Parkinson's reduces anticipatory eye responses, implicit timing unaffected

By Sarah Guy, medwireNews Reporter

The occurrence of anticipatory responses is decreased in the eyes of individuals with Parkinson's disease, indicate study results published in Neuropsychologia.

Despite this, these individuals' implicit timing is largely unaffected, report the researchers, who suggest their findings show that explicit and implicit timing are "encompassed by different neural structures."

Basal ganglia activation, which is reduced in Parkinson's disease, is known to play a role in explicit timing, but it has been hypothesized that it is less involved in implicit timing, explain Marcus Missal (Université catholique de Louvain, Brussels, Belgium) and colleagues.

To investigate further, the team assessed the influence of early Parkinson's disease and aging on the latency of anticipatory eye movements in eight patients, aged between 45 and 68 years, with early-stage disease. The results were compared with those from eight healthy age-matched individuals, 11 healthy young (21-43 years) individuals, and four healthy elderly (76-84 years) individuals.

After watching a constantly fixed target, participants observed it move "forward" along a circular path at one of two speeds, before suddenly changing direction and returning to its starting position.

Study participants with Parkinson's disease were able to pursue the moving target in 57% of "trials," compared with 73% of trials in healthy individuals, report Missal et al. They observed that participants with Parkinson's disease and healthy participants pursued the target with an average gain of approximately 0.9 ms, but 400 ms after motion onset, pursuit gain was significantly lower in the Parkinson's disease patients compared with their healthy counterparts.

"These results indicate that [Parkinson's disease] patients could accurately track the moving target but that initial eye acceleration was lower," say the authors, adding that target reversal anticipation also occurred less frequently in Parkinson's disease patients than healthy individuals.

Indeed, the change of eye movement direction after sudden target reversal occurred significantly later in Parkinson's disease patients than in controls, at 173 ms versus 159 ms, with the gain of pursuit at the beginning of the return movement also significantly lower in the former than the latter group.

"We suggest that longer latencies of smooth pursuit initiation after reversal reveal longer visual processing time in PD [Parkinson's disease] patients than in controls," say Missal and co-workers.

"However, implicit timing of target motion reversal was not affected in PD patients," they conclude.

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