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Textured backgrounds alter perceived speed
Vision Research 40 (2000) 629 – 638 www.elsevier.com/locate/visres
Textured backgrounds alter perceived speed Mark R. Blakemore, Robert J. Snowden * School of Psychology, Cardiff Uni6ersity, Cardiff, CF10 3YG, Wales, UK Received 11 August 1998; received in revised form 8 August 1999
Abstract Both the luminance contrast of an object, and the nature of the background texture over which it moves, are known to influence its perceived speed. In this study the effect of object contrast upon perceived speed was investigated for targets moving across textured patterns of various contrasts. Experiment 1 showed a strong effect of contrast for objects moving over homogenous backgrounds, that was reduced or abolished if the object moved over a textured background. A further experiment suggested that this reduction may be the result of an increase in target visibility, perhaps as a result of additional ‘second order’ motion signals produced by motion over texture backgrounds. A final experiment suggested that two processes were occurring: (1) higher contrast backgrounds appeared to increase the perceived speeds of all objects; and (2) that higher contrast backgrounds eliminated the contrast induced changes in perceived speed. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Perceived speed; Contrast; Background texture
1. Introduction In order to catch balls, drive cars or hurdle an obstacle, it would seem that we must acquire accurate information about the movements of the world and objects relative to ourselves. The visual sense of movement perception appears most adept at such tasks and might lead us to believe of the infallibility of our eyes in encoding the happenings of the real world. Yet perceptual psychologists are aware of alarming deviations from this ‘reality’. For instance as the clouds drift past the moon the sensation produced can be quite the opposite — that the moon appears to drift in the opposite direction over some stationary clouds. So whilst the fact that the motion is readily detected in such situations (indeed our detection of relative motion may be more acute than that of absolute motion — Snowden, 1992) the assignment of what is moving and what is stationary is less than trivial. A second example is known as the autokinetic effect. Here a small stationary light is observed in an otherwise dark room — however the observer perceives the light as wandering * Corresponding author. Tel.: +44-1222-874937; fax: +44-1222874858. E-mail address: [email protected] (R.J. Snowden)
around. Illumination of the background destroys that apparent wander and the light now appears stationary. Motion perception therefore depends not only upon what is moving, but also upon what is stationary. Landmarks have been found to decrease motion detection thresholds — the minimum amount of motion required to say that an object is moving or to describe the direction of that movement (Aubert, 1886; Leibowitz, 1955; Mates, 1969; Tyler & Torres, 1972; Legge & Campbell, 1981; Johnson & Scobey, 1982; Bonnet, 1984). In other words a slowly moving object may be seen as stationary on a blank background but moving if near some stationary reference mark(s). This in turn could be described as a speeding-up of perceived motion (some motion is faster than no motion). This effect of perceptual ‘speed-up’ has also been found to occur at much faster speeds. Brown (1931) found that simple dots appeared to move faster by 25% when moving across a ‘wallpapered’ background than across a blank field (for a recent review see McKee & Smallman, 1998). Gogel and McNulty (1983) used a tactile system to match the perceived speeds of points of light moving over reference marks. This system was used to reduce possible interactions that may occur when matching two visual stimuli. Their study found that an increase in reference mark density increased the appar-
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ent speed of the spot of light, with increases in speed of up to 42% when changing the density of reference marks from 0.10 to 0.65 marks per cm. They also found that the effects occurred at each of the speeds that they tested. Gogel and McNulty explained their findings by suggesting that the reference marks increase the relative motion cues, which in turn alter the perception of visual speed. They argued that spatial adjacency, as used by Taylor (1962) to explain the Oppel – Wundt illusion, could also explain the reference mark findings. In the Oppel–Wundt illusion, a space containing many reference marks appears larger than a space containing a smaller amount of reference points. Taylor (1962) argued that increasing reference mark density increases the perceived distance between reference points. Gogel and McNulty (1983) proposed that this perceived difference in size would interact with the perception of the moving stimuli, i.e. the moving stimuli are perceived to travel a greater distance over denser referenced areas and are therefore perceived to be travelling faster. Wallach (1959) separated the reference cues that affect visual judgements into two categories: cues which provide information about motion relative to the position of the observer — ‘subject-relative’ cues; and cues that provide information regarding the movement of an object relative to another object — ‘object-relative’ cues. Stationary landmarks are not the only ones that may influence our judgements of speed-moving ones can have equally profound effects (Tynan & Sekuler, 1975; Smeets & Brenner, 1995). For example, Norman, Norman, Todd and Lindsey (1996) presented central patches of moving random dots surrounded by an annulus of random dots. When the dots in the surrounding annulus moved at a faster speed than the central patch, the central patch was perceived to be slower than a standard stimulus without an annulus. When the dots in the annulus moved more slowly than the central patch then the central patch was perceived as faster than a standard patch without an annulus (these results occurred irrespective of whether the direction of motion was the same or opposite to the central dots). A non-direction specific motion contrast model was capable of explaining these results. When the dots in the annulus were stationary the central patches were perceived as faster than patches without any annulus, thereby supporting the earlier reference mark finding (Gogel & McNulty, 1983). Whilst these studies have demonstrated the quite profound (if rather complicated) effects that backgrounds may have upon perceived speed it is worth noting that there are some exceptions. For example, Peterken, Brown and Bowman (1991) found no significant effect of reference marks when judging the time to contact of a stimulus. However the critical factor in such studies appears to be the prediction interval (when
the stimulus is not visible), a period when relative motion may be of little importance. An earlier study of background texture and time to contact (Reynolds, 1968) found a trend for an effect of referencing but not at a significant level. Whilst the above studies have either reported that a textured backgrounds either increase perceived speed of the object moving across it (or have no effect), a rather paradoxical finding occurs for the speed of pursuit eye-movement to such a target-background texture decreases the speed of pursuit eye-movement (Raymond, Shapiro & Rose, 1984; Keller & Khan, 1986; Howard & Marton, 1992; Kimmig, Miles & Schwarz, 1992; Niemann, Ilg & Hoffmann, 1994). The relationship between motion perception and speed of pursuit eyemovement is currently the focus of debate (see for example McPeek, Verstraten & Cavanagh, 1997) here we simply note that the effects of background patterns are complex and not yet clearly understood. Background pattern is not the only factor that influences our perception of speed. A well reported phenomenon is that of the influence of contrast on perceived speed (Thompson, 1982; Stone & Thompson, 1992). Gratings of low contrast are perceived as moving more slowly than gratings of high contrast when travelling at the same physical speed-as this was originally reported by Thompson (1976) we shall term this the ‘Thompson effect’. This is found to be the case at slower speeds/temporal frequencies (B 8 Hz). At higher speeds (8–10 Hz) the effect has been reported to reverse with low contrast gratings perceived as faster than higher contrast gratings (Thompson, 1982). Other work suggests that this finding is not limited to grating stimuli; a single dot, disc or random dot pattern also appears to demonstrate a change in speed as a function of contrast (Blakemore & Snowden, 1999). They can even occur in ‘real-life’ situations such as driving in fog (Snowden, Stimpson & Ruddle, 1998). Our first experiment tested whether the visual system incorporated relative cues into its processing of visual stimuli. Was this well reported misperception altered by the relative motion cues?
2. Experiment 1 In this experiment the effect of contrast upon perceived speed was investigated for targets moving across background Julesz patterns of varying contrast. We may expect the background texture to affect perceived speed in one of two ways. Firstly, the effects of background texture and target contrast could be additive. If this is the case we would still expect to find differences in the perceived speed of the stimuli at different contrasts. This would suggest that these are two independent effects occurring at different stages of processing.
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Secondly the effects of target contrast may be modified by the texture pattern. This would suggest that the two variables interact in some manner and would suggest a common site at which the effects are determined.
2.1. Method 2.1.1. Apparatus The stimuli were generated using a Cambridge Research Graphics board (type 2.3) housed by a Research Machines 486/66 PC. They were presented to the observer upon a Mitsubishi 20× monitor (36 × 28 cm active area) with a mean screen luminance of 10 cd/m2 and frame rate of 120 Hz. The stimulus was
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composed of two circular discs, moving across the frontoparallel plane in a linear fashion. Disc stimuli were used, rather than windowed gratings, so that they could move and occlude texture elements. The discs moved horizontally in opposite directions, one disc positioned 1.12° above a fixation point and the other 1.12° below. Each disc had a diameter of 1°. The discs had positional offsets in their starting positions from trial to trial (in the horizontal direction). The background patterns were composed of Julesz patterns at four possible contrast (Michaelson) levels; 0, 5, 50 or 90%. The individual pixels of the Julesz patterns were squares of side 0.05° and the patterned filled the active area of the screen. The standard disc had a set contrast of 80% (dark relative to the background) and a set speed of either 1.5 or 6°/s. Within a single block of trials four different test stimuli were presented, on randomly interleaved trials, whose contrasts (dark relative to the background) were 4, 16, 50 or 80%. Each of the four different background contrasts (0, 5, 50 and 90%) were run in separate blocks of trials. Observers completed three to five sessions at each of the four background levels with the ordering of the blocks being chosen from a random distribution.
