Visual functions of microsaccade transients

Microsaccades, the microscopic and fast gaze relocations occurring while we attempt to maintain steady fixation, cause both spatial and temporal changes in the input to the retina. Despite much progress in understanding the spatial functions of these small eye movements during the last decade, it remains unclear whether the temporal modulations resulting from microsaccades are also beneficial for vision. This dissertation describes three studies aimed at providing answers to the following fundamental questions: (1) What are the space-time characteristics of the input to the retina at the time of saccades and microsaccades? Spectral analyses of the retinal input during free-viewing of natural images show that luminance modulations resulting from saccades and microsaccades redistribute the power of an otherwise stationary stimulus in a way that contributes more temporal power than ocular drift within a range of low spatial frequencies. These results suggest a specific role for saccadic eye movements in the encoding of low spatial frequencies. (2) We measured how microsaccade transients affect human contrast sensitivity at different spatial frequencies. We showed that contrast thresholds remain highly similar in the presence and absence of microsaccades below 30'. However, an improvement in sensitivity to low spatial frequency stimuli was found for saccades with amplitudes larger than 30'. Furthermore, saccades of all amplitudes, including microsaccades, were strongly suppressed during exposure to the stimuli. (3) What are the dynamics of visual sensitivity around the time of occurrence of microsaccades? We show that sensitivity is reduced at the time of microsaccades and small saccades, similar to what previously reported for saccades. Moreover, sensitivity is not homogeneous within the fovea but decreases with increasing eccentricity. These results clarify the importance of microsaccades to vision. They show that the luminance modulations resulting from both microsaccades and saccades play an important role in representation of visual information and affect our perception in a systematic way.2018-12-06T00:00:00