The effects of hippocampal system lesions on a novel temporal discrimination task for rats

A novel, appetitive, Pavlovian conditioning task was used to assess interval timing. Experiment 1 showed that normal rats could discriminate between tones of 1.5 s and 0.5 s duration, or between tones of 12.0 s and 3.0 s duration. Learning was demonstrated by a greater duration of magazine responding in the period before the delivery of a food reward and after cessation of the CS+ compared to the same time period after cessation of the CS−. Learning was, however, asymmetric as it was much quicker when the CS+ was the longer of the two durations (1.5 s and 12.0 s, respectively). Experiment 2 assessed the impact of fornix lesions on the acquisition of one version of this task (CS+ 1.5 s, CS− 0.5 s). No evidence was found of a change in discrimination learning following surgery. Experiment 3 examined whether rats with either fornix or hippocampal lesions affected discriminations between 12.0 s and 3.0 s stimuli. Again, there was no evidence of a lesion-induced deficit. T-maze alternation training confirmed the effectiveness of these lesions. The results not only reveal that neither the fornix nor the hippocampus is necessary for distinguishing temporal intervals within the ranges tested but also showed how under some circumstances these lesions can leave trace conditioning intact.