Temporal discrimination learning in severe amnesic patients reveals an alteration in the timing of eyeblink conditioned responses.

This study investigated whether the hippocampal system plays a modulatory role in the timing of conditioned responses (CRs) in eyeblink classical conditioning. Seven bitemporal amnesic patients and 7 controls were randomly presented 2 tone conditioned stimuli (CSs) that were individually paired with two different interstimulus intervals (ISIs) in a delay conditioning task. It was found that amnesic patients' CRs occurred significantly earlier than control participants' CRs at the longer ISI. Amnesic patients also produced significantly more nonadaptive CRs than did control participants, their level of acquisition was less than that of control participants after equating for ISI, and they did not show extinction with the longer ISI. These data suggest a role of the hippocampal system in controlling the precise timing of conditioned eyeblink responses and in acquiring and extinguishing responses within the context of a temporal discrimination task.