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Failure to find an absolute retinal limit of a putative short-range process in apparent motion
on4249X9 x3 s3.00 + o.tKI Copyright c 1983 Pergamon Pms Ltd
Vision Res.Vol. 23.No. 12.pp. 1663-1670. 1983 Printedm Great Britain.All rightsreserved
FAILURE TO FIND AN ABSOLUTE RETINAL LIMIT OF A PUTATWE SHORT-RANGE PROCESS IN APPARENT MOTION J. TIMOTHY PETERSIK’.~, RANDALL PUFAHL’ and ELIZABETH KRASWFF ‘Department of Psychology, Ripon College, Ripon. WI 54971. U.S.A.. and ‘New England Institute of Applied Biophysics, 59 North Ashland Ave.. Worcester. MA 01609. U.L.4. (Received
22 September
1982; in recked j&m
IO Jmwnr~~ 1983)
Abstract-Previous
studies of apparent movement have concluded that the short-range process does not operate when stimulus displacements exceed 15-20’ arc. In the present experiments. we studied a bistable apparent-movement display, one of whose perceptual organizations is mediated by short-range process. In both experiments it was found that the perceptual organization mediated by the short-range process could be made dominant at stimulus displacements well in excess of the proposed spatial limit, provided the stimulus elements were made larger. It is concluded that the spatial limit of the short-range process is a relative. not absolute. one. Current knowledge regarding the short-range process is reviewed. and an hypothesis regarding the functional utility of the short-range process in compensatmg for the effects of small eye tremors is advanced. Apparent movement
Bistable display
INTRODUCTlON Within the last 10 years, a relatively sizeable body of psychophysical evidence has accumulated to suggest that human visual motion perception is mediated by at least two distinct processes, one of which operates over a relatively small spatial range and another which operates over larger spatial ranges. These processes also differ with respect to the temporal ranges over which they operate (see below). The present paper focuses primarily on what has come to be known as the “short-range” process and the problem of how to describe its apparent spatial integration limit.
Spatiul limit qf’tlte short -runge process us studied rundom -do I displuys
with
Braddick ( 1974) first suggested that the perceptual segregation of correlated. but displaced, areas in alternating random-dot patterns was due to the activity of a “short-range” process. His studies (Braddick, 1973, 1974) showed that the short-range process matches individual elements in two frames provided that (a) their spatial displacement is IS’ arc or less, (b) the interstimulus interval (ISI) is less than about 70msec (for a IOO-msec exposure duration), (c) the frames are not presented dichoptically. and (d) the field presented during the IS1 is dark. Braddick contrasted the short-range process with the “long-range.’ process which apparently operates in classical
phi-movement
phenomena
Visual motion perception
Short-range process
and which is
maximally stimulated by greater spatial and temporal disparities. operates with dichoptic stimulation. and is insensitive to the luminance of the ISI. As early as 1976. Lappin and Bell had argued that the spatial limit of the short-range process was a relative, not absolute. one. determined by the size of the display and its elements. Lappin and Bell (1976) suggested that Braddick’s discovery of a “retinal” limit to the short-range process W;IS based upon a statistical confound produced by a variable signal/noise ratio. Hence. Baker and Braddick f 19x2) have extended the original findings in studies that covdried dot density. patch size (i.e. size of the area correlated and displaced between frames). dot spacing, and displacement. Based on their findings. they have concluded that the displacement limit of the short-range process is indeed retinal. but that the limit may grow with greater retinal eccentricities. However, Baker and Braddick f 1987) used a constant ISI. and therefore Wed to jnc~ude the ~ssibiljty of a spatiotemporal tradeoff in the determination of a limit to the short-range process. Furthermore. the authors do not indicate how or why a greater spatial limit in the periphery might induce perceptual correlations over correspondingly greater spatial ranges in the fovea. as apparently happens when large displaced patches are viewed.* Therefore, the issue regarding the nature of the spatial limit of the short-range process cannot be considered closed. Sputiui
limit
u hi.Whli~
*We are indebted to an anonymous reviewer for making this point.
of’ tlw short -rurgc* process
uppuriwt
-m~~twniv~t
us strtdied
with
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PBntle and Picciano (1976) and later Petersik and Pantle (1979) studied a histable apparent-movement
1663
1664
J. TIMOTHY PETERSIKPI d.
(1979) called the underlying process the percept c-process. The c -process
dtsplay similar to that shown in Fig. I and whose alternative organizations they identified with the short- and long-range processes. Under certain stimulus conditions. most notably short ISIS (generally less than 70 msec). alternation of the stimulus frames shoun m Fig. I gives rise to the perception of two statIonat-> center bars with a third bar moving back and tbrth around them. This type of percept is known as “end-to-end movement.” and Petersik and Pantle “The ~\\uc I-cg,lrdlng what the short-range process actually contrlhutc\ IO the perceptual experience of end-to-end mo\cmcnt ha\ been discussed by Pantle and Petersik I ILJhll~.,III~ II I\ worthwhile to quote them directly here: “App.~rcntI! the short-range process which mediates \cqc~;ll’l’” .~nd movement in random-dot patterns c~~ntr~htr~~\ to the (end-to-end) movement sensation oni! h! gcncr;ltln: a sIgnal for the inner lines of the LII+.I!. 11)~wu ho\c pomts overlap or nearly overlap in WLUX~I\L~ lrame\. The signal generated is one of ‘no I~O~~CIII~ u hen the points of the overlapping lines C\;ICII! ~~~rc\pond and one of small local movement \\ hen the\ nc;~~-l! correspond. that is. are slightly pcrlurhc2 Ths \I~I~;II Ihr the movement of the end line in ~ICI~CIIImo\cmcnt must be gcncrlrted by some other pr<~cr\ pr~~h.~hl! the same ;‘-process which accounts lor the pc~-cc~~sd displacements termed group moveIllc‘lll (p :I /I ” .A slmllar pomt was made by Braddick and ~~dl
h!
process that mediated the segregation ofcorrelated. but displaced. regions in alternating random-dot patterns on the basis of the following evidence:? an! manipulation ot ISI. the luminance of the ISI. stimulus contrast. or type of viewing (binocular vs dichoptic) that decreases the tendency of subjects to perceive the of correlated areas In alternating segregation random-dot patterns also produce5 fewer reports ol end-to-end movement with the t!pe ofdispla! shown in Fig. I. Furthermore. Pantle and Petersik (1980. see below) reported that end-to-end movement could not be perceived at stimulus displacement beyond about 16’ arc. This limit corresponds to the 15’ displacement limit reported by Braddick (1974) and supported the suggestion that the perceptual process mediating end-to-end movement was the same shortrange process studied by Braddick. Under other stimulus conditions. most notabll long ISIS. alternation of the frames shown in Fig. I gives rise to the perception of three bars moving together as a whole from left to right and back again. This movement is known as “group movement.” and Petersik and Pantle (1979) termed the underlying perceptual process ;*-process. This process \vas identified with the classical long-range process on the basis of the following evidence: group mo\‘ement occurs over larger spatial ranges and depends upon long frame durations (preferabl) greater than 100-200 msec) and long ISIS (preferably greater than SO-70 msec). These stimulus requirements are the same as those required to obtain apparent movement in other displays in which clusters of elements are identified on the basis of global form cues (Pantle. those authors
FIN. I. An example of the type of stimulus frames used in the present experiments. In this example. the edges of the t%o leftmost bars m the top frame are perfectly aligned with the edges of the two rightmost bars in the bottom frame. The edges of these bars were systematically misaligned (i.e. shifted) to form the stimuli used in the present experiments.
that mediated was thought
to be the same short-range
1973;
Ramachandran
group
movement
lation
and
ISI
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is insensltive luminance.
