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M E McCourt

Publications and source records attributed to M E McCourt.

8 recordsLinked to original sources
PubMed1990

Disappearance of grating induction at scotopic luminances.

The dependence of grating induction magnitude on retinal illuminance was examined in two subjects. Grating induction magnitude, as determined using the cancellation technique of McCourt, declines monotonically with decreasing retinal illuminance, effectively disappearing at a value of 0.3-0.5 phot td. In a second experiment, sensitivity differences for test lights of 500 and 600 nm were measured as a function of background illuminance in order to gauge the luminance operating range for grating induction with respect to duplex photoreceptor function. Cancelling contrast (and hence grating induction magnitude) fell below detection threshold contrast at retinal illuminances coinciding with the transition from photopic to scotopic visual function. In a third experiment, spatial contrast sensitivity was measured using both spatially extended (10 degrees) and truncated (2 degrees) sinewave gratings at frequencies below 2 c/deg, at three values of retinal illuminance. Illuminance values corresponded to those where grating induction magnitude was, as determined from the first experiment, either maximal, intermediate or negligible. Similar to grating induction, the strength of lateral inhibition, as indexed by the slope of the low-frequency decline in contrast sensitivity, is progressively reduced with decreasing retinal illuminance, particularly for the 2 degree field. There was, however, using the same criteria, evidence of lateral inhibition at a value of retinal illuminance which did not support grating induction. The implications of these results are discussed with respect to classical brightness contrast phenomena, recent neuroanatomical and neurophysiological evidence of segregated parvo- and magnocellular mediated contrast processing systems, and with results from previous studies of the grating induction effect.

Contrast Sensitivity↗
PubMed1990

Properties of area 17/18 border neurons contributing to the visual transcallosal pathway in the cat.

In a series of physiological experiments, a total of 203 neurons at the Area 17/18 border were recorded with a callosal link either demonstrated by antidromic or transsynaptic activation from stimulating electrodes located in the homotopic contralateral hemisphere (CH), or in the splenial segment of the corpus callosum (CC). Forty-four percent of the transcallosal cells could also be driven from stimulating electrodes in or just above the lateral geniculate nucleus (OR1). The majority (69%) of transcallosal neurons were classifiable as belonging to the complex family (B and C cells) and most of these were found in the supragranular laminae and in lamina 4A. The ocular dominance distribution of transcallosal cells was trimodal, consisting of roughly equal numbers of monocularly dominated and binocularly balanced neurons. Estimates of conduction time and synaptic delay were obtained for neurons driven from CH, CC, and from OR1, and in most instances the response latency was short enough to suggest a monosynaptic input from either the ipsi- or contra-lateral hemisphere. The distribution of transcallosal conduction times showed that S cells, as a class, had significantly faster conduction than cells of the complex family but otherwise there was no obvious signs of multimodality in the distribution curve. An analysis of the synaptic delays in transcallosal activation produced a mean of 0.6 to 0.7 ms but some were too short to be consistent with a transsynaptic drive, suggesting that some cells with an antidromic drive may have been included in the transsynaptic category. Results are interpreted in terms of the contribution made by the corpus callosum to stereoscopic vision.

Animals↗
PubMed1988

Physiological studies on the feedback connection to the striate cortex from cortical areas 18 and 19 of the cat.

The functional characteristics of the feedback connections from areas 18 and 19 to area 17 in the cat have been examined with electrophysiological techniques. The experiments involved single unit recording in laminae 2 and 3 of area 17 while stimulating electrically a small region of area 18 or 19. It was found that a precise retinotopic correspondence between the sites of recording and stimulation was necessary before neurons of area 17 could be activated by electrical stimulation in extrastriate areas. Latencies were long compared to those obtained after stimulation of the optic radiation. The mean latency for orthodromic drive from area 19 was 10.4 ms and 6.1 ms from area 18, suggesting that the conduction velocities in these pathways are of the order of 1 m/s. The jitter of the latency after repeated orthodromic stimulation was often shorter than 0.3 ms, indicating that a large number of the sampled neurons received a direct drive from area 18 or from area 19. The functional properties of neurons driven from area 19 were different from those of cells driven from area 18. Thus, most striate neurons orthodromically driven from area 19 were of the SH and S type whereas the cells activated by area 18 stimulation belonged to the C and B categories.