2.1.2. Procedure The subjects sat 114 cm away from the monitor and fixated upon a cursor to avoid eye movements. The task of speed matching the motion of the disc patterns used a two-option forced-choice methodology. This required the observer to state which of the two discs, the top one or the bottom one, moved fastest. This was signalled using a mouse press. In each session measurement of perceived speed for the four different test contrasts were interleaved using an adaptive staircase technique (QUEST — Watson & Pelli, 1983) . The initial speed of each of the test patterns was set to be 3–5 dB slower than the standard speed. The QUEST then altered the speed (in 1 dB steps) of the test via a staircase in accordance with the previous response of the observer to this contrast level. Each QUEST was run for 24 trials (making 96 trials in each session). The data were then analysed with a probit analysis (Finney, 1971) program to deduce the point at which the observer judged there to be no difference between the test and standard pattern speeds. Fig. 1. The two graphs show subject LDT’s raw data for the two speed conditions of Experiment 1. The contrast of the test pattern (in dB relative to the standard pattern contrast of 80%) is plotted as a function of the test speed (in dB relative to the standard speed of 1.5 or 6°/s). The regression lines were constrained to have intercepts of 0. These graphs were produced for all of the subjects in the study and the slopes of these fitted regression lines were used for the Fig. 2.
2.1.3. Subjects Four observers were used in this study. KF and LDT had normal vision. MRB and GS had corrected to normal vision. All of the observers except MRB were naive as to the purpose of the study.
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Fig. 2. This graph shows the data derived from the methods described in Fig. 1 for all four subjects in the study at the two standard speed speeds. The graphs show the slope of the fitted lines of regression (y-axis) plotted as a function of background contrast (x-axis). A schematic representation of the display is also shown (the elements size is arbitrary).
2.2. Results Data from the experiment for one observer is illustrated in Fig. 1. The contrast of the test pattern in dB (re: the contrast of the standard pattern, so that 80% contrast= 0 dB, and 4% contrast= − 26 dB) is plotted as a function of the speed of the test pattern in dB (re: the speed of the standard pattern, so a veridical match = 0 dB). Note that points above a speed of 0 dB indicate that the test pattern had to be moved faster than the standard pattern in order to have the same perceived speed. The upper part of the figure is for a standard speed of 1.5°/s and the lower section is for a standard speed of 6.0°/s. Following the lead of Gegenfurtner and Hawken (1996) and Hawken, Gegenfurtner and Tang (1994) who showed that perceived speed is related to stimulus contrast by a power function, we have quantified the effect of contrast upon speed by fitting the data with lines of linear regression in a logarithmic scale (dB). We also constrained our regression lines to have an intercept of 0, as the patterns to be matched are identical in this case and so any deviation from 0 could only be due to noise. From inspection of the data and regression lines in the upper part of the figure (data for standard speed of 1.5°/s) we can see that our data shows the ‘Thompson’ effect — the test
pattern had to be made faster as the contrast was decreased in order to match the apparent speed of the standard pattern, replicating our previous findings using simple disc stimuli (Blakemore & Snowden, 1999). We see that the Thompson effect decreases as the background effect increases, such that for a high contrast background there is now little or no effect of test contrast. A similar analysis was carried out on the data for the other three subjects in this study and the slopes of all these functions are plotted in the left panel of Fig. 2. We can see that there is a consistent pattern of behaviour in that the slope of the function relating contrast to speed decreases as the background contrast increases. For the standard speed of 6.0°/s we get little or no sign of the Thompson effect with no background. This is consistent with a range of studies showing little or no effect for gratings moving at medium to fast speeds (e.g. Stone & Thompson, 1992). For high contrast patterns this subjects appears to show a trend for decreasing target contrast to decrease perceived speed. A similar analysis was carried out on the data for the other three subjects in this study and the slopes of these functions are plotted in the right panel of Fig. 2. Three of the four subjects showed a small negative slope at the higher background contrast (Fig. 2 — right panel).
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2.3. Discussion We obtained an effect of object contrast for the slower speed (the Thompson effect) that was abolished by a high contrast background pattern. Therefore the effect of background is not merely additive1 to the perceived speed of the object but interacts with the object’s contrast. Why is this? We would like to consider three possible reasons in turn: (1) contrast normalisation; (3) the ‘visibility’ of stimuli; and (3) reference marks and the validity of motion estimation.
2.3.1. Contrast normalisation? A popular model of the initial extraction of motion signals has been the ‘energy detector’ proposed by Adelson and Bergen (1985). This model, and many similar ones, are inherently contrast dependent and so do not give a reliable estimate of the speed. In order to get such a reliable measure it has been proposed that there may be a further stage of ‘contrast normalisation’ whereby a signal from somewhere else (such as the output from a ‘stationary’ channel or a generalised signal from many types of detector — see for example Simoncelli & Heeger, 1998) divides the response from any particular motion detector (for a discussion see Thompson, Stone & Swash, 1996). Such a scheme should then produce a speed signal that is not contrast dependent. This would be true, however, only if the motion detector and the putative normalisation signal shared the same contrast dependence. If they do not then the output signal may still have some contrast dependence. This difference in contrast dependency could arise due to differential spatial pooling. One possibility is that the normalisation signal is pooled from a larger area than the motion signal and therefore distorts the ratio appropriately (Stone & Thompson, 1992). What does this theory predict for our experiments? Under conditions where there is no background pattern the motion energy signal for each disc depends upon both the speed and contrast of each disc individually. The normalisation signal, however, takes into account the contrast of both discs (due to its larger pooling area). Thus the normalisation signal is not appropriate to the contrast of the individual discs (the high contrast disc is normalised by a too small signal, whilst the low contrast one is normalised by a too large signal). On high contrast backgrounds the normalisation signal for both the patterns should now have a greater value (as this high contrast background will be included for both targets) and hence both of the discs should 1 Note that as both the test and standard patterns moved across the background any additive effects that were present should be equal and would therefore not be detected using the present paradigm. We shall return to whether there are any additive effects in Experiment 3.
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appear reduced in apparent speed. The exact prediction about how the ratio of speeds of the test and standard should change depends upon the details of the implementation of the model — however given simple assumptions2 one might expect that the low contrast target to be even more slowed than the high contrast one — i.e. the Thompson effect should be exaggerated. Results do not seem to fit this idea — firstly, we found that the background did have a strong effect on the ratio of the perceived speeds but it was to decrease the Thompson effect not increase it; secondly previous studies have found that reference marks increase (rather than decrease) perceived speed (we shall demonstrate this in Experiment 3); and thirdly more direct tests, where the test and standard patterns are surrounded by very different contrasts, found no evidence to support the normalisation model (Thompson et al., 1996).
2.3.2. Visibility of target stimuli? When the contrast of a grating or disc is reduced it becomes harder to see. This apparently trivial statement has nevertheless being somewhat ignored in considerations of the Thompson effect. For it might be that the effect is one of reduced visibility rather than reduced contrast per se. For this to be true then other factors that reduce visibility should produce similar effects as reduced contrast. This appears to have some prima facie support. Other conditions that reduce visibility such as more peripheral viewing are also capable of reducing apparent speed (Czermak, 1857; Campbell & Maffei, 1981; Tynan & Sekuler, 1982; Johnston & 2
For example let us consider an example of two targets moving at speed S, one of contrast C1 and one of contrast C2, moving on a background of contrast Cback. Let us make some simple assumptions about the area of contrast pooling such that it is confined to the target area for the motion energy signal, but that for the normalisation process it is essentially infinite. The speed of pattern 1 will be: S C1/(C2 +C1 +Cback) and that of pattern 2 S C2/(C1 +C2 +Cback) and therefore the ratio will be simply C1/C2. Note that for a background that is constant this ratio is unchanged by background contrast. However this large pooling is unrealistic so instead the equations change so that the speed of the first pattern will now be S C1/(C1 +d1 · C2 +d2 · Cback) and the second pattern S C2/(d1 · C1 +C2 +d2 · Cback) where d1 and d2 B1.0 and represents the weighting factors that would depend upon the area of pooling and the spatial parameters of the experiment. The ratio of speeds is now always less than C1/C2 (i.e. this produces a smaller and more realistic Thompson effects than the infinite pooling model) but that must increase towards the limiting value C1/C2 as Cback \ \C1 or C2 (i.e. the Thompson effect increases at greater background contrasts).
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Wright, 1986). However one is then left with the issue of what exactly one means by ‘visibility’ and what aspects of the image (such as its presence, motion, direction, etc.) should govern this idea. Nevertheless we would like to note that ‘visibility’ nevertheless does change for our disc stimuli as we alter the contrast of the background. Consider a disc of 0% contrast — clearly this is invisible when viewed upon a blank background, but stands out as an unmodulated area when viewed upon a high contrast textured background. Hence it is now highly visible. Note that in many ways this stimulus resembles those termed ‘second-order’ patterns (see Smith, 1994) in that the stimulus is defined in this extreme case not by its difference in luminance from the background but by its difference in contrast (a low contrast target on a high contrast background). It has been noted that second order patterns can also show the Thompson effect (Ledgeway & Smith, 1995), and that it may even be greater than that of first order patterns (Gegenfurtner & Hawken, 1996). Hence as the contrast of the background rises this should serve to produce a greater second order motion signal which might alleviate the loss of motion in the first-order system, thus giving a possible explanation of the current results.