as
1973). to
Furthermore.
dichoptic
is classical
stimu-
apparent
movement. In an en‘ort to determine the spatial-displacement range of the short-range process. Pantle and Petersik (1980) used a display like that shown in Fig. I but consisting of three evenlq spaced narrokv linesegments instead of bars. They systematically displaced the positions of the center two (“overlapping”) lines in the second frame and presented the display at ISIS ordinarily suitable for the production of end-to-end movement. Pantle and Petersik found that relative displacements of 16’ virtually abolished reports of end-to-end movement. Therefore. they agreed with Braddick’s (1974) conclusion that the short-range process operates over ;I spatial range no greater than Is’- 20’ arc. However, Pantle and Petersik had Failed to vary the size of the stimuli. Thus. if the spatial limit of the short-range process were relative. they would have failed to observe that relationship. Using stimuli that vary m siLc and spatial frequency, in the present experiments we show that with
Shorl-range
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large stimuli. sensations of end-to-end movement can be produced with spatial displacements that are intolerable when smaller stimuli are used and that indeed the spatial limit of the short-range process may be a relative one. Stimuli like those shown in Fig. I were professionally drafted in India ink on matte white cards. Twelve sets of such stimuli. formed by the factorial combination of three bar widths (spatial frequencies) and four relative displacements of the stimulus in the second frame. were prepared. Table I shows the parameters of the stimuli when they were viewed in our apparatus at a distance of 153 cm. EXPERIMENT
Pantle
and
1
Picciano (1976). Petersik and Pantle and Adlard (1978) and others have shown that there is a gradual increase in the percentage of group-movement responses produced with the end-to-end/group movement display (hereafter, EC display) as ISI is increased over a range from 0 to approx. 80msec (keeping in mind that previous studies have employed stimuli that did not vary in size). Such an increase suggests a gradual trade-off between the underlying short- and long-range processes. Therefore. in the present experiment. for each subject at each relative displacement of the stimuli. we sought to determine the SO”,, temporal equilibrium point. or transition point; i.e. that ISI at which the subject would see group movement and end-to-end movement SO”,, of the time. It is at this ISI that we assume that the short-range process is beginning to lose its perceptual dominance (see Petersik and Grassmuck. 19x1). Since the present display is perceptuully bistable. and since the frequency with which the alternative organizations are reported is assumed to be based upon a trade-off between the underlying processes. it may not in fact be possible to measure the precise point (in space or time) at which the short-range process ceases to operate. Therefore. we chose a constant criterion (i.e. the SO”,, ISI) as an index of short-range-process strength. Notice that this criterion is temporal. and therefore reflects spatio-temporal trade-offs. If there were an absolute spatial limit to the c-process. we would expect the temporal equilibrium point to vary with absolute displacement only. irrespective of the size of the stimuli. On the other hand. if the spatial limit is relative. the SO”,, ISI should vary
(1979). Braddick
process
with the function percent different
Ita
percentage shift of the stimuli. and the relating the temporal equilibrium pomt to shift should be the same for stimuli of sizes.
Stinndi md uppm~ru.~.Each of the two frames of a display was presented through one channel of a three-channel tachistoscope (Scientitic Prototype. Model GB). At a viewing distance of IS.1 cm. each frame subtended a visual angle of 6.6 horizontally and 4.8 vertically. The luminance of the white portion of each frame was 4.10 cd m’: of the darh bars. 0.65 cd,m-‘. These values resulted in ;I Michelson contrast of 0.73. There was no illumination during the ISI. Suhjwts. Thirty undergraduate psychology students between the ages of I8 and 21 yr participated in Experiment I. Each subject began with IO min of practice during which the subject made judgements of group and end-to-end movement with well-aligned (i.e. 0”” shift) stimuli presented at a number of ISIS. These stimuli were not subsequently used during data collection (see Table I). Prodwe. Each subject dark adapted IO min and viewed the stimuli from a light-tight booth. Since dark adaptation is known to favor the end-to-end movement sensation (Petersik and Pantle. 1979). our experimental condition produced a bias ilr foror of’the short-range process. It was hoped this would allow us to make estimates of the spatial limit under conditions favorable to the short-range process. On each trial a subject viewed seven repetitions of the EC display (frame duration = 200 msec) and immediately afterward reported whether he had perceived group or end-to-end movement. A modified method of limits was used to determine the temporal equilibrium point. On an ascending series. the tirst IS1 used was IOmsec. The ISI for each subsequent trial increased by 5 msec. An ascending series was terminated only after a subject had made at least two consecutive end-to-end-movement responses. followed by two group-movement responses. The midpoint of the interval between the last end-to-end movement response and the Hurst group-movement response was used as the estimate of the temporal equilibrium point. A t/~.~c~clitr~ series was run in a similar manner. beginning with an X0-msec IS1 and decreasing ISI in S-msec steps until two consecutive end-to-end-movement responses were produced. If no group-movement responses were obtained in an ascending series. the temporal equilibrium point was arbitrarily recorded as 85 msec. If no end-to-endmovement responses were ohtained in a descending series, the temporal equilibrium point was recorded as 5 msec. A trial with a randomly selected ISI was used approximately every fourth trial. The data from such trials were not considered part of a series. although they were retained. For each subject at each percent shift of the stirnull. two ascending and two
J.
1666
TIMOTHY
PETERSK
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75
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Fig. 2. (A) Equilibrium point (SOY,transition ISI) plotted as a function of the absolute shift of overlapping bars in the EG display. Bar size is the parameter. A: data for stimuli subtending 0.24 deg of arc; n : data for 0.48-deg stimuli: 0: 0.72-deg stimuli. (B) Equilibrium point plotted as a function of the percent shift of overlapping bars in the EG display. Bar size is again the parameter. Symbols used as in (A).
descending series were run in a random order. The mean of the estimated temporal equilibrium points obtained in the four series was the datum for each subject in each condition. For each trial, the subject directed his gaze to the center of the display while at the same time attending to the entire display. No fixation point was used. The control of eye movements was not deemed necessary for the following reason: with specially designed stimuli we have shown that either group movement or end-to-end movement can be perceived in opposite directions at the same time. However, the two movements are never perceived simultaneously in such displays. Furthermore. temporal equilibrium points do not change under such conditions. Therefore, we conclude that eye movements do not significantly influence the production of group and end-to-endmovement in our typical experimental conditions.
