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and other research, but may have to be raised if very rapid pursuit or

nystagmus is being recorded.

The limit is set in degrees per second. An example of this command is:

saccade_pursuit_fixup = 60

4.3.7 Fixation Updates

Monitoring eye position or pupil size during fixations usually requires

processing all samples produced by the tracker. This is acceptable for file data,

but is expensive for real-time systems using data sent via the link. Fixation

updates solve this problem by sending updates on eye position, pupil size,

velocity etc. at regular intervals during a fixation. The first update is sent one

update interval after the start of the fixation, and the last is sent at the end of

the fixation. Data is aggregated over a preset period, which lowers data noise.

The interval between updates and the data accumulation period can both be

set.

Fixation updates are most useful for real-time display paradigms. In some

studies, the subject is required to fixate a target while stimuli are presented.

Fixation updates can be used to check gaze position every 100 msec or so.

Computer interfaces operated via eye movements is a paradigm dramatically

simplified by fixation updates. Actions are triggered by gaze on an active area of

the screen for a critical duration. This is implemented simply by counting

sequential fixation updates that fall within the area.

Two commands set the fixation update interval and data accumulation period in

milliseconds. Usually these are set to the same value. An interval of zero

disables fixation update. An update interval of 50 or 100 msec is a good choice:

fixation_update_interval = 100

fixation_update_accumulate = 100

4.3.8 Other Parameters

The EyeLink 1000 PARSER.INI configuration file contains other commands that

configure the parser. These are of several types:

• Verification delays. These set the time in milliseconds that the parser

requires a detector output (saccadic velocity or acceleration thresholds, or

missing pupil for blink) to be stable before the parser changes its state and

sends events to the data file or link. These values have been determined

empirically, and there is little advantage to changing them.

• Parser filter types. Two velocity filters are available: fast and slow. The fast

filter works better in most cases. The slow filter is less noise sensitive, but

increases saccade duration and decreases sensitivity slightly.