Sequences assessed by declarative and procedural tests of memory in amnesic patients with hippocampal damage

Previous research indicates that amnesic subjects tested on sequential learning or serial reaction time tasks can learn a repeated procedural sequence but are unable to explicitly recall the correct sequence when asked to generate the sequence. Rats with hippocampal lesions are also able to learn and remember procedural or implicit sequences but were impaired for declarative sequences. We used analogous procedures used in rats to assess the role of the hippocampus in the acquisition of declarative and procedural sequences in amnesic and control participants. Amnesic participants with damage restricted to the hippocampus and control participants were administered analogous tasks of declarative and procedural sequential learning using a computer version of the radial arm maze. The amnesic participants had slower response times during the acquisition of procedural sequences, but were not impaired compared to controls when switched to a random sequence, suggesting that both groups learned the sequence. Alternatively, the amnesic but not control participants were significantly impaired in the declarative sequence task. Our findings provide support for evolutionary continuity in cognitive function of the hippocampus in rats and humans and the dissociation between the declarative and procedural sequential learning. The performance differences on the two sequence learning tasks are likely due to the use of different strategies associated with learning sequences based on procedural versus declarative knowledge.