Biomedical subjects

R P Kohly

Publications and source records attributed to R P Kohly.

3 recordsLinked to original sources

PubMed2001

Long-distance interactions in Cyclopean vision.

We report on evidence for selective long-distance interactions in Cyclopean binocular vision. When presented with a pair of Cyclopean test bars observers could discriminate trial-to-trial uncorrelated variations in the mean orientation, orientation difference, separation and mean location of the test bars while ignoring random variations in the orientation, width and location of a third bar placed between the two test bars. We propose that the human visual system contains Cyclopean long-distance comparators (i) that compare the outputs of two narrow receptive fields some distance apart while being insensitive to stimuli located between those receptive fields, and (ii) the outputs of which carry orthogonally labelled indicators of orientation difference, mean orientation, separation and mean location. In the evolutionary context, one role for the proposed mechanisms might be to encode information about the silhouettes of animals whose camouflage is broken by the binocular vision of predators.

Adult↗

PubMed2000

Coincidence detectors: visual processing of a pair of lines and implications for shape discrimination.

The psychophysical data reported here bear on how the boundaries of an object's retinal image are processed in early vision. We propose that the visual system contains a mechanism sensitive to four relationships between two local stimuli some distance apart and that the output of this hypothetical mechanism encodes and labels orthogonally the four relationships. We measured the just-noticeable difference in the orientation difference between two test lines as well as the just-noticeable differences in their mean orientation, mean location, and separation. A pair of noise lines was located between the two test lines. By arranging that trial-to-trial variations in the orientation difference, mean orientation, mean location and separation of the test lines had zero correlation with each other and with trial-to-trial variations in the corresponding variables for the two noise lines we could demonstrate that psychophysical responses were based on the task-relevant variable and that, for each of the four task-relevant variables, all task-irrelevant variables were ignored. The finding that responses to the test lines were unaffected by the noise lines implies that discriminations were not influenced by first-stage spatial filters with strictly local receptive fields that responded to both test lines. Because these findings held for a presentation duration of 20 ms we can exclude the possibility that observers compared the two test lines by shifting either fixation or attention. We propose that, rather than by attending to two different locations, the test lines were selected by attending to the output of the long-distance comparator whose 'separation' label signaled the largest magnitude. The above proposals can account for several previously reported phenomena. More generally, an array of the proposed long-distance comparators constitutes a system that may be capable of specifying the shape, size, location and implicit orientation of an object's retinal image.

Adult↗

PubMed1999

Evidence for a mechanism sensitive to the speed of cyclopean form.

We measured Weber fractions for discriminating the speed of cyclopean gratings and Weber fractions for discriminating the speed of luminance gratings. Of our 14 observers, five were unable to see the cyclopean grating sufficiently well to discriminate its speed. One observer experienced great difficulty in discriminating the speed of cyclopean gratings, even though her threshold for detecting cyclopean gratings was low, and even though she discriminated the speed of luminance gratings on the basis of the task-relevent variable. But several observers based their speed discriminations on trial-to-trial variations of the task-relevent variable while ignoring associated trial-to-trial variations in all task-irrelevant variables (specifically: displacement; temporal frequency; spatial frequency; and presentation duration). We conclude that the visual systems of these observers contain a specialized neural mechanism for the speed of cyclopean gratings that supports acute discriminations of speed (Weber fractions were as low as 0.05-0.07).

Differential Threshold↗