Loss of relative-motion sensitivity in the monkey superior colliculus after lesions of cortical area MT.

Many cells in the superficial layers of the monkey superior colliculus are sensitive to relative motion. The response to a small stimulus moving through a cell's receptive field is strongly modulated by the relative motion between the stimulus and a textured pattern moving through the surrounding visual field; modulation is independent of absolute direction and speed of the stimulus. To determine whether cortical visual area MT is essential for this type of relative-motion sensitivity, colliculus cells were studied in the anesthetized, immobilized preparation after ablation of area MT. Unilateral MT lesions were made by either aspiration, kainic acid injection, or a combination of both methods. Data from the lesioned animals were compared with those from intact animals. Ipsilateral to the lesions, colliculus cells showed an almost total loss of sensitivity to relative motion. This loss was related neither to inadvertent injury of cortical areas neighboring MT nor to incidental optic radiation damage. Two other forms of motion-dependent, center-surround interactions were still present in the colliculus after the cortical lesions. These were a rudimentary sensitivity to differential motion between stimulus and background, which occurs for only one direction of stimulus movement, and a nonselective center-surround suppression, which is induced by movement of a background stimulus in any direction. Visual responsiveness, ocular dominance, and flash-evoked responses were also unaffected by the cortical lesions. We conclude that input from area MT is crucial for relative-motion sensitivity, but not for other response properties, in the superficial layers of the monkey colliculus.

Effect of corticotectal tract lesions on relative motion selectivity in the monkey superior colliculus.

Many cells in the superficial layers of the monkey superior colliculus are sensitive to the relative motion between a small target moving through the classic receptive field and a textured, moving background pattern that fills the visual field beyond the classic receptive field. The cells respond well when motion of the target differs from that of the background, but their responses are suppressed when the target moves in phase with the background. To determine whether this relative motion sensitivity depends on input to the colliculus from visual cortex, we studied colliculus cells in immobilized, anesthetized monkeys after unilateral thermocoagulation, or anesthetic blockade, of the corticotectal tract at the level of the pulvinar. In the colliculus ipsilateral to the corticotectal tract lesions, relative motion sensitivity was significantly reduced when compared either with the colliculus in intact animals or with the colliculus contralateral to the lesion. However, a moving-background stimulus still had a modest suppressive effect compared with a stationary background ("background motion sensitivity"), as is the case for intact animals. Anesthetic blockade of the corticotectal tract had similar effects; relative motion sensitivity, but not background motion sensitivity, was lost following injection of mepivacaine or bupivacaine. Pulvinar cell loss alone, induced by kainic acid injection, had no effect on relative motion sensitivity in the colliculus. The corticotectal tract lesions, but not the anesthetic injections, also had minor effects on flash-evoked responses and spontaneous discharge rates; these effects may reflect a retrograde response of some tectopulvinar cells to injury of their axons by the corticotectal tract lesions. In the colliculus opposite the corticotectal tract lesion, relative motion sensitivity was similar to that in normal animals. However, responses in the presence of a moving background were enhanced, suggesting that removal of cortical input to one colliculus may disinhibit the contralateral colliculus, a phenomenon reminiscent of the Sprague effect in the cat. We conclude that while cortical input to the colliculus may contribute little to the classic receptive field properties of superficial-layer cells, it clearly does contribute to relative motion sensitivity.