Inhibitory and facilitatory spatial interactions in retinal receptive fields

Luminance processing in apparent motion, Vernier offset and stereoscopic depth

We obtained (apparently) linear responses to luminance from three special displays of apparent motion, Vernier offset and stereoscopic depth. In our motion stimulus a dark and a light bar exchanged luminances repetitively on a grey surround. Motion was attributed to the bar that differed more from the surround, that is, on a dark surround the light bar appeared to jump, and on a light surround the dark bar appeared to jump. The apparent motion disappeared when the luminance of the surround lay halfway between that of the bars--on a linear, not a logarithmic scale. Similar results were obtained for special Vernier offset and stereo stimuli. These results cannot be explained if all luminances are processed within the same luminance pathway and that pathway transforms input luminance using non-linear compression. However, the apparent linearity of our results could arise from opposite and equal non-linearities cancelling out within separate ON- and OFF-spatial luminance pathways. A second set of experiments presented one bar separately into each eye on different surrounds (dichoptic presentation of competing apparent motion signals) or manipulated the display spatially so that different surrounds were associated with different bars (binocular presentation of competing Vernier targets). Results showed that apparent motion and Vernier signals of equal Weber contrast (normalisation of linear difference to surround luminance) evoked equal-motion and equal Vernier offset strengths. Given that motion and Vernier strength followed Weber's law, we infer that the ON- and OFF-pathways transform luminance non-linearly. Our third experiment presents an example of a brightness bisection task in which we were able to influence the bisection steps, to follow either a linear or non-linear series. The benefits of parsing the visual scene so that visual information is processed within two opposite luminance pathways is discussed.

Spatiotemporal adaptation model for retinal ganglion cells

An adaptation model for the level of the ganglion cell in the retina is presented. The model assumes separate adaptation mechanisms for each of the receptive field (RF) regions, i.e., before edge detection. According to the model, the decay in the response time course of each RF region reflects its adaptation process. A mathematical description of adaptation that includes its temporal properties is developed through the change in the semisaturation constant theta in the Naka-Rushton equation. The model and its simulations show a good agreement with a wide variety of physiological studies.

Inhibition of cuneate neurones: its afferent source and influence on dynamically sensitive "tactile" neurones

1. Responses were recorded in decereberate, unanaesthetized cats from individual cuneate neurones in order to determine firstly, the afferent sources of inhibition on cuneate neurones and secondly, the influence of afferent-induced inhibition on those response features of dynamically sensitive tactile neurones which determine their capacity to code information about parameters of tactile stimuli.2. For all cuneate neurones which displayed afferent-induced inhibition from areas surrounding or within their excitatory receptive field (71% of the sample) it was consistently found that 300 Hz vibration at low amplitudes (< 25-50 mum) which selectively engages Pacinian corpuscles was an effective source of inhibition. In contrast, steady indentation which activates slowly adapting tactile afferents was quite ineffective, as was low frequency vibration (30 Hz) at amplitudes of < 50-100 mum. The latter stimulus can be used to engage rapidly adapting receptors either within glabrous skin (presumed to be Meissners corpuscles) or in association with hair follicles. It is concluded that afferents from Pacinian corpuscles are the dominant or exclusive source of afferent-induced inhibition of cuneate neurones.3. For dynamically sensitive neurones responsive to low frequency cutaneous vibration (30 Hz) there was a reduction in the slope of stimulus-response relations with afferent-induced inhibition, but no expansion of the range of stimulus amplitudes over which the neurone responded.4. The influence of afferent-induced inhibition on the phase-locking of impulse activity to a cutaneous vibratory wave form was examined by constructing post-stimulus time histograms and cycle histograms. Measures of dispersion of impulse activity around the preferred point of firing in the vibratory waveform indicated that the capacity of individual cuneate neurones to code information about the frequency of the cutaneous vibration was not systematically changed in the presence of afferent-induced inhibition.

Surround contribution to light adaptation in cat retinal ganglion cells

1. The sensitivity of a cat's retinal ganglion cell to a small, dim, spot flashed upon the middle of the receptive field depends upon the size of a concentric steady background: sensitivity is reduced monotonically with background area. All backgrounds which equal or exceed in size the central summing area of the ganglion cell produce an equivalent reduction of sensitivity, even though only backgrounds which extend outside the central summing area depress the maintained discharge. 2. If a small background lies upon the middle of the receptive field, and the test spot is made intense enough to evoke a strong response, steady illumination of the periphery may make the response larger. 3. This change in response is not due to an enhancement of centre sensitivity by the surround, but is readily understood if steady illumination of the periphery adapts out the surround's antagonism of the centre's response to the test flash. 4. The failure of steady stimulation of the surround to alter centre sensitivity implies that signals from the surround subtract from, or add to, those from the centre.

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.

Flux, not retinal illumination, is what cat retinal ganglion cells really care about

1. Evidence was obtained that the impulse/quantum (I/Q) ratio of the central response mechanism of retinal ganglion cells in the cat is controlled by the steady effective retinal flux of the background.2. One experiment revealed that the I/Q ratio was decreased as the area of an adapting spot, of constant illumination, was increased. The curve relating the I/Q ratio to background flux was the same regardless of the size of the adapting spot.3. The effective central summing area of many retinal ganglion cells was determined. For the same cells, the transition level (Enroth-Cugell & Shapley, 1973) of the impulse/quantum curve was also measured. Diffuse illumination at the transition level was inversely proportional to the effective summing area, when variation in dark-adapted sensitivity between cells was taken into account.4. Therefore, retinal ganglion cells with large central summing areas are more light-adapted by any given diffuse background than cells with small centres.