2.3.3. Reference marks allow us more 6eridical motion perception? As apparent speed appears to change systematically with contrast we are left with the question of ‘at which contrast do we get a veridical perception of speed?’ It is tempting (though perhaps false) to suggest that our perceptions at high contrast are correct and that those at low contrast are an ‘illusion’. For example Campbell and Maffei (1979, 1981) have shown that in extreme cases that a rotating disc may appear to stop altogether at low contrasts — clearly this is an illusory state. From this idea of low contrasts providing an illusion we can then suggest that reference marks allow us to be more veridical in our perception of motion by providing further information about motion, or by stimulating relative motion sensors that were not excited by absolute motion alone. We note the similarity of these ideas to those of ‘object-referencing’ discussed in the introduction (Wallach, 1959). Hence the background has its strongest effects where the illusion is at its strongest — they serve to speed up patterns of low contrast but have little effect at high contrast. This discussion then leads to two plausible explanations for the results of Experiment 1 — that the background provides increased visibility of the moving discs, or that the reference marks allow for a more veridical perception of speed. Experiment 2 sought to distinguish between these possibilities.
3. Experiment 2 In this experiment the landmarks were still present but they were not occluded by the moving disc. The dot background patterns were presented on the screen but were a small distance from the moving disc patterns; which travelled along zero contrast rectangular pathways from one side of the monitor to the other. Hence any ‘second order’ components that contributed in the first experiment were eliminated yet the disc still moved close by reference marks.
3.1. Method 3.1.1. Stimuli and procedure These were the same as Experiment 1 save that the discs did not move over the texture backgrounds but through strips between the texture patterns. The pathways were constructed by setting the contrast of the elements in the strips to 0% (i.e. mean luminance). The strips extended the length of the screen and were 1.5° wide centred on the position of the disc. Thus the top and bottom edges of the discs were 0.25° from the edge of the texture background. 3.1.2. Subjects Three observers took part in this study. LDT and GS had normal or corrected to normal vision and were naive to the purpose of the study. MRB had corrected to normal vision and was aware of the purpose of the study. 3.2. Results The data were analysed identically to Experiment 1. The slopes of the fitted functions are illustrated in Fig. 3. For a standard speed of 1.5°/s (left panel) we can see that all slopes were strongly negative indicating that the lower contrast patterns appeared to have slower speeds (the Thompson effect) at all background contrast. The data for a speed of 6.0°/s (right panel) show little sign of any systematic positive or negative slope (i.e. there is no Thompson effect) at any background contrast.
3.3. Discussion The ‘pathway’ matching experiment seems to suggest that the background must be occluded by the discs in order to influence the size of the Thompson effect. This suggests that a objects-reference theory may have trouble explaining the texture background findings. However it could be argued that the references may only interact with moving objects locally over a small scale and that this experiment used reference points that were beyond the scale of such a system. Norman et al. (1996) has demonstrated similar findings with annulus stimuli. These stimuli were composed of a central portion
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(whose speed was judged) and the surrounding annulus (containing the relative motion aspect of the stimulus). If a gap was produced between these two components and then progressively extended, the ‘distortion’ in the perceived speed of the central stimulus was reduced. Norman et al. deduced that the spatial area over which relative motion can influence is limited. Though we note that the gap between the disc and the texture in our experiment was very small (0.25°). The visibility explanation appears to have support from the experimental data. This model suggests that the low contrast discs are ‘corrected’ in regards to perceived speed because the texture pattern increases their visibility — perhaps via the ‘second order’ mechanism postulated earlier. If the background texture no longer defines the disc outline, visibility is unchanged and perceived speed is still influenced by disc contrast.
4. Experiment 3 In the previous experiments we have shown that movement over a textured background can eliminate the effects of stimulus contrast upon perceived speed, and that the moving pattern must actually move over the background rather than nearby it in order for the effect to occur. If we view the reduction in perceived speed as
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contrast is reduced as an ‘illusion’ or ‘error’ then we could regard the effect of the background as serving to eliminate this ‘error’ and produce a more veridical estimate of speed. Many previous studies (see Section 1) have shown that background patterns can increase the apparent speed of a moving target. This might arise either because the perceived speed without the textured background was in error (i.e. too slow), or because the texture simply produces a perceptual ‘speed-up’ irrespective of whether the speed without the texture was in error. In this experiment the background texture was split into two contrast sections, one of a high contrast and one of a low contrast. In each section was a disc that moved and the observers matched the two discs for perceived speed. We were interested to see now if discs of the same contrast would be matched veridically or if the differing backgrounds would produce changes in perceived speed. Additionally, this split background experiment provides a further test of the ‘contrast normalisation’ explanation of the Thompson effect (Stone & Thompson, 1992; Thompson et al., 1996).
4.1. Methods 4.1.1. Stimuli The same disc stimuli were used as in Experiments 1 and 2. This background pattern was composed of a Julesz
Fig. 3. This graph shows the results of Experiment 2. The data were once again derived as Fig. 1 and are plotted in an analogous manner to Fig. 2.
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Fig. 4. This graph shows the speed matching results of Experiment 3. The data for all three subjects have been plotted separately for when the pattern moved over the low contrast background (filled symbols) and when it moved over the high contrast background (open symbols). The test speed (relative to a standard of 1.5°/s) is plotted as a function of test contrast (relative to a standard of 50%). Linear regression was again used to fit the lines for each condition, although both the intercept and slopes were free to vary.
pattern which covered the whole screen, split horizontally into two different contrast sections; a 10% contrast section and a 90% contrast section. The position of these contrast sections, i.e. top or bottom half of the screen was randomised from one set of trials to another. Within a single session the standard disc at 50% contrast and a set speed of 1.5°/s, was matched for perceived speed to three different, randomly interleaved test pattern discs. These varied in their contrast, set at either 10, 50 or 90%. The vertical position of the standard and test discs was dependent upon the randomised position of the background contrast sections. In the study the standard to test speed match was carried out under two conditions. The first condition had the 50% standard moving over the 10% contrast dot background whilst being matched to the test discs moving over the 90% background. The second condition was a reversal of this, i.e. the 50% standard disc moving over the 90% background whilst being matched for speed to the test discs moving over the 10% background.
4.1.2. Procedure These were as for Experiments 1 and 2. 4.1.3. Subjects Three observers were used in this study. LDT had normal vision. MRB and GS had corrected to normal vision. All of the observers except MRB were naive as to the purpose of the study. 4.2. Results The data from this experiment are plotted in Fig. 4. The data have been plotted separately for when the test
pattern moved over the low contrast background (filled symbols) and when it moved over the high contrast background (open symbols). The data for each condition were fitted by linear regression — but we now allowed both the intercept and slope to vary as there is now no condition which is identical for both the test and standard pattern. When the test patterns are presented upon a low (10%) contrast background and matched to a 50% standard on the high (90%) contrast pattern matching speed is set to be greater than 0 dB — i.e. the pattern (test) moving across the low contrast background had to be made to move faster than that on the high contrast background in order to be a perceptual speed match. There appears to be little sign of the Thompson effect (what slope there is tends to be positive rather than negative) thus replicating the result of Experiment 1. When the opposite background conditions apply (i.e. the test is presented on the high contrast background and standard on the low contrast) then matching speed was set to be less than 0 dB — the test pattern was set to be slower than the standard pattern. Again there is no sign of the Thompson effect in these conditions.
4.3. Discussion The results of Experiment 3 using a split-background display indicate that the contrast of the background has important effects upon the perceived speed of the disc patterns. This effect of background contrast also appears to be independent of the disc contrast level. Thus it appears that background patterns can have two separate effects: (1) increasing perceived speed of all patterns (see also Brown, 1931; Gogel & McNulty, 1983; Snowden, 1997); and (2) eliminating contrast induced changes in perceived speed. One may argue that the backgrounds of different contrast may effect perceived size and/or depth (O’Shea, Blackburn & Ono, 1994) and therefore perceived speed. The findings of Kanecko and Uchikawa (1993) using moving stimuli of different graduated sizes, and therefore different perceived depths, suggested that depth has an influence on perceived speed. The results of Experiment 3 appear to reject that possible explanation in this case. If stimuli moving upon a low contrast background are perceived as further away then, for a given retinal rate, the notion of velocity constancy (though see McKee & Welch, 1989) would suggest they be perceived as faster. Our result show it is the target moving over the high contrast background that appears to move faster. The present results also have implications for explanations of the Thompson effect based on a contrast normalisation process (see Section 2.3). The theory predicts that patterns moving on a high contrast background should appear to move more slowly than those on a low contrast background. However our results show quite the opposite relationship.
