EEG (electroencephalography) captures electric activity in the brain via electrodes placed on the scalp. This electric activity is temporally very precise and provides important information about when different computations are happening in the brain. For example, we can investigate the earliest time at which our brain can distinguish familiar or preferred sounds from unfamiliar or less preferred sounds. Due to distortions arising from the conductive properties of the skin, bone, and brain tissue, the EEG technique has relatively poor spatial resolution – it is not so great at telling where exactly in the brain the activation is coming from. For the same reason, EEG is better at picking up activity from the outer parts of the brain, such as the cortex. The cortex is heavily involved in processes such as perception from all sensory modalities, planning and initiating movements or abstract reasoning and planning ahead. These processes are well suited for an investigation with EEG. Other complex processes in the brain, such as emotions, motivation, or certain aspects of memory lie in deep brain structures and are more easily observed using techniques such as fMRI (functional magnetic resonance imaging).
Eye-tracking involves observing the various movements of the eyes using a very fast and accurate camera. Here, we are particularly interested in changes of the pupil size. A diverse array of processes can lead to increases of pupil size, such as a high attentive state, positive emotions, or performing difficult mental tasks.