Distinctive Features of Saccadic Intrusions and Microsaccades in Progressive Supranuclear Palsy

Abstract

The eyes do not stay perfectly still during attempted fixation; fixational eye movements and saccadic intrusions (SIs) continuously change the position of gaze. The most common type of SI, square-wave jerk (SWJ), consists of saccade pairs that appear purely horizontal on clinical inspection: the first saccade moves the eye away from the fixation target and, after a short interval, the second saccade brings it back towards the target. SWJs are prevalent in certain neurological disorders, including progressive supranuclear palsy (PSP). Here we developed an objective method to identify SWJs. We found that SWJs are more frequent, larger and more markedly horizontal in PSP patients than in healthy human subjects. Further, the loss of a vertical component in fixational saccades and SWJs was the eye movement feature that best distinguished PSP patients from controls. We moreover determined that in PSP patients and controls, the larger the saccade the more likely it was part of a SWJ. Further, saccades produced by PSP patients had equivalent properties whether they were part of a SWJ or not, suggesting that normal fixational saccades (microsaccades) are rare in PSP. We propose that fixational saccades and SIs are generated by the same neural circuit, and that, both in PSP patients and in controls, SWJs result from a coupling mechanism that generates a second corrective saccade shortly after a large fixation saccade. Due to brainstem and/or cerebellum impairment, fixational saccades in PSP are abnormally large, and thus more likely to trigger a corrective saccade, giving rise to SWJs.

Suppressive interactions underlying visually evoked fixational saccades

“…1B). This suggests many fixational saccades occurred to correct the current fixation error introduced by previous fixational saccades or drift, consistent with previous reports (Cornsweet, 1956; Engbert & Kliegl, 2004; Otero-Millan et al, 2011b; Poletti & Rucci, 2010). Therefore, the average gaze position did not move toward the parafoveal stimulus as its contrast increased.…”

Micro and regular saccades across the lifespan during a visual search of “Where’s Waldo” puzzles

“…They occur in healthy adults but increase in frequency with neurological diseases such as Parkinsonism and cerebellar ataxias (Leigh & Zee, 2006). Recent studies have pointed out the numerous similarities between microsaccades and SWJ, arguing that they are closely related, involuntary saccades that utilize the same underlying saccade generator neural circuitry (Gowen et al, 2007; Otero-Millan et al, 2011). …”

Fixational eye movements in Tourette syndrome

“…The finding of reduced microsaccade amplitude and frequency in TS differed from that in subjects with disorders of visual system such as amblyopia [37], refractive error [38], or degenerative disorders such as Parkinson’s disease or progressive supranuclear palsy [26, 27]. Disorders of visual system such as amblyopia increases the amplitude of microsaccades and reduces its frequency; refractive error affects the amplitude of microsaccades [37, 38]; the degenerative disorder such as Parkinson’s disase and progressive supranuclear palsy increases the amplitude of microsaccades and increases their frequency [26, 27].…”

“…Mental fatigue and hypoxia also influence drift velocity [23, 25]. In hypokinetic rigid syndromes like parkinsonism, microsaccade amplitude is increased [26, 27]. TS is a disorder with “dopamine hyper-expression” in the fronto-striatal-thalamic pathways.…”