Stuart Anstis

PHI MOVEMENT AS A SUBTRACTION PROCESS

Abstract How similar must two successively presented patterns be for phi movement to be perceived between them ? Phi movement between two granular patterns, one being the photographic negative of the other, appeared to be reversed , towards the direction of the earlier stimulus. Moving objects, displayed on a TV picture which was made positive and negative on alternate frames, appeared to move backwards. (The backward movement could generate its own after effect of movement.) Conclusion: phi movement was perceived between nearby points of similar brightness, irrespective of form or colour. Phi movement was studied between two positive random-dot Julesz patterns. Pairs that gave stereo when presented dichoptically also gave phi movement when presented alternatively to one eye. When one pattern was degraded with noise, both stereo and phi broke down at the same noise level. Conclusion: phi, like stereo, depended upon point-by-point comparison of brightness between two patterns. It could precede the perception of form.

The motion aftereffect

The motion aftereffect is a powerful illusion of motion in the visual image caused by prior exposure to motion in the opposite direction. For example, when one looks at the rocks beside a waterfall they may appear to drift upwards after one has viewed the flowing water for a short period-perhaps 60 seconds. The illusion almost certainly originates in the visual cortex, and arises from selective adaptation in cells tuned to respond to movement direction. Cells responding to the movement of the water suffer a reduction in responsiveness, so that during competitive interactions between detector outputs, false motion signals arise. The result is the appearance of motion in the opposite direction when one later gazes at the rocks. The adaptation is not confined to just one population of cells, but probably occurs at several cortical sites, reflecting the multiple levels of processing involved in visual motion analysis. The effect is unlikely to be caused by neural fatigue; more likely, the MAE and similar adaptation effects provide a form of error-correction or coding optimization, or both.

Equiluminance: spatial and temporal factors and the contribution of blue-sensitive cones

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.

Illusory Displacement of Equiluminous Kinetic Edges

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.

Illusory reversal of visual depth and movement during changes of contrast

Abstract The visual system usually sees phi apparent movement when two similar pictures are exposed successively, and stereoscopic depth when the pictures are exposed one to each eye. But when a picture was followed via a dissolve by its own photographic negative, overlapping but displaced, strong apparent movement was seen in the opposite direction to the image displacement (“reversed phi”). When both eyes saw a positive picture, and one eye also saw an overlapping low-contrast negative containing binocular disparity, “reversed stereo” was seen, with the apparent depth opposite to the physical disparity. Results were explained with a model of spatial summation by visual receptive fields.

Movement aftereffects contingent on color, intensity, and pattern

We have found contingent movement aftereffects (CMAEs) lasting several days, contingent upon the color, intensity, and stripe width of moving patterns. Ss adapted for 10 min to a patterned disk rotating clockwise under red light, alternating every 10 sec with counterclockwise under green light. When stopped, the disk then appeared to rotate counterclockwise under red light and clockwise under green light. The effect lasted only a second or two, reappearing each time the field’s color was changed. But it increased in strength over the first 1/2 hand was still present 1 or 2 days later. Color transposition effects were found: after adaptation to red-clockwise (long wavelength) alternating with green-counterclockwise (short wavelength), a stationary yellow (long wavelength) test field appeared to rotate counterclockwise and a blue (short wavelength) field appeared to rotate clockwise. Relative, not absolute, color of the test triggered the CMAE. Similar CMAEs and transposition effects were produced by pairing direction of movement with intensity, with width of moving stripes and with orientation of a stationary grating projected onto a rotating patterned disk.

Extrapolation of motion path in human visual perception

According to Newton’s First Law of Motion. a physical object moving at uniform velocity in one direction will persevere in its state of uniform motion unless acted upon by an external force to change that state (Newton. 1687). Smce the visual system has evolved to process information from the physical world. one might expect to find a similar principle of “inertia” in the visual perception of moving objects. Using dot displays (Fig. 11 we have found that any object which moves in one du-ection at uniform velocity will tend to be perceived as continuing its motion in that direction (Ramachandran and Anstis. 1981). This might be regarded as a perceptual equivalent of Newton’s first law. If two spatialI> separated spots of light (Fig. la) are presented to the retina in rapid succession the spot will appear to move from the first point to the second. as commonly seen m neon advertisement signs (Korte. 1915; Kolers. 1971: Anstis. 1970. 1978; Julesz. 1971: Burt and Sperlmg. 19X1 ). If a single spot is follow,ed h! two flanking spots (Fig. I b) which appear on either side of it simultaneously. it is almost always seen to “split” and to move simultaneously in opposite directions (Ullman. 1980). This predilection for splitting can be ovcrcomc by placing one of the flanking spots nearer to the tirst spot. in which case it will always attract the apparent motion. We shall call this the “proxunit!” rule. FIgtIre Ic shows a matrix of dots (Gengerelli. 1948) forming the four corners of a diamond. This display (ah well as subsequent ones described in this paper) ~a’\ generated on ;I pCphosphor CRT using an “Apple 2” microcomputer and viewed from a distace of I m. The dots were arranged in a diamond with oblique sides because a square array with vertical sides shows an unwanted preponderance of vertical rather than horizontal apparent motion. possibly because of inter-hemispheric delays across the visual midline. The sides of the square subtended 1 and the dots themselves were about 4min of arc in diameter. The number by each dot refers to the time at which it is presented. If u and h. the sides of the square. are of equal length the display will be ambiguous and always seen as clearly bistable. The two possible per-

Perception of Fourier and non-Fourier motion by larval zebrafish

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.

Perceptual Organization in Multistable Apparent Motion

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.

Aftereffects from jogging

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.