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Biomedical subjects

B Fenelon

Publications and source records attributed to B Fenelon.

At least 19 recordsLinked to original sources
PubMed1991

Discrimination of motion in depth trajectory following acute alcohol ingestion.

Evoked potentials were recorded at four scalp sites O1, O2, T5, T6; before and after the consumption of one of two alcohol dose levels, to the occurrence of disparate stimuli in dynamic random dot stereograms. Twenty young adult subjects, all of whom had vision which was normal or corrected to normal, were randomly assigned to the two dosage groups. Subjects viewed stimuli which embodied crossed disparity and followed one of three trajectories in depth. The task was to distinguish between and identify, by specific button presses, these trajectories. One half of the stimuli were presented with monocular cues to motion in depth (dark condition); in the remainder only stereoscopic depth information was available (uniform condition). Responses to both conditions showed alcohol-related reduction of evoked responses across all sites. It was concluded that the visual systems associated with the processing of motion in depth were susceptible to alcohol effects, as revealed by reductions in the amplitude of evoked responses.

Adult↗
PubMed1991

Patterns of neuropsychological impairment after severe blunt head injury.

A consecutive series of 100 subjects with severe blunt head injuries was followed up 6 years after trauma. Neuropsychological test performances of 82 subjects and of a noninjured control group were analyzed by two principal components analyses (PCAs). Each PCA extracted 15 factors relating to a range of cognitive impairments, as well as neuropsychological features consistent with posttraumatic personality changes. Measures identified by the PCAs were applied to 85 head-injured subjects in the series to examine the incidence of impairment in four neuropsychological areas: disorders of learning and memory, neuropsychological features consistent with posttraumatic personality change, slowness in rate of information processing, and a range of basic neuropsychological skills. Overall, impairments occurred in 70% of the series. Disorders of learning and memory were the most common type of deficit (56.5%), with disturbances in basic neuropsychological skills the least frequent (16.5%). Variability among subjects with respect to the types and combinations of neuropsychological impairments was a characteristic feature of this clinical group, but the largest proportion (one-third) demonstrated isolated impairments. The implications of the incidence and selectivity of neuropsychological impairments are discussed.

Adolescent↗
PubMed1988

Scalp response topography to dynamic random dot stereograms.

Responses were recorded to dynamic random dot stereograms from 32 sites (acquired in separate 8-channel montages). The disparate stimulus comprised a central, 3 degrees square of 30 arc min crossed disparity, superimposed on a 10.7 degrees square background field. Stimulus onset occurred 100 msec into a 500 msec recording epoch, with a 100 msec stimulus duration. Wave forms for 8 subjects were normalised across the 100 msec pre-stimulus period. Average amplitudes were calculated for the post-onset intervals: 120-140, 140-180, 180-220, 220-260 and 260-300 msec. Contour lines of evoked response amplitude were plotted from these data. These plots are compared with theoretical plots generated from a '3 concentric sphere' model of the head. The results lend support to the proposal that preliminary stereoscopic processing takes place in the primary visual cortex, followed by secondary processing in more anterior cortical regions.

Adult↗
PubMed1988

Visual evoked potentials to stimuli in apparent motion.

Following an extensive single subject pilot study 12 Ss viewed a continuously cycling (3 Hz), two stimulus, apparent motion (AM) display. The "AM breakdown effect" was utilised to gather visual evoked potentials (VEPs) from sites O1, O2, T5, T6, P3, and P4 during periods in which the display elicited either: the percept of motion; or the percept of two discrete alternating stimuli. VEPs displayed components of larger amplitude during motion periods in all right (but no left) hemisphere sites during periods 60-82 msec post left field stimulus onset (positive component) and 100-126 msec post right field stimulus onset (negative component). Results were interpreted as indicating initial extraction of motion information within the occipital lobe, with further motion processing taking place in temporal and parietal lobes.

Adult↗
PubMed1988

The determinants of the non-motoric CNV in a complex, variable foreperiod, information processing paradigm.

The aim of the present study was to examine the determinants of the non-motoric CNV or E-wave elicited in a complex, variable foreperiod information processing task. Subjects were required to determine whether two sets of tones, one set presented back-to-back (S1/S2), the other separated by a variable foreperiod (S3/S4), were either matching or mismatching. Data were collected over two recording sessions; a baseline and an experimental run. The experimental session comprised three conditions; a match, a mismatch and no-response conditions. Mean amplitude and factor score comparisons showed the parietally-dominant E-wave to be dependent on the task relevance of the stimulus which it precedes. Differing midline distributions for the negative afterwave and the E-wave with increased processing requirements was taken to suggest that the two responses reflect functionally distinct phenomena.

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↗
PubMed1987

Effects of movement in the background field on long-range apparent motion.

Two experiments are reported using time-till-breakdown as a measure of apparent motion (AM) in untrained subjects. In Experiment 1 centrally viewed dynamic random dot stereograms and non-disparate stimuli in dynamic random dot fields yielded higher optimal frequencies than did a standard binocular condition. In Experiment 2 a higher optimal frequency was observed for disparate dynamic random dot stimuli compared with both standard binocular and non disparate stimuli presented on a static random dot field. Interaction between short- and long-range AM processes is considered in the interpretation of the findings.

Depth Perception↗
PubMed1987

Individual differences in responses of untrained observers to stroboscopic apparent motion.

Two large groups of inexperienced subjects (n = 208 and n = 50) and a small group of experienced subjects (n = 5) were tested using time-till-breakdown as a measure of long-range apparent motion across a range of temporal frequencies. One group of inexperienced subjects was retested after one week and demonstrated quite stable patterns of response. Large intersubject variability was observed in terms of the amount of motion seen, with most inexperienced subjects reporting very little apparent motion. A raster display produced a peak frequency 1 Hz higher than a standard tachistoscope display. The role of experience was also examined with a small group of inexperienced subjects (n = 8) tested once daily over five consecutive days. There was high intersubject variability and intrasubject consistency, demonstrating little influence of learning and experience. The results are discussed in terms of current ideas on the breakdown effect.

Humans↗
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↗
PubMed1986

Global stereopsis in stroke patients.

Contradictory evidence has been presented in the literature on the existence of a right-hemisphere mechanism subserving global stereopsis. In the present study evoked responses to dynamic random-dot stereograms were recorded from International 10-20 scalp sites O2, P4, T6, T4 and homotopic sites over the left hemisphere. Stroke patients with unilateral lesions were selected on the basis of satisfactory performance on ocular screening tests and on clinical or objective indications that dementia was absent. Control subjects were found among the relatives of patients. The stereopsis test schedule included left, right and centre field presentations of 30 arc min, disparities. The dot density of the disparity, relative to that of the remainder of the random dot field, was manipulated to provide a full contrast stimulus and a no-contrast uniform field stimulus, thus varying the availability of monocular cues provided by the contrast factor. Latency and amplitude of the evoked potentials were computed and submitted to analyses of variance. No significant results were found which supported the proposition of a lateralized mechanism for global stereopsis. This conclusion is in conflict with the findings of some earlier reports. It is suggested that the techniques used in the earlier experiments may not have been adequate for the study of the questions involved.

Aged↗
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↗