A computational model was previously developed to investigate the role of parallel basal ganglia-thalamocortical loops in solving tasks that rely on working memory. Different lesions are applied to the model in order to investigate the working memory deficits observed in Parkinson's disease and schizophrenia. The simulations predict that the working memory deficits observed in Parkinson's disease result from a local dysfunction within the brain due to a problem in the disinhibitory process arising from the basal ganglia. They also predict that the working memory deficits observed in schizophrenia involve many cortical and subcortical areas and result from a problem in selecting items in working memory which are stored in basal ganglia-thalamocortical loops. The simulations predict the temporal unfolding of neuronal activity in different brain regions, both in the normal case and in the two disease states. A specific event-related functional magnetic resonance imaging study was elaborated to test some of those predictions.