W.R. Levick

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.

Responses to single quanta of light in retinal ganglion cells of the cat.

Abstract Under the best conditions, single ganglion cells of the cats retina give one extra impulse (average) for less than 3 quanta (average) at the cornea. A stimulus containing about 15.50 quanta at the cornea is required to reliably modulate the maintained discharge. Calculations suggest that when a single quantum is absorbed in the retina several extra impulses must be initiated. This is confirmed by the statistics of the response. For quanta absorbed in receptors in the periphery of the receptive field fewer impulses result, and this is confirmed by reduced variability of response.

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.

Analysis of orientation bias in cat retina

1. Responses of cat retinal ganglion cells to a drifting sinusoidal grating stimulus were measured as a function of the grating orientation and spatial frequency.

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.

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 (Webers 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.

Variation in the response latency of cat retinal ganglion cells

Abstract Methods are described and compared for assessing the intrinsic timing variability in responses of a cat retinal ganglion cell to a series of identical, brief flash-stimuli. The standard deviation of the distribution of latencies to half-height of the smoothed representation of individual responses was a consistent and reproducible measure of the variability of response timing. It was about1/20 to 1/10 of mean latency at various levels of stimulus intensity and background and its value (under scotopic conditions) lay in the range 1–6 msec. The relation of this measure to psychophysical estimates of the accuracy of judgements of temporal order of visual stimuli is discussed.

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.

Crossed and uncrossed representation of the visual field by brisk-sustained and brisk-transient cat retinal ganglion cells

Abstract The projection of the catsmonocular visual fieldto the contralateral and ipsilateral optic tractsdiffered for thebrisk-sustainedand brisk-transientclasses of retinal ganglion cell. For both types all units recorded with receptive field centres more than about 0.5° into the temporal visual hemifield hadcrossedaxons. The transition from crossed coverage of temporal hemifield to uncrossedcoverage of nasal hemifield was complete at about 0.5° nasal for the brisk-sustained units but crossed brisk-transient units were found with receptive fields as far nasal as 15.8°. The 50% crossed/50% uncrossed zone for brisk-transient units was located 1–2° nasal of that for brisk-sustained units.

The crossed or uncrossed destination of axons of sluggish-concentric and non-concentric cat retinal ganglion cells, with an overall synthesis of the visual field representation

Abstract Two distinct patterns of representationof the visual fieldby the slowly conducting ganglion cellsof the cats retinawere observed. Both differed from the classical arrangement ofcrossedcells for temporal hemifield and uncrossedfor nasal hemifield. The first, exemplified by sluggish-sustained cells, involved a limited spread of crossed units into the nasal hemifield. The second, found with sluggish-transient units, local-edge-detectors and direction-selective units involved a preponderant representation of the entire nasal hemifield by crossed units.