Reward-related neuronal activity in the rat superior colliculus

The activity of single units in the intermediate and deep layers of the superior colliculus was recorded while rats performed an operant conditioning task. On all trials, each animal pressed a bar and then inserted his snout into a food cup; on half of the trials, food reinforcement was available. To test for tactile sensitivity, on half of the trials the rats received a puff of air to the face when the snout entered the food cup. Activity of most cells was correlated with the motor activity of inserting the snout into the food cup, even when reinforcement was not available. For many cells, a larger burst of activity was seen on the reinforced trials than on trials when rats made the same movements without the presence of reward. There was no evidence that an increase in tactile sensitivity occurred when the animal retrieved the reinforcement. These results suggest that cells in the superior colliculus have an increase in activity associated with reward retrieval, which for some neurons is not dependent on simple sensory or motor factors.