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W R Levick

Publications and source records attributed to W R Levick.

At least 37 records · Page 2Linked to original sources
PubMed1980

Ocular pigmentation in white and Siamese cats.

Ocular pigmentation in white cats with blue and yellow eyes and in Siamese cats was examined ophthalmoscopically and histologically. Yellow-eyed white cats had entirely normal ocular pigmentation. Blue eyes of white cats had normal pigmentation of the iridial and retinal pigment epithelia but no stromal pigmentation of the iris or choroid. This deficit is apparently due to the absence of stromal pigment cells, certainly in the iris. As a general rule, the blue eye of white cats had no tapetum. Siamese cats had reduced pigmentation of the iridial and retinal pigment epithelia and no stromal pigmentation of the iris or choroid. The lack of pigmentation is apparently due to the inability of stromal pigment cells to produce pigment, certainly in the iris. We conclude that the abnormality of visual pathways previously described in the Siamese cat is not due simply to a deficiency of pigment of cells of neural crest origin.

Animals↗
PubMed1976

Lateral geniculate relay of slowly conducting retinal afferents to cat visual cortex.

1. Lateral geniculate neurones of the cat were studied in terms of the latency for activation by local electrical stimulation of the retina, the latency of electrical activation from the visual cortex and properties of receptive fields. Most of the units were relay cells (antidromic activation from visual cortex) but a small proportion were trans-synaptically activated from the cortex. The latter group included units with on-off, on-centre or off-centra receptive fields. 2. Direct activation of lateral geniculate neurones from local electrical stimulation of retinal ganglion cells or their axons in the retina was identified by the sharpness of timing of the elicited impulses. This procedure revealed the existence of slowly conducting axons relaying in the lateral geniculate nucleus. 3. The distribution of latencies for direct activation from the retina was bimodal with an extended tail of long values. It is similar to the distribution of antidromic latencies of retinal ganglion cells following stimulation of the optic tract. 4. There was a tendency for geniculate neurones with fast input from the retina to have fast axons to the visual cortex and correspondingly for medium-speed and slow input. 5. The previous classification of geniculate receptive fields into sustained and transient types was extended to include commonly encountered 'brisk' and uncommonly encountered 'sluggish' varieties of each. The extension was based on visual properties and latency for direct electrical activation from the retina. Units with receptive fields differing from the familiar on-centre or off-centre concentric pattern were encountered rarely; they included colour-coded fields, local-edge-detectors and one edge-inhibitory off-centre type.

Animals↗
PubMed1976

Threshold setting by the surround of cat retinal ganglion cells.

1. The slope of curves relating the log increment threshold to log background luminance in cat retinal ganglion cells is affected by the area and duration of the test stimulus, as it is in human pyschophysical experiments. 2. Using large area, long duration stimuli the slopes average 0-82 and approach close to 1 (Weber's Law) in the steepest cases. Small stimuli gave an average of 0-53 for on-centre units using brief stimuli, and 0-56 for off-centre units, using long stimuli. Slopes under 0-5 (square root law) were not found over an extended range of luminances. 3. On individual units the slope was generally greater for larger and longer test stimulus, but no unit showed the full extent of change from slope of 0-5 to slope of 1. 4. The above differences hold for objective measures of quantum/spike ratio, as well as for thresholds either judged by ear or assessed by calculation. 5. The steeper slope of the curves for large area, long duration test stimuli compared with small, long duration stimuli, is associated with the increased effectiveness of antagonism from the surround at high backgrounds. This change may be less pronounced in off-centre units, one of which (probably transient Y-type) showed no difference of slope, and gave parallel area-threshold curves at widely separated background luminances, confirming the importance of differential surround effectiveness in changing the slope of the curves. 6. In on-centre units, the increased relative effectiveness of the surround is associated with the part of the raised background light that falls on the receptive field centre. 7. It is suggested that the variable surround functions as a zero-offset control that sets the threshold excitation required for generating impulses, and that this is separate from gain-setting adaptive mechanisms. This may be how ganglion cells maintain high incremental sensitivity in spite of a strong maintained excitatory drive that would otherwise cause compressive response non-linearities.

Action Potentials↗
PubMed1975

Form and function of cat retinal ganglion cells.

Recent explorations of the morphology of retinal neurones, combined with neurophysiological recordings have made it possible to link specific anatomical types with particular physiological classes. At the same time, the relatively complete anatomical mapping of the retina has revealed some bias in the sampling of neurones by electrophysiological techniques.

Animals↗
PubMed1975

The distribution of the alpha type of ganglion cells in the cat's retina.