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5. General discussion
References
We have found that the change in perceived speed as a function of contrast (the Thompson effect) can be eliminated when the target patterns move across a textured background. The Thompson effect returned if the target stimuli were provided with narrow untextured alleyways over which to move so that they no longer occluded/revealed areas of texture. Finally increasing background contrast was also shown to have an absolute effect on perceived speed such as those moving across a high contrast background appear faster than those on a low contrast background. In this study we used a disc moving over a textured background whilst most other studies of the Thompson effect have been limited to sinusoidal gratings (see Blakemore & Snowden, 1999). Such a disc contains many spatial frequencies and therefore changes in contrast will alter the effective spatial frequency contact (due to the modulation transfer function of the visual system). However we note that any such effect of contrast upon the effective spatial frequency spectrum should be present for all background conditions. The results from the first two experiments taken together seem to suggest that the crucial element in eliminating the Thompson effect is that the targets actually occlude/reveal areas of texture rather than move close by them. We suggest that this provides a ‘second-order’ signal as to the presence of the target which may provide by itself a motion signal (Chubb & Sperling, 1988) or allow the engagement of other processes such as attentive tracking (Cavanagh, 1992) that are less contrast dependent. Contrast is not the only variable that is capable of affecting perceived speed. Many other variables such as size (Brown, 1931), density (Watamaniuk, Grzywacz & Yuille, 1993), spatial frequency (Diener, Wist, Dichgans & Brandt, 1976; Smith & Edgar, 1990) and absence of luminance information (equiluminance — Cavanagh, Tyler & Favreau, 1984) all have strong effects on perceived speed. All these studies have so far only examined these variable with respect to movement over a blank background — or more typically within a confined window. It would be illuminating to see if these effects were also reduced or eliminated for stimuli moving through more complex environments as we have demonstrated here for changes in contrast.
Adelson, E. H., & Bergen, J. R. (1985). Spatiotemporal energy models for the perception of motion. Journal of the Optical Society of America A, 2, 284 – 299. Aubert, H. (1886). Die Bewegungsempfindung. Pflugers Archi6 fur die Gesamte Physiologie, 39, 347 – 370. Blakemore, M. R., & Snowden, R. J. (1999). The effect of contrast upon perceived speed: a general phenomenon? Perception, 28, 33 – 48. Bonnet, C. (1984). Discrimination of velocities and mechanisms of motion perception. Perception, 13, 275 – 282. Brown, J. F. (1931). The visual perception of velocity. Psychologische Forschung, 14, 199 – 232. Campbell, F. W., & Maffei, L. (1979). Stopped visual motion. Nature, 278, 192. Campbell, F. W., & Maffei, L. (1981). The influence of spatial frequency and contrast on the perception of moving patterns. Vision Research, 21, 713 – 721. Cavanagh, P. (1992). Attention-based motion perception. Science, 257, 1563 – 1565. Cavanagh, P., Tyler, C. W., & Favreau, O. E. (1984). Perceived velocity of moving chromatic patterns. Journal of the Optical Society of America A, 1, 893 – 899. Chubb, C., & Sperling, G. (1988). Drift-balanced random stimuli: a general basis for studying non-Fourier motion perception. Journal of the Optical Society of America A, 5, 1986 – 2006. Czermak, J. (1857). Ideen zu einer Lehre vom Zeitseinn. Gesellschaft Schriften, 1, 421. Diener, H. C., Wist, E. R., Dichgans, J., & Brandt, T. (1976). The spatial frequency effect on perceived velocity. Vision Research, 16, 169 – 176. Finney, D. J. (1971). Probit analysis. Cambridge: Cambridge University Press. Gegenfurtner, K. R., & Hawken, M. J. (1996). Perceived velocity of luminance, chromatic and non-Fourier stimuli: Influence of contrast and temporal frequency. Vision Research, 36, 1281–1290. Gogel, W. C., & McNulty, P. (1983). Perceived velocity as a function of reference mark density. Scandina6ian Journal of Psychology, 24, 257 – 265. Hawken, M. J., Gegenfurtner, K. R., & Tang, C. (1994). Contrast dependence of colour and luminance motion mechanisms in human vision. Nature, 367, 268 – 270. Howard, I. P., & Marton, C. (1992). Visual pursuit over textured backgrounds in different depth planes. Experimental Brain Research, 90, 625 – 629. Johnson, C. A., & Scobey, R. P. (1982). Effects of reference lines on displacement thresholds at various durations of movement. Vision Research, 22, 819 – 821. Johnston, A., & Wright, M. J. (1986). Matching velocity in central and peripheral vision. Vision Research, 26, 1099 – 1109. Kanecko, H., & Uchikawa, K. (1993). Apparent relative size and depth of moving objects. Perception, 22, 537 – 547. Keller, E. L., & Khan, N. S. (1986). Smooth-pursuit initiation in the presence of a textured background in monkey. Vision Research, 26, 943 – 955. Kimmig, H. G., Miles, F. A., & Schwarz, U. (1992). Effects of stationary textured backgrounds on the initiation of pursuit eye movements in monkeys. Journal of Neurophysiology, 68, 2147– 2164. Ledgeway, T., & Smith, A. T. (1995). The perceived speed of secondorder motion and its dependence on stimulus contrast. Vision Research, 35, 215 – 224. Legge, G. E., & Campbell, F. W. (1981). Displacement detection in human vision. Vision Research, 21, 205 – 214. Leibowitz, H. W. (1955). Effect of reference lines on the discrimination of movement. Journal of the Optical Society of America, 45, 829 – 830.
Acknowledgements MRB was supported by a SERC research studentship (946030978).
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Mates, B. (1969). Effects of reference marks and luminance on discrimination of movement. Journal of Psychology, 73, 209 – 221. McKee, S. P., & Smallman, H. S. (1998). Size and speed constancy. In V. Walsh, & J. J. Kulikowski, Perceptual constancies: why things look like they do (pp. 373–408). New York: Cambridge University Press. McKee, S. P., & Welch, L. (1989). Is there a constancy for velocity? Vision Research, 29, 553–561. McPeek, R. M., Verstraten, F. A. J., & Cavanagh, P. (1997). Target position, not target velocity, drives sustained smooth pursuit. In6estigati6e Ophthalmology and Visual Science Supplement, 38, S932. Niemann, T., Ilg, U. J., & Hoffmann, K. (1994). Eye movements elicited by transparent stimuli. Experimental Brain Research, 98, 314 – 322. Norman, H. F., Norman, J. F., Todd, J. T., & Lindsey, D. T. (1996). Spatial interactions in perceived speed. Perception, 25, 815 – 830. O’Shea, R. P., Blackburn, S. G., & Ono, H. (1994). Contrast as a depth cue. Vision Research, 34, 1595–1604. Peterken, C., Brown, B., & Bowman, K. (1991). Predicting the future position of a moving target. Perception, 20, 5–16. Raymond, J. E., Shapiro, K. L., & Rose, D. J. (1984). Optokinetic backgrounds affect perceived velocity during ocular tracking. Perception and Psychophysics, 36, 221–224. Reynolds, H. N. (1968). Temporal estimation in the perception of occluded motion. Perceptual and Motor Skills, 26, 407–416. Simoncelli, E. P., & Heeger, D. J. (1998). A model of neuronal responses in visual area MT. Vision Research, 38, 743–761. Smeets, J. B. J., & Brenner, E. (1995). Perception and action are based on the same visual information: distinction between position and velocity. Journal of Experimental Psychology: Human Perception and Performance, 21, 19–31. Smith, A. T. (1994). The detection of second-order motion. In A. T. Smith, & R. J. Snowden, Visual Detection of Motion (pp. 145 – 176). New York: Academic Press. Smith, A. T., & Edgar, G. K. (1990). The influence of spatial frequency on perceived temporal frequency and perceived speed. Vision Research, 30, 1467–1474.
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Snowden, R. J. (1992). Sensitivity to relative and absolute motion. Perception, 21, 563 – 568. Snowden, R. J. (1997). Perceived speed: effects of field size and background texture. In6estigati6e Ophthalmology and Visual Science Supplement, 38, 1167. Snowden, R. J., Stimpson, N., & Ruddle, R. A. (1998). Speed perception fogs up as visibility drops. Nature, 392, 450. Stone, L. S., & Thompson, P. (1992). Human speed perception is contrast dependent. Vision Research, 32, 1535 – 1549. Taylor, M. M. (1962). Figural after-effects: a psychophysical theory of the displacement effect. Canadian Journal of Psychology, 16, 247 – 277. Thompson, P. (1976). Velocity after effects and the perception of mo6ement. Ph.D. thesis, University of Cambridge, UK. Thompson, P. (1982). Perceived rate of movement depends on contrast. Vision Research, 22, 377 – 380. Thompson, P., Stone, L. S., & Swash, S. (1996). Speed estimates from grating patches are not contrast-normalized. Vision Research, 36, 667 – 674. Tyler, C. W., & Torres, J. (1972). Frequency response characteristics for sinusoidal movement in the fovea and periphery. Peception and Psychophysics, 12, 232 – 236. Tynan, P., & Sekuler, R. (1975). Simultaneous motion contrast: Velocity, sensitivity and depth response. Vision Research, 15, 1231 – 1238. Tynan, P. D., & Sekuler, R. (1982). Motion processing in peripheral vision: reaction time and perceived velocity. Vision Research, 22, 61 – 68. Wallach, H. (1959). The perception of motion. Scientific American, 201, 2 – 6. Watamaniuk, S. N. J., Grzywacz, N. M., & Yuille, A. L. (1993). Dependence of speed and direction perception on cinematogram dot density. Vision Research, 33, 849 – 859. Watson, A. B., & Pelli, D. G. (1983). QUEST: a Bayesian adaptive psychometric method. Perception and Psychophysics, 33, 113– 120.
