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W R Levick

Publications and source records attributed to W R Levick.

At least 55 records · Page 3Linked to original sources
PubMed1973

Properties of sustained and transient ganglion cells in the cat retina.

1. The functional basis for a sustained/transient classification of cat retinal ganglion cells has been strengthened by quantitative measurements of the sizes of the centre and surround components of receptive fields. Transient cells had larger surrounds than sustained; the distributions were non-overlapping. Although the distributions of centre sizes overlapped, the transient cells had very significantly larger centres on average.2. There were characteristic differences in area-threshold and annulus-threshold curves and shapes of responses to brief and long flashes of light, and relative differences in the nature of surround adaptation and maintained discharge rates.3. The distinction of sustained from transient units survived over a wide range of backgrounds including the two principle reorganizations of function: relative weakening of surround components at low background; Purkinje shift at high.4. Sustained and transient units were differentially distributed in the retina: there was an overwhelming preponderance of sustained units in the area centralis.5. It is proposed that the transient units are the so-called multidendritedeep cells and the sustained units are the variously styled small ganglion cells.

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PubMed1971

Sustained and transient neurones in the cat's retina and lateral geniculate nucleus.

1. Cat retinal ganglion cells may be subdivided into sustained and transient response-types by the application of a battery of simple tests based on responses to standing contrast, fine grating patterns, size and speed of contrasting targets, and on the presence or absence of the periphery effect. The classification is equivalent to the ;X'/;Y' (linear/nonlinear) subdivision of Enroth-Cugell & Robson which is thus confirmed and extended.2. The sustained/transient classification applied to both on-centre and off-centre cells.3. Lateral geniculate neurones may be similarly classified by the same tests. Occasional concentrically organized cells had a mixture of sustained and transient properties.4. A technique for simultaneous recording from a geniculate neurone and one or more retinal ganglion cells providing its excitatory input showed that the connexions were specific with respect to the sustained/transient classification as well as the on-centre/off-centre classification. Most geniculate neurones are excitatorily driven only by retinal ganglion cells of the same functional type. In a few cases the inputs were mixed but only with respect to the sustained/transient classification.5. Sustained retinal ganglion cells had slower-conducting axons than the transient type. The same was true for lateral geniculate neurones but in this case the distributions showed considerable overlap.6. The sustained/transient classification is the functional correlate for the well-known segregation of optic nerve fibres into two conduction groups.7. The pathways carrying sustained and transient information remain essentially separate from retina through the lateral geniculate nucleus to the striate cortex.

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PubMed1970

Responses of cat retinal ganglion cells to brief flashes of light.

1. The responses of cat retinal ganglion cells to brief flashes of light have been illustrated and described with a view to providing material for comparison with psychophysical experiments in the scotopic (rod-dominated) range of performance.2. There is a minimum response duration of 50-70 msec no matter how brief the flash is made. This duration is reached with stimuli lasting 32 msec or shorter.3. Reducing the background illumination obviously increases the latency of responses to stimuli at 4 times threshold intensity (about 10 msec increment per log. unit decrement) but has no obvious effect on the minimum response duration.4. The relation between intensity and duration of a flash for threshold responses closely resembles that in human psychophysical experiments. The Bunsen-Roscoe law is applicable for flash durations up to 64 msec.5. If equal amounts of energy are delivered in the form of a pair of flashes of varying separation rather than by rectangular pulses, the shape of the response changes more abruptly with the temporal factor.6. Non-linear performance is apparent for stimuli as weak as 4 times threshold.7. A method is developed for quantitative analysis of individual responses. It is based upon cross-correlation of the train of impulses with a Gaussian smoothing function and represents local impulse frequency as a smooth function of time. The method also improves the signal-to-noise ratio of post-stimulus time-histograms of the sum of many responses.8. The measurement of variability of individual responses is the main result of the method; its magnitude indicates that it is a significant new factor limiting temporal resolution with suprathreshold stimuli.

Animals↗
PubMed1969

Rabbit lateral geniculate nucleus: sharpener of directional information.

Direction-selective neurons in the rabbit lateral geniculate nucleus signal movement-direction more precisely than their retinal counterparts. This increased selectivity arises from the interaction of retinal inputs to the geniculate cell. A direction-selective geniculate neuron is fed by at least two retinal direction-selective cells, whose preferred directions are 180 degrees apart. One retinal input is excitatory to the geniculate cell and the other is inhibitory.

