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M E McCourt

Publications and source records attributed to M E McCourt.

33 records · Page 2Linked to original sources
PubMed1993

The effect of edge blur on grating induction magnitude.

In order to assess the contribution of high spatial frequency channels (i.e. local, edge-dependent mechanisms) to the grating induction effect, grating induction magnitude was measured as a function of systematic amounts of blurring of the inducing/test field boundary for four test field heights which spanned a two octave range (0.25-2.0 degrees). Measurements were obtained from two subjects using both the cancelling procedure of McCourt [(1982) Vision Research, 22, 119-134] and a contrast matching paradigm. The two measures yielded similar outcomes: consistent with previous results, both matching and cancelling contrast decreased monotonically with increasing test field height. The effect of blurring the edge was to produce a small (eta 2 = 0.6-5.1%), but significant (P < 0.001), increase in grating induction magnitude. A second experiment utilized the contrast matching paradigm to investigate the effect of edge blur at extreme values (i.e. zero blur and maximum blur) on grating induction magnitude across a four octave range of spatial frequency (0.0625-1.0 c/deg). The results of the matching procedure were again consistent with those obtained previously using the cancelling technique: grating induction magnitude decreased monotonically with increasing spatial frequency. The effect of blurring was to produce a modest (eta 2 = 1.0-2.8%), but significant (P < 0.001), elevation in induction magnitude. These results lead to the conclusion that, unlike some other brightness effects, visual spatial filters selectively sensitive to high spatial frequencies, or to the edges which they sharpen, are not essential for the production of the grating induction effect.

Contrast Sensitivity↗
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↗
PubMed1984

Refractive state, depth of focus and accommodation of the eye of the California ground squirrel (Spermophilus beecheyi).

Retinoscopy and electrophysiological refraction were performed on 55 and 24 far-accommodated eyes of California ground squirrels (Spermophilus beecheyi), respectively. These two indices were highly correlated, revealing the eye of this animal to be roughly emmetropic (-0.25 to -0.13 D). Depth of focus was assessed by measuring the effect which defocusing produced on the spatial resolving power of 32 optic nerve fibers. Depth of focus of the ground squirrel eye for a pupil diameter of 2.5 mm is estimated to be +/- 1.6 D, but will increase rapidly for smaller pupils. Accommodation in eleven ground squirrels ranged from 2 to 6 D, with a mean value of 3.9 D.

Accommodation, Ocular↗
PubMed1984

Spatial filter characteristics of optic nerve fibers in California ground squirrel (Spermophilus beecheyi).

The spatial response properties of single optic nerve fibers in the California ground squirrel (Spermophilus beecheyi) were investigated. Results are reported for each of several response classes. Resolution limits determined for 165 spectrally nonopponent optic nerve fibers ranged from 0.1 to 4.2 cycles/degree (c/d), with a mean resolution limit of 1.49 c/d. Directionally selective units possessed the highest resolution limits (mean = 1.91 c/d.) Sustained and transient units possessed mean resolution limits of 1.44 and 1.09 c/d, respectively. The correlation between response and sensitivity measures of spatial tuning was examined for 29 units. Optimal spatial frequency and spatial frequency bandwidth estimates derived from the two measures were highly correlated. As measured from response functions, optimal spatial frequencies of 97 spectrally nonopponent units ranged from 0.1 to 2.2 c/d (mean = 0.44 c/d). Directionally selective units possessed the highest optimal spatial frequencies (mean = 0.60 c/d). Mean optimal spatial frequencies for sustained and transient units were 0.32 and 0.27 c/d, respectively. Mean half-amplitude response bandwidth for 96 optic nerve fibers was 2.80 octaves. Directionally selective units were narrowly tuned, possessing a mean half-amplitude bandwidth of 1.99 octaves. Sustained and transient units had mean half-amplitude response bandwidths of 3.56 and 2.80 octaves, respectively. The response bandwidths of directionally selective optic nerve fibers were highly negatively correlated with optimal spatial frequency; no significant correlation between these measures existed for sustained or transient units. Peak contrast sensitivity in 88 optic nerve fibers ranged from 3.0 to 55.0. Mean contrast sensitivity was 16.7 and did not differ between fiber response classes. Peak contrast sensitivity was not correlated with optimal spatial frequency. Poststimulus time histograms of unit responses to sinusoidal luminance modulation revealed that some sustained units modulate their discharge rates around spontaneous levels and show predominant Fourier amplitude at the fundamental frequency. Transient units possess low levels of spontaneous discharge and respond abruptly to the passage of grating half-cycles, resulting in a characteristically dispersed spectrum of Fourier amplitude. Directionally selective units respond to sinusoidal luminance gratings with an elevation of maintained firing rate and possess moderate Fourier amplitudes corresponding to the fundamental and second harmonic frequencies.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1984

Directional filter characteristics of optic nerve fibers in California ground squirrel (Spermophilus beecheyi).