Textured backgrounds alter perceived speed Mark R. Blakemore, Robert J. Snowden * School of Psychology, Cardiff Uni6ersity, Cardiff, CF10 3YG, Wales, UK Received 11 August 1998; received in revised form 8 August 1999
Abstract Both the luminance contrast of an object, and the nature of the background texture over which it moves, are known to influence its perceived speed. In this study the effect of object contrast upon perceived speed was investigated for targets moving across textured patterns of various contrasts. Experiment 1 showed a strong effect of contrast for objects moving over homogenous backgrounds, that was reduced or abolished if the object moved over a textured background. A further experiment suggested that this reduction may be the result of an increase in target visibility, perhaps as a result of additional ‘second order’ motion signals produced by motion over texture backgrounds. A final experiment suggested that two processes were occurring: (1) higher contrast backgrounds appeared to increase the perceived speeds of all objects; and (2) that higher contrast backgrounds eliminated the contrast induced changes in perceived speed. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Perceived speed; Contrast; Background texture
1. Introduction In order to catch balls, drive cars or hurdle an obstacle, it would seem that we must acquire accurate information about the movements of the world and objects relative to ourselves. The visual sense of movement perception appears most adept at such tasks and might lead us to believe of the infallibility of our eyes in encoding the happenings of the real world. Yet perceptual psychologists are aware of alarming deviations from this ‘reality’. For instance as the clouds drift past the moon the sensation produced can be quite the opposite — that the moon appears to drift in the opposite direction over some stationary clouds. So whilst the fact that the motion is readily detected in such situations (indeed our detection of relative motion may be more acute than that of absolute motion — Snowden, 1992) the assignment of what is moving and what is stationary is less than trivial. A second example is known as the autokinetic effect. Here a small stationary light is observed in an otherwise dark room — however the observer perceives the light as wandering * Corresponding author. Tel.: +44-1222-874937; fax: +44-1222874858. E-mail address: [email protected] (R.J. Snowden)
around. Illumination of the background destroys that apparent wander and the light now appears stationary. Motion perception therefore depends not only upon what is moving, but also upon what is stationary. Landmarks have been found to decrease motion detection thresholds — the minimum amount of motion required to say that an object is moving or to describe the direction of that movement (Aubert, 1886; Leibowitz, 1955; Mates, 1969; Tyler & Torres, 1972; Legge & Campbell, 1981; Johnson & Scobey, 1982; Bonnet, 1984). In other words a slowly moving object may be seen as stationary on a blank background but moving if near some stationary reference mark(s). This in turn could be described as a speeding-up of perceived motion (some motion is faster than no motion). This effect of perceptual ‘speed-up’ has also been found to occur at much faster speeds. Brown (1931) found that simple dots appeared to move faster by 25% when moving across a ‘wallpapered’ background than across a blank field (for a recent review see McKee & Smallman, 1998). Gogel and McNulty (1983) used a tactile system to match the perceived speeds of points of light moving over reference marks. This system was used to reduce possible interactions that may occur when matching two visual stimuli. Their study found that an increase in reference mark density increased the appar-
0042-6989/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 6 9 8 9 ( 9 9 ) 0 0 2 1 4 - X
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ent speed of the spot of light, with increases in speed of up to 42% when changing the density of reference marks from 0.10 to 0.65 marks per cm. They also found that the effects occurred at each of the speeds that they tested. Gogel and McNulty explained their findings by suggesting that the reference marks increase the relative motion cues, which in turn alter the perception of visual speed. They argued that spatial adjacency, as used by Taylor (1962) to explain the Oppel – Wundt illusion, could also explain the reference mark findings. In the Oppel–Wundt illusion, a space containing many reference marks appears larger than a space containing a smaller amount of reference points. Taylor (1962) argued that increasing reference mark density increases the perceived distance between reference points. Gogel and McNulty (1983) proposed that this perceived difference in size would interact with the perception of the moving stimuli, i.e. the moving stimuli are perceived to travel a greater distance over denser referenced areas and are therefore perceived to be travelling faster. Wallach (1959) separated the reference cues that affect visual judgements into two categories: cues which provide information about motion relative to the position of the observer — ‘subject-relative’ cues; and cues that provide information regarding the movement of an object relative to another object — ‘object-relative’ cues. Stationary landmarks are not the only ones that may influence our judgements of speed-moving ones can have equally profound effects (Tynan & Sekuler, 1975; Smeets & Brenner, 1995). For example, Norman, Norman, Todd and Lindsey (1996) presented central patches of moving random dots surrounded by an annulus of random dots. When the dots in the surrounding annulus moved at a faster speed than the central patch, the central patch was perceived to be slower than a standard stimulus without an annulus. When the dots in the annulus moved more slowly than the central patch then the central patch was perceived as faster than a standard patch without an annulus (these results occurred irrespective of whether the direction of motion was the same or opposite to the central dots). A non-direction specific motion contrast model was capable of explaining these results. When the dots in the annulus were stationary the central patches were perceived as faster than patches without any annulus, thereby supporting the earlier reference mark finding (Gogel & McNulty, 1983). Whilst these studies have demonstrated the quite profound (if rather complicated) effects that backgrounds may have upon perceived speed it is worth noting that there are some exceptions. For example, Peterken, Brown and Bowman (1991) found no significant effect of reference marks when judging the time to contact of a stimulus. However the critical factor in such studies appears to be the prediction interval (when
the stimulus is not visible), a period when relative motion may be of little importance. An earlier study of background texture and time to contact (Reynolds, 1968) found a trend for an effect of referencing but not at a significant level. Whilst the above studies have either reported that a textured backgrounds either increase perceived speed of the object moving across it (or have no effect), a rather paradoxical finding occurs for the speed of pursuit eye-movement to such a target-background texture decreases the speed of pursuit eye-movement (Raymond, Shapiro & Rose, 1984; Keller & Khan, 1986; Howard & Marton, 1992; Kimmig, Miles & Schwarz, 1992; Niemann, Ilg & Hoffmann, 1994). The relationship between motion perception and speed of pursuit eyemovement is currently the focus of debate (see for example McPeek, Verstraten & Cavanagh, 1997) here we simply note that the effects of background patterns are complex and not yet clearly understood. Background pattern is not the only factor that influences our perception of speed. A well reported phenomenon is that of the influence of contrast on perceived speed (Thompson, 1982; Stone & Thompson, 1992). Gratings of low contrast are perceived as moving more slowly than gratings of high contrast when travelling at the same physical speed-as this was originally reported by Thompson (1976) we shall term this the ‘Thompson effect’. This is found to be the case at slower speeds/temporal frequencies (B 8 Hz). At higher speeds (8–10 Hz) the effect has been reported to reverse with low contrast gratings perceived as faster than higher contrast gratings (Thompson, 1982). Other work suggests that this finding is not limited to grating stimuli; a single dot, disc or random dot pattern also appears to demonstrate a change in speed as a function of contrast (Blakemore & Snowden, 1999). They can even occur in ‘real-life’ situations such as driving in fog (Snowden, Stimpson & Ruddle, 1998). Our first experiment tested whether the visual system incorporated relative cues into its processing of visual stimuli. Was this well reported misperception altered by the relative motion cues?
2. Experiment 1 In this experiment the effect of contrast upon perceived speed was investigated for targets moving across background Julesz patterns of varying contrast. We may expect the background texture to affect perceived speed in one of two ways. Firstly, the effects of background texture and target contrast could be additive. If this is the case we would still expect to find differences in the perceived speed of the stimuli at different contrasts. This would suggest that these are two independent effects occurring at different stages of processing.
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Secondly the effects of target contrast may be modified by the texture pattern. This would suggest that the two variables interact in some manner and would suggest a common site at which the effects are determined.
2.1. Method 2.1.1. Apparatus The stimuli were generated using a Cambridge Research Graphics board (type 2.3) housed by a Research Machines 486/66 PC. They were presented to the observer upon a Mitsubishi 20× monitor (36 × 28 cm active area) with a mean screen luminance of 10 cd/m2 and frame rate of 120 Hz. The stimulus was
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composed of two circular discs, moving across the frontoparallel plane in a linear fashion. Disc stimuli were used, rather than windowed gratings, so that they could move and occlude texture elements. The discs moved horizontally in opposite directions, one disc positioned 1.12° above a fixation point and the other 1.12° below. Each disc had a diameter of 1°. The discs had positional offsets in their starting positions from trial to trial (in the horizontal direction). The background patterns were composed of Julesz patterns at four possible contrast (Michaelson) levels; 0, 5, 50 or 90%. The individual pixels of the Julesz patterns were squares of side 0.05° and the patterned filled the active area of the screen. The standard disc had a set contrast of 80% (dark relative to the background) and a set speed of either 1.5 or 6°/s. Within a single block of trials four different test stimuli were presented, on randomly interleaved trials, whose contrasts (dark relative to the background) were 4, 16, 50 or 80%. Each of the four different background contrasts (0, 5, 50 and 90%) were run in separate blocks of trials. Observers completed three to five sessions at each of the four background levels with the ordering of the blocks being chosen from a random distribution.
