Binocular interaction in the superior colliculus of chronic cats

Velocity response profiles of collicular neurons: parallel and convergent visual information channels

We have recorded from single neurons in the retinorecipient layers of the superior colliculus of the cat. We distinguished several functionally distinct groups of collicular neurons on the basis of their velocity response profiles to photic stimuli. The first group was constituted by cells responding only to photic stimuli moving at slow-to-moderate velocities across their receptive fields (presumably receiving strong excitatory W-type input but not, or only subthreshold, Y-type input). These cells were recorded throughout the stratum griseum superficiale and stratum opticum and constituted 50% of our sample. The second group of cells exhibited excitatory responses only at moderate and fast velocities (presumably receiving excitatory Y-type but not W-type input). These cells constituted only about 7% of the sample and were located principally in the lower stratum griseum superficiale. The third group of cells was constituted by cells excited over the entire range of velocities tested (1-2000 /s) and presumably received substantial excitatory input from both W- and Y-channels. These cells constituted almost 26% of our sample and were located in the lower stratum griseum superficiale, stratum opticum and the upper part of the stratum griseum intermediale. Overall, cells receiving excitatory Y-type input, i.e. the sum of group two and group three cells, constituted about a third of the sample and their excitatory discharge fields were significantly larger than those of cells receiving only W-type input. A fourth distinct group of collicular neurons was also constituted by cells responding over a wide range of stimulus velocities. These cells were excited by slowly moving stimuli, while fast-moving photic stimuli evoked purely suppressive responses. The excitatory discharge fields of these cells (presumably, indicating the spatial extent of the W-input) were located within much larger inhibitory fields, the extent of which presumably indicates the spatial extent of the Y-input. These low-velocity-excitatory/high-velocity-suppressive cells were recorded from the stratum griseum superficiale, stratum opticum and stratum griseum intermediale and constituted about 17% of the sample. The existence of low-velocity-excitatory/high-velocity-suppressive cells in the mammalian colliculus has not been previously reported. Low-velocity-excitatory/high-velocity-suppressive cells might play an important role in activating "fixation/orientation" and "saccade" premotor neurons recorded by others in the intermediate and deep collicular layers. Overall, in the majority (57%) of collicular neurons in our sample there was no indication of a convergence of W- and Y-information channels. However, in a substantial minority of collicular cells (about 43% of the sample) there was clear evidence of such convergence and about 40% of these (low-velocity-excitatory/high-velocity-suppressive cells) appear to receive excitatory input from the W-channel and inhibitory input from the Y-channel.

Alterations in receptive field properties of superior colliculus cells produced by visual cortex ablation in infant and adult cats

To determine if functional alterations in the superior colliculus might account for recovery of visual behaviors following visual cortex removal in infant cats, the receptive field characteristics of single units in the superior colliculus of cats whose visual cortex was removed within the first week of life were compared with those of cats which sustained visual cortex lesions in adulthood and with those of normal cats. In the normal superior colliculus, 90% of all cells responded to moving stimuli irrespective of shape or orientation. Sixty-four percent of these units were directionally selective, responding well to movement in one direction but poorly or not at all to movement in the opposite direction. Ninety percent of units were binocular, the vast majority of these responding equally to stimulation of either eye or showing only slight preference for stimulation of the contralateral eye. Responses to stationary flashes of light were observed in only 33% of all visually activated cells in the normal superior colliculus. After visual cortex ablation in adult cats, only six percent of movement sensitive cells were directionally selective. Binocular preference was shifted following adult visual cortex lesions such that sixty percent of all cells responded exclusively or predominantly to stimulation of the contralateral eye. Seventy-one percent of all visually responsive units responded to stationary lights flashed on or off within their receptive field boundaries. Lesions limited primarily to area 17 had the same effect as larger lesions of visual cortex. Infant visual cortex lesions resulted in receptive field alterations similar to those observed after adult ablation. Only fifteen percent of motion sensitive units were directionally selective. Seventy-one percent responded exclusively or predominantly to stimulation of the contralateral eye. Seventy-six percent of visually responsive cells were activated by stationary light. Lesions largely confined to area 17 produced the same alterations as more extensive lesions of visual cortex. Thus, no evidence was found that the superior colliculus is involved in the functional reorganization presumed to occur following visual cortex ablation in infant cats. Recovery of visual behaviors following neonatal injury may therefore not involve alterations in the receptive fields of single cells.

Binocular interaction in the cat's superior colliculus

1. Binocularly driven neurones with small receptive fields near the area centralis were recorded in the cat's superior colliculus. 2. Binocular interaction was tested by stimulating both eyes simultaneously with a single moving stimulus at various retinal disparities. 3. Collicular cells in general showed strong summation or even facilitation when the images of the stimulus were in exact correspondence on the receptive fields, sometimes with occlusion when they were out of register. The range of retinal disparity over which there was additive interaction could be as little as 1 or 2 deg, almost as narrow as for the most precisely tuned neurones in the visual cortex. Even cells with large receptive fields sometimes showed a narrow range of binocular interaction. 4. Non-directional cells generally exhibited weaker summation and broader disparity selectivity than did direction-selective cells. 5. Some neurones with virtually no response to a stimulus in one of the eyes can exhibit marked binocular interaction. Other apparently monocular cells show little or no binocular interaction. 6. The disparity of the centres of the receptive fields was measured after correcting for small eye movements, which were assessed by two different techniques. For 132 cells the measured distribution of horizontal disparity (range 4.5 deg; S.D. 0.93 deg) was significantly broader than that of vertical disparity (range 2.2 deg; S.D. 0.52 deg). Sources of error in these measurements are considered. 7. The results are discussed in relation to the known connexions between visual cortex and superior colliculus and the possible role of the latter in the regulation of eye movements.