There is a distinct class of large ganglion cell bodies observable in whole-mount preparations of the cat's retina stained with cresyl violet. Measurements of perikaryal size and its variation with eccentricity from the central area support the identification of the large cells with the class of alpha cells previously defined in Golgi-stained preparations. A complete enumeration of alpha cells in one retina yielded a total of 6212. They were encountered in all parts of the retina and had a peak density within the central area ( 200/mm2). The retinal distribution was displayed as a contour map of alpha-cell density. Away from the central area, the lines of iso-density had the form of a 4-pointed star with rather blunt points corresponding to horizontal and vertical ridges of augmented density. Along horizontal, vertical and oblique strips through the central area, alpha-cell density was an approximately constant fraction of total ganglion cell-density. The average value of the fraction was 3.3% from which it has been calculated that the cat's retina may contain as many as 190,000 ganglion cells.

Animals↗
PubMed1975

Physiological identification of a morphological class of cat retinal ganglion cells.

1. Small selected patches of retina (approximately 1 mm2) were exhaustively searched electrophysiologically to identify every brisk-transient unit present. The patches were then marked by electrolytic lesions. 2. Whole-mount preparations were made from which the distribution of alpha cells within and around the marked areas could be obtained. 3. A one-to-one correspondence could be demonstrated between brisk-transient units and alpha cells. 4. The correspondence was maintained within a patch of retina including the central area. 5. The correspondences were subject to about 10% of uncertainty attributed to technical limitations of the method.

Animals↗
PubMed1974

Brisk and sluggish concentrically organized ganglion cells in the cat's retina.

1. Nine hundred and sixty cat retinal ganglion cells were evaluated with respect to receptive-field organization and latency to antidromic activation of their axons from optic-tract and mid-brain positions.2. The vast majority (92%) had the familiar concentric centre/surround organization. As in earlier work these could be classed as sustained or transient, independently of the centre type. About 13% of the concentric cells were characterized by relatively sluggish responses to conventional visual stimuli which yielded brisk responses from the others. The sluggish cells constituted a previously unspecified class of concentric receptive fields.3. The responses of brisk and sluggish cells to a variety of stimuli were described with a view to developing procedures for distinguishing them on functional grounds.4. Measurements of latency to antidromic activation of retinal axons confirmed earlier work in showing that cells classed as brisk-transient had the shortest conduction times from the optic tract. Cells classed as brisk-sustained had intermediate conduction times and from earlier work would constitute an important input to the lateral geniculate nucleus. A proportion of the brisk-sustained axons reached the pretectal region (especially on-centre types) and a small minority reached the superior colliculus (especially off-centre types).5. Sluggish cells had generally slower antidromic conduction times; despite some overlap with the brisk-sustained class, the slower conduction provided independent support for the functional differentiation. Sluggish axons reached the pretectal region and superior colliculus.6. The brisk-sustained cells constituted the majority of the recordings in the area centralis.7. A comparison with the morphological data of Boycott & Wässle is made which suggests that the brisk-transient units corresponded with alpha cells, the brisk-sustained with beta cells, and the sluggish units were included amongst the gamma cells.

Animals↗
PubMed1974

Properties of rarely encountered types of ganglion cells in the cat's retina and an overall classification.

1. In a reference sample of 960 cat retinal ganglion cells, seventy-three had receptive fields departing from the concentric centre-surround pattern.2. Five classes were distinguished among the subset: local edge detectors, direction-selective cells, colour-coded cells, uniformity detectors, edge inhibitory off-centre cells.3. Local edge detectors (forty-five) possessed a radially symmetrical pattern of responses to both centrifugal and centripetal movements of both black and white small targets, an on-off receptive field with a silent inhibitory surround and a low or zero maintained discharge. Their operation could be interpreted as the detection of a contrasting border confined to a small region of the visual field.4. With direction-selective units (eleven) it was possible to find an axis through the receptive field along which sharply different responses could be obtained for opposite directions of movement of small black or white targets.5. Colour units (six) were mostly of the single opponent type having excitatory input from blue-sensitive cones and inhibitory input from long wave-length cones. Both inputs coexisted at the centre of the field and either could be spatially more extensive than the other. One example changed over to rod input under scotopic conditions, another did not.6. Uniformity detectors (five) had a brisk maintained discharge which was reduced or abolished temporarily by all forms of visual stimulation.7. Edge inhibitory off-centre units (three) behaved like uniformity detectors for small targets and fine gratings but like off-centre on-surround units for large targets. Their receptive fields consisted of three concentric regions: a small sized, central edge inhibitory region; a larger zone of off-responsiveness; and an outlying annulus of on-responsiveness.8. It is argued that the above physiological types belong to the morphologically heterogeneous class of cells called gamma cells. The argument is based on similarity in the sizes of receptive fields and dendritic trees and on evidence that the axons are thinner than those of the brisk-sustained and brisk-transient ganglion cells.9. The physiological classification of cat retinal ganglion cells developed in this paper and the preceding one is summarized in a Table.10. It now appears that cat and rabbit possess a qualitatively similar complement of receptive field types among their ganglion cells; the differences reside in the quantitative expression of the various classes.

Animals↗