Are the visual transients from microsaccades helpful? Measuring the influences of small saccades on contrast sensitivity

Abstract

Like all saccades, microsaccades cause both spatial and temporal changes in the input to the retina. In space, recent studies have shown that these small shifts precisely relocate a narrow (smaller than the foveola) high-acuity retinal locus on the stimulus. However, it has long been questioned whether the temporal modulations resulting from microsaccades are also beneficial for vision. To address this question, we combined spectral analysis of the visual input to the retina with measurements of contrast sensitivity in humans. Estimation of how different types of eye movements redistribute the power of an otherwise stationary stimulus shows that small saccades contribute more temporal power than ocular drift in the low-frequency range, suggesting a specific role for these movements in the encoding of low spatial frequencies. However, an influence on contrast sensitivity was only found for saccades with amplitudes larger than 30′. Contrast thresholds remained highly similar in the presence and absence of smaller saccades. Furthermore, saccades of all amplitudes, including microsaccades, were strongly suppressed during exposure to the stimulus. These findings do not support an important function of the visual transients caused by microsaccades.

Changes in visibility as a function of spatial frequency and microsaccade occurrence

“…First, we analyzed the global average microsaccade magnitudes for the different SFs and found no significant differences across the SFs tested (Fig. ; F 4, 56 = 2.180, P = 0.083, MSE = 0.0004, ), an apparently null result ostensibly consistent with (Mostofi et al ., ; Spotorno et al ., ). Yet, when we analyzed microsaccade magnitudes in the peak interval (i.e.…”

“…; F 4,56 = 1.71, P = 0.16, MSE = 0.008, ). This apparently null result is consistent with a prior study (Mostofi et al ., ), and provides a plausible explanation for previous failures to find a significant relationship between microsaccade production and visual perception as a function of SF. Next, we examined the timing of microsaccades with respect to perceptual fluctuations, by locking variations in microsaccade rates to perceptual transition reports.…”

“…Recent work on the contrast sensitivity thresholds of low and high SF targets found little or no link between (micro)saccade production and perception (Mostofi et al ., ; Spotorno et al ., ). These previous studies related overall (micro)saccade parameters (i.e.…”

“…Our study goes beyond previous research that did not time‐lock microsaccade occurrence to perceptual transitions on target visibility (Mostofi et al ., ; Spotorno et al ., ). Prior work measured grating discrimination (Spotorno et al ., ) or contrast sensitivity (Mostofi et al ., ) variations as a function of SF, and compared such metrics to global microsaccade rates (or occurrence) and global microsaccade magnitudes.…”

“…Mostofi et al . () concluded that microsaccades (defined as saccades < 0.5°) had little impact on contrast sensitivity (a similar, but not identical concept to visibility) at both low [0.8 cycles per degree (cpd)] and high SFs (10 cpd); yet, spectral analyses suggested a microsaccadic contribution to enhancing low‐frequency vision. Spotorno et al .…”