With Extended Methodological Discussion
An article published in Journal of Experimental Psychology: General investigates effects of word predictability on eye movements (EM) and fixation-related brain potentials (FRPs) during natural sentence reading. Electroencephalogram (EEG) and EM were recorded simultaneously, allowing for direct comparisons between EEG measures and oculomotor behavior. Within a methodological review section the authors discuss the technical and data-analytical issues related to simultaneous EM-EEG recordings during normal viewing.
Pioneer Study on co-registration in natural reading
The researchers co-registered EM and EEG in an unconstrained viewing situation in order to directly compare fixation behavior to brain activity measured across the entire scalp. The method reveals the basic relationship and temporal contingency between the dependent variables in EM research (e.g., fixation duration) and ERP research (e.g., component amplitude).
By time-locking the EEG to fixations on words that were expected to various degrees in the sentence context, researchers could replicate robust effects of word predictability on behavior and concurrent brain activity and compare the latencies at which both types of measures are influenced by predictability.
Discussion of technical and data-analytical issues
Within an extended methodological review section the following technical and data-analytical issues are discussed that need to be addressed when FRPs are recorded during free viewing (e.g., reading, visual search, or scene perception):
- The co-registration of precise gaze position without interference
- The correction of corneoretinal and myogenic eye movement artifacts
- Temporal overlap between successive FRPs as well as between FRPs and background ERPs
- Variability of saccade-related brain potentials according to low-level visuomotor factors
Co-Registration with SMI Eye Tracking in Natural Reading
High resolution recording of eye movements with single-letter accuracy required for reading analysis was conducted with the SMI Hi-Speed eye tracking device.
Synchronization of EM and EEG records was achieved with the flexible analogue and digital interfaces of the SMI solution. A shared TTL trigger pulse was sent at the beginning and end of each trial from the stimulus presentation PC and looped through to two additional PCs recording EMs and EEG. To double-check synchronization, the analog-to-digital converter in the eye tracker was used to feed an analog copy of the gaze position signal as additional channels into the EEG headbox.
To reduce pressure artifacts from contact between electrodes and the eye tracker the electrodes on the forehead were foam-cushioned.
Muscle artifacts were minimized through careful adaptation of the subject’s sitting position.
To test for electromagnetic artifacts, the EEG spectrum during steady fixation was compared while the eye tracker was either recording or disconnected from power.
Authors mention remote eye tracking devices as a feasible alternative for co-registration studies in order to minimize pressure, electromagnetic and muscle artifact