Search PubMed⌕ Search

Biomedical subjects

R A Neill

Publications and source records attributed to R A Neill.

25 records · Page 2Linked to original sources
PubMed1986

Evoked potentials to dynamic random dot stimuli with varying dot density ratios of disparity to background.

A systematic study was made of visual evoked responses to dynamic random dot stimuli containing controllable, monocularly visible contrast cues. Ratios of dot densities for the centrally presented, disparate figure and the background were varied in steps of 1/8 maximum density between 0/8 and 8/8. The figure was either a square or an equivalent area of nebulous shape. A 30-arc min disparity was compared with binocular nondisparate and monocular conditions. Evoked responses (scalp sites 02, 01, T6, T5) were averaged for each of 24 disparity-contrast-shape conditions. At all contrast levels, response amplitudes and latency over the left hemisphere was significantly greater than over the right hemisphere. For 30 arc min disparity, amplitudes in the 8/8 condition were significantly smaller than in conditions where stimulus/background contrast could afford monocular depth cues. Hemisphere amplitude differences diminished as contrast decreased. Factor analyses isolated two overlapping components in the response to disparate stimuli. The earlier, at 236 ms latency, may index the stereoscopic stimulus features. The later, at 295 ms, peaking at maximum contrast and present in all suprathreshold nondisparity conditions, may index contrast features of the stimulus. The results indicate the importance of controlling dot density ratios in electrophysiological studies of the stereoscopic response to random dot stimuli.

Adult↗
PubMed1986

Evoked potentials to dynamic random dot stereograms in upper, center and lower fields.

Dynamic random dot stereograms consisting of square, crossed disparities subtending 3 degrees, exposure duration 100 ms, were presented in the midplane centrally (surrounding the fixation point) and in the upper and lower fields at 3.5 degrees vertical displacement from fixation. Stereoscopic cerebral evoked potentials (EPs) recorded from scalp sites 02, 01, T6 and T5 (10-20 system) were analyzed in a group of normal young adults (N = 28). Fourteen were able to perceive and report the stereophenomenon (perceivers); the other half were unable to do so (nonperceivers). Mean response amplitudes were significantly larger to center field stimuli than to those in the upper field. Responses were larger in perceivers than nonperceivers, maximal at T5, and larger to stimuli with definite than indefinite boundaries. Peak EP latencies were shortest to lower field stimuli (243 ms) but not significantly different than to center (247 ms) or upper (253 ms) field stimuli. A separate experiment comparing EP amplitudes and latencies with crossed and uncrossed disparities of varying magnitudes confirmed greater responses to the crossed.

Adult↗
PubMed1984

Stereoscopic cerebral evoked potentials of Air Force pilots and civilian comparison groups.

Dynamic random-dot stereograms were presented to Air Force jet pilots (N = 14) through a new visual display system. Games which simulated target-detection exercises were included in the test sequence. Event-related potentials were recorded at scalp sites 01, 02, T5, T6, referenced to an anterior midline site. Comparisons were made where possible with the data from recent studies on three civilian groups. A pattern of response was revealed in the event-related potential measures. In general, left-hemisphere amplitudes exceeded right and the predominant response was recorded at the left-temporal site. Amplitudes of responses in subjects able to describe the stimuli in subjective report (perceivers) exceeded those of non-perceivers. Stimuli with definite boundaries evoked stronger and earlier-latency responses than stimuli with nebulous boundaries. The response patterns of subjects make a contribution to normative data on the cerebral electrical indices of stereoscopic vision.

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↗
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↗