The biasing of weight judgments in Alzheimer's and Huntington's disease: a priming or programming phenomenon

The performances of Huntington's disease (HD) patients and patients with dementia of the Alzheimer type (DAT) were compared on an adaptation-level task involving the judgment of weights. In this task, subjects were first exposed to either a relatively heavy (heavy bias) or a relatively light (light bias) set of weights, and were later asked to rate the heaviness of a standard set of 10 weights using a 9-point scale. Patients with DAT and intact control subjects both perceived the standard set of weights as heavier following the light bias trials and lighter following the heavy bias trials, despite the DAT patients' poor explicit memory for the initial biasing session. In contrast, the weight judgments of the HD group as a whole was not significantly influenced by prior exposure to relatively heavy or light weights, and the size of the bias effect for this group was significantly correlated with the severity of their dementia. It is suggested that the impaired biasing performance of the HD patients, like their previously demonstrated impairment in motor skill learning, is due to a motor programming deficit resulting from neostriatal dysfunction.