Investigating the relationship between microsaccades and oscillations in the human visual cortex

Neural oscillations play important roles in vision and attention. Most studies of oscillations use visual fixation to control the visual input. Small eye movements, called microsaccades, occur involuntarily ~ 1-2 times per second during fixation and they are also thought to play important roles in vision and attention. The aim of the work described in this thesis was to explore the relationship between microsaccades and oscillations in the human visual cortex. In Chapter 2, I describe how remote video eye tracking can be used to detect and characterize microsaccades during MEG recordings. Tracking based on the pupil position only, without corneal reflection, and with the participant’s head immobilized in the MEG dewar, resulted in high precision gaze tracking and enabled the following investigations. In Chapter 3, I investigated the relationship between induced visual gamma oscillations and microsaccades in a simple visual stimulation paradigm. I did not find evidence for the relationship. This finding supports the view that sustained gamma oscillations reflect local processing in cortical columns. In addition, early transient gamma response had a reduced amplitude on trials with microsaccades, however the exact nature of this effect will have to be determined in future studies. In Chapter 4, I investigated the relationship between alpha oscillations and microsaccades in covert spatial attention. I did not find evidence for a relationship between hemispheric lateralization of the alpha amplitude and the directional bias of microsaccades. I propose that microsaccades and alpha oscillations represent two independent attentional mechanisms - the former related to early attention shifting and the latter to maintaining sustained attention. In Chapter 5, I recorded, for the first time, microsaccade-related spectral responses. Immediately after their onset, microsaccades increased amplitude in theta and beta bands and this effect was modulated by stimulus type. Moreover, microsaccades reduced alpha amplitude ~ 0.3 s after their onset and this effect was independent of stimulus type. These results have important implications for the interpretation of the classical oscillatory effects in the visual cortex as well as for the role of microsaccades in vision and attention