Arbitrary visuo-motor mapping during object manipulation in mild cognitive impairment and Alzheimer's disease: a pilot study.

Empirical evidence for an essential role of the hippocampal system in arbitrary visuo-motor mapping suggests that acquisition and retrieval of arbitrary visuo-motor mapping might be impaired in mild cognitive impairment (MCI) and Alzheimer's disease (AD). The present pilot study investigated whether MCI of amnestic type or AD impact upon the capacity to scale grip force in a predictive manner to the mass of an object to be lifted based on learned associations between arbitrary colour cues and mass. Patients with MCI (n=8) and AD (n=8) grasped and lifted two different masses (400g and 600g) in random order using a precision grip between index finger and thumb. In a "no cue" experiment, a non-informative neutral visual stimulus was presented prior to each lift, thereby disallowing any prediction about which of the two masses was going to be lifted in the next trial. In a "cue" experiment an arbitrary colour cue provided advance information about which of the two masses to be lifted. In the "no cue" condition patients scaled their grip force according to the mass of the preceding lift. In the "cue" experiment neither patients with amnestic MCI nor those with AD were able to adjust their grip force based on visuo-motor mappings with arbitrary colour cues. These preliminary data suggest that the hippocampal system plays an essential role for arbitrary visuo-motor mapping in the grip-lift task.