Time's role for information, processing, and normalization.

This paper describes the first of two complementary studies designed to identify and to investigate the properties and likely functional significance of independently generated components of scalp-recorded responses evoked by stationary patterns. These experiments compared the influence of various stimulus parameters, including site of stimulation, pattern form, nature of background field and several binocular and monocular depth cues on a single subject's visual evoked potentials. The results revealed the presence, inter alia, of two topographically distinct components with the following properties. The earlier component (C2), whose polarity depends on the stimulus location in the visual field, is: contour-specific; best evoked by discrete pattern elements, but not gratings, in the central few degrees of the visual field; insensitive to any depth cues. By contrast, the later (consistently) negative potential (LNP) is not dependent on the form of the stimulus and is larger for paracentrally (beyond 1.5 degrees) than centrally located stimuli. It is also selectively enhanced by both monocular and binocular depth-cue stimuli, including the simulated forward movement of a pattern relative to a steady textured background; a stimulus which evokes no C2. The respective response properties of these scalp potentials suggest that there is parallel processing of depth- and contour-related features of stationary stimuli in anatomically separate regions of the human visual cortex.

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PubMed1996

Evidence for a neural mechanism that encodes angles.

We measured the discrimination threshold (delta theta)Th for angle theta, where theta was either the angle of a Vee composed of two straight lines contained within the frontoparallel or the angle intersection of two straight lines contained within the frontoparallel plane. The two-line pattern was rotated bodily through a random angle between trials with the aim of eliminating the absolute orientation of one or the other line as a reliable cue to the task. We report evidence that this aim was achieved. Our main conclusion is that the ability to discriminate a change in angle theta cannot entirely be explained in terms of the ability to discriminate changes in the orientations of the individual lines that comprise the Vee. We propose that the human visual pathway contains a neural mechanism that encodes the difference in the orientations of two simultaneously-presented straight lines. Discrimination threshold for angle (delta theta)Th is roughly twice orientation discrimination threshold for an isolated line. When subjects cannot use the orientation of one or another line as a cue to the task, the plot of (delta theta)Th vs theta is approximately flat between the delta = 20 and 160 deg.

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