Control of visually guided saccades in multiple sclerosis: Disruption to higher-order processes

Ocular motor abnormalities are a common feature of multiple sclerosis (MS), with more salient deficits reflecting tissue damage within brainstem and cerebellar circuits. However, MS may also result in disruption to higher level or cognitive control processes governing eye movement, including attentional processes that enhance the neural processing of behaviourally relevant information. The attentional control of eye movement was investigated in 25 individuals with MS and a comparable number of neurologically healthy individuals matched for age and IQ. This entailed an evaluation of distractor-related effects on the generation of both unpredictable and predictable visually guided saccades, as well as an evaluation of the effects of presenting endogenous cues prior to target onset. For unpredictable saccades, we revealed an exaggerated distractor effect in MS, with saccade latencies prolonged and endpoints less accurate in the presence of a visual distractor. Predictable saccades tended to be hypometric for MS patients, although we found no significant distractor effects. For endogenously cued saccades, we found no group differences in latency following a valid cue, but an exaggerated increase in latency following invalid cues for MS patients. MS patients also generated a significantly greater proportion of erroneous responses to cue stimuli. These ocular motor characteristics demonstrate considerable sensitivity with respect to evaluating attentional deficits in MS, evident even in the absence of clinical signs of disease.