Figure 2(a) shows how the temporal equilibrium point varies as a function of absolute displacement for stimuli of each bar-size. Each curve is a montonic
PI u/
declining function of displacement. meaning that the sensation of end-to-end movement gives way IO the sensation of group movement at shorter and shorter ISIS as displacement is increased. The curves are non-overlapping, and the slope of each curve becomes shallower as stimulus size increases. These curves suggest that there is no absolute spatial limit to the short-range process. To understand this interpretation. consider three data points: when one frame of the small (0.24 bar width) stimuli was displaced 10.7’ arc (a 75”,, shift) the two percepts are in equilibrium at an ISI of 29 msec (i.e. group movement and end-to-end movement are reported 50”,, of the time). On the other hand, shifts of the large (0.72 ) stimuli of 21.5’-32.2’ (shifts of 50-75”,,) yield temporal equilibrium points in much the same range. The same point is made in Fig. 2(b) where the data are replotted as a function of relative shift. Here it can be seen that the data overlap to a significant extent so that they can be fitted by a single regression line. Y’ = 64.0 - 0.524s. This regression equation accounts for 93.4”” of the variation in the data. The predicted Y-intercept (64msec) agrees well with the empirically determined dark-adapted temporal equilibrium point (62 msec) for aligned stimuli (Petersik and Pantle. 1979) as well as with observations made by the present authors. The data in Fig. 2 show that for misaligned stimuli in the EG display, temporal equilibrium point varies with the percent shift of the stimuli between frames and not directly as a function of the size of the stimuli. This finding was veritied in a mixed analysis of variance of the temporal equilibrium-point data with percent shifi and stimulus six as factors. Percent shift was found to have a significant main effect. F(3, 81) = 51.95. P < 0.0001. whereas neither the main effect of size nor the size by shift interaction was significant. F(2. 27) = 0. I02 and F(6, 81) = I .90 respectively. both P, > 0.05. From these data we infer that the short-range process can operate over considerable spatial extents. provided the stimuli are large enough. Furthermore, weakening the r-process with a spatial displacement of stimuli can be compensated for by reducing ISI. This confirms the finding of Petersik and Grassmuck (1981) that spatial and temporal factors interact m then intluence on the (- and y-processes. In an effort to check the validity of the estimates of the temporal equilibrium points obtained by the method of limits, we plotted the percentage of groupmovement responses (over all subjects in a condition) obtained at each IS1 as functions of stimulus size and percent shift. For each experimental condition. we then used interpolation to produce a new estimate of the temporal equilibrium point. These estimates are shown along with those obtained by the method of limits in Table 2. Interpolation was not possible with the data obtained at loo”,, shifts because the curves did not cross the 50”,,-group movement line. As can he seen. the two sets of estimates are in good agree-
1667
Short-range process Table 2. Estimated equilibrium points (in msec) obtained directly with the method of limits or through graphic interpolation Stimulus size 0.48
0.24
0.72 .-.____.__ 75
Percent shift
25
50
7s
loo
2s
50
75
100
2s
50
Limits Interpolation
41.1 44.5
42.0 38.0
29.3 26.0
6.9 -
53.2 52.0
41.1 35.0
22.3 17.4
12.6 -
52.8 50.0
26.9 21.0
ment. This finding suggests that our conclusions based upon the temporal equilibrium points obtained with the method of limits are warranted. Given the linear relationship found in Fig. 2(b), the relative spatial limit of the short-range process can be estimated to be a 1239, displacement of stimuli of any size. This figure suggests that the results of Pantie and Petersik (1980) should show a significant decrease in the influence of the short-range process for linesegment displacements of only 4.5’. and indeed such a weakening was found although its magnitude was not as great as one would expect on the basis of the present study. However, this may bc due to the difference between the spatial-frequency spectrum of the Pantle and Petetsik stimuli and those of the present experiment. At any rate, it is important to note that the estimated displacement limit of 123% seems to apply equally well to stimuli confined to the fovea (e.g. the smallest display used in the present experiment. as well as the line-segments used by Pantle and Petersik, 1980) and those extending into the periphery (e.g. the larger displays in the present experiment). EXPERIMENT 2
In order to further examine the behaviors of the end-to-end and group-movement percepts as they vary with spatial shifts, and to attempt a replication of the findings of Experiment I, five practiced psychophysical observers made judgments of group and end-to-end movement with the same stimuli at ISIS ranging from IO to 80 msec. In Experiment 2, stimuli were presented by the method of constant stimuli rather than by the method of limits.
Stinruii unri uppurutus. The stimuli and apparatus were the same as in Experiment 1. S&jec& Five practiced psychophysical observers, four males and one female between the ages of 18-l I yr. participated in the present experiment. Three observers were reimbursed for their participation; the other two were volunteers. Each subject spent 5 hr making group- and end-to-endmovement judgments with perfectly aligned stimuli prior to the onset of Experiment 2. P~~~c~~~i~re. The viewing conditions were all the same as reported for Experiment I. Each observer served in each of the 96 experimental conditions formed by the factorial combination of three stimulus
26.4 15.0
___ 100 13.6 -
sizes, four shifts, and eight ISIS (IO, 20, 30. 40, 50, 60, 70 and 80msec) a total of IO times. At the beginning of each experimental session, a stimulus condition consisting of a randomly selected stimulus size and percent shift was chosen. Following this, the stimuli were presented at each of the eight ISIS in a randomized order. At each ISI, the subject observed eight alternations of the stimulus frames and immediately afterward reported whether group or end-to-end movement had been perceived. Next, a new combination of stimulus size and shift was chosen, and the above procedure was repeated. Each experimental session lasted approx. 50min. For each subject, approx. 30 sessions were required to complete the experiment. Results and Discussion
Figures 3-5 show, each for stimuli of a different size (bar widths of 0.24”, 0.48” and 0.72” respectively), mean percentages of group-movement responses as functions of ISI. Each curve in the figures shows the results obtained with stimuli of a given percent shift. Standard errors of the points shown in these figures were generally low, ranging from 0.7 to 9.356, with an average of 4.1. The first finding revealed in these figures is that, irrespective of stimulus size, for stimuli of 25, 50 and 75% shifts, the percentage of group-movement responses increases montonically as a function of ISI.
.’ 0
to
20
I
I
I
.I
I
I
30
40
50
60
70
80
ISI
(msec)
Fig. 3. Percentage of grou~movement responsesas a function of ISI (in msec) for stimulus displays whose bar widths were 0.24deg. Percent shift of the overlapping bars is the parameter. A: 257; shift; n : 50% shift; +: 75% shift; 0: 100% shift. Data are averaged over ten trials at each ISI for each of five trained psychophysical observers.
A’ 0
IO
20
I
I
I
I
I
I
30
40
50
60
70
60
ISI
(msec)
Fig. 4. Percentage of group-movement responses as a function of IS1 for stimulus displays whose bar width was 0.48deg. Other conventzons as in Fig. 3.
This replicates the findings of Pantle and Picciano (1976) and Petersik and Pantle (1979). obtained with perfectly aligned stimuli, and suggests that for stimuli of any size at these shifts, there is a gradual changeover from dominance of the ~-process to dominance of the y-process as ISI is increased. Stimuli with a loo”, shift never showed a dominance of end-to-endmovement responses. regardless of ISI, under the conditions of this experiment. The main effect of IS1 was shown to be significant in a 3 x 4 x 8 (size by shift by ISI) repeated measures ANOVA of these data, 1;(7,28) = 39.79, P < 0.0001. The second major finding of this study was that at the shorter ISIS, curves for the stimuli with greater displacements lie above the curves for stimuli with smaller displacements (e.g. for the data obtained with the 0.24 stimuli, points on lOO’;b;-shift curve fall above those on the 75”<-shift curve, which in turn are above those on the 50’:;-shift curve, etc.. up to an IS1 of about SOmsec). This finding suggests that up to some limiting IS1 (30-6Om~c), at which temporal factors become relatively more important than spatial factors in the mediation of group- and end-to-endmovement responses. successively greater spatial displacements produce a gradual changeover from dominance of the c-process to dominance of the :*-process. The main effect of percent shift was also found to be significant. F(3. 12) = 54.54. P < 0.0001. The next new finding in these data was that the slopes of the curves shown in Figs 3-5 become more shallow as a function of increasing size: for the 0.24 stimuli. the slope of the function relating ISI to percent group-movement responses (over all percent shifts excluding loo’,‘,) is 1.28; for the 0.48’ stimuli. the slope is I. 19: for the 0.72 stimuli, 0.89. This tinding indicates that as the size of the stimuli increases. the changeover from dominance of the c-process to dominance of the y-process with increasing IS1 is slower or more gradual as though larger stimuli were more preferred in the matching
process. The finding of a gradual change in the slopes of the curves with increasing 1st also accounts for our finding of a signiticnnt size by ISI interaction. F( 14. 56) = 2.68, P -C0.025. However. as in Experiment 1. the main etfect of size was not significant. F(2.8) < 2. Note also that ~t.jr~~~~~ each figure. the slopes of the curves become more shailo% with increasing percent shift. This finding can best be attributed to the fact. already noted, that at shorter ISls the curves for the greater shifts tend to fail above the curves for the smaller shifts. along with the ceiling eflect seen at the longer ISIS. This finding also accounts for the significant ISI by shift interaction found in the data, P(2l. 84) = 11.37. P -c 0.0001. Together the change in slope with size and with percent shift accounts for interaction. size b> shift the significant F(6.24) = 4.09. P < 0.01. Taken together. these resuits show that the trade-off between the 6 and ;’ processes is not a simple one determined exclusively by the temporal frequency of the display. Rather. the trade-off is determined by the joint effects of at least two spatial dimensions (stimulus size and degree of overlap) along with ~liternation rate (see also Petersik and Grassmuck. 19gi ). As mentioned in the introduction, other factors (e.g. stimulus contrast) also contribute to the determination of such a tradeoff. These factors should be kept in mind by researchers attempting to isolate and study either the long-range or short-range process in their studies of apparent movement.
