movement Time. According to model, the RT when dots move at 1 deg,sec should be this same Movement Time, 201.2. plus 16 x 2.37. This predicted RT. 239.1 msec. is within 2 msec of the obtained mean, 241.1 msec. But using the same approach, the model predicts a mean RT to 0.25 deg/sec of 352.9 msec. nearly 40msec longer than the mean RT actually obtained. 312.5 msec. Predictions for mean RT to 0.25 deg/sec are even further off (approx. 75 msec) when applied to the smallest stimulus patch we used. Generally, the claim that RT depends upon the traversal of some critical distance does not provide a satisfactory fit to our results. Returning to our initial comments. our goal was to test an hypothesis about psychophysical parallels to the retinopic distributions of neural cells whose temporal responses differ from one another. Some effects we obtained do parallel the retinopic distributions of neural cells that respond best to low rates of temporal modulation and of neural cells that respond best to higher rates of modulation (Fukuda and Stone. 1976; Kirk PI ul., 1978). Obviously, our results need to be followed up with measurements of response to other kinds of temporally modulated stimuli at various eccentricities. Presumably such stimuli should include spatially localized targets whose eccentricity is easier to specify unequivocally. A~knorc,/~tl~rn~rnrs--Research supported by U.S. Army contract MDA903-79-M-3971. We thank Drs Gordon Shulman and Randolph Blake for most helpful comments on earlier versions of this paper. REFERENCES
Ball K. and Sekuler R. (1980) Models of stimulus uncertainty in motion perception. Psycho/. Rtr. 87, 4355469. Ball K. and Sekuler R. (1980) Human vision favours centrifugal motion. Pr,c,rprion 9, 317-325. Borkenhagen J. M. (1974) Rotary acceleration of a subject inhibits choice reaction time to motion in peripheral vision. J. e.up. Ps~~hol. 102, 484487. Breitmeyer B. G. and Ganz L. (1976) Implications of sustained and transient channels for theories of visual pattern masking. saccadic suppression. and information processing. Ps~~c~/w/. Rrr. 83, I-36.
J. F. (1931) The visual perception of velocity. PsvForsch. 14, 199-232. Campbell F. W. and Maffei L. (1979) Stopped visual motion. Ntrrurr 278, 192. Diener H. C.. Wist E. R.. Dichgans J. and Brandt T. (1976) The spatial frequency effect of perceived velocity. Vision Rex 16. 169-176. Engen T. and Levy N. (1955) The Influence of standards on psychophysical judgments. Pcwepr. mol. Ski/. 5, 193 197. Fankhauser R. and Schmidt T. (1960) Die optimalen Bedingungen fur die Untersuchung der raumlichen Summation mit stehender Reizmarke nach der Methode der quantitativen Lichtsehnperimetrie. Ophthtrlmoloyicrr 139, 409 -423. Fukuda Y. and Stone J. (1974) Retinal distribution and central projections of Y-. X- and W-cells of the cat’s retina. J. Neurophwiol. 37, 749-772. Graham C. H. (19651 Perception of movement. In Vision cd C’iwul Purcrprion (Edited by Graham C. H. cr ul.). pp. 575-588. Wiley. New York. Harweth R. S. and Levi D. M. (1978) Reaction time as a measure of suprathreshold grating detection. Vision Rex 18, 1579-1586. Kinchla R. A. (1976) A psychophysical model of visual movement-perception. In Err M0wmenf.s ctntl P.s~c/Iohqicctl Procrssr.s (Edited by Monty R. A. and Senders J. W.). pp. 2633276. Wiley. New York. Kirk D. L.. Levick W. R. and Cleland B. G. (1976) Crossed and uncrossed representation of the visual field by brisksustained and brisk-transient cat retinal ganglion cells. Vision Rr.s. 16, 2255232. Kowler E. and Steinman R. M. (1981) The effect of expectations on slow oculomotor control-Ill. Guessing unpredictable target displacements. Vi.Gon Rex 21, Brown dud.
19lL204.
LeGrand Y. (1967) Form trntl Spwe Vi.sion. Indiana University Press. Lichtenstein M. (1963) Spatio-temporal factors in cessation of smooth apparent motion. J. opf. Sot. Ant. 53, 3022306. Mates B. and Graham C. H. (1970) Effect of rectangle length on velocity thresholds for real movement. Proc. w/n. Actrtl. %i. L’.S.A. 65, 5 I6 520. McGolgm F. H. (1960) Movement thresholds in peripheral vision. J. opr. Sot. Am. SO, 774.-779. Sekuler R. and Tynan P. (1981) S~rrre~~ of femporol ,jkror.\ tmfecriny riwtrf tlispltrl~ cuptrhilirirs. Report for U.S. Army. Thompson P. G. (1976) C>/ocir\. &rc/jkr.s untl r/w perwprkm c~J’mo~~n~rnt Ph.D. Dissertation. Cambridge University. Verheijen F. J. (1961) A simple after-image method demonstrating the invsoluntary. multi-directional eye movements during fixation. Oprktrl Acttr 8, 309-31 I. Wtlson H. R. (1978) Quantitative characterization of two types of spread function near the fovea. Vision Rrs. 18, 971-982.