Biomedical subjects

F MacCana

Publications and source records attributed to F MacCana.

4 recordsLinked to original sources

PubMed1988

The effects of spatial frequency and temporal waveform on three measures of temporal processing.

The effects of spatial frequency and temporal transition of sine-wave grating onset and offset were assessed using measures of reaction time, visual persistence, and temporal order judgements. The stimuli were lateralized fields, separated by 1 degree of visual angle. Slow temporal transition resulted in significantly poorer performance than did abrupt onset and offset, but spatial frequency had a minimal effect. Thus, the latency, temporal resolution, and temporal ordering of events are mediated by a mechanism that is sensitive to abrupt temporal transients. The stimulus conditions employed did not result in a shift in the point of subjective simultaneity.

Attention↗

PubMed1983

Contribution of two movement detecting mechanisms to central and peripheral vision.

Two mechanisms, one for the detection of fast, and the other for slow movement of a sinusoidal grating are identified, and investigated under central, parafoveal, and peripheral viewing conditions. The fast movement data is considered in terms of the Reichardt model, in which signals from two adjacent inputs are cross-correlated leading to halving of the spatial resolving power for movement detection, compared with that for pattern detection. The mechanism underlying slow movement detection is regarded as being closely related to pattern detection, probably at the single unit level. The characteristics of this mechanism are discussed in the light of recent electrophysiological experiments describing clusters of simple cells in the visual cortex with "directional preference" properties.

Form Perception↗

PubMed1983

Changes in spatial resolution for pattern and movement detection in clinical cases.

A simple and relatively fast method was used to evaluate visual performance: gratings of sinusoidal luminance profile, generated on the face of an oscilloscope, were reversed in contrast at a rate of 2 Hz. Under these conditions the patients could set separate contrast thresholds for seeing either the structure of a grating or its apparent movement. The grating resolution limit (maximum spatial frequency) for each task was determined by extrapolation from thresholds measured at several spatial frequencies. The resulting ratios of pattern/movement resolution limits (P:M) are usually between 1.6 and 2 for normals and for patients with pathological changes located distally (probably prior to the chiasma). On the other hand in patients suspected of multiple sclerosis the P:M ratios were reduced to below 1.3. It is considered that such results may suggest a central site for the damage, although they do not exclude pre-chiasmal defects which may occur simultaneously during the acute stages of the disease.

Adult↗

PubMed1983

Flash and pattern VEPs: examples of cases of hysterical amblyopia and provoked visual impairment (Uhthoff's sign).

The diagnostic application of averaged visual evoked potentials (VEPs) is illustrated in the investigation of hysterical amblyopia and multiple sclerosis. Averaged VEPs elicited either by a flash of light or a presentation of a grating lasting 100 ms were recorded from the occiput of patients. All the VEPs in hysterical amblyopia were normal. In multiple sclerosis the flash VEPs looked normal for both eyes, whereas the VEPs elicited by grating (2 and 5 c/deg) showed an increase in the latency of the first positive wave, again for both eyes. After physical exercise the VEPs from the right eye were dramatically reduced in amplitude (Uhthoff's sign).

Adolescent↗