Fixation Related Potentials (FRP)
An FRP is the average brain response elicited when natural eye-fixations are made to visual stimuli, such as the mouth or eyes in a face, or a word written in a sentence. We measure FRPs using a method developed by our collaborators in Salzburg. In this system an eye-tracker monitors and records eye-movements, and sends codes to the ERP system at the beginning of a stimulus presentation, or when the eyes fixate on a pre-defined area of a visual scene for more than 35 ms. We use these codes and the record of eye-movements to calculate the average brain activity in response to the presentation of a visual stimulus, as well as subsequent fixations made to one or more areas of the stimulus. For example, we can calculate the average brain response to the eyes in a face and the average brain response to the mouth in the same face, by directing initial fixations to be over specific regions of a stimulus when presented. The full FRP paradigm then allows us to record the brain activity that occurs when participants' eyes then fixate on various parts of the face during natural scanning. This allows us to observe brain activity during dynamic visual processing, and is a very useful technique if you want to measure brain responses to stimuli presented in real-world situations such as words within sentences, eyes in faces, or (perhaps) musical notes in scores.
EyeLink 1000 tower-mounted system
The tower-mounted infra-red EyeLink 1000 eye-tracking system by SR Research provides the greatest spatial and temporal resolution. It operates at 2000Hz (monocular), and is capable of sampling eye-position from corneal reflection, as well as pupillometry data. The participant's head is stabilized on a chin-rest in a tower. The camera is positioned above the chinrest and monitors their eye-movements via a mirror placed between the participant and the computer screen. The mirror reflects infra-red light but visible light passes through, so the participant’s view of the computer screen is not affected. Our tower-mounted eye-tracker is integrated with a Neuroscan EEG system, allowing brain responses to be time-locked to fixation on particular areas of interest on the screen.