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Some new, or newish, illusions demonstrate that motion signals in the early parts of the visual system are profoundly altered by stimulus luminance and contrast. I shall show that contrast affects:
1. Motion strength in Time till breakdown. 2. Motion strength in Crossover motion. 3. Speed in The Footsteps illusion. 4. Direction in The Plaid-motion illusion. 5. Direction: Split dots:
I shall then consider how it is that higher perceptual processes massage these neural motion signals into the perception of moving objects. For instance, moving line terminators help to solve the aperture problem. But these solutions are modified by stimulus contrast in the Plaid-motion illusion and in the Peripheral-oblique illusion. In the Chopstick and Sliding Rings illusion, the motion of terminators propagates along straight lines and is blindly (and incorrectly) assigned to the motion of the central intersection. :
Finally, a new display of moving dots alternates perceptually between two radically different perceptual interpretations. Usually the Local percept (trees) is seen first, but the Global interpretation (forest) gradually takes over in the course of time:
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Anstis SM (in press) Footsteps and inchworms: illusions show that contrast modulates motion salience. Accepted by Perception.
A horizontal grey bar that drifts horizontally across a surround of black and white vertical stripes appears to stop and start as it crosses each stripe. A dark bar appears to slow down on a black stripe, where its edges have low contrast, and to accelerate on a white stripe, where its edges have high contrast. A light grey bar appears to slow down on a white stripe and to accelerate on a black stripe. If the background luminances at the leading and trailing edges of the moving bar are the same, the bar appears to change speed, and if they are different the bar appears to change in length. A plaid surround can induce 2-D illusions that modulate the apparent direction, not just the speed, of moving squares. Thus, the motion salience of a moving edge depends critically on its instantaneous contrast against the background. :
Orger MB, Smear MC, Anstis SM, Baier H. (2000) Perception of Fourier and non-Fourier motion by larval zebrafish.Nat Neurosci Nov;3(11):1128-33
A moving grating elicits innate optomotor behavior in zebrafish larvae; they swim in the direction of perceived motion. We took advantage of this behavior, using computer-animated displays, to determine what attributes of motion are extracted by the fish visual system. As in humans, first-order (luminance-defined or Fourier) signals dominated motion perception in fish; edges or other features had little or no effect when presented with these signals. Humans can see complex movements that lack first-order cues, an ability that is usually ascribed to higher-level processing in the visual cortex. Here we show that second-order (non-Fourier) motion displays induced optomotor behavior in zebrafish larvae, which do not have a cortex. We suggest that second-order motion is extracted early in the lower vertebrate visual pathway. :
Anstis SM. (2000) Monocular lustre from flicker. Vision Res;40(19):2551-6
A spot that flickers at 16 Hz between two luminance levels (on a grey surround) has an appearance of metallic lustre, which we call 'monocular lustre'. Binocular and monocular lustre were measured in comparable conditions by a rating procedure, and both were reported only when the light and dark values of the flickering (or binocularly fused) spot straddled the surround luminance, so that the spot was alternately brighter and darker than the surround. We attribute lustre to competition between ON and OFF visual pathways. :
Becker MW, Pashler H, Anstis SM (2000) .The role of iconic memory in change-detection tasks. Perception;29(3):273-86
In three experiments, subjects attempted to detect the change of a single item in a visually presented array of items. Subjects' ability to detect a change was greatly reduced if a blank interstimulus interval (ISI) was inserted between the original array and an array in which one item had changed ('change blindness'). However, change detection improved when the location of the change was cued during the blank ISI. This suggests that people represent more information of a scene than change blindness might suggest. We test two possible hypotheses why, in the absence of a cue, this representation fails to produce good change detection. The first claims that the intervening events employed to create change blindness result in multiple neural transients which co-occur with the to-be-detected change. Poor detection rates occur because a serial search of all the transient locations is required to detect the change, during which time the representation of the original scene fades. The second claims that the occurrence of the second frame overwrites the representation of the first frame, unless that information is insulated against overwriting by attention. The results support the second hypothesis. We conclude that people may have a fairly rich visual representation of a scene while the scene is present, but fail to detect changes because they lack the ability to simultaneously represent two complete visual representations. :
Anstis SM, Smith DR, Mather G.Luminance processing in apparent motion, Vernier offset and stereoscopic depth. Vision Res 2000;40(6):657-75
