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Biomedical subjects

B Breitmeyer

Publications and source records attributed to B Breitmeyer.

11 recordsLinked to original sources
PubMed1997

Backward masking performance in unaffected siblings of schizophrenic patients. Evidence for a vulnerability indicator.

BACKGROUND: Visual masking is a procedure that is used to assess the earliest components of visual processing. In backward masking, the identification of an initial stimulus (the target) is disrupted by a later stimulus (the mask). The masking function can be divided into an early component (e.g., up to about 60 ms) that reflects the involvement of sensory-perceptual processes, and a later component that reflects susceptibility to attentional disengagement as the mask diverts processing away from the representation of the target. Schizophrenic patients show anomalies on both masking components. It is not known whether backward masking deficits reflect enduring genetic vulnerability to schizophrenia. METHODS: We assessed 32 unaffected siblings of schizophrenic patients and 52 normal control subjects on the early and late components of 4 masking conditions. The conditions differentially involved the sustained and transient visual pathways. RESULTS: The unaffected siblings showed poorer overall performance than control subjects on the masking procedures. More specifically, siblings showed anomalies on the early, sensory-perceptual component, but not on the later, attentional disengagement component. CONCLUSIONS: The backward masking performance deficits that have been observed in schizophrenic patients appear to reflect enduring vulnerability to the disorder rather than only the symptoms of the illness. This vulnerability appears to be associated with early, sensory-perceptual processes.

Adolescent↗
PubMed1987

Mechanisms of visual attention revealed by saccadic eye movements.

This paper summarizes recent data on the initiation of saccadic eye movement in relation to the mechanisms of visual attention. In particular, the occurrence of express saccades, defined by their extremely short reaction times, is discussed on the basis of the observation that these saccades do not occur when the subjects (man or monkey) are attending to either a fixation point or to any other visual stimulus in the periphery of their field of view including the "future" saccade target location. It is concluded that the system of visual attention can be in two states: engaged or disengaged. In order to generate a saccade or to move attention from one point to another visual attention must be in the disengaged state. The disengagement takes some time which is or is not included in the saccadic reaction time depending on whether or not visual attention is engaged at the time of the onset of the saccade target. During engaged visual attention saccades are inhibited thereby providing steady central fixation or the absence of saccades during directed peripheral attention.

Attention↗
PubMed1977

Existence and implications of a tilted binocular disparity space.

The existence of a tilted binocular disparity space was experimentally established. The results of the experiment showed that a texture surface which under binocular viewing appears to be oriented vertically and perpendicular to the line of sight in fact is tilted such that the portions of the texture surface above and below fixation incline toward and away from the observer, respectively. Under monocular viewing such a discrepancy between apparent and objective vertical tilt is not observed. This suggests that under binocular viewing the cortical representations of the upper and lower visual hemifields are biased toward uncrossed and crossed disparities, respectively. The tilted binocular disparity space is explained in terms of an experientially induced disparity gradient effected during development of the binocular system.

Cues↗
PubMed1976

U-shaped backward contour masking during stroboscopic motion.

Two stationary and spatially separated visual stimuli, presented briefly and successively in time, are known to produce stroboscopic motion whose vividness is a U-shaped function of the stimulus onset asynchrony. Contour masking is also known to occur under such stimulus conditions. The findings show that the contour masking is confined to only the first stimulus and that it, like metacontrast, is a backward U-shaped function of the stimulus onset asynchrony. A simple model, based on known psychophysical and neurophysiological properties, is proposed to explain these results.

Female↗
PubMed1976

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 anistropy (and left-right isotropy) in spatial resolution. Introduction of monocular clues into the stereograms tended to eliminate the anisotropies. This implies that the anisotropies 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.

Depth Perception↗
PubMed1975

Dynamic random-dot stereograms reveal up-down anisotropy and left-right isotropy between cortical hemifields.

With the use of dynamic random-dot sterograms (which are devoid of all monocular depth cues), the temporal duration for detecting a small, briefly presented test square of different depth than the surround varied as a function of its location in the central portion of the visual field. Test squares presented in the upper hemifield were detectable at consistently shorter durations than those in the lower hemifield when the fixation marker was in front of the surround, and vice versa when the marker was behind. No such anisotropy was found for left and right hemifield. Esploratory studies suggested a similar up-down anisotropy and left-right isotropy in spatial resolution. Thus, the upper hemifield representation at the cortex shows a general superiority over the lower one for vinocular detectors tuned to uncrossed disparitites, and the lower hemifield shows superiority for those tuned to crossed disparities.

Brain Mapping↗