PubMed Health⌕ Search

Biomedical subjects

D B Dunlop

Publications and source records attributed to D B Dunlop.

7 recordsLinked to original sources
PubMed1992

Detection duration thresholds and evoked potential measures of stereosensitivity.

Visual evoked potentials have been proposed by some researchers to be more useful than behavioral techniques to evaluate stereo performance in children and certain clinical populations. Stimulus duration detection thresholds, visual evoked potentials, and scalp electrical potential distribution maps to dynamic random dot stereograms were studied. A high degree of correspondence was found between visual evoked potential amplitudes and behaviorally determined detection thresholds. Upper field stimuli had higher detection thresholds and generated lower-amplitude visual evoked potential responses than did centrally presented stimuli. For the most eccentrically presented stimuli, lower detection thresholds were found for stimuli presented in the right visual field than the left visual field. This finding was consistent with the pattern of VEP responses to be lateralized, with higher-amplitude responses recorded over left-hemisphere sites. The study examined a proposal that the major negative component of the stereoscopic visual evoked potential originates in cortical area V1. The results failed to support the proposal and were consistent with the main negative component of the VEP being generated in V2, rather than V1.

Adult↗
PubMed1988

Stereoscopic evoked responses to crossed and uncrossed disparity accompanying simulated refractive error.

Evoked potentials were recorded to the occurrence of a disparate stimulus in dynamic random dot stereograms. Seven adult males, all of whom had vision which was normal or corrected to normal, participated in the experiment. Subjects viewed 100 ms duration stimuli which embodied 30 arc min of either crossed or uncrossed disparity under four conditions of spherical overcorrection: -0.25, +1.0, +2.0, +3.0 dioptres. The first condition, essentially normal refraction, yielded reliable behavioural reports of the stimulus and clear evoked potentials to both crossed and uncrossed disparity. With increasing overcorrection the behavioural reports became less reliable, and the evoked potentials were degraded for both conditions of disparity. The responses to the crossed disparity condition, however, showed significantly less degradation in both behavioural and electrophysiological measures. The implications of this finding may be that there are separate cortical subsystems for the processing of crossed and uncrossed disparity and that the former is more robust under non-ideal viewing conditions.

Adult↗
PubMed1983

Evoked responses to distinct and nebulous stereoscopic stimuli.

When geometric or contoured shapes are generated as disparity regions in dynamic random-dot stereograms, the question of whether the subject's response is evoked largely by boundaries and corners of the disparate region arises. To examine this question a disparate square was shifted randomly within a focal region at one millisecond intervals, resulting in a nebulous stimulus which "welled up" in the centre of the field of view. The technique may be regarded as an analogue of the defocusing technique used to resolve similar questions about evoked potentials to patterned stimuli. Average visual evoked potentials (VEP) were recorded from homolateral pairs of occipital and posterior temporal electrodes referred to a common midline prefrontal site. The latencies and amplitudes of VEP to distinct and nebulous stimuli were compared in a group of young normal adults, of whom 14 succeeded in perceiving and 14 failed to perceive the presented stimuli. Latencies of maximal response to distinct and nebulous stimuli were closely similar in the two groups (range N242-N252). Mean amplitudes of the peak response differed between perceivers and non-perceivers. The responses to nebulous stimuli were smaller than those to distinct stimuli in site-for-site comparisons in both groups. Even in short averaging runs a substantial VEP (about 3 microvolt) was recorded to nebulous stimuli in the perceiver group. It is suggested that both disparity and boundary features contribute to the VEP when this particular display system is used.

Adult↗
PubMed1981

On the origin of visual dyslexia.

Analysis of the binocular co-ordination errors of a young dyslexic reveals a simple optical explantation for one of two types of letter confusion found in dyslexia. Although binocular control was grossly abnormal in reading, no ocular defect could be found in standard ophthalmic and orthoptic tests. Re-examination of the same patient six years later, again showed no ocular defect on normal testing but binocular movements in reading, while still abnormal, showed no trace of the striking original defect. This complete masking of the original condition with time must severely restrict the value of mass surveys of established dyslexics.

Adolescent↗
PubMed1980

Measurement of dynamic stereoacuity and global stereopsis.

Quantification of global stereopsis in addition to stereoacuity has been achieved by using dynamic random dot test patterns generated by inexpensive computer controlled integrated electronics. A pilot study revealed marked differences between the responses of a variety of clinically defective and normal subjects.

Adolescent↗