We obtained (apparently) linear responses to luminance from three special displays of apparent motion, Vernier offset and stereoscopic depth. In our motion stimulus a dark and a light bar exchanged luminances repetitively on a grey surround. Motion was attributed to the bar that differed more from the surround, that is, on a dark surround the light bar appeared to jump, and on a light surround the dark bar appeared to jump. The apparent motion disappeared when the luminance of the surround lay halfway between that of the bars--on a linear, not a logarithmic scale. Similar results were obtained for special Vernier offset and stereo stimuli. These results cannot be explained if all luminances are processed within the same luminance pathway and that pathway transforms input luminance using non-linear compression. However, the apparent linearity of our results could arise from opposite and equal non-linearities cancelling out within separate ON- and OFF-spatial luminance pathways. A second set of experiments presented one bar separately into each eye on different surrounds (dichoptic presentation of competing apparent motion signals) or manipulated the display spatially so that different surrounds were associated with different bars (binocular presentation of competing Vernier targets). Results showed that apparent motion and Vernier signals of equal Weber contrast (normalisation of linear difference to surround luminance) evoked equal-motion and equal Vernier offset strengths. Given that motion and Vernier strength followed Weber's law, we infer that the ON- and OFF-pathways transform luminance non-linearly. Our third experiment presents an example of a brightness bisection task in which we were able to influence the bisection steps, to follow either a linear or non-linear series. The benefits of parsing the visual scene so that visual information is processed within two opposite luminance pathways is discussed. :
Anstis S, Kontsevich L, Tyler C. (1999) Demonstrating the temporal modulation transfer function. Perception;28(5):623-6
A 120-frame movie, which can be downloaded from specified web sites, allows an observer to see the qualitative form of his or her temporal modulation transfer function. Results collected from two of the authors are presented: .
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Anstis S, Sturzel F, Spillmann L. (1999) Spatial distortions in rotating radial figures. Vision Res Apr;39(8):1455-63
A white sector on a black rotating disk appears spatially compressed. We found that apparent shrinkage: (1) for sectors ranging from 15 to 150 degrees and rotating at 1.25 rps varied in an inverted U-shaped manner from 3 to 16 degrees and back to 11 degrees (corresponding to 20, 16, and 7.5%, respectively); (2) increased with speed of rotation producing maximal compressions of between 7 and 30 degrees for velocities ranging from 0.8 to 2 rps; and (3) affected the leading and the trailing portions of the rotating sector equally, while allowing for apparent expansion of the middle region. Consistent with these findings we found that (4) two black lines 20 mm apart across the center of the rotating disk and extending outward towards the edge appeared to converge when they were actually parallel and were seen as parallel when their end points were physically diverged by 6 degrees. Our findings suggest a foreshortening process which ensures that the shapes of moving stimuli are perceived approximately correctly, irrespective of whether they are actually sharp or blurred. :
Anstis SM (1998) Picturing peripheral acuity. Perception
The grain of the retina becomes progressively coarser from the fovea to the periphery. This is caused by the decreasing number of retinal receptive fields and decreasing amount of cortex devoted to each degree of visual field (= cortical magnification factor) as one goes into the periphery. We simulate this with a picture that is progressively blurred towards its edges; when strictly fixated at its centre looks equally sharp all over. :
Anstis S, Ho A. (1998) Nonlinear combination of luminance excursions during flicker, simultaneous contrast, afterimages and binocular fusion. Vision Res Feb;38(4):523-39
The changes in apparent brightness or color, induced into a test spot by a surround, can be greatly enhanced either by flickering the test spot between two luminances, or by binocularly fusing a pair of test spots of different luminances. Simultaneous contrast, in which a white surround makes a grey spot look darker, is greatly enhanced if the spot (not the surround) flickers between black and white. Colour contrast is likewise enhanced by chromatic flicker: on a blue surround, a grey spot looks slightly yellowish, but a yellow/blue flickering spot looks strongly yellow. Temporal successive contrasts, or negative afterimages, are also enhanced by flickering the test field. The negative afterimage of a half-white, half-black rectangle looked dark grey and light grey when projected on a grey test field, but it looked almost black and almost white when projected on a test field that flickered between black and white. Coloured negative afterimages were also enhanced by projecting them on a chromatic flickering test field. We examined the combination rules for pairs of luminances which were presented either successively as flicker or else dichoptically (and fused binocularly). The brightness averaging functions for spatial increments (light spots) on dark surrounds were quasi-linear for binocular fusion but quadratic for flicker. For spatial decrements (dark spots) on white surrounds, the brightness averaging functions were strongly nonlinear winner-take-all for both binocular fusion and flicker. We also found temporal analogues of Fechner's [(1860). Elements of psychophysics. New York: Holt, Rinehart, Winston, 1966] paradox and Levelt's [(1965). British Journal of Psychology, 56, 1-13] dichoptic contour effect. We conclude that the visual rules for combining luminance excursions, whether in flicker or binocular fusion, favour disproportionately the spot with the higher contrast. :
Anstis S, Hutahajan P, Cavanagh P (1998) Optomotor test for wavelength sensitivity in guppyfish (Poecilia reticulata) Vision Res Jan;38(1):45-53.
