Biomedical subjects

M J Musselwhite

Publications and source records attributed to M J Musselwhite.

5 recordsLinked to original sources

PubMed1986

A visual evoked potential study of metacontrast masking.

In a comparison of the subjective appearance of, and the scalp-recorded potentials evoked by, the first of two successively presented patterns of spatially adjacent elements, we recorded typical U-shaped metacontrast masking functions but found no discernible modification of either of the two initial VEP components, C1 and C2. These results, together with the previously established properties of C1 and C2, suggest that the early stages of contour-specific processing in the visual cortex have relatively short response latencies and durations and are uninfluenced by the subsequent presentation of metacontrast masking stimuli. The VEP data would thus appear to conflict with the basic assumptions of inhibition theories of metacontrast.

Evoked Potentials, Visual↗

PubMed1985

The influence of spatial frequency on the reaction times and evoked potentials recorded to grating pattern stimuli.

The simple reaction times recorded to sine-wave and square-wave grating stimulus patterns of both constant physical contrast and of constant suprathreshold contrast were appreciably delayed by an increase in spatial frequency from 0.5 to 10 c/deg. There was no comparable increase, however, in the peak latency of the initial visual evoked potential component, C1, recorded to the same stimulus patterns. In view of the evidence that C1 has a striate cortical origin, these results suggest that the large spatial-frequency dependent variations in RT do not reflect delays of stimulus-induced neuronal responses in the primary visual pathway from retina to striate cortex.

Evoked Potentials, Visual↗

PubMed1984

The influence of background motion on the motion aftereffect.

The motion aftereffect caused by adaptation to moving bars is visible in a stationary test pattern consisting of static visual noise (texture). The aftereffect resulting from adaptation to moving bars presented on a background of texture is highly dependent on the direction and velocity of motion of the background during adaptation, and less dependent on the nature of the test pattern. Background motion in the same direction as bar motion during adaptation enhances the aftereffect, whilst a stationary background or background motion in the opposite direction suppresses, and in some cases reverses the direction of, the aftereffect. The influence of background motion is greatest using a textured test pattern, a low adapting texture velocity, and a low grating spatial frequency. The physiological implications of these results are discussed.

Adaptation, Ocular↗

PubMed1983

Visual evoked potentials to double-pulse pattern presentation.

The temporal resolution and summation characteristics of human cortical processes were investigated by recording an individual VEP component, C1, to the double-pulse presentation of pattern pairs of both the same (+ve/+ve; -ve/-ve) and of opposite (+ve/-ve; -ve/+ve) contrast polarity at varying onset-to-onset intervals (SOA). The results show that comparable limiting SOA values of 40-50 msec are needed for the C1 components to the two patterns to separate out to form a double-peaked response and for the stimulus to be seen as two distinct events. Also, the amplitude variations of the single-peaked response obtained at shorter SOAs show evidence of complete precortical response integration for SOA values below 5-10 msec and partial integration for values up to 30-40 msec, which again correspond to the results of related psychophysical studies. These VEPs show no reflection, however, of the inhibition/summation effects reported for subjective responses to pattern pairs of the same/opposite contrast polarity at SOA values between 40 and 70 msec. The implications of these findings are discussed.

Evoked Potentials, Visual↗

PubMed1982

Pattern-evoked potentials and Bloch's law.

Experiments are described which show that the latencies, unlike the amplitudes, of the C1 and C2 components of the human pattern-onset VEP do not conform to the contrast equivalent of Bloch's law. Whereas each component's amplitude is linearly related to the log of the (contrast X duration) product below a critical duration of 50 msec, its latency is independent of the stimulus pattern's duration and determined by its contrast alone; increasing by 30-35 msec for a 1.4 log unit contrast decrease. The relationship between these results and corresponding single-unit findings in the cat is discussed.

Evoked Potentials, Visual↗