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R J Snowden

Publications and source records attributed to R J Snowden.

42 records · Page 3Linked to original sources
PubMed1990

Differences in the processing of short-range apparent motion at small and large displacements.

Using random dot patterns we have compared performance on direction discrimination tasks for single and multi-step sequences of apparent motion at a range of displacement sizes. Performance was measured by varying the correlation between the frames. For "small" displacements we found that no improvement in performance occurs with stimulus duration (number of frames) if the movement of individual elements within the pattern was restricted to one step, whereas if elements undergo multiple steps, performance improves with duration. For "large" displacements, on the contrary, performance improves with increasing stimulus duration irrespective of whether individual elements are restricted to single steps. These results suggest that small and large displacements are processed in different ways. We review possible psychophysical and physiological correlates of this suggestion.

Discrimination, Psychological↗
PubMed1990

Suppressive interactions between moving patterns: role of velocity.

Detection and use of motion in complex environments requires movement measurements to be combined. The role of velocity in the suppressive interaction between patterns that move in orthogonal directions was assessed. When dmax was measured, it was found that a range of medium to high speeds of vertical motion all had a suppressive effect upon detecting horizontal motion. When dmin was measured, only a range of low velocities caused a disruption of performance. Thus, whilst velocity is an important parameter in determining the size of the suppressive effect, it has nonparallel influences at the upper and lower end of the displacement range.

Acceleration↗
PubMed1989

The combination of motion signals over time.

The improvement in performance with increasing number of frames in a random-dot kinematogram (temporal recruitment) was assessed by measuring threshold signal-to-noise ratios of direction discrimination. At fast frame presentation rates (50 Hz) thresholds fell sharply as the number of frames in the sequence increased, whereas at slow frame presentation rates (20 and 10 Hz) there was a less dramatic fall in thresholds. The similarity between the results at 20 and 10 Hz suggests that the mechanism of this less dramatic rise is relatively independent of temporal factors. The recruitment effect also does not appear to be limited by a maximum spatial range. We propose that temporal recruitment may occur via two mechanisms. One involves stimulating motion detectors with greater spans and delays, whilst the other involves the co-operative interaction of signals from units tuned to similar directions and have similar spans and delays. This distinction is supported by a further experiment which eliminates the first of these recruitment mechanisms by destroying possible correlations between non-adjacent frames.

Humans↗
PubMed1989

Extension of displacement limits in multiple-exposure sequences of apparent motion.

In order to examine the processes by which motion signals are combined over time, we presented subjects with random dot kinematograms which could vary in terms of the number of frames in the sequence and the duration between the onset of each stationary frame. Performance (as measured by the greatest displacement at which subjects could discriminate opposite directions of movement) improved with increasing number of displacements up to around 5 displacements, whilst manipulations of the frame duration had no affect upon this figure. Thus the results cannot be described in terms of a limited integration time. By creating sequences in which no dots underwent more than a single displacement we show that the improvement is not specific to individual dot paths. We suggest that these results could be accounted for in terms of a co-operative network in which mutual facilitation can propagate between detectors tuned to a common direction of motion.

Humans↗
PubMedDate not supplied

The effect of transiency on perceived velocity of visual patterns: a case of "temporal capture".

We measured the points of subjective equality of velocity for dynamic unidirectionally moving random-dot patterns with different amounts of transiency. The transiency was changed by varying the time a dot would move before being randomly replotted within the stimulus. The perceived velocity of patterns moving at intermediate velocities (4 or 6 deg/sec) was increased by decreasing the point lifetime while no speedup was observed at high velocities (12 deg/sec). A speedup was also observed when a few stationary points of short lifetime were introduced into a stimulus. The non-directional transiency generated by these flickering points seems to be captured by the moving pattern and biases the velocity estimate. We term this phenomenon "temporal capture". The results are in agreement with models that determine velocity by comparing the activity in lower and higher temporal frequency channels. Our stimuli would selectively increase activity in high temporal frequency channels and thus lead to an increase in perceived velocity.

Humans↗