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Wavelength sensitivity was measured in the guppyfish by means of optomotor responses to a special apparent-motion display. A set of red and green bars appeared to humans to move to the left if red was darker than green, but to the right if red was lighter than green. At equiluminance there was no apparent motion. By noting the direction in which the fish swam to follow the stripes we were able to record equiluminance points for red, green and blue. Store-bought guppies (Poecilia reticulata) were mildly protan compared with humans, and wild-strain guppies were strongly protan, being 50% more sensitive to short wavelengths and 67% more sensitive to medium wavelengths than human observers. We also measured optomotor responses to achromatic Michelson contrast: responses were maximum if the contrast exceeded 0.3. Finally, the optomotor threshold (signal/noise ratio) for motion coherence was 20% for fine dots and 40% for coarse dots. These stimuli should be easy to use on any non-verbal species. :
Spillmann L, Anstis S, Kurtenbach A, Howard I (1997).Reversed visual motion and self-sustaining eye oscillations. Perception;26(7):823-30
A random-dot field undergoing counterphase flicker paradoxically appears to move in the same direction as head and eye movements, i.e. opposite to the optic-flow field. The effect is robust and occurs over a wide range of flicker rates and pixel sizes. The phenomenon can be explained by reversed phi motion caused by apparent pixel movement between successive retinal images. The reversed motion provides a positive feedback control of the display, whereas under normal conditions retinal signals provide a negative feedback. This altered polarity invokes self-sustaining eye movements akin to involuntary optokinetic nystagmus. :
Anstis S. (1996) Adaptation to peripheral flicker. Vision Res Nov;36(21):3479-85
With strict fixation, a flickering disk presented in the peripheral retina rapidly appeared to lose contrast and stop flickering, owing to adaptation. Subjects measured this adaptation by continually adjusting the flicker amplitude of a peripherally viewed disk to hold it just at threshold. Results: (1) The contrast threshold for flicker increased logarithmically over time. (2) The slope of the temporal decay function increased with eccentricity (1-16 deg) and with decreasing disk size (8 deg-3.6 min arc). (3) M-scaling the stimulus size could abolish the dependence upon eccentricity for small disks, but not completely for large disks. (4) The temporal decay rate increased with flicker rate (3-15 Hz), as though each cycle of flicker elevated contrast threshold equally. :
Mather G, Anstis S. (1995) Second-order texture contrast resolves ambiguous apparent motion. Perception;24(12):1373-82
When a black and a white square on a grey surround exchange places, it was previously shown that on a dark surround it is the white square, and on a light surround it is the black square, that is seen in apparent motion (AM). Thus the higher-contrast square carries the AM. We now show that the same is true for second-order AM of texture-defined squares. Squares were defined by four different textures: by anisotropy (horizontal versus vertical random dashes), by alpha numeric letters, by hash marks, or by dot size. The result was that the square that differed more from the surround in texture properties carried the second-order AM. Judgments of texture salience revealed a high correlation between salience and apparent motion. In a third experiment, crossover AM between dissimilar textures was investigated, and it was found that the more salient textures carried the AM. Results cannot be explained by the concept of "texture activity', but instead indicate that the system extracts a measure of "texture contrast' prior to analysis of salience and apparent motion. :
Anstis S. (1995) Aftereffects from jogging. Exp Brain Res;103(3):476-8
After running on a treadmill, runners who attempted to jog in place on solid ground inadvertently jogged forwards. One-legged hopping on the treadmill produced an aftereffect in the same leg, but not in the other leg. This non-transfer suggests a peripheral neural site. Judgments of velocity and slope were affected; running on a backward-moving treadmill made a stationary test treadmill seem to move forwards, and running on an uphill-sloping treadmill made a horizontal test treadmill seem to slope downhill. These aftereffects suggest an automatic gain control process. :