GENERAL
DISCIISSION
The major finding of the present experiments is that the percept (i.e. end-to-end movement) associated with the short-range process continues to be elicited at stimulus displacements well in excess of the originally proposed spatial limit of 15’~20’. In this
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40
50
60
70
60
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Fig. 5. Percentage
(mseci
of group-movcmcnt rcsponsc~ its a function of ISI for stimulus displays whocc bar width was 0.72deg. Other conventions as in Figs 3 and 4.
Short-range process respect, our conclusion supports a major finding of Baker and Braddick (1982). namely, that with increasing size of a correlated area (in their case a patch of random dots; in ours. “overiapping” bars) the degree of stimulus displacement tolerated grows correspondingly. At a minimum. these results suggest that there is no single retinal limit over which the short-range process operates. Our conclusions differ from Baker and Braddick’s in the following ways. Baker and Braddick (1982) contend that the notion of a retinal limit is valid, but that the limit itself increases with increasing retinal eccentricity. We believe that since the functions relating stimulus displacement to temporal equilibrium point pass through roughly the same point on the abcissa irrespective of stimulus size (and largely irrespective of retinal location), the spatial limit of the short-range process is an approximately constant proportion of stimulus size. This conclusion is not inconsistent with the notion of a retinal limit, but it requires the additional assumption that retinal limit grows at a constant rate across the retina. Such an assumption would seriously hinder the theoretical usefulness of the notion of a retinal limit. Furthermore. our data indicate that in order to maintain a fixed criterion of strength of the short-range process (i.e. the 50”” point), ISI must be increased with increasing stimulus displacements. Baker and Braddick’s results did not take into account the potential for a spatio-temporal trade-off of this sort. The comparison between the EG display and the random-dot stimuli. and therefore the relevance of our results. could be criticized on the grounds that with each of our periodic stimulus sets there was stimulus overlap as well as stimulus displacement. On the other hand, with the stimuli of Pantle and Petersik (1980) there was no stimulus overlap after displacement (the stimuli were very thin line segments), With Baker and Braddick‘s (1982) random-dot stimuli. there was also no overlap of the individual elements that were shifted in alternate frames; however, there was overlap in the correlated (and displaced region as a whole. We re.ject the above criticisms on several grounds. First of all. as noted in the introduction. the stimulus conditions that both favor and constrain end-to-end movement are virtually the same as those that favor and constrain segregation of correlated patches in alternating random-dot patterns. Secondly, the information processing requirements for the two types of perceptual phenomena seem very similar. In end-toend movement. there is evidently a point-by-point matching of the entire black and white regions that define the center (“o~~erlapp~ng”) bars. This matching is most noticeable at the edger; of the bars, since, in end-to-end movement the left and right edges of the center bars always align themselves correctly as the stimulus frames alternate. Although this matching process results in a perceptual “jiggling” of the center bars during end-to-end movement. the center bars
I669
nevertheless perceptually remain in the center of the display while the third bar moves back and forth from end to end. Similarly. with alternating randomdot patterns there seems to be a point-by-point matching of correlated elements and a consequent perceptual jiggling the displaced region. It would appear that in both stimulus displays the short-range process serves to maintain perceptual coherence of slightly perturbed stimulus regions over time. In Experiment 2. it was found that the slopes of the curves relating percent group-movement responses to ISi become more shallow as the size of the stimulus bars increases. This finding suggests that larger stimuii are more favorable to the short-range process than smaller stimuli and is consistent with the results of Petersik and Grassmuck (1981). Those authors measured the XV’, temporal equilibrium point with EG stimuli that varied in fundamental spatial frequency. Petersik and Grassmuck concluded that decreases in fundamental spatial frequency favor the 6-process. The understanding of the short-range process in apparent movement that has emerged from the present studies and other recent investigations is as follows: since perceptual responses associated with the short-range process cannot be obtained with dichoptic stimulation (Braddick. 1974: Pantle and Picciano. 1976). it appears to be relatively low-level, perhaps “hard-wired”, process (Braddick, 1980). The notion that his low-level process is hard wired finds support in the fact that many subtle stimulus modifications disrupt its activity. For example. lightfilled ISIS (Braddick, 1973: Pantle and Picciano. 1976), contrast variations from frame to frame, light adaptation (both, Petersik and Pantle, 1979), orientation perturbations of successive stimuli, simple shifts of the overlapping elements in the EG display (both, Pantle and Petersik. 1980), and high fundamental spatial frequencies (Petersik and Grassmuck, 1981) ail tend to disrupt the short-range process. The short-range process tends to operate best with relatively small temporal and spatial separations, although its spatial limit is relative to the size of the stimuli being viewed (at least for stimuli that partially overlap). Its function appears to be to pair similar stimulus elements that may be slightly misaligned or mismatched (on the retina) in space and time. For this reason. we suggest that the short-range process (or c-process, as we have called it) may not have as its primary function the detection of motion per se. Rather, an alternate hypothesis to be considered should be that the function of the short-range process is to compensate for the mismatch of local (retinal) information that occurs with small eye tremors.
A~knoa,I~~JK~~rnl.s--The authors gratefully acknowledge the competent assistance of Craig DeLisle. Brenda Jones. Robert Malsh, and K. Patrick Yarbrough throughout various stages of the research.
1670
J. TIMOTHY PETERSIK er al. REFERENCES
Baker C. L. and Braddick 0. J. (1982) The basis of area and dot number effects in random dot motion perception. Vision Res. 22, 1253-1259. Braddick 0. J. (1973) The masking of apparent motion in random-dot patterns. Vision Res. 13, 355-369. Braddick 0. J. (1974) A short-range process in apparent motion. Vision Res. 14. 519-527. Braddick 0. J. Low-level and high-level processes in ;yrent motion. Phil. Truns. R. Sot. Land. E 290, 13?Braddick 0. J. and Adlard A. J. (1978) Apparent motion and the motion detector. In k’irual Ps,whoph.vsics and Ph~siolog.r (edited by J. Armington, J. Krasukopf and B. R. Wooten). Academic Press. New York. Lappin J. S. and Bell H. H. (1976) The detection of coherence m moving random-dot patterns. Vision Res. 16, 161-168.