2.1.2. Procedure The subjects sat 114 cm away from the monitor and fixated upon a cursor to avoid eye movements. The task of speed matching the motion of the disc patterns used a two-option forced-choice methodology. This required the observer to state which of the two discs, the top one or the bottom one, moved fastest. This was signalled using a mouse press. In each session measurement of perceived speed for the four different test contrasts were interleaved using an adaptive staircase technique (QUEST — Watson & Pelli, 1983) . The initial speed of each of the test patterns was set to be 3–5 dB slower than the standard speed. The QUEST then altered the speed (in 1 dB steps) of the test via a staircase in accordance with the previous response of the observer to this contrast level. Each QUEST was run for 24 trials (making 96 trials in each session). The data were then analysed with a probit analysis (Finney, 1971) program to deduce the point at which the observer judged there to be no difference between the test and standard pattern speeds. Fig. 1. The two graphs show subject LDT’s raw data for the two speed conditions of Experiment 1. The contrast of the test pattern (in dB relative to the standard pattern contrast of 80%) is plotted as a function of the test speed (in dB relative to the standard speed of 1.5 or 6°/s). The regression lines were constrained to have intercepts of 0. These graphs were produced for all of the subjects in the study and the slopes of these fitted regression lines were used for the Fig. 2.
2.1.3. Subjects Four observers were used in this study. KF and LDT had normal vision. MRB and GS had corrected to normal vision. All of the observers except MRB were naive as to the purpose of the study.
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Fig. 2. This graph shows the data derived from the methods described in Fig. 1 for all four subjects in the study at the two standard speed speeds. The graphs show the slope of the fitted lines of regression (y-axis) plotted as a function of background contrast (x-axis). A schematic representation of the display is also shown (the elements size is arbitrary).
2.2. Results Data from the experiment for one observer is illustrated in Fig. 1. The contrast of the test pattern in dB (re: the contrast of the standard pattern, so that 80% contrast= 0 dB, and 4% contrast= − 26 dB) is plotted as a function of the speed of the test pattern in dB (re: the speed of the standard pattern, so a veridical match = 0 dB). Note that points above a speed of 0 dB indicate that the test pattern had to be moved faster than the standard pattern in order to have the same perceived speed. The upper part of the figure is for a standard speed of 1.5°/s and the lower section is for a standard speed of 6.0°/s. Following the lead of Gegenfurtner and Hawken (1996) and Hawken, Gegenfurtner and Tang (1994) who showed that perceived speed is related to stimulus contrast by a power function, we have quantified the effect of contrast upon speed by fitting the data with lines of linear regression in a logarithmic scale (dB). We also constrained our regression lines to have an intercept of 0, as the patterns to be matched are identical in this case and so any deviation from 0 could only be due to noise. From inspection of the data and regression lines in the upper part of the figure (data for standard speed of 1.5°/s) we can see that our data shows the ‘Thompson’ effect — the test
pattern had to be made faster as the contrast was decreased in order to match the apparent speed of the standard pattern, replicating our previous findings using simple disc stimuli (Blakemore & Snowden, 1999). We see that the Thompson effect decreases as the background effect increases, such that for a high contrast background there is now little or no effect of test contrast. A similar analysis was carried out on the data for the other three subjects in this study and the slopes of all these functions are plotted in the left panel of Fig. 2. We can see that there is a consistent pattern of behaviour in that the slope of the function relating contrast to speed decreases as the background contrast increases. For the standard speed of 6.0°/s we get little or no sign of the Thompson effect with no background. This is consistent with a range of studies showing little or no effect for gratings moving at medium to fast speeds (e.g. Stone & Thompson, 1992). For high contrast patterns this subjects appears to show a trend for decreasing target contrast to decrease perceived speed. A similar analysis was carried out on the data for the other three subjects in this study and the slopes of these functions are plotted in the right panel of Fig. 2. Three of the four subjects showed a small negative slope at the higher background contrast (Fig. 2 — right panel).
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2.3. Discussion We obtained an effect of object contrast for the slower speed (the Thompson effect) that was abolished by a high contrast background pattern. Therefore the effect of background is not merely additive1 to the perceived speed of the object but interacts with the object’s contrast. Why is this? We would like to consider three possible reasons in turn: (1) contrast normalisation; (3) the ‘visibility’ of stimuli; and (3) reference marks and the validity of motion estimation.
2.3.1. Contrast normalisation? A popular model of the initial extraction of motion signals has been the ‘energy detector’ proposed by Adelson and Bergen (1985). This model, and many similar ones, are inherently contrast dependent and so do not give a reliable estimate of the speed. In order to get such a reliable measure it has been proposed that there may be a further stage of ‘contrast normalisation’ whereby a signal from somewhere else (such as the output from a ‘stationary’ channel or a generalised signal from many types of detector — see for example Simoncelli & Heeger, 1998) divides the response from any particular motion detector (for a discussion see Thompson, Stone & Swash, 1996). Such a scheme should then produce a speed signal that is not contrast dependent. This would be true, however, only if the motion detector and the putative normalisation signal shared the same contrast dependence. If they do not then the output signal may still have some contrast dependence. This difference in contrast dependency could arise due to differential spatial pooling. One possibility is that the normalisation signal is pooled from a larger area than the motion signal and therefore distorts the ratio appropriately (Stone & Thompson, 1992). What does this theory predict for our experiments? Under conditions where there is no background pattern the motion energy signal for each disc depends upon both the speed and contrast of each disc individually. The normalisation signal, however, takes into account the contrast of both discs (due to its larger pooling area). Thus the normalisation signal is not appropriate to the contrast of the individual discs (the high contrast disc is normalised by a too small signal, whilst the low contrast one is normalised by a too large signal). On high contrast backgrounds the normalisation signal for both the patterns should now have a greater value (as this high contrast background will be included for both targets) and hence both of the discs should 1 Note that as both the test and standard patterns moved across the background any additive effects that were present should be equal and would therefore not be detected using the present paradigm. We shall return to whether there are any additive effects in Experiment 3.
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appear reduced in apparent speed. The exact prediction about how the ratio of speeds of the test and standard should change depends upon the details of the implementation of the model — however given simple assumptions2 one might expect that the low contrast target to be even more slowed than the high contrast one — i.e. the Thompson effect should be exaggerated. Results do not seem to fit this idea — firstly, we found that the background did have a strong effect on the ratio of the perceived speeds but it was to decrease the Thompson effect not increase it; secondly previous studies have found that reference marks increase (rather than decrease) perceived speed (we shall demonstrate this in Experiment 3); and thirdly more direct tests, where the test and standard patterns are surrounded by very different contrasts, found no evidence to support the normalisation model (Thompson et al., 1996).
2.3.2. Visibility of target stimuli? When the contrast of a grating or disc is reduced it becomes harder to see. This apparently trivial statement has nevertheless being somewhat ignored in considerations of the Thompson effect. For it might be that the effect is one of reduced visibility rather than reduced contrast per se. For this to be true then other factors that reduce visibility should produce similar effects as reduced contrast. This appears to have some prima facie support. Other conditions that reduce visibility such as more peripheral viewing are also capable of reducing apparent speed (Czermak, 1857; Campbell & Maffei, 1981; Tynan & Sekuler, 1982; Johnston & 2
For example let us consider an example of two targets moving at speed S, one of contrast C1 and one of contrast C2, moving on a background of contrast Cback. Let us make some simple assumptions about the area of contrast pooling such that it is confined to the target area for the motion energy signal, but that for the normalisation process it is essentially infinite. The speed of pattern 1 will be: S C1/(C2 +C1 +Cback) and that of pattern 2 S C2/(C1 +C2 +Cback) and therefore the ratio will be simply C1/C2. Note that for a background that is constant this ratio is unchanged by background contrast. However this large pooling is unrealistic so instead the equations change so that the speed of the first pattern will now be S C1/(C1 +d1 · C2 +d2 · Cback) and the second pattern S C2/(d1 · C1 +C2 +d2 · Cback) where d1 and d2 B1.0 and represents the weighting factors that would depend upon the area of pooling and the spatial parameters of the experiment. The ratio of speeds is now always less than C1/C2 (i.e. this produces a smaller and more realistic Thompson effects than the infinite pooling model) but that must increase towards the limiting value C1/C2 as Cback \ \C1 or C2 (i.e. the Thompson effect increases at greater background contrasts).
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Wright, 1986). However one is then left with the issue of what exactly one means by ‘visibility’ and what aspects of the image (such as its presence, motion, direction, etc.) should govern this idea. Nevertheless we would like to note that ‘visibility’ nevertheless does change for our disc stimuli as we alter the contrast of the background. Consider a disc of 0% contrast — clearly this is invisible when viewed upon a blank background, but stands out as an unmodulated area when viewed upon a high contrast textured background. Hence it is now highly visible. Note that in many ways this stimulus resembles those termed ‘second-order’ patterns (see Smith, 1994) in that the stimulus is defined in this extreme case not by its difference in luminance from the background but by its difference in contrast (a low contrast target on a high contrast background). It has been noted that second order patterns can also show the Thompson effect (Ledgeway & Smith, 1995), and that it may even be greater than that of first order patterns (Gegenfurtner & Hawken, 1996). Hence as the contrast of the background rises this should serve to produce a greater second order motion signal which might alleviate the loss of motion in the first-order system, thus giving a possible explanation of the current results.