Arnold K, Anstis S. (1993) Properties of the visual channels that underlie adaptation to gradual change of luminance. Vision Res Jan;33(1):47-54
Following adaptation to a spatially uniform patch of light that is gradually brightening (or dimming), a steady test patch appears to be gradually dimming (or brightening). We measured this ramp aftereffect with a nulling method, as a function of the amplitude and temporal repetition rate of the adapting sawtooth waveform and at various retinal eccentricities and levels of dark adaptation. We conclude that the underlying visual channels respond best to large-amplitude sweeps in luminance of at least 20 dB (1 log unit); but they are fairly insensitive to the temporal rate of this sweep. The channels are present out to an eccentricity of at least 40 degrees but they almost disappear during dark adaptation. The ramp aftereffects were asymmetrical: the subjectively darkening aftereffect produced by a brightening adapting ramp was slightly stronger than vice versa. :
Reed MJ, Steinbach MJ, Anstis SM, Gallie B, Smith D, Kraft S. (1991) The development of optokinetic nystagmus in strabismic and monocularly enucleated subjects. Behav Brain Res Dec 13;46(1):31-42
Asymmetries of monocular optokinetic nystagmus (OKN) following anomalous visual experience are thought to be due to disruption at the cortical level. Visual disruption usually results from eye suture (in animals), unilateral dense and central cataracts or strabismus (in humans). Many form-deprived animals and humans frequently show a residual strabismus after lid opening (animals) or cataract extraction and optical correction (humans). We wanted to determine whether strabismus was unique in causing monocular asymmetries of OKN. Two independent observers rated eye movement videotapes of 20 normal subjects, the non-deviating eye of 25 unilateral strabismic subjects and 29 unilaterally eye-enucleated subjects, who were watching either a nasally directed square wave grating, a temporally directed square wave grating, or a blank field. Observers rated the proportion of trials where OKN occurred, the duration of OKN in each trial and the number of beats of OKN within each trial. Monocular OKN was symmetrical in normal subjects for the proportion and duration measures, but half the normal group showed small but significant asymmetries for the beats measure. Subjects in both enucleate and strabismic groups showed asymmetries of OKN favouring nasally directed stimulation, but only the early onset strabismics (as a group) showed asymmetries that were significantly greater (P less than 0.05) than the normal group. Asymmetry scores correlated significantly with age of diagnosis of strabismus for the strabismic group but not with age of enucleation for the enucleate group. The results are discussed in terms of binocular competition. :
Cavanagh P, Anstis S. (1991) The contribution of color to motion in normal and color-deficient observers. Vision Res;31(12):2109-48
By opposing drifting luminance and color gratings, we have measured the "equivalent luminance contrast" of color, the contribution that color makes to motion. We found that this equivalent contrast was highest (greater than 10%) for low spatial and temporal frequencies and was higher for red/green than for blue/yellow stimuli. Equivalent luminance contrast was about 4% for a green/purple stimulus that fell along the tritan confusion line, indicating a modest input to the motion pathway from the short wavelength-sensitive cones (B-cones). Contrast thresholds for the discrimination of the direction of motion showed that the contribution of color to motion was about the same (within a factor of 2) as that for luminance in terms of multiples of threshold contrast. These responses to moving, chromatic gratings could be mediated by any of several factors that can create a residual response in a luminance pathway: temporal phase lag between the responses to the colors of the stimuli, second harmonic distortion in the response and variability in equiluminance points across units. Each of these factors was evaluated experimentally and their combined effect could account for only a small portion of the contribution of color to motion. As a result, we attribute the perception of the motion of equiluminous stimuli to an opponent-color input to directionally selective cortical units. Chromatic stimuli had little or no equivalent contrast for color-deficient observers, whether the stimulus was red/green, which they discriminate less well than normals, or blue/yellow, which they discriminate almost as well as normals. The equivalent contrast measure provided an excellent basis for classifying normal, protan and deutan observers. :
