Size Increase in Panum’s Fusional Range Is Driven by Eccentricity and Not Separation

Previous research has demonstrated that Panum’s fusional range increases in the periphery, and this increase is usually attributed to eccentricity. However, it is unclear whether the increase in the periphery is driven by eccentricity or separation between the stimulus and the central fixation marker. In Experiment 1, we independently measured the effects of eccentricity and stimulus separation on Panum’s fusional range for horizontal disparities. We observed significant increases in Panum’s range as eccentricity increased independently of stimulus separation. Experiment 2 revealed that the extent of Panum’s fusional range for vertical disparities increased with eccentricity independently of stimulus separation. Taken together, these results strongly support previously held conclusions that Panum’s fusional range for both horizontal and vertical disparities increases with increasing eccentricity and is little impacted by stimulus separation.

Warning Signals Influence Motor Processing

When observers initiate responses to visual targets, they do so sooner when a preceding stimulus indicates that the target will appear shortly. This consequence of a warning signal may change neural activity in one of four ways. On the sensory side, the warning signal may speed up the rate at which the target is registered by the brain or enhance the magnitude of its signal. On the motor end, the warning signal may lower the threshold required to initiate a response or speed up the rate at which activity accumulates to reach threshold. Here, we describe which explanation is better supported. To accomplish this end, monkeys performed different versions of a cue-target task while we monitored the activity of visuomotor and motor neurons in the superior colliculus. Although the cue target task was designed to measure the properties of reflexive spatial attention, there are two events in this task that produce nonspecific warning effects: a central reorienting event (brightening of central fixation marker) that is used to direct attention away from the cue, and the presentation of the cue itself. Monopolizing on these tendencies, we show that warning effects are associated with several changes in neural activity: the target-related response is enhanced, the threshold for initiating a saccade is lowered, and the rate at which activity accumulates toward threshold rises faster. Ultimately, the accumulation of activity toward threshold predicted behavior most closely. In the discussion, we describe the implications and limitations of these data for theories of warning effects and potential avenues for future research.

Integration of Disparity Information across Multiple Fixations

We measured stereo thresholds for observers viewing noisy, dynamic random-dot stereograms (DRDS) containing either a spatial or a temporal step in depth. Stereograms were presented either with or without a small fixation cross at zero disparity. When the fixation marker was present observers were instructed to maintain fixation throughout the presentation. For spatial steps in depth, the lowest thresholds were found when there was no fixation marker and observers were free to look around the stimulus. Thresholds were higher with a fixation marker in the stimulus. The opposite pattern of results was found for stimuli that contained a temporal step in depth, ie the lowest thresholds were found with a fixation marker. Furthermore, presentation of a single depth region with a fixation marker gave the lowest thresholds of any configuration tested. The results with temporal changes in depth are reminiscent of the findings of Westheimer (1979 Experimental Brain Research36 585 – 597). Westheimer argued that sequential presentation in the same location might depress sensitivity to changes in depth, but in DRDS no single dot changes in depth without also changing in retinal location, so this explanation is not applicable. We conclude that, with a spatial step in depth, it is advantageous to foveate both sides of the step, so multiple fixations help. With a temporal step in depth, the spatial structure of the stimulus is uniform, so multiple fixations may not necessarily help, especially if they result in uncertainty about vergence position.

Binocular-Disparity-Dependent Upper—Lower Hemifield Anisotropy and Left—Right Hemifield Isotropy as Revealed by Dynamic Random-Dot Stereograms

Dynamic random-dot stereograms devoid of all monocular depth cues were used to measure the limits of temporal and spatial resolution in the center of the visual field. The temporal durations for detecting a small, briefly presented test square of different binocular disparity than the surround varied as a function of its location and binocular disparity. The test squares presented in the upper hemifield were detectable at consistently shorter durations than those presented in the lower hemifield for a surround disparity which was uncrossed relative to the fixation marker. For crossed surround disparity this preference reversed, resulting in a superiority of the lower hemifield. The anisotropy diminished for zero surround disparity. No such anisotropy was found when left and right visual hemifields were compared. It was also shown that this upper—lower temporal anisotropy (and left—right isotropy) is paralleled by a similar disparity-dependent upper—lower anisotropy (and left—right isotropy) in spatial resolution. Introduction of monocular clues into the stereograms tended to eliminate the anisotropics. This implies that the anisotropics reflect the spatiotemporal properties and distribution of binocular disparity detectors in the human cortex and result in a tilted surface that pivots around the horizontal midline in the space of binocular depth perception.

Effect of Reference Points and Masking on Tachistoscopic Pattern Perception

4 experiments examined S's tendency to reproduce more accurately elements about reference points, i.e., fixation (fixation facilitation) and extreme positions (end-segregation), of binary patterns. Exp. I varied pattern length from 4 to 28 elements. As predicted, end-segregation was greatest for intermediate lengths, and absent for longer patterns. Unexpected results were an end-segregation effect for shortest patterns and no fixation facilitation. In Exp. II, pairs of elements at all eccentric positions were more accurately reproduced when presented alone. Exp. III found no fixation facilitation with 28-element patterns and fixation-reference markers on the responding templates. Exp. IV tested the hypothesis that the fixation marker had masked central elements in Exps. I and III. Fixation facilitation appeared with 28- and 12-element patterns when a smaller marker was used. These experiments show that mnemonic factors affect differential perceptual accuracy among elements in tachistoscopic patterns.