Simple Eye Movement Task Reveals Impairment in Schizophrenia

Many studies have documented oculomotor abnormalities in patients with schizophrenia, typically focusing on impairments in smooth pursuit or antisaccade performance. Both of these tasks involve complex neurophysiological processes mediated by multiple brain regions, making interpretation challenging due to potentially confounding factors like motivation, intelligence, and medication effects.

In the recent paper, Eye Movement Task Reveals an Impairment in Schizophrenia, Vanteemar Sreera, from the National Institute of Mental Health and Neurosciences in India, and his colleagues investigated performance on a simple fixation stability task as a potential endophenotype marker for schizophrenia (SCZ). Fixation stability primarily assesses basic visuomotor processing and sustained attention, particularly the control of reflexive saccades. Critically, the study included antipsychotic-naïve/free SCZ patients and their first-degree relatives (FDRs), thereby mitigating the common confounding effects of chronic illness and medication. The researchers hypothesized that both SCZ patients and FDRs would show poorer fixation stability than healthy controls (HCs), with deficits being more pronounced when visual distractors were introduced. They also explored how distractor features, like distance and laterality, would influence fixation stability performance.

Fixation Stability Cross-Sectional Design

The study employed a cross-sectional design involving 69 antipsychotic-naïve/free SCZ patients, 49 FDRs, and 76 HCs.

The researchers used an EyeLink 1000 eye tracker(SR Research, Canada) to record monocular high-frequency eye tracking data at 1000 Hz. Participants were asked to gaze at a central circular target for 5 seconds. In some trials, identical distractors appeared at near (1.43°) or far (2.86°) distances on the left or right, and subjects were instructed to ignore them and maintain their gaze on the central target. This high-precision recording system was essential for deriving detailed metrics of fixation stability, including:

• Bivariate Contour Ellipse Area
• Fixation frequency
• Mean and median fixation duration
• Saccade frequency and amplitude
• Scanpath length

Eye Tracking Measure for Schizophrenia

The study yielded several key findings:

• Inversed Laterality Effect: A significant and particularly interesting finding was the rightward bias in attentional distraction for both SCZ and FDRs, where right-sided distractors caused greater impairment. HCs, conversely, showed the expected leftward bias. This suggests an attenuation or reversal of typical visual functional asymmetry in SCZ vulnerability, which is linked to atypical lateralized brain function.
• Impaired Fixation Stability in SCZ (Target-Only Trials): Even without distractors, the SCZ group showed significantly higher fixation frequency, saccade amplitude, and scanpath length compared to HCs. This indicates poorer gaze control and impaired basic visuomotor processing, uncontaminated by medication effects.
• Worsening with Distractors: Introducing distractors further worsened performance across almost all fixation stability measures in the SCZ group, highlighting an inability to filter out irrelevant visual stimuli and a specific deficit in reflexive saccade inhibition.
• Preserved Fixation Stability in FDRs (Target-Only Trials): FDRs showed intact fixation stability in trials without distractors compared to HCs, which contrasts with some previous findings and suggests that the core fixation stability deficit may not fully capture the genetic mechanisms of the endophenotype, although a distractor effect was noted for mean saccade amplitude in FDRs.

Schizophrenia Biomarkers

The fixation stability task, facilitated by the high-resolution EyeLink eye tracking, proved to be a simple yet sensitive measure for differentiating antipsychotic-naïve/free SCZ patients from healthy controls. The pronounced fixation instability and the unique rightward laterality bias in SCZ and their FDRs suggest a fundamental disruption in visuomotor filtering and functional brain asymmetry. The study highlights the potential of fixation stability as a marker of early, trait-level neurophysiological changes in SCZ, warranting it further evaluation as a potential biomarker for the disorder.

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