Mather G, O'Halloran A, Anstis S. (1991) The spacing illusion: a spatial aperture problem? Perception;20(3):387-92
A geometrical illusion in which the horizontal spacing between adjacent parallel lines in a row is underestimated when the lines are tilted away from vertical in a chevron configuration was investigated in two experiments. The perceived spacing was found to decrease as the tilt angle increased, consistent with the idea that separation judgements are influenced by the normal spacing between lines ie at right angles to the line orientation. It is proposed that this illusion reveals an analogue in spatial perception to the well-known aperture problem in motion perception. In establishing the separation of nearby or overlapping shapes in an image, the visual system cannot only rely upon the normal separation of contours belonging to each shape (as would be visible through small spatial apertures or receptive fields), since this varies with contour orientation. The system is therefore faced with a spatial aperture problem. The spacing illusion may arise because information usually available to solve the problem is absent in the illusion figure, or it may reflect a bias in favour of the orthogonal, which is adopted in the face of the ambiguity. :
Anstis S. (1990) Motion aftereffects from a motionless stimulus. Perception;19(3):301-6
Dimming or brightening regions superimposed, slightly out of register, on static light or dark blobs, give rise to apparent motion. When these regions are replaced by apparent brightening or dimming produced by ramp aftereffects, a directional motion aftereffect is perceived. It is concluded that filters sensitive to temporal derivative signals of net brightening or dimming Back
provide an input into the motion pathways. :
Ramachandran VS, Anstis SM (1990).Illusory displacement of equiluminous kinetic edges. Perception;19(5):611-6
A stationary window was cut out of a stationary random-dot pattern. When a field of dots was moved continuously behind the window (a) the window appeared to move in the same direction even though it was stationary, (b) the position of the 'kinetic edges' defining the window was also displaced along the direction of dot motion, and (c) the edges of the window tended to fade on steady fixation even though the dots were still clearly visible. The illusory displacement was enhanced considerably if the kinetic edge was equiluminous and if the 'window' region was seen as 'figure' rather than 'ground'. Since the extraction of kinetic edges probably involves the use of direction-selective cells, the illusion may provide insights into how the visual system uses the output of these cells to localize the kinetic edges. :
Maurer D, Lewis TL, Cavanagh P, Anstis S (1989).A new test of luminous efficiency for babies. Invest Ophthalmol Vis Sci Feb;30(2):297-303
We used the minimum motion method devised by Anstis and Cavanagh (1983) to measure the luminous efficiency of red and green and of yellow and blue for "normal" 1- 3-month-old babies and for one 3-month-old boy destined to be color-deficient because of a deutan mother. Subjects watched a display which created apparent motion, the direction of which depended on the relative luminance of the colors. To determine the equiluminant points, we observed the optokinetic nystagmus elicited by the display as the relative luminance of the colors was varied. The equiluminant points of the normal mothers and their infants were similar to each other but different from those of the deutan mother and her son. Our new method demonstrates the early maturation of input from red and green cones into achromatic pathways. It can also be used to identify some color-deficient infants. :
Giaschi D, Anstis S. (1989) The less you see it, the faster it moves: shortening the "on-time" speeds up apparent motion Vision Res;29(3):335-47.
