Fos expression in the prefrontal cortex and nucleus accumbens following exposure to retrospective timing tasks.

The dorsal striatum and prefrontal cortex have been implicated in interval timing. We examined whether performance of temporal discrimination tasks is associated with increased neuronal activation in these areas, as revealed by Fos expression, a marker for neuronal activation. In Experiment 1, rats were trained on a discrete-trials temporal discrimination task in which a light (22 cd/m²) was presented for a variable time, t (2.5-47.5 s), after which levers A and B were presented. A response on lever A was reinforced if t < 25 s, and a response on lever B was reinforced if t > 25 s. A second group was trained on a light-intensity discrimination procedure, in which a light of variable intensity, i (3.6-128.5 cd/m²) was presented for 25 s. A response on lever A was reinforced if i < 22 cd/m², and a response on lever B was reinforced if i > 22 cd/m². In Experiment 2, bisection procedures were used to assess temporal (200-800 ms, 22 cd/m²) and light-intensity (3.6-128.5 cd/m², 400 ms) discrimination. The increase in proportional choice of lever B as a function of stimulus duration or intensity conformed to a two-parameter logistic equation. Fos expression in the prefrontal cortex and nucleus accumbens was higher in rats performing temporal discrimination tasks than in those performing light-intensity discrimination tasks, indicating greater neuronal activation in these areas during temporal discrimination tasks. Fos expression in the dorsal striatum did not differ between rats performing temporal and light-intensity discrimination tasks. These results suggest that the prefrontal cortex and nucleus accumbens are involved in temporal discrimination.