Single Unit Activity in the Rat Superior Colliculus During Reward Magnitude Task Performance

Long-Evans hooded rats were trained to run an alternation pattern in a t-maze with low reward (1 pellet) in the food cup of one arm and high reward (3 pellets) in the food cup of the other arm. Single units in the intermediate and deep layers of the superior colliculus were recorded using moveable microelectrodes. Most units showed activity that was associated with food retrieval, with a phasic increase in activity either upon the cessation of running or upon the insertion of the snout into the food cup. Some units (16%) showed different activity patterns upon retrieval of the two magnitudes of reward. Although 70% of the units showed either unimodal or multimodal sensory sensitivity, usually in response to tactile stimulation of the snout, this property was not related to activity associated with food retrieval. These data support previous observations that cells in the rat SC exhibit activity related to food retrieval, and indicate that for some of these neurons the coding reflects the characteristics of the reward stimulus.