The apparent motion (AM) created by two spots illuminated in alteration looks faster when there is dark temporal interval (ISI) between the offset of one spot and the onset of the other than when the spots are presented immediately after one another (no ISI), even though the temporal frequency and the spatial separation between spots are held constant. AMISI looks 18.6% faster than AMnoISI at temporal frequencies between 1.5 and 4.5 Hz. Reducing the duty cycle from 0.5 to 0.05 increases the apparent speedup to 30%. This difference in subjective speed is not due to differential saturation of velocity detectors, nor to the apparent spatial separation between spots, nor to differences in the time-averaged luminance of the stimuli. It is the "on-time", the time for which the spot is visible in one position, that determines the subjective speed. The longer the on-time, the slower the spot appears to move. :
Anstis S, Harris J. (1987) Magnification factor for adaptation of a visual transient mechanism. : J Opt Soc Am A Aug;4(8):1688-98
After adaptation by an observer to a patch of gradually increasing (or decreasing) luminance, a steady test patch appeared to be gradually dimming (or brightening). These aftereffects did not transfer interocularly. Adaptation to a checkerboard, in which the white squares gradually dimmed while the black squares gradually brightened, gave an aftereffect that was a pattern of intersecting diagonal lines, that is, an extremely blurred checkerboard. The larger the squares of the checkerboard were, the farther into the periphery the aftereffect extended, because small squares were blurred out by the summation areas of the underlying visual channels, which were larger at increasing eccentricities and had diameters of 20 times the resolvable dot separation. The estimated visual acuity of these channels was as low as 20/400. These estimates were confirmed by manipulating separately the local and space-averaged luminances of the adapting stimulus. :
Cavanagh P, MacLeod DI, Anstis SM. (1987) Equiluminance: spatial and temporal factors and the contribution of blue-sensitive cones. J Opt Soc Am A Aug;4(8):1428-38
Equiluminance ratios for red/green, red/blue and green/blue sine-wave gratings were determined by using a minimum-motion heterochromatic matching technique that permitted reliable settings at temporal frequencies as low as 0.5 Hz. The red/green equiluminance ratio was influenced by temporal but not spatial frequency, the green/blue ratio was influenced by spatial but not temporal frequency, and the red/blue ratio was influenced by both. After bleaching of the blue-sensitive cones, there was no change in equiluminance ratios, indicating no contribution of the blue-sensitive cones to the luminance channel even at low temporal and spatial frequencies. The inhomogeneity of yellow pigmentation within the macular region was identified as the source of the spatial-frequency effect on the blue/green ratio. :
Anstis S, Ramachandran VS. (1987) Visual inertia in apparent motion. Vision Res;27(5):755-64
Four dots in an imaginary diamond were flashed in succession to give ambiguous apparent motion (AM). The top and bottom dots were flashed at time t1 and replaced by the left and right dots at time t2. If two priming dots were flashed beforehand at time t0 in line with two parallel sides of the diamond, AM was seen along those sides. We call this "visual inertia". The amount of visual inertia (measured by a null method) fell off with increasing angle between the priming dot and the side of the diamond. Visual inertia was still seen when the priming dots were presented to one eye and the test dots to the other, so the effect must be partly central. The timing and length of the priming path made little difference to visual inertia. However, static priming dots were ineffective. We conclude that the visual system was examining at least three successive time frames in deciding which items in one frame correspond with which items in succeeding frames. :
Ramachandran VS, Anstis SM. (1986) The perception of apparent motion. Sci Am Jun;254(6):102-9
Cavanagh P, Anstis SM. (1986) Brightness shift in drifting ramp gratings isolates a transient mechanism. Vision Res;26(6):899-908
A brightness shift is demonstrated for moving ramp stimuli. The data suggest that the transient visual response to changing luminance is combined with the sustained response to produce an overall impression of brightness. We attribute the brightness shift to the saturation of the transient response: a limitation on the maximum transient when responding to rapid brightening or dimming. The brightness shift provides a new technique for isolating the responses of the transient mechanisms. :
Anstis S. (1986) Recovering motion information from luminance. Vision Res;26(1):147-59
