Although natural vision involves an active sampling of the environment with several saccadic eye movements per second, electroencephalographic (EEG) correlates of visual cognition are predominantly recorded under artificial conditions of prolonged fixation. An alternative approach to EEG analysis, explored in the present thesis, is to time-lock the signal not to passive stimulations, but to the on- or offsets of naturally occurring eye movements, yielding saccade- and fixation-related potentials (SRPs/FRPs). Using simultaneous high-resolution eye-tracking (ET), this technique was applied in two contexts. The first part of the thesis (publications 1 & 2) investigated brain-electric correlates of microsaccades, small involuntary eye movements, which occur despite attempted fixation during traditional EEG paradigms. In a series of experiments, we show that SRPs from microsaccades present a significant, but normally hidden source of visuocortical potentials that is active in most trials and can confound the interpretation of stimulus-locked data under specific conditions. The second part of the thesis assessed the feasibility and utility of using FRPs in the study of natural reading. Publication 3 provides a review of artifact sources, low-level factors, and high-level influences determining the FRP waveform in free viewing and proposes methods to optimize signal quality. We then replicate the N400 word predictability effect, a cornerstone of neurolinguistic research, in left-to-right sentence reading and relate N400 amplitude to measures of fixation time. In publication 4, the FRP technique was combined with gaze-contingent display manipulations to investigate the depth of parafoveal preprocessing in fluent reading. Our results show that simultaneous recordings improve the understanding of electrophysiological data recorded during fixation, extend the EEG's methodological scope to naturalistic viewing scenarios, and help to integrate findings from EEG and ET research.
Keywords: EEG, eye-tracking, fixation-related potentials, free viewing, reading, microsaccade, saccadic eye movements