A model-based theory on the signal transformation for microsaccade generation

The eyes are continuously fluctuating even during fixation. The fluctuations are called miniature eye movements and consist of microsaccades, drifts, and tremors. It has been revealed that these miniature eye movements aid our vision; they improve the visibility of high spatial frequency components, and prevent retinal adaptation during fixation. Although the functional roles of the miniature eye movements have gradually been uncovered, their generation mechanism remains a mystery. Here, we focused on microsaccades, and constructed a neuronal network model to explore their generation mechanism. Several lines of evidence ensure that microsaccades and saccades share the same neuronal circuitry because they fall on the same main sequence, a relationship between their amplitudes and peak velocities. In the saccade pathway, saccade commands generated in the superior colliculus are relayed to motoneurons via burst neurons (BNs) and the integrator network. The BNs are inhibited by omnipause neurons (OPNs) except when saccades are generated. We configured a model for microsaccades based on the well-defined saccade neuronal pathway including tonic neurons, BNs, OPNs, the integrator network, and the eye plant. The model successfully reproduced various characteristics of microsaccade: square-wave jerk, single-sided microsaccades, and the main sequence. Moreover, during microsaccades, BNs showed low-rate spikes due to a partial release from the OPN inhibition. These results suggest that microsaccades are generated when BNs are partially, but not completely, released from tonic inhibition by OPNs during fixation, in contrast to the generation of ordinary saccades in which OPNs pause firing and release BNs from their strong inhibition.