Pantle A. J. fl977) Stroboscopic n~~~~ernentbased upon global information in successively; presented vtsual parterns. ./. opr. SOL..,4nt. 63. 1780 (abstr.). Pantle A. J. and Petersik J. T. (1980) Effects of spatial parameters on the perceptual organtzatton of a histahle motion display. Pcrce~pr. P.r~~c/tophr.\.27. 307-3 I?. Pantle A. J. and Picciano L. (1976). A multistable movcment display: evidence for two separate motion systems in human vision. .S&twc 193. 500-502. Petersik J. T. and Grassmuck J. (14X1) High fundamental spatial frequencies and edges have diherent perceptual consequences in the “grouplend-to-end” movement phenomenon. Pvrceplion i0, 375.182. Petersik J. T. and Pantle A. J. (1979) Factors controlhng the competing sensations produced b! ;I bistable strohoscoptc motion display. ?%on Rm. 19, 143-l 54. Ramachandran V. S.. Madhusudan Rao V. and Vidyasapar T. (1973) Apparent movement wtth suhjecttve contours. Vision Res. 13, I399- 1401.
Vision Res.Vol. 23.No. 12.pp. 1663-1670. 1983 Printedm Great Britain.All rightsreserved
FAILURE TO FIND AN ABSOLUTE RETINAL LIMIT OF A PUTATWE SHORT-RANGE PROCESS IN APPARENT MOTION J. TIMOTHY PETERSIK’.~, RANDALL PUFAHL’ and ELIZABETH KRASWFF ‘Department of Psychology, Ripon College, Ripon. WI 54971. U.S.A.. and ‘New England Institute of Applied Biophysics, 59 North Ashland Ave.. Worcester. MA 01609. U.L.4. (Received
22 September
1982; in recked j&m
IO Jmwnr~~ 1983)
Abstract-Previous
studies of apparent movement have concluded that the short-range process does not operate when stimulus displacements exceed 15-20’ arc. In the present experiments. we studied a bistable apparent-movement display, one of whose perceptual organizations is mediated by short-range process. In both experiments it was found that the perceptual organization mediated by the short-range process could be made dominant at stimulus displacements well in excess of the proposed spatial limit, provided the stimulus elements were made larger. It is concluded that the spatial limit of the short-range process is a relative. not absolute. one. Current knowledge regarding the short-range process is reviewed. and an hypothesis regarding the functional utility of the short-range process in compensatmg for the effects of small eye tremors is advanced. Apparent movement
Bistable display
INTRODUCTlON Within the last 10 years, a relatively sizeable body of psychophysical evidence has accumulated to suggest that human visual motion perception is mediated by at least two distinct processes, one of which operates over a relatively small spatial range and another which operates over larger spatial ranges. These processes also differ with respect to the temporal ranges over which they operate (see below). The present paper focuses primarily on what has come to be known as the “short-range” process and the problem of how to describe its apparent spatial integration limit.
Spatiul limit qf’tlte short -runge process us studied rundom -do I displuys
with
Braddick ( 1974) first suggested that the perceptual segregation of correlated. but displaced, areas in alternating random-dot patterns was due to the activity of a “short-range” process. His studies (Braddick, 1973, 1974) showed that the short-range process matches individual elements in two frames provided that (a) their spatial displacement is IS’ arc or less, (b) the interstimulus interval (ISI) is less than about 70msec (for a IOO-msec exposure duration), (c) the frames are not presented dichoptically. and (d) the field presented during the IS1 is dark. Braddick contrasted the short-range process with the “long-range.’ process which apparently operates in classical
phi-movement
phenomena
Visual motion perception
Short-range process
and which is
maximally stimulated by greater spatial and temporal disparities. operates with dichoptic stimulation. and is insensitive to the luminance of the ISI. As early as 1976. Lappin and Bell had argued that the spatial limit of the short-range process was a relative, not absolute. one. determined by the size of the display and its elements. Lappin and Bell (1976) suggested that Braddick’s discovery of a “retinal” limit to the short-range process W;IS based upon a statistical confound produced by a variable signal/noise ratio. Hence. Baker and Braddick f 19x2) have extended the original findings in studies that covdried dot density. patch size (i.e. size of the area correlated and displaced between frames). dot spacing, and displacement. Based on their findings. they have concluded that the displacement limit of the short-range process is indeed retinal. but that the limit may grow with greater retinal eccentricities. However, Baker and Braddick f 1987) used a constant ISI. and therefore Wed to jnc~ude the ~ssibiljty of a spatiotemporal tradeoff in the determination of a limit to the short-range process. Furthermore. the authors do not indicate how or why a greater spatial limit in the periphery might induce perceptual correlations over correspondingly greater spatial ranges in the fovea. as apparently happens when large displaced patches are viewed.* Therefore, the issue regarding the nature of the spatial limit of the short-range process cannot be considered closed. Sputiui
limit
u hi.Whli~
*We are indebted to an anonymous reviewer for making this point.
of’ tlw short -rurgc* process
uppuriwt
-m~~twniv~t
us strtdied
with
iii.vplrt,l*
PBntle and Picciano (1976) and later Petersik and Pantle (1979) studied a histable apparent-movement
1663
1664
J. TIMOTHY PETERSIKPI d.
(1979) called the underlying process the percept c-process. The c -process
dtsplay similar to that shown in Fig. I and whose alternative organizations they identified with the short- and long-range processes. Under certain stimulus conditions. most notably short ISIS (generally less than 70 msec). alternation of the stimulus frames shoun m Fig. I gives rise to the perception of two statIonat-> center bars with a third bar moving back and tbrth around them. This type of percept is known as “end-to-end movement.” and Petersik and Pantle “The ~\\uc I-cg,lrdlng what the short-range process actually contrlhutc\ IO the perceptual experience of end-to-end mo\cmcnt ha\ been discussed by Pantle and Petersik I ILJhll~.,III~ II I\ worthwhile to quote them directly here: “App.~rcntI! the short-range process which mediates \cqc~;ll’l’” .~nd movement in random-dot patterns c~~ntr~htr~~\ to the (end-to-end) movement sensation oni! h! gcncr;ltln: a sIgnal for the inner lines of the LII+.I!. 11)~wu ho\c pomts overlap or nearly overlap in WLUX~I\L~ lrame\. The signal generated is one of ‘no I~O~~CIII~ u hen the points of the overlapping lines C\;ICII! ~~~rc\pond and one of small local movement \\ hen the\ nc;~~-l! correspond. that is. are slightly pcrlurhc2 Ths \I~I~;II Ihr the movement of the end line in ~ICI~CIIImo\cmcnt must be gcncrlrted by some other pr<~cr\ pr~~h.~hl! the same ;‘-process which accounts lor the pc~-cc~~sd displacements termed group moveIllc‘lll (p :I /I ” .A slmllar pomt was made by Braddick and ~~dl
h!
process that mediated the segregation ofcorrelated. but displaced. regions in alternating random-dot patterns on the basis of the following evidence:? an! manipulation ot ISI. the luminance of the ISI. stimulus contrast. or type of viewing (binocular vs dichoptic) that decreases the tendency of subjects to perceive the of correlated areas In alternating segregation random-dot patterns also produce5 fewer reports ol end-to-end movement with the t!pe ofdispla! shown in Fig. I. Furthermore. Pantle and Petersik (1980. see below) reported that end-to-end movement could not be perceived at stimulus displacement beyond about 16’ arc. This limit corresponds to the 15’ displacement limit reported by Braddick (1974) and supported the suggestion that the perceptual process mediating end-to-end movement was the same shortrange process studied by Braddick. Under other stimulus conditions. most notabll long ISIS. alternation of the frames shown in Fig. I gives rise to the perception of three bars moving together as a whole from left to right and back again. This movement is known as “group movement.” and Petersik and Pantle (1979) termed the underlying perceptual process ;*-process. This process \vas identified with the classical long-range process on the basis of the following evidence: group mo\‘ement occurs over larger spatial ranges and depends upon long frame durations (preferabl) greater than 100-200 msec) and long ISIS (preferably greater than SO-70 msec). These stimulus requirements are the same as those required to obtain apparent movement in other displays in which clusters of elements are identified on the basis of global form cues (Pantle. those authors
FIN. I. An example of the type of stimulus frames used in the present experiments. In this example. the edges of the t%o leftmost bars m the top frame are perfectly aligned with the edges of the two rightmost bars in the bottom frame. The edges of these bars were systematically misaligned (i.e. shifted) to form the stimuli used in the present experiments.
