Biomedical subjects

S A Beardsley

Publications and source records attributed to S A Beardsley.

3 recordsLinked to original sources

PubMed1999

The perception and discrimination of speed in complex motion.

Random dot kinematograms were used to simulate radial, rotational and spiral optic flow. The stimuli were designed so that, while dot speed increased linearly with distance from the centre of the display, the density of dots remained uniform throughout their presentation. In two experiments, subjects were required to perform a temporal 2AFC speed discrimination task. Experiment 1 measured the perceived speed of a range of optic flow patterns against a rotational comparison stimulus. Radial motions were found to appear faster than rotations by approximately 10%, with a smaller but significant effect for spirals. Experiment 2 measured discrimination thresholds for pairs of similar optic flow stimuli identical in all respects except mean speed. No consistent differences were observed between the speed discrimination thresholds of radial, rotational and spiral motions and a control stimulus with the same speed profile in which motion followed fixed random trajectories. The perceived speed results are interpreted in terms of a model satisfying constraints on motion-in-depth and object rigidity, while speed discrimination appears to be based upon the pooled responses of elementary motion detectors.

Differential Threshold↗

PubMed1998

Computational modelling of optic flow selectivity in MSTd neurons.

In neurophysiological experiments examining the selectivity of MSTd neurons to visual motion components of optic flow stimuli in monkeys, Duffy and Wurtz (1991) reported cells with double-component (plano-radial and plano-circular) and triple-component (plano-radial-circular) selectivities, while Graziano et al (1994) reported neurons selective to a continuum of optic flow stimuli including spiral motion. Here, we address these reported findings under simulated experimental conditions by examining the development of optic flow selectivity in the hidden units of a two-layer back-propagation network. We also examine network motion sensitivity during simulated psychophysical tests via the addition of a competitive decision layer. Network analysis with neurophysiological stimuli identified a majority of hidden units whose position invariance and motion selectivity were consistent with MSTd responses to the visual motion components of optic flow stimuli reported by Duffy and Wurtz and Graziano et al. Furthermore, the hidden units developed a continuum of optic flow selectivities independent of the biases associated with the specification of the motion selectivity in the output layer. During psychophysical testing, network responses showed motion sensitivities which met or exceeded human performance. Within the limitations imposed by the learning algorithm, the psychophysical results were consistent with a model of global motion perception via local integration along complex motion trajectories.

Animals↗

PubMedDate not supplied

A laterally interconnected neural architecture in MST accounts for psychophysical discrimination of complex motion patterns.

The complex patterns of visual motion formed across the retina during self-motion, often referred to as optic flow, provide a rich source of information describing our dynamic relationship within the environment. Psychophysical studies indicate the existence of specialized detectors for component motion patterns (radial, circular, planar) that are consistent with the visual motion properties of cells in the medial superior temporal area (MST) of nonhuman primates. Here we use computational modeling and psychophysics to investigate the structural and functional role of these specialized detectors in performing a graded motion pattern (GMP) discrimination task. In the psychophysical task perceptual discrimination varied significantly with the type of motion pattern presented, suggesting perceptual correlates to the preferred motion bias reported in MST. Simulated perceptual discrimination in a population of independent MST-like neural responses showed inconsistent psychophysical performance that varied as a function of the visual motion properties within the population code. Robust psychophysical performance was achieved by fully interconnecting neural populations such that they inhibited nonpreferred units. Taken together, these results suggest that robust processing of the complex motion patterns associated with self-motion and optic flow may be mediated by an inhibitory structure of neural interactions in MST.

Animals↗