Cue and reward signals carried by monkey entorhinal cortex neurons during reward schedules

Ablation of entorhinal/perirhinal cortices prevents learning associations between visual stimuli used as cues in reward schedules and the schedule state. Single neurons in perirhinal cortex are sensitive to associations between the cues and the reward schedules. To investigate whether neurons in the entorhinal cortex have similar sensitivities, we recorded single neuronal activity from two rhesus monkeys while the monkeys performed a visually cued reward schedule task. When the cue was related to the reward schedules, the monkeys made progressively fewer errors as the schedule state became closer to the reward state, showing that the monkeys were sensitive to the cue and the schedule state. Of 75 neurons recorded in the entorhinal cortex during task performance, about 30% responded. About half of these responded after cue presentation. When the relation of the cue to the reward schedules was random, the cue-related responses disappeared or lost their selectivity for schedule states. The responses of the entorhinal cortex neurons are similar to responses of perirhinal cortex neurons in that they are selective for the associative relationships between cues and reward schedules. However, they are particularly selective for the first trial of a new schedule, in contrast to perirhinal cortex where responsivity to all schedule states is seen. A different subpopulation of entorhinal neurons responded to the reward, unlike perirhinal neurons which respond solely to the cue. These results indicate that the entorhinal signals carry associative relationships between the visual cues and reward schedules, and between rewards and reward schedules that are not simply derived from perirhinal cortex by feed-forward serial processing.