Animals↗
PubMed1988

Factors governing the adaptation of cells in area-17 of the cat visual cortex.

Neurons in area 17 of the cat visual cortex adapt when stimulated by drifting patterns of optimal orientation, spatial frequency and temporal frequency (Ohzawa et al. 1982; Albrecht et al. 1984; Ohzawa et al. 1985). A component of this adaptation has been attributed to a contrast gain-control mechanism, rather than to neural fatigue, and results in enhanced differential sensitivity around the adapting contrast level (Ohzawa et al. 1982; Albrecht et al. 1984; Ohzawa et al. 1985). Experiments described here suggest that neural response rate, the directional selectivity of the cell, and the temporal frequency of the stimulus, are the principal determinants of adaptation, irrespective of other stimulus parameters such as contrast, velocity, or spatial frequency. The present results can nevertheless accommodate the results of previous studies of adaptation, and additionally provide scope for the resolution of apparent contradictions between results from psychophysical and neurophysiological studies of adaptation.

Adaptation, Physiological↗
PubMed1986

Layering in lamina 6 of cat striate cortex.

Retrograde transport after the injection of the tracer, wheatgerm agglutinin-horseradish peroxidase, into different neural sites revealed a layering of labelled cells in lamina 6 of the striate cortex of the cat. Depending on their destination, efferent cells were clustered at different levels in lamina 6 so that cells projecting to the claustrum congregated in the lower half of the lamina while those projecting to other parts of the visual cortex, in either ipsi- or contralateral hemispheres, were found principally in the upper half and the cells with axons passing to the lateral geniculate nucleus occupied the central expanse (middle three-fifths) of the lamina.

Animals↗
PubMed1985

Spatial frequency interference on grating-induction.

Spatial frequency interference and facilitation in suprathreshold vision were studied using the grating-induction effect [McCourt, Vision Res. 22, 119-134 (1982)] as a sensitive probe. The effects on grating-induction magnitude produced by variations in "interfering" and "inducing" grating spatial frequency, contrast and phase were examined in four experiments. A limited range of high spatial frequency interfering gratings reduced the contrast of gratings induced by spatially coextensive lower frequency inducing gratings. Both phase-dependent and phase-independent interference was observed. Facilitation of grating-induction was produced by interfering gratings of lower frequency than the inducing grating. It is hypothesized that the grating-induction interference effect is due to inhibition of the low spatial frequency selective mechanisms responsible for induction, by channels tuned to higher frequencies. The functional significance of induction and spatial frequency inhibition is discussed, and a mathematical description of the results is presented.

Form Perception↗
PubMed1985

Visual grating induction.

If a homogeneous illuminated test field is inserted within a sine-wave grating, an opposite phase grating will be perceived in the test field under a wide range of conditions. A cancellation technique was used to measure the magnitude of grating induction. The manner in which the effect depends on eye movements, inducing frequency, test-field height, inducing-field height, inducing amplitude, test-field luminance, and test-field width was determined in four experiments. Mathematical equations that describe these results are presented. It is shown that linear filters whose spatial weighting functions resemble receptive fields of the most common types of visual cell do not produce outputs with the properties of induced gratings. However, linear filters with highly elongated negative end zones and a small positive center produce opposite phase gratings in the test field, and an array of such filters of different sizes can account for several properties of induced gratings. There are other properties of the effect that are highly nonlinear. A second model, which is nonlinear and based on the properties of hypercomplex cells, is suggested that may encompass both the linear and the nonlinear properties of the effect.

Adaptation, Physiological↗