Binocular quantification and characterization of microsaccades

Abstract

Background. The significance of microsaccades in the visual process has been discussed for more than 50 years. However, only a few studies have measured microsaccades binocularly, and detailed quantification and characterization of these small movements are needed in order to further understand their nature.

Method. The amplitude, velocity, acceleration and direction of microsaccades were quantified binocularly in 10 normal test persons during a 40-s fixation task, using an infrared recording technique.

Results. All microsaccades for all test persons were performed simultaneously and individually with an almost identical amplitude in the right and left eye (a range of 0.003–0.042 deg between right and left eye mean values). The mean microsaccadic amplitude for the test persons was within a range of 0.223–1.079 deg. The directional difference between simultaneously-performed right and left eye microsaccades was less than 22.5 deg for 84.8% of the saccades, indicating that the majority of microsaccades are conjugated. Three different fixation patterns were identified and characterized: (1) a classic interplay between easily identified drifts and medium-sized microsaccades (mean amplitude range 0.328–0.413 deg); (2) long intersaccadic intervals (4–5 s) with almost absent drifts, followed by three or four large microsaccades (mean amplitude range 0.755–1.079 deg); and (3) low-amplitude drift movements interrupted by low-amplitude microsaccades (mean amplitude range 0.231–0.265 deg).

Conclusion. Microsaccades are involuntary, predominantly conjugated, simultaneously performed, and of almost identical amplitude in the right and left eye, suggesting a central control mechanism for microsaccades at subcortical level.