that mediated was thought
to be the same short-range
1973;
Ramachandran
group
movement
lation
and
ISI
CI
t/l..
is insensltive luminance.
as
1973). to
Furthermore.
dichoptic
is classical
stimu-
apparent
movement. In an en‘ort to determine the spatial-displacement range of the short-range process. Pantle and Petersik (1980) used a display like that shown in Fig. I but consisting of three evenlq spaced narrokv linesegments instead of bars. They systematically displaced the positions of the center two (“overlapping”) lines in the second frame and presented the display at ISIS ordinarily suitable for the production of end-to-end movement. Pantle and Petersik found that relative displacements of 16’ virtually abolished reports of end-to-end movement. Therefore. they agreed with Braddick’s (1974) conclusion that the short-range process operates over ;I spatial range no greater than Is’- 20’ arc. However, Pantle and Petersik had Failed to vary the size of the stimuli. Thus. if the spatial limit of the short-range process were relative. they would have failed to observe that relationship. Using stimuli that vary m siLc and spatial frequency, in the present experiments we show that with
Shorl-range
Shifl Bar SIC 0 72 IO 69 c dCfl 0 4x I I 0.4c dcg) 0 24 12 ox c de&?)
Is”..
W..
l?‘_
loo”.,
0.1x
0.36
0.54
1IO7’)’
(21.5’1
132.2
0.14 (14 3’) 0.11 (7 I’)
0.36 (?I 4’) 0.18 (IO 7’)
0 72 (42 Y’I 0 4x (2X 5’1 0.24 (14.3 )
0 II I71 1 0.06 (3 0’1
)
lThe\e \alue, arc rcporwd for grrater prcc,s,on
large stimuli. sensations of end-to-end movement can be produced with spatial displacements that are intolerable when smaller stimuli are used and that indeed the spatial limit of the short-range process may be a relative one. Stimuli like those shown in Fig. I were professionally drafted in India ink on matte white cards. Twelve sets of such stimuli. formed by the factorial combination of three bar widths (spatial frequencies) and four relative displacements of the stimulus in the second frame. were prepared. Table I shows the parameters of the stimuli when they were viewed in our apparatus at a distance of 153 cm. EXPERIMENT
Pantle
and
1
Picciano (1976). Petersik and Pantle and Adlard (1978) and others have shown that there is a gradual increase in the percentage of group-movement responses produced with the end-to-end/group movement display (hereafter, EC display) as ISI is increased over a range from 0 to approx. 80msec (keeping in mind that previous studies have employed stimuli that did not vary in size). Such an increase suggests a gradual trade-off between the underlying short- and long-range processes. Therefore. in the present experiment. for each subject at each relative displacement of the stimuli. we sought to determine the SO”,, temporal equilibrium point. or transition point; i.e. that ISI at which the subject would see group movement and end-to-end movement SO”,, of the time. It is at this ISI that we assume that the short-range process is beginning to lose its perceptual dominance (see Petersik and Grassmuck. 19x1). Since the present display is perceptuully bistable. and since the frequency with which the alternative organizations are reported is assumed to be based upon a trade-off between the underlying processes. it may not in fact be possible to measure the precise point (in space or time) at which the short-range process ceases to operate. Therefore. we chose a constant criterion (i.e. the SO”,, ISI) as an index of short-range-process strength. Notice that this criterion is temporal. and therefore reflects spatio-temporal trade-offs. If there were an absolute spatial limit to the c-process. we would expect the temporal equilibrium point to vary with absolute displacement only. irrespective of the size of the stimuli. On the other hand. if the spatial limit is relative. the SO”,, ISI should vary
(1979). Braddick
process
with the function percent different
Ita
percentage shift of the stimuli. and the relating the temporal equilibrium pomt to shift should be the same for stimuli of sizes.
Stinndi md uppm~ru.~.Each of the two frames of a display was presented through one channel of a three-channel tachistoscope (Scientitic Prototype. Model GB). At a viewing distance of IS.1 cm. each frame subtended a visual angle of 6.6 horizontally and 4.8 vertically. The luminance of the white portion of each frame was 4.10 cd m’: of the darh bars. 0.65 cd,m-‘. These values resulted in ;I Michelson contrast of 0.73. There was no illumination during the ISI. Suhjwts. Thirty undergraduate psychology students between the ages of I8 and 21 yr participated in Experiment I. Each subject began with IO min of practice during which the subject made judgements of group and end-to-end movement with well-aligned (i.e. 0”” shift) stimuli presented at a number of ISIS. These stimuli were not subsequently used during data collection (see Table I). Prodwe. Each subject dark adapted IO min and viewed the stimuli from a light-tight booth. Since dark adaptation is known to favor the end-to-end movement sensation (Petersik and Pantle. 1979). our experimental condition produced a bias ilr foror of’the short-range process. It was hoped this would allow us to make estimates of the spatial limit under conditions favorable to the short-range process. On each trial a subject viewed seven repetitions of the EC display (frame duration = 200 msec) and immediately afterward reported whether he had perceived group or end-to-end movement. A modified method of limits was used to determine the temporal equilibrium point. On an ascending series. the tirst IS1 used was IOmsec. The ISI for each subsequent trial increased by 5 msec. An ascending series was terminated only after a subject had made at least two consecutive end-to-end-movement responses. followed by two group-movement responses. The midpoint of the interval between the last end-to-end movement response and the Hurst group-movement response was used as the estimate of the temporal equilibrium point. A t/~.~c~clitr~ series was run in a similar manner. beginning with an X0-msec IS1 and decreasing ISI in S-msec steps until two consecutive end-to-end-movement responses were produced. If no group-movement responses were obtained in an ascending series. the temporal equilibrium point was arbitrarily recorded as 85 msec. If no end-to-endmovement responses were ohtained in a descending series, the temporal equilibrium point was recorded as 5 msec. A trial with a randomly selected ISI was used approximately every fourth trial. The data from such trials were not considered part of a series. although they were retained. For each subject at each percent shift of the stirnull. two ascending and two
J.
1666
TIMOTHY
PETERSK
60
A
‘\
\ 0
$
‘-.
.
I
I
I
I
I
I
I
I
5
IO
15
20
25
30
35
40
Absolute
;;
-1
‘m
shift
I 45
( m,” CIK1
50-
E ; B 5 2
40-
30
-
zo-
= z w
IO01
I
I
I
I
25
50
75
100
Percent
shlf
t
Fig. 2. (A) Equilibrium point (SOY,transition ISI) plotted as a function of the absolute shift of overlapping bars in the EG display. Bar size is the parameter. A: data for stimuli subtending 0.24 deg of arc; n : data for 0.48-deg stimuli: 0: 0.72-deg stimuli. (B) Equilibrium point plotted as a function of the percent shift of overlapping bars in the EG display. Bar size is again the parameter. Symbols used as in (A).
descending series were run in a random order. The mean of the estimated temporal equilibrium points obtained in the four series was the datum for each subject in each condition. For each trial, the subject directed his gaze to the center of the display while at the same time attending to the entire display. No fixation point was used. The control of eye movements was not deemed necessary for the following reason: with specially designed stimuli we have shown that either group movement or end-to-end movement can be perceived in opposite directions at the same time. However, the two movements are never perceived simultaneously in such displays. Furthermore. temporal equilibrium points do not change under such conditions. Therefore, we conclude that eye movements do not significantly influence the production of group and end-to-endmovement in our typical experimental conditions.
