5 Working with Events, Samples, and Interest Areas

The EyeLink Data File (EDF file) contains two streams of data: eye-position samples (up to 2000 per second produced from the EyeLink tracker, depending on the system model) and events (eye-movement events such as saccades and fixations, subject responses, and synchronizing events from the experimental application). Both streams are timesynchronized for easy analysis. The file is organized into trials of data, one for each of the start and stop of tracker recording. Each trial may have samples, events, or both.

Samples are time-stamped in milliseconds and contain monocular or binocular eyeposition data - for now Data Viewer only report the eye position in screen coordinate

(GAZE) while other data formats (PUPIL or HREF) are ignored. Pupil sizes as area or diameter are also recordable. Samples may also contain eye-movement resolution (used to compute true velocity or saccadic amplitudes).

Eye-movement events record eye position changes identified by the EyeLink tracker's online parser, such as fixations, blinks, and saccades. Both the onset and end of these events are marked, allowing samples to be assigned to eye-movement periods without complex algorithms. Important data for analysis such as average position for fixations and peak velocity for saccades is also recorded in the events. Other events record subject responses

(such as button presses) and synchronization and data messages from applications. These can be used to record the time of a change in the display, or an experimental condition.

Events are generated by the EyeLink eye tracker (EyeLink I, EyeLink II, and EyeLink

CL) in real-time based on an internal heuristic saccade detector (using a velocity and acceleration-based saccade detection method). A blink is defined as a period of saccadedetector activity with the pupil data missing for three or more samples in a sequence. A fixation event is defined as any period that is not a blink or saccade. To detect a saccade, for each data sample, the parser computes instantaneous velocity and acceleration and compares these to the velocity and acceleration thresholds. If either is above threshold, a saccade signal is generated. The parser will check that the saccade signal is on or off for a critical time before deciding that a saccade has begun or ended. A detailed discussion on event parsing can be found in section "4. Data Files" of the EyeLink II or EyeLink 1000

User Manual.

In addition to providing a file-based and a trial-based summary of performance, the

EyeLink Data Viewer allows a detailed examination of event data such as fixations, saccades, blinks, buttons, messages, as well as raw sample data.

5.1 Common Operations

EyeLink Data Viewer

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