2.3.3. Reference marks allow us more 6eridical motion perception? As apparent speed appears to change systematically with contrast we are left with the question of ‘at which contrast do we get a veridical perception of speed?’ It is tempting (though perhaps false) to suggest that our perceptions at high contrast are correct and that those at low contrast are an ‘illusion’. For example Campbell and Maffei (1979, 1981) have shown that in extreme cases that a rotating disc may appear to stop altogether at low contrasts — clearly this is an illusory state. From this idea of low contrasts providing an illusion we can then suggest that reference marks allow us to be more veridical in our perception of motion by providing further information about motion, or by stimulating relative motion sensors that were not excited by absolute motion alone. We note the similarity of these ideas to those of ‘object-referencing’ discussed in the introduction (Wallach, 1959). Hence the background has its strongest effects where the illusion is at its strongest — they serve to speed up patterns of low contrast but have little effect at high contrast. This discussion then leads to two plausible explanations for the results of Experiment 1 — that the background provides increased visibility of the moving discs, or that the reference marks allow for a more veridical perception of speed. Experiment 2 sought to distinguish between these possibilities.
3. Experiment 2 In this experiment the landmarks were still present but they were not occluded by the moving disc. The dot background patterns were presented on the screen but were a small distance from the moving disc patterns; which travelled along zero contrast rectangular pathways from one side of the monitor to the other. Hence any ‘second order’ components that contributed in the first experiment were eliminated yet the disc still moved close by reference marks.
3.1. Method 3.1.1. Stimuli and procedure These were the same as Experiment 1 save that the discs did not move over the texture backgrounds but through strips between the texture patterns. The pathways were constructed by setting the contrast of the elements in the strips to 0% (i.e. mean luminance). The strips extended the length of the screen and were 1.5° wide centred on the position of the disc. Thus the top and bottom edges of the discs were 0.25° from the edge of the texture background. 3.1.2. Subjects Three observers took part in this study. LDT and GS had normal or corrected to normal vision and were naive to the purpose of the study. MRB had corrected to normal vision and was aware of the purpose of the study. 3.2. Results The data were analysed identically to Experiment 1. The slopes of the fitted functions are illustrated in Fig. 3. For a standard speed of 1.5°/s (left panel) we can see that all slopes were strongly negative indicating that the lower contrast patterns appeared to have slower speeds (the Thompson effect) at all background contrast. The data for a speed of 6.0°/s (right panel) show little sign of any systematic positive or negative slope (i.e. there is no Thompson effect) at any background contrast.
3.3. Discussion The ‘pathway’ matching experiment seems to suggest that the background must be occluded by the discs in order to influence the size of the Thompson effect. This suggests that a objects-reference theory may have trouble explaining the texture background findings. However it could be argued that the references may only interact with moving objects locally over a small scale and that this experiment used reference points that were beyond the scale of such a system. Norman et al. (1996) has demonstrated similar findings with annulus stimuli. These stimuli were composed of a central portion
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(whose speed was judged) and the surrounding annulus (containing the relative motion aspect of the stimulus). If a gap was produced between these two components and then progressively extended, the ‘distortion’ in the perceived speed of the central stimulus was reduced. Norman et al. deduced that the spatial area over which relative motion can influence is limited. Though we note that the gap between the disc and the texture in our experiment was very small (0.25°). The visibility explanation appears to have support from the experimental data. This model suggests that the low contrast discs are ‘corrected’ in regards to perceived speed because the texture pattern increases their visibility — perhaps via the ‘second order’ mechanism postulated earlier. If the background texture no longer defines the disc outline, visibility is unchanged and perceived speed is still influenced by disc contrast.
4. Experiment 3 In the previous experiments we have shown that movement over a textured background can eliminate the effects of stimulus contrast upon perceived speed, and that the moving pattern must actually move over the background rather than nearby it in order for the effect to occur. If we view the reduction in perceived speed as
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contrast is reduced as an ‘illusion’ or ‘error’ then we could regard the effect of the background as serving to eliminate this ‘error’ and produce a more veridical estimate of speed. Many previous studies (see Section 1) have shown that background patterns can increase the apparent speed of a moving target. This might arise either because the perceived speed without the textured background was in error (i.e. too slow), or because the texture simply produces a perceptual ‘speed-up’ irrespective of whether the speed without the texture was in error. In this experiment the background texture was split into two contrast sections, one of a high contrast and one of a low contrast. In each section was a disc that moved and the observers matched the two discs for perceived speed. We were interested to see now if discs of the same contrast would be matched veridically or if the differing backgrounds would produce changes in perceived speed. Additionally, this split background experiment provides a further test of the ‘contrast normalisation’ explanation of the Thompson effect (Stone & Thompson, 1992; Thompson et al., 1996).
4.1. Methods 4.1.1. Stimuli The same disc stimuli were used as in Experiments 1 and 2. This background pattern was composed of a Julesz
Fig. 3. This graph shows the results of Experiment 2. The data were once again derived as Fig. 1 and are plotted in an analogous manner to Fig. 2.
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Fig. 4. This graph shows the speed matching results of Experiment 3. The data for all three subjects have been plotted separately for when the pattern moved over the low contrast background (filled symbols) and when it moved over the high contrast background (open symbols). The test speed (relative to a standard of 1.5°/s) is plotted as a function of test contrast (relative to a standard of 50%). Linear regression was again used to fit the lines for each condition, although both the intercept and slopes were free to vary.
pattern which covered the whole screen, split horizontally into two different contrast sections; a 10% contrast section and a 90% contrast section. The position of these contrast sections, i.e. top or bottom half of the screen was randomised from one set of trials to another. Within a single session the standard disc at 50% contrast and a set speed of 1.5°/s, was matched for perceived speed to three different, randomly interleaved test pattern discs. These varied in their contrast, set at either 10, 50 or 90%. The vertical position of the standard and test discs was dependent upon the randomised position of the background contrast sections. In the study the standard to test speed match was carried out under two conditions. The first condition had the 50% standard moving over the 10% contrast dot background whilst being matched to the test discs moving over the 90% background. The second condition was a reversal of this, i.e. the 50% standard disc moving over the 90% background whilst being matched for speed to the test discs moving over the 10% background.
4.1.2. Procedure These were as for Experiments 1 and 2. 4.1.3. Subjects Three observers were used in this study. LDT had normal vision. MRB and GS had corrected to normal vision. All of the observers except MRB were naive as to the purpose of the study. 4.2. Results The data from this experiment are plotted in Fig. 4. The data have been plotted separately for when the test
pattern moved over the low contrast background (filled symbols) and when it moved over the high contrast background (open symbols). The data for each condition were fitted by linear regression — but we now allowed both the intercept and slope to vary as there is now no condition which is identical for both the test and standard pattern. When the test patterns are presented upon a low (10%) contrast background and matched to a 50% standard on the high (90%) contrast pattern matching speed is set to be greater than 0 dB — i.e. the pattern (test) moving across the low contrast background had to be made to move faster than that on the high contrast background in order to be a perceptual speed match. There appears to be little sign of the Thompson effect (what slope there is tends to be positive rather than negative) thus replicating the result of Experiment 1. When the opposite background conditions apply (i.e. the test is presented on the high contrast background and standard on the low contrast) then matching speed was set to be less than 0 dB — the test pattern was set to be slower than the standard pattern. Again there is no sign of the Thompson effect in these conditions.
4.3. Discussion The results of Experiment 3 using a split-background display indicate that the contrast of the background has important effects upon the perceived speed of the disc patterns. This effect of background contrast also appears to be independent of the disc contrast level. Thus it appears that background patterns can have two separate effects: (1) increasing perceived speed of all patterns (see also Brown, 1931; Gogel & McNulty, 1983; Snowden, 1997); and (2) eliminating contrast induced changes in perceived speed. One may argue that the backgrounds of different contrast may effect perceived size and/or depth (O’Shea, Blackburn & Ono, 1994) and therefore perceived speed. The findings of Kanecko and Uchikawa (1993) using moving stimuli of different graduated sizes, and therefore different perceived depths, suggested that depth has an influence on perceived speed. The results of Experiment 3 appear to reject that possible explanation in this case. If stimuli moving upon a low contrast background are perceived as further away then, for a given retinal rate, the notion of velocity constancy (though see McKee & Welch, 1989) would suggest they be perceived as faster. Our result show it is the target moving over the high contrast background that appears to move faster. The present results also have implications for explanations of the Thompson effect based on a contrast normalisation process (see Section 2.3). The theory predicts that patterns moving on a high contrast background should appear to move more slowly than those on a low contrast background. However our results show quite the opposite relationship.
