Temporal variation in the control of goal-directed visuospatial attention in basal ganglia disorders.

A predictive central-cueing paradigm was used to compare visuospatial deficits in patients with Parkinson's (PD) and Huntington's diseases (HD), employing directionally valid and invalid visual cues over a range of stimulus onset asynchronies (SOA) to elicit a saccadic response. Compared to age-matched control groups, both PD and HD patients responded erroneously to cue stimuli more frequently, increasing significantly over longer SOAs. Both valid and invalid cues resulted in elevated latencies compared to un-cued visually guided saccades, the associated 'cost' of invalid cueing significantly greater than that for valid cueing, over all SOAs. Unlike control subjects, PD and HD patients demonstrated temporal variation with cue presentation. For PD patients, latencies following directionally invalid cues were significantly longer for intermediate SOAs, suggesting difficulty overcoming a build-up of inhibitory activity over time. No validity effect was found at 1000 ms. For HD patients, latencies for validly cued saccades with 150 ms SOAs were shorter than latencies for no-cue trials, representing a 'benefit' of valid cueing. For 500 ms SOAs latencies were comparably elevated following directionally valid and invalid cues, and for 1000 ms SOAs neither cue condition resulted in increased latencies. These findings reflect the consequence of disruption to the balance of activity over the basal ganglia facilitatory and inhibitory pathways. Imbalance in PD resulted in difficulty sustaining goal-directed behaviour, and in HD, difficulty gating inappropriate behaviour.