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Biomedical subjects

S M Anstis

Publications and source records attributed to S M Anstis.

At least 19 recordsLinked to original sources
PubMed1991

The development of optokinetic nystagmus in strabismic and monocularly enucleated subjects.

Assymmetries 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.

Adolescent↗
PubMed1990

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.

Attention↗
PubMed1987

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.

Color↗
PubMed1986

Brightness shift in drifting ramp gratings isolates a transient mechanism.

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.

Form Perception↗
PubMed1986

Illusory continuous motion from oscillating positive-negative patterns: implications for motion perception.

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.

Color Perception↗
PubMed1985

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.

Fixation, Ocular↗
PubMed1985

Kinetic occlusion by apparent movement.

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.

Cognition↗
PubMed1985

A moving display which opposes short-range and long-range signals.

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.

Adaptation, Physiological↗
PubMed1985

Effects of luminance and contrast on direction of ambiguous apparent motion.

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.

Adaptation, Physiological↗
PubMed1983

Displacement thresholds for coherent apparent motion in random dot-patterns.

Two correlated random-dot patterns (A and B) were generated on a CRT screen and presented in rapid alternation; (B) was shifted horizontally by varying amounts in relation to (A) so that coherent apparent motion was seen. We found that larger shifts were tolerated if (i) the stimulus onset asynchrony (SOA) was longer; (ii) if the patterns were optically blurred; and (iii) if there were fewer dots on the screen. Hence apparent motion in random-dot patterns may involve a global pattern matching operation as in stereopsis. Two uncorrelated random-dot patterns were alternated to produce incoherent dynamic "noise". A low spatial frequency sine wave grating was then projected on this "noise" and moved in step with the alternating random-dot patterns. This resulted in "motion-capture"--i.e. all the dots now seemed to move synchronously with the moving grating. The effect could not be obtained with high spatial frequency gratings or with stationary dots. As a tentative solution to the "correspondence problem" it is suggested that low spatial frequencies are matched first and these matches impose constraints on subsequent high frequency matches--thus allowing the system to home in on a unique solution.

Form Perception↗
PubMed1983

Low spatial frequencies dominate apparent motion.

Experiments are reported which have been designed to establish what features of a pair of figures can be used as an input for apparent motion. The display consisted of a central figure, A, which appeared briefly and was followed immediately afterwards by two figures, B and C, which appeared on either side of the original location of A. Figure A can thus move towards either B or C. When A was a low-pass filtered square it moved towards C (a low-pass filtered square that was similar to A but 'rotated' by 45 degrees) rather than toward B (a high-pass filtered square identical to A in orientation and size). When A was an unfiltered square it moved towards C (a low-pass filtered square of identical orientation) rather than towards B (a high-pass filtered square of identical orientation). Lastly, when A was a 'solid' square it moved towards C (a solid circle) rather than towards B (an outline square). All three experiments suggest that the direction of perceived movement is determined exclusively by low spatial frequencies rather than by similarity of oriented edges, especially when speed of alternation is rapid.

Discrimination Learning↗
PubMed1980

The perception of apparent movement.

When two similar pictures, overlapping but slightly displaced, were projected on a screen in alternation, apparent movement could be seen. How similar must successive pictures be to give apparent movement? This is the 'correspondence problem'. Manipulations of the local and global correspondences between pictures included motion phenomena such as reversed apparent movement; a four-stroke oscillatory cycle which gave an illusion of continuous motion in one direction; edges defined by texture, stereoscopic depth, or flicker, kinetic edges; and wave motion. It was concluded that human motion perception may comprise two separate mechanisms. Local point-by-point correlations between pictures are detected by a relatively peripheral system, probably based on directonally selective neural units. More subtle global correspondences are analysed by a more cognitive system which extracts edges before it process motion.

Afterimage↗
PubMed1979

Interactions between simultaneous contrast and adaptation to gradual change of luminance.

Following adaptation to a field of light which was modulated by a rising ramp so that it repetitively grows gradually brighter, a steady test field of light appears to be gradually growing dimmer. In this study, if a small grey spot of constant luminance was centered in the brightening field, it appeared to be growing gradually dimmer by simultaneous contrast. This apparent dimming led to a brightening aftereffect in the spot. It was shown that this spot aftereffect had two independent components: the apparent dimming of the adapting spot produced its own aftereffect (contrast produced an aftereffect) and also the dimming aftereffect in the surround field spatially induced an aftereffect into the spot during the test period (aftereffect produced simultaneous contrast). Thus simultaneous contrast can both precede and follow successive contrast in the visual system.

Adaptation, Ocular↗