The contribution of acetylcholine and dopamine to subprocesses of visual working memory - What patients with amnestic mild cognitive impairment and Parkinson's disease can tell us

Attentional selection, i.e. filtering out of irrelevant sensory input and information storage are two crucial components of working memory (WM). It has been proposed that the two processes are mediated by different neurotransmitters, namely acetylcholine for attentional selection and dopamine for memory storage. However, this hypothesis has been challenged by others, who for example linked a lack in dopamine levels in the brain to filtering deficits. Here we tested the above mentioned hypothesis in two patient cohorts which either served as a proxy for a cholinergic or a dopaminergic deficit. The first group comprised 18 patients with amnestic mild cognitive impairment (aMCI), the second 22 patients with Parkinson's disease (PD). The two groups did not differ regarding their overall cognitive abilities. Both patient groups as well as a control group without neurological deficits (n=25) performed a visuo-spatial working memory task in which both the necessity to filter out irrelevant information and memory load, i.e. the number of items to be held in memory, were manipulated. In accordance with the primary hypothesis, aMCI patients displayed problems with filtering, i.e., were especially impaired when the task required ignoring distracting stimuli. PD patients on the other hand showed difficulties when memory load was increased suggesting that they mainly suffered from a storage deficit. In sum, this study underlines how the investigation of neurologic patients with a presumed neurotransmitter deficit can aid to clarify these neurotransmitters' contribution to specific cognitive functions. (C) 2014 Elsevier Ltd. All rights reserved.