Biomedical subjects

J E Raymond

Publications and source records attributed to J E Raymond.

3 recordsLinked to original sources

PubMed1992

Temporary suppression of visual processing in an RSVP task: an attentional blink? .

Through rapid serial visual presentation (RSVP), we asked Ss to identify a partially specified letter (target) and then to detect the presence or absence of a fully specified letter (probe). Whereas targets are accurately identified, probes are poorly detected when they are presented during a 270-ms interval beginning 180 ms after the target. Probes presented immediately after the target or later in the RSVP stream are accurately detected. This temporary reduction in probe detection was not found in conditions in which a brief blank interval followed the target or Ss were not required to identify the target. The data suggest that the presentation of stimuli after the target but before target-identification processes are complete produces interference at a letter-recognition stage. This interference may cause the temporary suppression of visual attention mechanisms observed in the present study.

Adult↗

PubMed1992

Visual accommodation and sustained visual resolution in multiple sclerosis.

Many multiple sclerosis (MS) patients frequently experience transient blurring. We investigated the possibility that this symptom is due to the inability of patients to sustain an accommodative response to stimuli viewed at distances nearer than or farther from the individual tonus position of accommodation. In a group of MS patients and age-matched healthy control subjects, we measured (1) accommodative range and tonus position; (2) reaction time (RT) to detect a change in a small optotype (viewed in a Badal lens system) as a function of viewing distance; and (3) contrast sensitivity at a fixed viewing distance. MS patients did not differ significantly from healthy controls on near point, far point, pupil size, accommodative range, or tonus position measures. However, as a group, MS patients showed significantly slower RTs than controls to detect optotype changes for stimuli viewed at distances nearer to or farther from the individual tonus position of accommodation. All subjects showed significantly slower RTs to detect changes in optotypes viewed at extreme near and far optical distances compared to RTs to detect changes in stimuli viewed at the tonus position. This difference was significantly larger for MS patients than for controls. These data also suggest that dynamic dioptric factors contribute to the magnitude of contrast sensitivity deficits in this patient population and indicate that the relationship between the individual tonus accommodation position and viewing distance is an important variable in CS testing.

Accommodation, Ocular↗

PubMed1990

The effect of local luminance contrast on induced motion.

Recently it has been suggested that the magnocellular, as opposed to the parvocellular, subsystem of the primary visual pathway of primates subserves motion perception. This suggestion is partly based on the observation that both the visual responses of magnocellular neural units and certain motion perception phenomena have high contrast sensitivity and are only dependent on luminance contrast for a narrow range of low contrasts. Parvocellular units have low contrast sensitivity and are dependent on contrast for a wide range of values. In the present experiment, the effect of local luminance contrast on induced motion was measured using a nulling procedure to quantify the magnitude, of illusory motion perceived in a centre grating which was viewed against a moving surround grafting. Centre grating contrast was either matched to the surround or maintained at a low (2.5%) or high (60%) value. Surround contrast ranged from 2.5% to 60%. It was found that (1) centre contrast had no observable effect on the magnitude of the illusion, (2) induced motion was marginal or absent with low contrast but detectable surrounds, and (3) induced motion increased as contrast in the surround increased for the range of contrasts tested. This contrast response function is more similar to that of parvocellular than magnocellular units and therefore suggests that the parvocellular stream may play a role in some aspects of motion processing.

Adult↗