Implicit and explicit memory after focal thalamic lesions

Background: Lesions of the thalamus interfere with cognitive functions main ly in the area of declarative learning and memory. Little is known about th e role the thalamus plays in implicit learning. Objective: To study explici t and implicit learning and memory in subjects with thalamic lesions and to analyze the influence of lesion characteristics on cognitive performance. Methods: The authors studied the performance of 15 subjects with focal thal amic infarction or hemorrhage on a comprehensive neuropsychological test ba ttery focusing on tests of explicit memory and learning of a nondeclarative motor skill. Subjects with thalamic lesions were compared to 15 healthy ma tched control subjects and to a clinical control group of 22 subjects who h ad sustained basal ganglia lesions. Results: Subjects with thalamic lesions showed well-preserved intellectual and executive functions but demonstrate d deficits on measures of attention and psychomotor speed, explicit memory, and implicit visuomotor sequence learning. Lesion size in the thalamus was clearly related to subjects' long-term explicit memory performance. Howeve r, few of the neuropsychological deficits found seemed specific to the long -term neuropsychological outcome of focal thalamic infarctions. Subjects wi th lesions in the basal ganglia demonstrated similar deficits. Conclusions: Focal subcortical lesions in the thalamus and the basal ganglia lead to a similar profile of neuropsychological deficits. Lesions in the thalamus not only affect declarative memory but also interfere with nondeclarative moto r skill learning.