Button Data; Events; Messages - SR Research EyeLink Portable Duo User Manual

those factors not being explicitly manipulated must be equated across
conditions to achieve a valid measure of pupil size. Lastly, since pupil size is
recorded in arbitrary units that are not calibrated across participants,
measures of pupil size are best recorded as percent change relative to a baseline
period.

4.4.4 Button Data

The state of up to 8 buttons or input port bits may be recorded in each sample.
Button ports, bits, and polarity may be set in the EyeLink tracker configuration
file BUTTONS.INI.
The button data consists of two 8-bit fields, recorded as a 16-bit number. The
lower 8 bits contain the current status of the 8 buttons (bit = 0 if off, 1 if
pressed). Each of the upper 8 bits will be set to 1 if its button has changed
since the last sample. The least-significant bit in each byte corresponds to
button 1, and the most-significant to button 8.
4.5

Events

One of the most significant aspects of the EyeLink tracking system and the EDF
file format is its on-line processing of eye-movement data to identify and record
events such as fixations and saccades. This eliminates the need for recording of
sample data for many types of research, and achieves a data compression of
20:1 or greater. Samples need only be recorded for data validation or if sample-
by-sample eye position or velocity is required.
Events can record application data such as the time of a display change and
experimental conditions, or real-time events such as button presses. Events
also define the start and end of blocks of data in the EDF file, allowing
applications to process data recorded with different data types.
Each event contains one or two timestamps (in milliseconds) and several data
fields. Data for each event are compressed, and an extendable data format
allows compatibility with future expanded file formats.
Note that not all the event data listed here are available through the EDF2ASC
translator program.

4.5.1 Messages

The most flexible event type is the message event. A message is most often text,
but can contain any type of binary data as well, up to a maximum of 300 bytes.
Messages are created by the application software, and sent over the link to the
EyeLink tracker, which timestamps the data and writes it to the EDF file. The
application does not need precise time keeping, since link delays are usually
very low (on the order of 1 or 2 milliseconds).
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