Abstract
During attempted visual fixation, saccades of a range of sizes occur. These “fixational saccades” include microsaccades, which are not apparent in regular clinical tests, and “saccadic intrusions”, predominantly horizontal saccades that interrupt accurate fixation. Square-wave jerks (SWJs), the most common type of saccadic intrusion, consist of an initial saccade away from the target followed, after a short delay, by a “return saccade” that brings the eye back onto target. SWJs are present in most human subjects, but are prominent by their increased frequency and size in certain parkinsonian disorders and in recessive, hereditary spinocerebellar ataxias. Here we asked whether fixational saccades showed distinctive features in various parkinsonian disorders and in recessive ataxia. Although some saccadic properties differed between patient groups, in all conditions larger saccades were more likely to form SWJs, and the intervals between the first and second saccade of SWJs were similar. These findings support the proposal of a common oculomotor mechanism that generates all fixational saccades, including microsaccades and SWJs. The same mechanism also explains how the return saccade in SWJs is triggered by the position error that occurs when the first saccadic component is large, both in the healthy brain and in neurological disease.
Highlights
During attempted visual fixation of a stationary target, saccadic intrusions and fixational eye movements continuously change the position of gaze [1,2,3]
We set out to determine if this coupling mechanism might apply to Parkinson’s disease (PD), to other parkinsonian disorders, and to recessive ataxia: if so, it would be an important piece of evidence supporting our proposal that a common saccade generation mechanism can explain some of the oculomotor deficits associated with each of these diseases
The present results extend the correlation between saccade size and Square-wave jerks (SWJs) coupling, previously found in progressive supranuclear palsy (PSP) patients and healthy controls [12], to all subject groups, including PD, multiple system atrophy (MSA), corticobasal syndrome (CBS) and SCASI patients
Summary
During attempted visual fixation of a stationary target, saccadic intrusions (predominantly horizontal saccades that ‘‘intrude on’’ or interrupt accurate fixation) and fixational eye movements (including microsaccades, drift and tremor) continuously change the position of gaze [1,2,3]. Microsaccades, which counteract perceptual fading resulting from sensory adaptation [4,5,6,7,8], are too small (often ,0.5 deg) to be evident during clinical examination. Square-wave jerks (SWJs), a type of saccadic intrusion consisting of a small saccade away from the fixation target, followed by a corrective saccade back towards the target, can be large enough to be evident clinically. SWJs occur in normal, healthy humans [1,9], but are a clinically prominent feature–due to increased frequency and magnitude–in a number of neurological diseases, especially parkinsonian disorders [9] and recessive spinocerebellar ataxias [10]. We recently studied microsaccades and SWJs in healthy controls and patients with progressive supranuclear palsy (PSP), a parkinsonian disorder in which SWJs are a feature of the clinical syndrome [11,12].
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