Detection of object displacement during a saccade is prioritized by the oculomotor system

The application of saccades to assess cognitive impairment among older adults: a systematic review and meta-analysis

BACKGROUND

Saccade is a novel and feasible method for cognition assessment and has potential to screen older people with cognitive impairment.

OBJECTIVES

To systematically summarize the evidence and determine whether different saccade parameters can effectively identify patients with mild cognitive impairment (MCI) and Alzheimer's disease (AD).

METHODS

English and Chinese databases were searched until 19 April 2022. Studies analyzing saccade parameters in older adults with normal cognition, MCI, or AD were included. Two researchers independently performed the screening, data extraction, and quality appraisal. Meta-analyses were conducted and standard mean differences and 95% confidence intervals were estimated with a random effects model.

RESULTS

Thirty-five studies were included, and 26 studies were pooled for the meta-analysis. The results demonstrated that patients with cognitive impairment exhibited longer latency and lower accuracy rates in the prosaccade and antisaccade tasks, along with lower corrected error rates in the antisaccade tasks. However, the pooled results for antisaccades were more stable, providing the ability to distinguish patients with cognitive impairment among older adults. The results of the subgroup analyses revealed that only the accuracy rates of the antisaccades differed significantly between people with MCI and AD. Regarding the differences between older adults with normal cognition and those with MCI, the effect sizes of latency and the accuracy rates of saccades as well as the corrected error rates of antisaccades were significant.

CONCLUSIONS

Saccades, especially antisaccades, are a potential screening and assessment tool for distinguishing older adults with MCI or AD from those with normal cognition.

Rapid spatial oculomotor updating across saccades is malleable

The oculomotor system uses a sophisticated updating mechanism to adjust for large retinal displacements which occur with every saccade. Previous studies have shown that updating operates rapidly and starts before saccade is initiated. Here we used saccade adaptation to alter life-long expectations about how a saccade changes the location of an object on the retina. Participants made a sequence of one horizontal and one vertical saccade and ignored an irrelevant distractor. The time-course of oculomotor updating was estimated using saccade curvature of the vertical saccade, relative to the distractor. During the first saccade both saccade targets were shifted on 80% of trials, which induced saccade adaptation (Experiment 1). Critically, since the distractor was left stationary, successful saccade adaptation (e.g., saccade becoming shorter) meant that after the first saccade the distractor appeared in a different hemifield than without adaptation. After adaptation, second saccades curved away only from the newly learned distractor location starting at 80 ms after the first saccade. When on the minority of trials (20%) the targets were not shifted, saccades again first curved away from the newly learned (now empty) location, but then quickly switched to curving away from the life-long learned, visible location. When on some trials the distractor was removed during the first saccade, saccades curved away only from the newly learned (but empty) location (Experiment 2). The results show that updating of locations across saccades is not only fast, but is highly malleable, relying on recently learned sensorimotor contingencies.