We review evidence that visual transient channels responding to temporal change of luminance provide inputs to motion mechanisms, and also play a part in judgments of static brightness. These channels can be adapted to give after effects of apparent dimming or brightening. Nonlinearity in these channels causes a sawtooth grating to look dark (or light) while it is moving to the left (or right). The perceptual outcome in a competitive motion situation is governed by the larger temporal change in luminance: when a white bar and a black bar suddenly change places, on a dark (light) surround it is the white (black) bar that appears to move. The motion system responds to linear, not log luminance. If a black and white picture dissolves (fades) to its own photographic negative which if shifted a few min arc to the right "reversed apparent motion" is seen toward the left. These results constrain possible models of motion perception. :
Ramachandran VS, Anstis S (1986) Figure-ground segregation modulates apparent motion. Vision Res;26(12):1969-75
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We explored the relationship between figure-ground segmentation and apparent motion. Results suggest that: static elements in the surround can eliminate apparent motion of a cluster of dots in the centre, but only if the cluster and surround have similar "grain" or texture; outlines that define occluding surfaces are taken into account by the motion mechanism; the brain uses a hierarchy of precedence rules in attributing motion to different segments of the visual scene. Being designated as "figure" confers a high rank in this scheme of priorities. :
Anstis S, Ramachandran VS. (1986) Entrained path deflection in apparent motion. Vision Res;26(10):1731-9
A dot jumping back and forth between two positions would normally appear to jump along a straight line. But when surrounded by dots which jumped through three positions arranged in a V, it also appeared to jump along a V-shaped trajectory. :
Anstis SM, Rogers BJ. (1986) Illusory continuous motion from oscillating positive-negative patterns: implications for motion perception. Perception;15(5):627-40
A black and white (positive) grating pattern was superimposed in exact register on its own photographic negative. Four operations were repetitively applied to this positive pattern so that it moved fractionally to the right, grew dimmer, moved back to the left, and grew brighter again. This sequence produced a strong illusion of continuous apparent motion to the right for as long as the cycle was repeated. The small relative motion between the two patterns generated two new illusory effects: enhanced real movement (ERM) and reversed real movement (RRM). The dimming and brightening phases gave rise to reversed apparent movement (RAM). All three effects are attributed to spatial filtering by neural mechanisms, which shifts the effective position of the positive-negative contours. :
Anstis S, Giaschi D, Cogan AI. (1985) Adaptation to apparent motion. Vision Res;25(8):1051-62
A spot alternating between two positions can produce apparent motion (AM). Following prolonged inspection, the AM degenerates into flicker. This adaptation effect was found to depend on spacing and timing; the probability of seeing motion during a 30-sec inspection period declined linearly with log spatial separation (over a range from 0.1 to 1 deg), and with log alternation rate (over a range from 2 to 4.5 Hz). Cross-adaptation, in which subjects were adapted to one alternation rate and tested at another, showed that low alternation rates gave stronger motion signals than high rates did. Adaptation to real motion (RM) strongly suppressed AM, which suggests that AM must be stimulating the same neural pathways as RM. Flickering spots (i.e. in-phase flicker) produced less adaptation than did a spot alternating between two positions (i.e. counterphase flicker), so the adapting mechanism must be responding to relative temporal phase. Embedding the adapting spots in configurations of other spots, which altered the pattern of perceived adapting motion without altering the local retinal stimulation, minimized the adaption, so the adapting mechanism must be responding to the path of seen motion. Adaptation can be used to measure the strength of AM and shows that AM is strongest for small separations, low alternation rates and high luminance contrast. :
Anstis SM, Mather G. (1985) Effects of luminance and contrast on direction of ambiguous apparent motion. Perception;14(2):167-79
A study is reported of the role of luminance and contrast in resolving ambiguous apparent motion (AM). Different results were obtained for the short-range (SR) and the long-range (LR) motion-detecting processes. For short-range jumps (7.5 min arc), the direction of ambiguous AM depended on brightness polarity, with AM only from white to white and from black to black. But for larger jumps, or when an interstimulus interval (ISI) was introduced, AM was less dependent on polarity, with white often jumping to black and black jumping to white. Two potential AMs were pitted against each other, one carried by a light stimulus and the other by a dark stimulus. The stimulus whose luminance differed most from the uniform surround captured the AM. Visual response to luminance was linear, not logarithmic. When the stimulus was modified to give continuous AM in one direction it was followed by a negative aftereffect of motion only when the spatial displacement was 1 min arc. A larger displacement (10 min arc) gave good AM but no motion aftereffect. Thus only short-range motion adapts motion-sensitive channels. :