Directional units in the optic nerve of the California ground squirrel (Spermophilus beecheyi) were studied with respect to their response to diffuse light, preferred directions of motion, tuning for preferred direction, the relationship between spatial and directional tuning characteristics, and receptive-field size and areal summating properties. Directional units in the ground squirrel optic nerve are of the "on-off" type. No purely on or off units were encountered in a sample of 356 directionally selective fibers. The distribution of preferred directions of image motion for 356 units was significantly anisotropic; greater than 50% of the directional units prefer motion in the direction of the superior-nasal visual quadrant. Mean directional bandwidth, measured at half-amplitude response, for 39 units was 88.5 degrees. The distribution of directional bandwidths suggests that two subpopulations of directional units may exist a broadly tuned (106.4 degrees bandwidth) group preferring image motion in the superior-nasal direction, and a narrowly tuned group (59.9 degrees bandwidth) with a uniform distribution of preferred direction. Tuning for direction of motion and for spatial frequency were significantly positively correlated in a sample of 35 directional units. Area-vs.-response measures for directional units show that they possess excitatory discharge centers with a concentric antagonistic surround, plus a larger suppressive surround activated specifically by moving luminance contours, which may be asymmetric. Critical activation areas for directional units, as measured along orthogonal orientations, were highly positively correlated. This suggests that these receptive fields possess the property of linear spatial summation, not of luminance flux, but of areas of moving luminance contours.

Animals↗
PubMed1983

Effects of photic environment on the development of spectral response properties of optic nerve fibers in the ground squirrel.

The California ground squirrel (Spermophilus beecheyi) has two classes of cone photopigments (lambda max = 440 and 525 nm). Under photopic conditions about 30% of all optic nerve fibers receives inputs from both cone classes, the remainder are driven solely by the 525 nm cone. Recordings were made from optic nerves in young ground squirrels to trace the development of their spectral responsiveness. Animals were reared from birth in one of three photic environments: white light, darkness, or red light. In the youngest ground squirrels examined (ca. 50 days old) many units receiving inputs from the 525 nm cone were found, but the proportion of units receiving inputs from both cone classes was significantly lower than that of adults. From these initial low levels the proportion of such units increases gradually. Adult proportions were achieved at varying rates which depended on the photic environment in which the animal was reared: animals reared in white light achieved the adult standard earliest, those reared in darkness somewhat later, and the animals reared in red light required much longer to achieve the adult organization. We conclude that: (a) the neural substrates for normal color vision in this species develop to some extent postnatally, and (b) the normal sequence of development can be significantly extended by spectral environments which provide a highly biased stimulation of the two cone mechanisms.

Animals↗
PubMed1983

Brightness induction and the Café Wall illusion.

The Café Wall illusion is a distortion illusion in which the parallel lines of a chessboard-like figure consisting solely of parallel and perpendicular line elements appear to converge in alternating rows, creating a wedge distortion similar to that of the well-known Zöllner illusion. Gregory and Heard have formulated an explanation for the Café Wall illusion which relies upon the operation of a 'border-locking mechanism' in the visual system. The results of the present experiment suggest an alternative explanation in which the operation of brightness induction within the mortar regions of the Café Wall produces a series of 'twisted cords' or slanted line elements akin to those of the Fraser or Zöllner figures. A series of such 'twisted cords' is shown to be capable of itself to produce an illusory convergence like that of the Café Wall. Manipulations of the luminance of discrete regions in the mortar lines of the Café Wall, designed either to augment or cancel the effects of brightness induction in the production of these slanted line elements, are successful in enhancing or reducing, respectively, the wedge distortion of this visual illusion.

Adult↗
PubMed1982

A spatial frequency dependent grating-induction effect.

An inducing field containing a vertical sinewave luminance grating which surrounds a test field of similar space-average luminance induces within the homogeneous test field the appearance of a second sinewave grating of equal spatial frequency, but of opposite phase. The perceived contrast of the induced grating in the test field, as measured by a cancellation technique, can reach 90% that of the actual luminance contrast of the inducing grating. The perceived contrast of the induced grating decreases with increases in the spatial frequency of the inducing grating, and with increases in the dimension of the test field parallel to the orientation of the inducing grating. Square wave inducing gratings produce weaker induction effects than sinewave inducing gratings of the same spatial frequency and contrast. Additional observations indicate that the neural locus of this induction effect is cortical, lying at or beyond the level of spatial frequency selective channels.

Humans↗