Abnormal explicit but normal implicit sequence learning in premanifest and early Huntington's disease.

Learning may occur with or without awareness, as explicit (intentional) or implicit (incidental) learning. The caudate nucleus and the putamen, which are affected early in Huntington's disease (HD), are thought to be essential for motor sequence learning. However, the results of existing studies are inconsistent concerning presence/absence of deficits in implicit and explicit motor sequence learning in HD. We assessed implicit and explicit motor sequence learning using sequences of equivalent structure in 15 individuals with a positive HD genetic test (7 premanifest; 8 early stage disease) and 11 matched controls. The HD group showed evidence of normal implicit motor sequence learning, whereas explicit motor sequence learning was impaired in manifest and premanifest HD gene carriers, with progressive decline with progressive disease. Explicit sequence learning may be a useful cognitive biomarker for HD progression.