Mather G, Cavanagh P, Anstis SM (1985).A moving display which opposes short-range and long-range signals. Perception;14(2):163-6
A novel display is described which stimulates both the long-range and the short-range motion detecting processes simultaneously, but with opposing directions of movement. The direction in which the stimulus appears to move depends on retinal eccentricity and element size, but adaptation to the display always produces a motion aftereffect (MAE) direction opposite to the direction of the short-range component. The display may offer insights into the properties of the two-process motion detecting system. :
Anstis SM, Ramachandran VS (1985).Kinetic occlusion by apparent movement. Perception;14(2):145-9
A small square and a large triangle below it were presented in the first frame. These were switched off and replaced by a triangle alone in the second frame, shifted horizontally and upwards. The triangle appeared to move obliquely, as expected, but most observers also saw the square moving horizontally and hiding behind the triangle, although there was no stimulus corresponding to it in the second frame. The visual system invokes the occlusion 'hypothesis' in order to explain the otherwise mysterious disappearance of the square. The experiment suggests that apparently intelligent solutions can be rapidly computed by the visual system. :
Ramachandran VS, Anstis SM. (1985) Perceptual organization in multistable apparent motion. Perception;14(2):135-43
Is motion perception based on a local piecemeal analysis of the image or do 'global' effects also play an important role? Use was made of bistable apparent-motion displays in trying to answer this question. Two spots were flashed simultaneously on diagonally opposite corners of a 1 deg wide square and then switched off and replaced by two spots appearing on the other two corners. One can either see vertical or horizontal oscillation and the display is bistable just as a Necker cube is. If several such bistable figures are randomly scattered on the screen and presented simultaneously, then one usually sees the same motion axis in all of them, suggesting the presence of field-like effects for resolving ambiguity in apparent motion. While viewing a single figure observers experience hysteresis: they tend to adhere to one motion axis or the other and can switch the axis only by looking away and looking back after 10-30 s have elapsed. The figure can be switched off and made to reappear at some other random location on the screen and it is then always found to retain its motion axis. Several such demonstrations are presented to show that spatial induction effects in metastable motion displays may provide a particularly valuable probe for studying 'laws' of perceptual organization. :
Cavanagh P, Anstis S, Mather G. (1984) Screening for color blindness using optokinetic nystagmus. Invest Ophthalmol Vis Sci Apr;25(4):463-6
Red/green luminosity ratios were determined by a new method. A special colored grating of red and green bars appeared to move to the left (or right) when the red bars were darker (or lighter) than the green bars. Optokinetic nystagmic eye movements (OKN) elicited by this stimulus could be measured photoelectrically, or by directly watching the subject's eyes. When the red/green ratio passed through equiluminance, the direction of apparent movement (AM) and of OKN reversed in direction. Protans needed more red than normals to reach equiluminance, and deutans needed more green. This OKN method might be applied to nonverbal subjects such as infants and animals. :
Ramachandran VS, Anstis SM (1983).Perceptual organization in moving patterns. Nature Aug 11-17;304(5926):529-31
Is human motion perception based on a local piecemeal analysis of the image or do global effects play an important role? We used metastable apparent motion displays (Fig. 2) to try to answer this question. Two spots were flashed simultaneously on diagonally opposite corners of a square and then switched off and replaced by two spots appearing on the remaining corners. One could either see vertical or horizontal oscillation of the spots and the display was bistable just as a Necker cube is. We found that if several such bistable figures were randomly scattered on the screen (Fig. 3b), and presented simultaneously, then one always saw the same motion-axis in all of them, suggesting the presence of global field-like effects for resolving ambiguity in apparent motion. Surprisingly, the appearance of these displays could not be influenced by voluntary effort unless the speed of alternation was very slow. (Less than 3 frames per second.) It may be that if the events in the module that computes apparent motion are too rapid then it cannot be coupled with the "will" mechanism, which may have a long time constant. :
Anstis SM, Tassinary L. (1983) Pouting and smiling distort the tactile perception of facial stimuli. : Percept Psychophys Mar;33(3):295-7
Anstis, S. M. and Ramachandran, V. S. (1995) At the edge of movement. In: The artful eye. Gregory, R. L., Harris, J., Heard, P. and Rose, D. (Eds). Oxford University Press, pp. 232-248. .