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5. General discussion
References
We have found that the change in perceived speed as a function of contrast (the Thompson effect) can be eliminated when the target patterns move across a textured background. The Thompson effect returned if the target stimuli were provided with narrow untextured alleyways over which to move so that they no longer occluded/revealed areas of texture. Finally increasing background contrast was also shown to have an absolute effect on perceived speed such as those moving across a high contrast background appear faster than those on a low contrast background. In this study we used a disc moving over a textured background whilst most other studies of the Thompson effect have been limited to sinusoidal gratings (see Blakemore & Snowden, 1999). Such a disc contains many spatial frequencies and therefore changes in contrast will alter the effective spatial frequency contact (due to the modulation transfer function of the visual system). However we note that any such effect of contrast upon the effective spatial frequency spectrum should be present for all background conditions. The results from the first two experiments taken together seem to suggest that the crucial element in eliminating the Thompson effect is that the targets actually occlude/reveal areas of texture rather than move close by them. We suggest that this provides a ‘second-order’ signal as to the presence of the target which may provide by itself a motion signal (Chubb & Sperling, 1988) or allow the engagement of other processes such as attentive tracking (Cavanagh, 1992) that are less contrast dependent. Contrast is not the only variable that is capable of affecting perceived speed. Many other variables such as size (Brown, 1931), density (Watamaniuk, Grzywacz & Yuille, 1993), spatial frequency (Diener, Wist, Dichgans & Brandt, 1976; Smith & Edgar, 1990) and absence of luminance information (equiluminance — Cavanagh, Tyler & Favreau, 1984) all have strong effects on perceived speed. All these studies have so far only examined these variable with respect to movement over a blank background — or more typically within a confined window. It would be illuminating to see if these effects were also reduced or eliminated for stimuli moving through more complex environments as we have demonstrated here for changes in contrast.
Adelson, E. H., & Bergen, J. R. (1985). Spatiotemporal energy models for the perception of motion. Journal of the Optical Society of America A, 2, 284 – 299. Aubert, H. (1886). Die Bewegungsempfindung. Pflugers Archi6 fur die Gesamte Physiologie, 39, 347 – 370. Blakemore, M. R., & Snowden, R. J. (1999). The effect of contrast upon perceived speed: a general phenomenon? Perception, 28, 33 – 48. Bonnet, C. (1984). Discrimination of velocities and mechanisms of motion perception. Perception, 13, 275 – 282. Brown, J. F. (1931). The visual perception of velocity. Psychologische Forschung, 14, 199 – 232. Campbell, F. W., & Maffei, L. (1979). Stopped visual motion. Nature, 278, 192. Campbell, F. W., & Maffei, L. (1981). The influence of spatial frequency and contrast on the perception of moving patterns. Vision Research, 21, 713 – 721. Cavanagh, P. (1992). Attention-based motion perception. Science, 257, 1563 – 1565. Cavanagh, P., Tyler, C. W., & Favreau, O. E. (1984). Perceived velocity of moving chromatic patterns. Journal of the Optical Society of America A, 1, 893 – 899. Chubb, C., & Sperling, G. (1988). Drift-balanced random stimuli: a general basis for studying non-Fourier motion perception. Journal of the Optical Society of America A, 5, 1986 – 2006. Czermak, J. (1857). Ideen zu einer Lehre vom Zeitseinn. Gesellschaft Schriften, 1, 421. Diener, H. C., Wist, E. R., Dichgans, J., & Brandt, T. (1976). The spatial frequency effect on perceived velocity. Vision Research, 16, 169 – 176. Finney, D. J. (1971). Probit analysis. Cambridge: Cambridge University Press. Gegenfurtner, K. R., & Hawken, M. J. (1996). Perceived velocity of luminance, chromatic and non-Fourier stimuli: Influence of contrast and temporal frequency. Vision Research, 36, 1281–1290. Gogel, W. C., & McNulty, P. (1983). Perceived velocity as a function of reference mark density. Scandina6ian Journal of Psychology, 24, 257 – 265. Hawken, M. J., Gegenfurtner, K. R., & Tang, C. (1994). Contrast dependence of colour and luminance motion mechanisms in human vision. Nature, 367, 268 – 270. Howard, I. P., & Marton, C. (1992). Visual pursuit over textured backgrounds in different depth planes. Experimental Brain Research, 90, 625 – 629. Johnson, C. A., & Scobey, R. P. (1982). Effects of reference lines on displacement thresholds at various durations of movement. Vision Research, 22, 819 – 821. Johnston, A., & Wright, M. J. (1986). Matching velocity in central and peripheral vision. Vision Research, 26, 1099 – 1109. Kanecko, H., & Uchikawa, K. (1993). Apparent relative size and depth of moving objects. Perception, 22, 537 – 547. Keller, E. L., & Khan, N. S. (1986). Smooth-pursuit initiation in the presence of a textured background in monkey. Vision Research, 26, 943 – 955. Kimmig, H. G., Miles, F. A., & Schwarz, U. (1992). Effects of stationary textured backgrounds on the initiation of pursuit eye movements in monkeys. Journal of Neurophysiology, 68, 2147– 2164. Ledgeway, T., & Smith, A. T. (1995). The perceived speed of secondorder motion and its dependence on stimulus contrast. Vision Research, 35, 215 – 224. Legge, G. E., & Campbell, F. W. (1981). Displacement detection in human vision. Vision Research, 21, 205 – 214. Leibowitz, H. W. (1955). Effect of reference lines on the discrimination of movement. Journal of the Optical Society of America, 45, 829 – 830.
Acknowledgements MRB was supported by a SERC research studentship (946030978).
638
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Mates, B. (1969). Effects of reference marks and luminance on discrimination of movement. Journal of Psychology, 73, 209 – 221. McKee, S. P., & Smallman, H. S. (1998). Size and speed constancy. In V. Walsh, & J. J. Kulikowski, Perceptual constancies: why things look like they do (pp. 373–408). New York: Cambridge University Press. McKee, S. P., & Welch, L. (1989). Is there a constancy for velocity? Vision Research, 29, 553–561. McPeek, R. M., Verstraten, F. A. J., & Cavanagh, P. (1997). Target position, not target velocity, drives sustained smooth pursuit. In6estigati6e Ophthalmology and Visual Science Supplement, 38, S932. Niemann, T., Ilg, U. J., & Hoffmann, K. (1994). Eye movements elicited by transparent stimuli. Experimental Brain Research, 98, 314 – 322. Norman, H. F., Norman, J. F., Todd, J. T., & Lindsey, D. T. (1996). Spatial interactions in perceived speed. Perception, 25, 815 – 830. O’Shea, R. P., Blackburn, S. G., & Ono, H. (1994). Contrast as a depth cue. Vision Research, 34, 1595–1604. Peterken, C., Brown, B., & Bowman, K. (1991). Predicting the future position of a moving target. Perception, 20, 5–16. Raymond, J. E., Shapiro, K. L., & Rose, D. J. (1984). Optokinetic backgrounds affect perceived velocity during ocular tracking. Perception and Psychophysics, 36, 221–224. Reynolds, H. N. (1968). Temporal estimation in the perception of occluded motion. Perceptual and Motor Skills, 26, 407–416. Simoncelli, E. P., & Heeger, D. J. (1998). A model of neuronal responses in visual area MT. Vision Research, 38, 743–761. Smeets, J. B. J., & Brenner, E. (1995). Perception and action are based on the same visual information: distinction between position and velocity. Journal of Experimental Psychology: Human Perception and Performance, 21, 19–31. Smith, A. T. (1994). The detection of second-order motion. In A. T. Smith, & R. J. Snowden, Visual Detection of Motion (pp. 145 – 176). New York: Academic Press. Smith, A. T., & Edgar, G. K. (1990). The influence of spatial frequency on perceived temporal frequency and perceived speed. Vision Research, 30, 1467–1474.
.
Snowden, R. J. (1992). Sensitivity to relative and absolute motion. Perception, 21, 563 – 568. Snowden, R. J. (1997). Perceived speed: effects of field size and background texture. In6estigati6e Ophthalmology and Visual Science Supplement, 38, 1167. Snowden, R. J., Stimpson, N., & Ruddle, R. A. (1998). Speed perception fogs up as visibility drops. Nature, 392, 450. Stone, L. S., & Thompson, P. (1992). Human speed perception is contrast dependent. Vision Research, 32, 1535 – 1549. Taylor, M. M. (1962). Figural after-effects: a psychophysical theory of the displacement effect. Canadian Journal of Psychology, 16, 247 – 277. Thompson, P. (1976). Velocity after effects and the perception of mo6ement. Ph.D. thesis, University of Cambridge, UK. Thompson, P. (1982). Perceived rate of movement depends on contrast. Vision Research, 22, 377 – 380. Thompson, P., Stone, L. S., & Swash, S. (1996). Speed estimates from grating patches are not contrast-normalized. Vision Research, 36, 667 – 674. Tyler, C. W., & Torres, J. (1972). Frequency response characteristics for sinusoidal movement in the fovea and periphery. Peception and Psychophysics, 12, 232 – 236. Tynan, P., & Sekuler, R. (1975). Simultaneous motion contrast: Velocity, sensitivity and depth response. Vision Research, 15, 1231 – 1238. Tynan, P. D., & Sekuler, R. (1982). Motion processing in peripheral vision: reaction time and perceived velocity. Vision Research, 22, 61 – 68. Wallach, H. (1959). The perception of motion. Scientific American, 201, 2 – 6. Watamaniuk, S. N. J., Grzywacz, N. M., & Yuille, A. L. (1993). Dependence of speed and direction perception on cinematogram dot density. Vision Research, 33, 849 – 859. Watson, A. B., & Pelli, D. G. (1983). QUEST: a Bayesian adaptive psychometric method. Perception and Psychophysics, 33, 113– 120.