The effect of microsaccades in the primary visual cortex: a two-phase modulation in the absence of visual stimulation

Our eyes are never still. Even when we attempt to fixate, the visual gaze is never motionless, as we continuously perform miniature oculomotor movements termed as fixational eye movements. The fastest eye movements during the fixation epochs are termed microsaccades (MSs), that are leading to continual motion of the visual input, affecting mainly neurons in the fovea. Yet our vision appears to be stable. To explain this gap, previous studies suggested the existence of an extra-retinal input (ERI) into the visual cortex that can account for the motion and produce visual stability. Here, we investigated the existence of an ERI to V1 fovea in behaving monkeys while they performed spontaneous MSs, during fixation. We used voltage-sensitive dye imaging (VSDI) to measure and characterize at high spatio-temporal resolution the influence of MSs on neural population activity, in the foveal region of the primary visual cortex (V1). In the absence of a visual stimulus, MSs induced a two-phase response modulation: an early suppression transient followed by an enhancement transient. A correlation analysis revealed an increase in neural synchronization around ∼100 ms after MS onset. Next, we investigated the MS effects in the presence of a small visual stimulus, and found that this modulation was different from the non-stimulated condition yet both modulations co-existed in the fovea. Finally, the VSD response to an external motion of the fixation point could not explain the MS modulation. These results support an ERI that may be involved in visual stabilization already at the level of V1.