Biomedical subjects

A P Aitsebaomo

Publications and source records attributed to A P Aitsebaomo.

4 recordsLinked to original sources

PubMed2000

Saccadic and psychophysical discrimination of double targets.

Saccades are rapid, conjugate eye movements that shift the foveas onto peripheral targets of interest. However, the initial saccades to a peripheral pair of targets have been reported to direct the foveas between the targets, despite instructions to look at one or the other. This phenomenon, referred to as saccadic averaging, can occur for target separations of 10 degrees or more and is usually interpreted as evidence that the spatial information available to the saccadic system is of inherently poor resolution. We compared the amplitudes of initial saccades to single peripheral targets and to pairs of targets, with and without prior auditory cues that provided unambiguous information about which target represented the saccadic goal. Target duration ranged from 33 to 100 ms, followed by a mask that was intended to limit neural processing time. The results show that supplementary auditory cues reduce the extent of saccadic averaging to target separations as small as 2 degrees. Saccadic averaging is more pronounced for targets of brief duration, consistent with the degradation of psychophysical resolution that occurs for targets of short duration. We conclude that saccadic averaging results in large part because of ambiguity about which of two targets represents the saccadic goal and, to a lesser extent, because of the limited time that is available before the saccade to process target position.

Adult↗

PubMed1992

Psychophysical and saccadic information about direction for briefly presented visual targets.

For saccades, the difference between desired and actual eye movement (constant error), as well as the variability in amplitude on repeated trials (variable error), presumably represent a combination of errors in processing target position (sensory error) and errors in execution (motor error). To examine whether the saccadic system uses the same information about target position as visual perception, subjects made saccades to and psychophysical judgments about the location of targets presented for 17-200 msec. Mean saccadic amplitude markedly decreased and inter-trial variability increased when saccadic targets were presented briefly and followed by a spatial mask. Judged target position (from psychophysical vernier and bisection tasks) also shifted to lesser eccentricities and was more variable. Although qualitatively alike, changes with duration in saccadic constant errors were larger than could be accounted for by the psychophysical results, suggesting similar but separate processing of position information for the saccadic and perceptual systems. Differences in the sizes of collicular receptive fields that preferentially respond to targets at short and long durations can account qualitatively for the observed changes in saccadic amplitude as well as the common occurrence of saccadic undershoots.

Adult↗

PubMed1992

Spatial localization without visual references.

To explain the veridical percept of the spatial ordering of objects and the generation of eye movements to peripheral targets, Lotze (1885 Microcosmos. Edinburgh: T. & T. Clark) proposed that there is a position label (local sign) for each retinal element. To estimate the precision of local sign information, we measured absolute localization thresholds at various eccentricities in the nasal visual field, in the complete absence of visual references. To eliminate perception of the visual surround, observers viewed a large display screen through a neutral density filter (2.0 log unit) in a dark room. The fixation target was extinguished at various times (interstimulus intervals or ISIs) prior to the onset of the test stimulus. In general, our results show that localization thresholds are proportional to the target eccentricity at all ISIs. At each eccentricity, localization thresholds are elevated after the extinction of the visual reference compared to thresholds when the reference is present. However, relative to the referenced threshold, unreferenced thresholds are elevated by a greater proportion at smaller eccentricities than at larger eccentricities. Our threshold vs ISI data can be adequately modeled on the basis of an intrinsic positional uncertainty, which increases with eccentricity, and additive and multiplicative sources of noise. The additive noise appears to reflect primarily the increasing scatter in eye position when the fixation target is extinguished. Our model's estimate of intrinsic positional uncertainty in the isoeccentric direction appears to reflect primarily the intrinsic positional uncertainty of the peripheral retina (the local sign), being very similar to cumulative cone position uncertainty and to the spacing between ON-P beta ganglion cells. In the isoeccentric direction, the estimated precision of the local sign mechanism across eccentricities is slightly better than the precision of saccadic endpoints, suggesting that noise in the motor system must also contribute to the scatter of saccadic endpoints in the isoeccentric direction. Interestingly, in the radial direction, we find a surprising similarity in our observers' positional uncertainty and the precision of saccadic endpoints.

Fovea Centralis↗

PubMed1985

Vernier acuity, crowding and cortical magnification.

When a vernier target is flanked by optimally positioned lines, foveal vernier discrimination is strongly degraded (Westheimer and Hauske, 1975). We confirmed this observation (Experiment I) and have mapped out a 2 dimensional "perceptive field" for crowding in the fovea using a 2 dot target (Experiment II). Crowding was also measured in peripheral vision, using either small flanking dots as masks (Experiment III), or using repetitive vernier gratings (Experiment IV). The results showed that when scaled in proportion to recent estimates of the cortical magnification factor, vernier acuity is as good in the periphery as it is centrally. Both centrally and peripherally, there appears to be a psychophysical processing module which we term a "perceptive hypercolumn". At all eccentricities vernier thresholds were found to be approximately 1/40 of the size of a perceptive hypercolumn and were elevated if interfering contours are present in the same (or adjacent) hypercolumns.

Form Perception↗