Biomedical subjects

L W Baitch

Publications and source records attributed to L W Baitch.

4 recordsLinked to original sources

PubMed1991

Image segmentation and VEP topography.

VEP scalp fields were measured for four pattern onset-offset stimuli identical except for the single contrast attribute that allowed a square-wave grating to be seen. The contrast attribute used for each stimulus was either texture, binocular disparity, luminance, or color. All stimuli were presented as dynamic random dot displays. Scalp potentials were sampled at 1.5 cm intervals by 66 electrodes over the posterior scalp. Customized rigid helmets were constructed for each subject to ensure reproducible electrode placement. The scalp fields due to the four stimuli were significantly different in all cross-comparisons. This implies that in the human visual cortex there exists regional selectivity for the processing of the four different contrast attributes.

Brain Mapping↗

PubMed1991

Binocular beat VEPs: losses of cortical binocularity in monkeys reared with abnormal visual experience.

Binocular beat VEPs were recorded from anesthetized macaque monkeys with diverse visual rearing histories, including surgically induced esotropia, optical prism dissociation, optical anisometropia, monocular form deprivation (MD), and normal rearing. Dichoptic visual stimulation was produced by temporally modulating the luminances of uniform fields presented to each eye. Five pairs of temporal frequencies were used, all of which had interocular differences of 2 Hz. While normally reared animals exhibited robust binocular beat responses strongly tuned to temporal frequency, the responses from monkeys with abnormal rearing experiences showed losses in beat signal-to-noise ratios that correlated with the age of onset or duration of the abnormal visual experience. Surgical esotropia induced early in life (2 months of age) produced a virtually complete loss of the binocular beat response; the cortical losses were less severe as the age of surgery rose to 10 months. Monkeys reared with either anisometropia or optical dissociation also manifested substantial reductions in evoked beat nonlinearity. MD monkeys sutured relatively late in development (8 and 25 months) showed mild reductions. The correspondence of these results to earlier psychophysical data obtained from these animals, and the similarity of these results to previous findings with binocularly normal and abnormal humans, supports the use of the binocular beat as an objective, noninvasive index of binocular neural integrity.

Amblyopia↗

PubMed1989

Binocular beats: psychophysical studies of binocular interaction in normal and stereoblind humans.

We describe a psychophysical method for assessing binocular integration using dichoptically-presented uniform fields. By temporally modulating uniform field luminances at different frequencies between the eyes, a rhythmic beat is produced--a visual percept characterized by undulations in luminance at a frequency equal to the arithmetic difference between the two monocular stimulus frequencies. Using a signal detection paradigm, we studied the beat as a function of modulation depth in normal and binocularly deficient subjects. Normal subjects easily detected the beat, even at low modulation depths, while stereoblind subjects (with stereoacuity worse than 2000 sec arc) failed to detect beats at any modulation depth or with any combination of stimulus frequencies tested. The beat provides evidence for the confluence of monocular signals into binocular integrating mechanisms. Our results therefore suggest that the status of functioning binocular mechanisms is related to the detectability of the beat. This uniform-field stimulus does not require accurate accommodation, fixation, vergence or high spatial resolution, thus making this technique particularly attractive for the study of binocular interaction in developing infants and in binocularly deficient adults.

Adult↗

PubMed1988

Evidence for nonlinear binocular interactions in human visual cortex.

Dichoptically presented uniform fields were sinusoidally modulated in luminance, with differing temporal frequencies between the eyes. This stimulus evokes a unique electrophysiological response termed "beats"; visual evoked potential (VEP) components reflecting nonlinear neural behavior and which could only arise from integrative binocular units. Individuals with strabismus or amblyopia, which accompany disrupted binocular function in the visual cortex manifest a severe reduction of this nonlinearity, suggesting an abnormality in the binocular cortical processes underlying this response.

Amblyopia↗