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S Anstis

Publications and source records attributed to S Anstis.

12 recordsLinked to original sources
PubMed1991

The contribution of color to motion in normal and color-deficient observers.

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.

Color↗
PubMed1991

The spacing illusion: a spatial aperture problem?

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.

Adult↗
PubMed1990

Motion aftereffects from a motionless stimulus.

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 provide an input into the motion pathways.

Attention↗
PubMed1989

The less you see it, the faster it moves: shortening the "on-time" speeds up apparent motion.

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.

Female↗
PubMed1989

A new test of luminous efficiency for babies.

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.

Color Perception↗
PubMed1987

Visual inertia in apparent motion.

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.

Humans↗
PubMed1987

Magnification factor for adaptation of a visual transient mechanism.

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.

Acclimatization↗
PubMed1986

Entrained path deflection in apparent motion.

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.

Eye Movements↗
PubMed1986

Recovering motion information from luminance.

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.

Adaptation, Ocular↗
PubMed1986

Figure-ground segregation modulates apparent motion.

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.

Form Perception↗
PubMed1985

Adaptation to apparent motion.

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.

Adaptation, Ocular↗