Biomedical subjects

S N Watamaniuk

Publications and source records attributed to S N Watamaniuk.

4 recordsLinked to original sources

PubMed1992

The human visual system averages speed information.

It has been known for many years that human observers are unable to detect modest accelerations and decelerations in moving visual stimuli. We find that human observers can integrate speeds over many dots, moving at different speeds, producing a global speed percept analogous to the global direction percept first reported by Williams, D. W. and Sekuler, R. (1984, Vision Research, 24, 55-62). We measured speed discrimination for random dot stimuli which contained many different speeds. Our results show that observers always base their discrimination on the mean speed of the stimulus; changes in other stimulus characteristics (e.g. mode) are not detected. Moreover, discrimination thresholds for the global mean speed derived from many different speeds are comparable to those obtained with stimuli in which all dots move at the same speed suggesting that the internal noise associated with the encoding of speed information is quite high.

Differential Threshold↗

PubMed1992

Temporal and spatial integration in dynamic random-dot stimuli.

Random-dot cinematograms comprising many different, spatially intermingled local motion vectors can produce a percept of global coherent motion in a single direction. Thresholds for discriminating the direction of global motion were measured under various conditions. Discrimination thresholds increased with the width of the distribution of directions in the cinematogram. Thresholds decreased as the duration of area of the cinematogram increased. Temporal integration for global direction discrimination extends over about 465 msec (9.3 frames) while the spatial integration limit is at least as large as 63 deg2 (circular aperture diameter = 9 deg). The large spatial integration area is consistent with the physiology of higher visual areas such as MT and MST.

Discrimination, Psychological↗

PubMed1992

Visible persistence is reduced by fixed-trajectory motion but not by random motion.

Despite the sluggish temporal response of the human visual system, moving objects appear clear and without blur, which suggests that visible persistence is reduced when objects move. It has been argued that spatiotemporal proximity alone can account for this modulation of visible persistence and that activation of a motion mechanism per se is not necessary. Experiments are reported which demonstrate that there is a motion-specific influence on visible persistence. Specifically, points moving in constant directions, or fixed trajectories, show less persistence than points moving with the same spatial and temporal displacements but taking random walks, randomly changing direction each frame. Subjects estimated the number of points present in the display for these two types of motion conditions. Under conditions chosen to produce 'good' apparent motion, ie small temporal and spatial increments, the apparent number of points for the fixed-trajectory condition was significantly lower than the apparent number in the random-walk condition. The traditional explanation of the suppression of persistence based on the spatiotemporal proximity of objects cannot account for these results. The enhanced suppression of persistence observed for a target moving in a consistent direction depends upon the activation of a directionally tuned motion mechanism extended over space and time.

Female↗

PubMed1989

Direction perception in complex dynamic displays: the integration of direction information.

We created random-dot cinematograms in which each dot's successive movements were independently drawn from a Gaussian distribution of directions of some characteristic bandwidth. Such a display, comprising many different, spatially intermingled local motion vectors, can produce a percept of global coherent motion in a single direction. Using pairs of cinematograms, direction discrimination of global motion was measured under various conditions of direction distribution bandwidth, exposure duration, and constancy of each dot's path. A line-element model gave an excellent account of the results: (i) over a considerable range, discrimination was unaffected by the cinematogram's direction distribution bandwidth; (ii) only for the briefest presentations did changes in duration have an effect; (iii) so long as the overall directional content of the cinematogram remained unchanged, the constancy or randomness of individual dots' paths did not affect discrimination. Finally, the line-element model continued to give a good account of the results when we made additional measurements with uniform rather than Gaussian distributions of directions.

Discrimination, Psychological↗