Neural correlates of the proximity and quantity of anticipated food rewards in the ventral striatum of domestic chicks.

To identify the neuro-cognitive substrates of valuation and choice, we analysed the neural correlates of anticipated food rewards in the ventral striatum of freely behaving chicks. One-week-old chicks were trained in a color-discrimination task using four color cues (red, yellow, green and blue), each of which was associated with a different food reward. Choosing a red bead was immediately rewarded with a large amount of food, choosing a yellow bead resulted in an immediate-small food reward, and choosing a green bead resulted in a late-large food reward. We selected chicks that consistently chose a large and immediate food reward (red over yellow, and red over green), with the proximity of the food valued higher than the size of the food reward (yellow over green). Of the 47 neurons recorded from the ventral striatum of these chicks, 20 neurons selectively showed cue-period responses to cues associated with food rewards. Five of these 20 neurons responded differentially during the cue period according to the expected delay to reward, and were thus assumed to code for the proximity of the reward. Additionally, three other neurons responded to the quantity of the reward. Furthermore, in the post-operant delay period, many of these 20 neurons showed reward-related activities that were linked to the proximity or presence of the food reward. We therefore propose that impulsive choice and behavioral perseveration observed after lesions of the ventral striatum could be due to impaired anticipation of rewards in the cue and delay periods, respectively.