Saccadic intrusions, predominantly horizontal saccades that interrupt accurate fixation, include square-wave jerks (SWJs; the most common type of saccadic intrusion), which consist of an initial saccade away from the fixation 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 set out to determine the characteristics of SWJs in normal rhesus macaques during attempted fixation. We found that primate and human SWJs shared comparable features overall, but the likelihood of a given saccade being part of a SWJ was lower for primates than for humans.
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