Fixational instability as a hidden source of visual brain signal in the EEG
Dimigen, O.1, Pajkert, A.1, Dambacher, M.2, Sommer, W.1, and Kliegl, R.3
1Institut für Psychologie, Humboldt Universität zu Berlin; 2Institut für Psychologie, Freie Universität Berlin; 3Institut für Psychologie, Universität Potsdam
Microsaccades are tiny (< 1°), involuntary flicks in eye position that occur once or twice per second during attempted fixation. Although monkey studies have shown that microsaccades modulate neuronal firing throughout the visual pathway, fixational eye movements are typically not recorded in human neuroimaging. We investigated the electrophysiological correlates of normal fixational instability by co-recording EEG and high-resolution eye movements during prolonged fixation (of checkerboards and faces) and in three common ERP paradigms: visual oddball, face classification, and RSVP reading (total N=79). We replicate recent findings that microsaccadic muscle spikes translate to gamma band spectral artifacts. Importantly, however, we also demonstrate that the retinal displacement generated by each microsaccade (as small as 0.1°) evokes a sizable visual response over occipital cortex after 100-130 ms. Visually-evoked cortical potentials from microsaccades were present in most (> 85%) stimulus-locked EEG segments. In two experiments, this overlap influenced the waveshape of late ERP components (P300/LPC).