Figure 2(a) shows how the temporal equilibrium point varies as a function of absolute displacement for stimuli of each bar-size. Each curve is a montonic
PI u/
declining function of displacement. meaning that the sensation of end-to-end movement gives way IO the sensation of group movement at shorter and shorter ISIS as displacement is increased. The curves are non-overlapping, and the slope of each curve becomes shallower as stimulus size increases. These curves suggest that there is no absolute spatial limit to the short-range process. To understand this interpretation. consider three data points: when one frame of the small (0.24 bar width) stimuli was displaced 10.7’ arc (a 75”,, shift) the two percepts are in equilibrium at an ISI of 29 msec (i.e. group movement and end-to-end movement are reported 50”,, of the time). On the other hand, shifts of the large (0.72 ) stimuli of 21.5’-32.2’ (shifts of 50-75”,,) yield temporal equilibrium points in much the same range. The same point is made in Fig. 2(b) where the data are replotted as a function of relative shift. Here it can be seen that the data overlap to a significant extent so that they can be fitted by a single regression line. Y’ = 64.0 - 0.524s. This regression equation accounts for 93.4”” of the variation in the data. The predicted Y-intercept (64msec) agrees well with the empirically determined dark-adapted temporal equilibrium point (62 msec) for aligned stimuli (Petersik and Pantle. 1979) as well as with observations made by the present authors. The data in Fig. 2 show that for misaligned stimuli in the EG display, temporal equilibrium point varies with the percent shift of the stimuli between frames and not directly as a function of the size of the stimuli. This finding was veritied in a mixed analysis of variance of the temporal equilibrium-point data with percent shifi and stimulus six as factors. Percent shift was found to have a significant main effect. F(3, 81) = 51.95. P < 0.0001. whereas neither the main effect of size nor the size by shift interaction was significant. F(2. 27) = 0. I02 and F(6, 81) = I .90 respectively. both P, > 0.05. From these data we infer that the short-range process can operate over considerable spatial extents. provided the stimuli are large enough. Furthermore, weakening the r-process with a spatial displacement of stimuli can be compensated for by reducing ISI. This confirms the finding of Petersik and Grassmuck (1981) that spatial and temporal factors interact m then intluence on the (- and y-processes. In an effort to check the validity of the estimates of the temporal equilibrium points obtained by the method of limits, we plotted the percentage of groupmovement responses (over all subjects in a condition) obtained at each IS1 as functions of stimulus size and percent shift. For each experimental condition. we then used interpolation to produce a new estimate of the temporal equilibrium point. These estimates are shown along with those obtained by the method of limits in Table 2. Interpolation was not possible with the data obtained at loo”,, shifts because the curves did not cross the 50”,,-group movement line. As can he seen. the two sets of estimates are in good agree-
1667
Short-range process Table 2. Estimated equilibrium points (in msec) obtained directly with the method of limits or through graphic interpolation Stimulus size 0.48
0.24
0.72 .-.____.__ 75
Percent shift
25
50
7s
loo
2s
50
75
100
2s
50
Limits Interpolation
41.1 44.5
42.0 38.0
29.3 26.0
6.9 -
53.2 52.0
41.1 35.0
22.3 17.4
12.6 -
52.8 50.0
26.9 21.0
ment. This finding suggests that our conclusions based upon the temporal equilibrium points obtained with the method of limits are warranted. Given the linear relationship found in Fig. 2(b), the relative spatial limit of the short-range process can be estimated to be a 1239, displacement of stimuli of any size. This figure suggests that the results of Pantie and Petersik (1980) should show a significant decrease in the influence of the short-range process for linesegment displacements of only 4.5’. and indeed such a weakening was found although its magnitude was not as great as one would expect on the basis of the present study. However, this may bc due to the difference between the spatial-frequency spectrum of the Pantle and Petetsik stimuli and those of the present experiment. At any rate, it is important to note that the estimated displacement limit of 123% seems to apply equally well to stimuli confined to the fovea (e.g. the smallest display used in the present experiment. as well as the line-segments used by Pantle and Petersik, 1980) and those extending into the periphery (e.g. the larger displays in the present experiment). EXPERIMENT 2
In order to further examine the behaviors of the end-to-end and group-movement percepts as they vary with spatial shifts, and to attempt a replication of the findings of Experiment I, five practiced psychophysical observers made judgments of group and end-to-end movement with the same stimuli at ISIS ranging from IO to 80 msec. In Experiment 2, stimuli were presented by the method of constant stimuli rather than by the method of limits.
Stinruii unri uppurutus. The stimuli and apparatus were the same as in Experiment 1. S&jec& Five practiced psychophysical observers, four males and one female between the ages of 18-l I yr. participated in the present experiment. Three observers were reimbursed for their participation; the other two were volunteers. Each subject spent 5 hr making group- and end-to-endmovement judgments with perfectly aligned stimuli prior to the onset of Experiment 2. P~~~c~~~i~re. The viewing conditions were all the same as reported for Experiment I. Each observer served in each of the 96 experimental conditions formed by the factorial combination of three stimulus
26.4 15.0
___ 100 13.6 -
sizes, four shifts, and eight ISIS (IO, 20, 30. 40, 50, 60, 70 and 80msec) a total of IO times. At the beginning of each experimental session, a stimulus condition consisting of a randomly selected stimulus size and percent shift was chosen. Following this, the stimuli were presented at each of the eight ISIS in a randomized order. At each ISI, the subject observed eight alternations of the stimulus frames and immediately afterward reported whether group or end-to-end movement had been perceived. Next, a new combination of stimulus size and shift was chosen, and the above procedure was repeated. Each experimental session lasted approx. 50min. For each subject, approx. 30 sessions were required to complete the experiment. Results and Discussion
Figures 3-5 show, each for stimuli of a different size (bar widths of 0.24”, 0.48” and 0.72” respectively), mean percentages of group-movement responses as functions of ISI. Each curve in the figures shows the results obtained with stimuli of a given percent shift. Standard errors of the points shown in these figures were generally low, ranging from 0.7 to 9.356, with an average of 4.1. The first finding revealed in these figures is that, irrespective of stimulus size, for stimuli of 25, 50 and 75% shifts, the percentage of group-movement responses increases montonically as a function of ISI.
.’ 0
to
20
I
I
I
.I
I
I
30
40
50
60
70
80
ISI
(msec)
Fig. 3. Percentage of grou~movement responsesas a function of ISI (in msec) for stimulus displays whose bar widths were 0.24deg. Percent shift of the overlapping bars is the parameter. A: 257; shift; n : 50% shift; +: 75% shift; 0: 100% shift. Data are averaged over ten trials at each ISI for each of five trained psychophysical observers.
A’ 0
IO
20
I
I
I
I
I
I
30
40
50
60
70
60
ISI
(msec)
Fig. 4. Percentage of group-movement responses as a function of IS1 for stimulus displays whose bar width was 0.48deg. Other conventzons as in Fig. 3.
