EyeTracking - Meg Wiki

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Eye tracking

The CBU currently has 4 eye trackers. One is in the MEG lab, one in the MRI scanner, and we have two separate eye trackers for use in other locations. All eye trackers are manufactured by SMI and use the same SMI software for controlling the eye tracking hardware, for stimulus presentation and for analysis of the eye tracking data. In addition, all trackers can also be used with E-Prime, if needed.

All 4 eye trackers use 'dark pupil' technology, where the gaze of the eye is tracked with an infra-red camera by identifying the pupil and the reflection of an infra-red light source on the cornea.

Eye trackers come in two varieties: 'contact' and 'remote'. With contact eye trackers the camera and light source are fixed to the head, or the head is on a chin rest to limit movements relative to the camera. In remote systems the camera and light source are in a fixed location, at some distance from the subject, enabling some head movements. Contact systems are more precise and reliable, in general.

Another important feature of an eye tracker is the sampling frequency. To be able to follow the eye during saccades the minimum frequency necessary is about 200 Hz. For 'heat maps', which map summed gaze duration by location, or for AOI dwell time analyses lower frequencies are sufficient.

All our eye trackers are able to output the x and y coordinates of the screen location the subject is looking at, and the diameter of the pupil. That last value will be 0 during a blink.

A very good online resource for anything related to eye tracking is [http://www.eyemovementresearch.com eyemovementresearch.com].

Eye trackers at the CBU

[:MegEyeTracker:MEG eye tracker] - 50 hz tracker.

[:MRIEyeTracker:MRI eye tracker] - 50 hz tracker, similar to MEG setup.

[:REDEyeTracker:RED eye tracker] - Remote system for behavioural tests.

[:HiSpeedEyeTracker:Hi-speed eye tracker] - High-speed tracking for more demanding behavioural tests.

Using eye trackers

Eye tracking is not as easy as it might look, and there's also quite a bit of variability between subjects. Most problems can be solved and with the vast majority of people you should be able to acquire decent quality eye tracking data.

[:SettingUpEyeTracker:Eye tracker setup]

[:EyeTrackingProblems:Common eye tracking problems]

Instructions for specific software packages

The SMI eye trackers come with their own stimulus presentation software, Experiment Center, and their own analyses tool, BeGaze. Experiments designed and executed in Experiment Center are very easy to analyse in BeGaze, as everything is recognised automatically. The trade-off is that Experiment Center only allows fairly simplistic experimental designs.

If you want to use other experiment software, you will need to customise your scripts.

[:EyeTrackingWithEprime:Eye tracking with Eprime]

[:EyeTrackingWithMatlab:Eye tracking with Matlab (Cogent / Psychtoolbox)]

Data Analysis

The eye tracking data can be analysed very easily with the SMI software package BeGaze. This will plot raw data, gaze paths, dwell times, heatmaps etc. at the click of a mouse. It cannot do statistical analyses, though, and you will have to export the data from BeGaze for that. BeGaze will allow you to create areas of interest, even moving ones in videos, and calculate total dwell time for all your AOI's.

[:BeGaze:Analysing Eye tracking data with BeGaze]

[:ExportingEyetrackingData:Exporting eye tracking data for custom analysis]