Biomedical subjects

John Cass

Publications and source records attributed to John Cass.

4 recordsLinked to original sources

PubMed2006

The mechanisms of collinear integration.

Low-contrast visual contour fragments are easier to detect when presented in the context of nearby collinear contour elements (U. Polat & D. Sagi, 1993). The spatial and temporal determinants of this collinear facilitation have been studied extensively (J. R. Cass & B. Spehar, 2005; Y. Tanaka & D. Sagi, 1998; C. B. Williams & R. F. Hess, 1998), although considerable debate surrounds the neural mechanisms underlying it. Our study examines this question using a novel stimulus, whereby the flanking "contour" elements are rotated around their own axis. By measuring contrast detection thresholds to a brief foveal target presented at various phases of flanker rotation, we find peak facilitation after flankers have rotated beyond their collinear phase. This optimal facilitative delay increases monotonically as a function of target-flanker separation, yielding estimates of cortical propagation of 0.1 m/s, a value highly consistent with the dynamics of long-range horizontal interactions observed within primary visual cortex (V1). A curious new finding is also observed: Facilitative peaks also occur when the target flash precedes flanker collinearity by 20-80 ms, a range consistent with contrast-dependent cortical onset latencies. Together, these data suggest that collinear facilitation involves two separate mechanisms, each possessing distinct dynamics: (i) slowly propagating horizontal interactions within V1 and (ii) a faster integrative mechanism, possibly driven by synchronous collinear cortical onset.

Contrast Sensitivity↗

PubMed2006

Evidence for two interacting temporal channels in human visual processing.

Previous studies have generally estimated that two independent channels underlie human temporal vision: one broad and low-pass, the other high, and band-pass. We confirm this with iso-oriented targets and masks. With orthogonal masks, the same high-frequency channel emerges but no low-pass channel is observed, indicating the high-frequency channel is orientation invariant, and possibly pre-cortical in origin. In contrast, orientation dependence for low frequencies suggests a cortical origin. Subsequent masking experiments using unoriented spatiotemporal-filtered noise demonstrated that high-frequency masks (>8Hz) suppress low-frequency targets (1 and 4Hz), but low frequencies do not suppress high frequencies. This asymmetry challenges the traditional assumption of channel independence. To explain this, we propose a two-channel model in which a non-orientation-selective high-frequency channel suppresses an orientation-tuned low-frequency channel. This asymmetry may: (i) equalise the over-representation of low temporal-frequency energy in natural stimuli (1/f power spectrum); (ii) contribute to motion deblurring.

Contrast Sensitivity↗

PubMed2005

Contour interactions between pairs of Gabors engaged in binocular rivalry reveal a map of the association field.

A psychophysical study was conducted to investigate contour interactions (the 'association field'). Two Gabor patches were presented to one eye, with random-dot patches in corresponding locations of the other eye so as to produce binocular rivalry. Perceptual alternations of the two rivalry processes were monitored continuously by observers and the two time series were cross-correlated. The Gabors were oriented collinearly, obliquely, or orthogonally, and spatial separation was varied. A parallel condition was also included. Correlation between the rivalry processes strongly depended on separation and relative orientation. Correlations between adjacent collinear Gabors was near-perfect and reduced with spatial separation and as relative orientation departed from collinear. Importantly, variations in cross-correlation did not alter the rivalry processes (average dominance duration, and therefore alternation rate, was constant across conditions). Instead, synchronisation of rivalry oscillations accounts for the correlation variations: rivalry alternations were highly synchronised when contour interactions were strong and were poorly synchronised when contour interactions were weak. The level of synchrony between these two stochastic processes, in depending on separation and relative orientation, effectively reveals a map of the association field. These association fields are not greatly affected by contrast, and can be demonstrated between contours that are presented to separate hemispheres.

Computer Graphics↗

PubMed2002

Win-shift and win-stay learning in the short-beaked echidna (Tachyglossus aculeatus).

Numerous previous investigators have explained species differences in spatial memory performance in terms of differences in foraging ecology. In three experiments we attempted to extend these findings by examining the extent to which the spatial memory performance of echidnas (or "spiny anteaters") can be understood in terms of the spatio-temporal distribution of their prey (ants and termites). This is a species and a foraging situation that have not been examined in this way before. Echidnas were better able to learn to avoid a previously rewarding location (to "win-shift") than to learn to return to a previously rewarding location (to "win-stay"), at short retention intervals, but were unable to learn either of these strategies at retention intervals of 90 min. The short retention interval results support the ecological hypothesis, but the long retention interval results do not.

Animals↗