This replicates the findings of Pantle and Picciano (1976) and Petersik and Pantle (1979). obtained with perfectly aligned stimuli, and suggests that for stimuli of any size at these shifts, there is a gradual changeover from dominance of the ~-process to dominance of the y-process as ISI is increased. Stimuli with a loo”, shift never showed a dominance of end-to-endmovement responses. regardless of ISI, under the conditions of this experiment. The main effect of IS1 was shown to be significant in a 3 x 4 x 8 (size by shift by ISI) repeated measures ANOVA of these data, 1;(7,28) = 39.79, P < 0.0001. The second major finding of this study was that at the shorter ISIS, curves for the stimuli with greater displacements lie above the curves for stimuli with smaller displacements (e.g. for the data obtained with the 0.24 stimuli, points on lOO’;b;-shift curve fall above those on the 75”<-shift curve, which in turn are above those on the 50’:;-shift curve, etc.. up to an IS1 of about SOmsec). This finding suggests that up to some limiting IS1 (30-6Om~c), at which temporal factors become relatively more important than spatial factors in the mediation of group- and end-to-endmovement responses. successively greater spatial displacements produce a gradual changeover from dominance of the c-process to dominance of the :*-process. The main effect of percent shift was also found to be significant. F(3. 12) = 54.54. P < 0.0001. The next new finding in these data was that the slopes of the curves shown in Figs 3-5 become more shallow as a function of increasing size: for the 0.24 stimuli. the slope of the function relating ISI to percent group-movement responses (over all percent shifts excluding loo’,‘,) is 1.28; for the 0.48’ stimuli. the slope is I. 19: for the 0.72 stimuli, 0.89. This tinding indicates that as the size of the stimuli increases. the changeover from dominance of the c-process to dominance of the y-process with increasing IS1 is slower or more gradual as though larger stimuli were more preferred in the matching
process. The finding of a gradual change in the slopes of the curves with increasing 1st also accounts for our finding of a signiticnnt size by ISI interaction. F( 14. 56) = 2.68, P -C0.025. However. as in Experiment 1. the main etfect of size was not significant. F(2.8) < 2. Note also that ~t.jr~~~~~ each figure. the slopes of the curves become more shailo% with increasing percent shift. This finding can best be attributed to the fact. already noted, that at shorter ISls the curves for the greater shifts tend to fail above the curves for the smaller shifts. along with the ceiling eflect seen at the longer ISIS. This finding also accounts for the significant ISI by shift interaction found in the data, P(2l. 84) = 11.37. P -c 0.0001. Together the change in slope with size and with percent shift accounts for interaction. size b> shift the significant F(6.24) = 4.09. P < 0.01. Taken together. these resuits show that the trade-off between the 6 and ;’ processes is not a simple one determined exclusively by the temporal frequency of the display. Rather. the trade-off is determined by the joint effects of at least two spatial dimensions (stimulus size and degree of overlap) along with ~liternation rate (see also Petersik and Grassmuck. 19gi ). As mentioned in the introduction, other factors (e.g. stimulus contrast) also contribute to the determination of such a tradeoff. These factors should be kept in mind by researchers attempting to isolate and study either the long-range or short-range process in their studies of apparent movement.
GENERAL
DISCIISSION
The major finding of the present experiments is that the percept (i.e. end-to-end movement) associated with the short-range process continues to be elicited at stimulus displacements well in excess of the originally proposed spatial limit of 15’~20’. In this
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Fig. 5. Percentage
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of group-movcmcnt rcsponsc~ its a function of ISI for stimulus displays whocc bar width was 0.72deg. Other conventions as in Figs 3 and 4.
Short-range process respect, our conclusion supports a major finding of Baker and Braddick (1982). namely, that with increasing size of a correlated area (in their case a patch of random dots; in ours. “overiapping” bars) the degree of stimulus displacement tolerated grows correspondingly. At a minimum. these results suggest that there is no single retinal limit over which the short-range process operates. Our conclusions differ from Baker and Braddick’s in the following ways. Baker and Braddick (1982) contend that the notion of a retinal limit is valid, but that the limit itself increases with increasing retinal eccentricity. We believe that since the functions relating stimulus displacement to temporal equilibrium point pass through roughly the same point on the abcissa irrespective of stimulus size (and largely irrespective of retinal location), the spatial limit of the short-range process is an approximately constant proportion of stimulus size. This conclusion is not inconsistent with the notion of a retinal limit, but it requires the additional assumption that retinal limit grows at a constant rate across the retina. Such an assumption would seriously hinder the theoretical usefulness of the notion of a retinal limit. Furthermore. our data indicate that in order to maintain a fixed criterion of strength of the short-range process (i.e. the 50”” point), ISI must be increased with increasing stimulus displacements. Baker and Braddick’s results did not take into account the potential for a spatio-temporal trade-off of this sort. The comparison between the EG display and the random-dot stimuli. and therefore the relevance of our results. could be criticized on the grounds that with each of our periodic stimulus sets there was stimulus overlap as well as stimulus displacement. On the other hand, with the stimuli of Pantle and Petersik (1980) there was no stimulus overlap after displacement (the stimuli were very thin line segments), With Baker and Braddick‘s (1982) random-dot stimuli. there was also no overlap of the individual elements that were shifted in alternate frames; however, there was overlap in the correlated (and displaced region as a whole. We re.ject the above criticisms on several grounds. First of all. as noted in the introduction. the stimulus conditions that both favor and constrain end-to-end movement are virtually the same as those that favor and constrain segregation of correlated patches in alternating random-dot patterns. Secondly, the information processing requirements for the two types of perceptual phenomena seem very similar. In end-toend movement. there is evidently a point-by-point matching of the entire black and white regions that define the center (“o~~erlapp~ng”) bars. This matching is most noticeable at the edger; of the bars, since, in end-to-end movement the left and right edges of the center bars always align themselves correctly as the stimulus frames alternate. Although this matching process results in a perceptual “jiggling” of the center bars during end-to-end movement. the center bars
I669
nevertheless perceptually remain in the center of the display while the third bar moves back and forth from end to end. Similarly. with alternating randomdot patterns there seems to be a point-by-point matching of correlated elements and a consequent perceptual jiggling the displaced region. It would appear that in both stimulus displays the short-range process serves to maintain perceptual coherence of slightly perturbed stimulus regions over time. In Experiment 2. it was found that the slopes of the curves relating percent group-movement responses to ISi become more shallow as the size of the stimulus bars increases. This finding suggests that larger stimuii are more favorable to the short-range process than smaller stimuli and is consistent with the results of Petersik and Grassmuck (1981). Those authors measured the XV’, temporal equilibrium point with EG stimuli that varied in fundamental spatial frequency. Petersik and Grassmuck concluded that decreases in fundamental spatial frequency favor the 6-process. The understanding of the short-range process in apparent movement that has emerged from the present studies and other recent investigations is as follows: since perceptual responses associated with the short-range process cannot be obtained with dichoptic stimulation (Braddick. 1974: Pantle and Picciano. 1976). it appears to be relatively low-level, perhaps “hard-wired”, process (Braddick, 1980). The notion that his low-level process is hard wired finds support in the fact that many subtle stimulus modifications disrupt its activity. For example. lightfilled ISIS (Braddick, 1973: Pantle and Picciano. 1976), contrast variations from frame to frame, light adaptation (both, Petersik and Pantle, 1979), orientation perturbations of successive stimuli, simple shifts of the overlapping elements in the EG display (both, Pantle and Petersik. 1980), and high fundamental spatial frequencies (Petersik and Grassmuck, 1981) ail tend to disrupt the short-range process. The short-range process tends to operate best with relatively small temporal and spatial separations, although its spatial limit is relative to the size of the stimuli being viewed (at least for stimuli that partially overlap). Its function appears to be to pair similar stimulus elements that may be slightly misaligned or mismatched (on the retina) in space and time. For this reason. we suggest that the short-range process (or c-process, as we have called it) may not have as its primary function the detection of motion per se. Rather, an alternate hypothesis to be considered should be that the function of the short-range process is to compensate for the mismatch of local (retinal) information that occurs with small eye tremors.
A~knoa,I~~JK~~rnl.s--The authors gratefully acknowledge the competent assistance of Craig DeLisle. Brenda Jones. Robert Malsh, and K. Patrick Yarbrough throughout various stages of the research.
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J. TIMOTHY PETERSIK er al. REFERENCES
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