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PubMed1969

Three factors limiting the reliable detection of light by retinal ganglion cells of the cat.

1. Responses of cat retinal ganglion cells have been examined with a view to specifying the characteristics that limit the detection of light stimuli.2. Threshold is defined as the weakest stimulus that can be reliably detected by examination of the output from a retinal ganglion cell; it depends upon (a) the quantum/spike ratio, which is the mean number of additional quantal absorptions required to produce an additional impulse, (b) the temporal course of the response, which determines the time interval within which the maintained discharge is modified, and (c) the statistical distribution of the number of impulses that occur in this time interval in the absence of the stimulus.3. The quantum/spike ratio changes greatly when adapting luminance is changed, and this is the predominant factor accounting for changes in increment threshold.4. The time course of the response changes with adaptation level and area of the stimulus. This may account for the changes in temporal integration that occur in analogous psychophysical experiments.5. Changes in the irregularity of the maintained discharge also affect the threshold of single ganglion cells. This is only a minor factor in the conditions of most of our experiments, but it may be important when unstabilized images and non-equilibrium adaptation conditions are encountered.

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PubMed1969

Changes in the maintained discharge with adaptation level in the cat retina.

1. The mean rate, impulse interval distribution, and pulse number distribution of the maintained discharge of ganglion cells in the retina of the cat have been studied after prolonged adaptation at different luminance levels.2. From a state of complete dark adaptation up to a luminance of about 5 x 10(-3) cd/m(2) (viewed through a 7 mm(2) pupil) the mean rate of on-centre units increases. From darkness up to a somewhat higher luminance the mean rate of some off-centre units decreases, but others show little change.3. Above 10(-2) cd/m(2) the mean rate ceases to show a regular relation with adaptation level. On units often decrease and then increase again at a higher level. Rhythmical fluctuations are common near 10(-2) cd/m(2), especially if the retina is not in first class condition. Mean rates above 70/sec and below 1/sec are very rare at any adaptation level.4. Over the range where mean rate increases monotonically with adaptation level, it is shown that the surround of on-centre units does not inhibit and off responses cannot be elicited. When the monotonic increase is slowed or reversed it becomes possible to elicit responses from the surround. We suspect that the surround exerts a sustained, tonic, inhibiting effect at higher adaptation levels, thus holding down the maintained discharge.5. There are rare units, forming less than 1% of those isolated, that show a regularly increasing frequency with increase of adaptation level above 10(-2) cd/m(2). These units have concentric ;on' and ;off' zones in their receptive fields, but the responses to rapidly changing stimuli are sluggish and they have an unusually regular maintained discharge. They have been tentatively called ;luminance units'.6. The impulse-interval and pulse-number distributions of on-centre units change predictably with adaptation level; the variance of the pulse number distribution is usually several times the mean at very low adaptation levels, but only a fraction of the mean at high levels. Off-centre units do not show predictable changes. Luminance units fire more regularly, and have narrower pulse-number and impulse interval distributions than other concentric units.7. A simple model is proposed to account for the statistical characteristics of the maintained discharge of on-centre units.

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PubMed1967

Receptive fields and trigger features of ganglion cells in the visual streak of the rabbits retina.

1. A survey of the properties of retinal ganglion cells in the central part of the rabbit retina has been carried out.2. The five types of unit previously encountered in the peripheral retina were also found in the central region. Their receptive fields were smaller, and tended to be oval-shaped with the long axis horizontal.3. In addition, three new types were discovered: orientation-selective cells, local-edge-detectors, uniformity-detectors.4. Orientation-selective cells were sensitive to either vertically or horizontally extended targets. Analysis suggested they were modified concentric units with an incomplete antagonistic surround.5. Local-edge-detectors responded to the appearance or movement of a contrasting border within the receptive field. They were inhibited by similar stimulation of the region surrounding the receptive field. Detailed attention was given to the demonstration of edge-detection.6. Uniformity detectors had a relatively high level of ongoing activity in the absence of stimulation. All forms of stimulation (lights flashed on or off, movement of darker or lighter targets) produced a diminution or cessation of ongoing activity.7. The results are compared with behaviour described in other species.

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