Impaired probabilistic category learning in hypoxic subjects with hippocampal damage.

Previous research has suggested that a probabilistic category learning task (e.g. weather prediction task) can be used to elucidate brain substrates of learning. We tested amnesic subjects with bilateral hippocampal damage due to hypoxia and matched controls on the weather prediction task and a variant, the "ice cream" task, which maintains a similar category structure. The hypoxic subjects were impaired relative to controls on both tasks; in the ice cream task, this difference was evident even early in training (first 50 trials). This finding is similar to functional neuroimaging (fMRI) studies in healthy subjects, which show medial temporal involvement even in early learning on this task. Additionally, strategy analysis of response patterns during learning suggest that the hypoxic group relied more heavily on simple, degraded learning strategies than did the control group. These results may suggest a qualification of the generally held conclusion that amnesic patients are not impaired at probabilistic category learning: at least under some circumstances, amnesic patients show an early and lasting deficit.