Understanding blink-saccade artefacts
The EyeLink system calculates gaze position by identifying the centre of the pupil. During a blink, the eyelids move rapidly to cover and uncover the eye. This fast movement causes the centre of the visible pupil to shift abruptly, which the system’s detection algorithm misinterprets as a high-velocity eye movement, or saccade.
Because of this shift, a blink is recorded as a specific sequence of events in the raw data. An "artefact" saccade is triggered as the eyelid closes and as it reopens, essentially wrapping the blink event within a saccade event. In the data file, this appears as:
SSACC (Start Saccade) → SBLINK (Start Blink) → EBLINK (End Blink) → ESACC (End Saccade)
Data Viewer processing
By default, "blink-saccades" are treated as valid saccade events by Data Viewer which can skew your saccade metrics if left unaddressed. To improve data integrity, Data Viewer provides options to merge these blink-saccades with the blinks they surround so that, for instance:
- samples marked as blinks in a Sample Report will more accurately reflect the total blink time including eyelid closure and reopening.
- motion artefacts are prevented from being incorrectly included in your saccade count during analysis.
(There are a few "blink saccade" preferences in Data Viewer's Preferences > Data Filters section)
Identifying "real" (physiological) blinks vs. data loss
It is important to note that EyeLink systems record a blink event (wrapped around a blink-saccade) whenever the pupil is lost. This includes not only real blinks (eyelid closures) but also any other loss of pupil such as when a participant moves out of the camera’s field of view or the pupil is obscured by other means.
In Data Viewer's Playback Animation view, a true physiological blink during a fixation typically appears as a rapid downward saccade from the fixated location that exits the screen boundaries, followed by an upward saccade (as the eye is uncovered) back to the fixated location - see the right-side of the video clip below.
The upward/downward saccade is the blink-saccade event. Note that this blink-saccade characteristic is typical only for blinks during a fixation and cannot, alone, be used to distinguish all physiological blinks from other data loss.
The EyeLink system calculates gaze position by identifying the centre of the pupil. During a blink, the eyelids move rapidly to cover and uncover the eye. This fast movement causes the centre of the visible pupil to shift abruptly, which the system’s detection algorithm misinterprets as a high-velocity eye movement, or saccade.
Because of this shift, a blink is recorded as a specific sequence of events in the raw data. An "artefact" saccade is triggered as the eyelid closes and as it reopens, essentially wrapping the blink event within a saccade event. In the data file, this appears as:
SSACC (Start Saccade) → SBLINK (Start Blink) → EBLINK (End Blink) → ESACC (End Saccade)
Data Viewer processing
By default, "blink-saccades" are treated as valid saccade events by Data Viewer which can skew your saccade metrics if left unaddressed. To improve data integrity, Data Viewer provides options to merge these blink-saccades with the blinks they surround so that, for instance:
- samples marked as blinks in a Sample Report will more accurately reflect the total blink time including eyelid closure and reopening.
- motion artefacts are prevented from being incorrectly included in your saccade count during analysis.
(There are a few "blink saccade" preferences in Data Viewer's Preferences > Data Filters section)
Identifying "real" (physiological) blinks vs. data loss
It is important to note that EyeLink systems record a blink event (wrapped around a blink-saccade) whenever the pupil is lost. This includes not only real blinks (eyelid closures) but also any other loss of pupil such as when a participant moves out of the camera’s field of view or the pupil is obscured by other means.
In Data Viewer's Playback Animation view, a true physiological blink during a fixation typically appears as a rapid downward saccade from the fixated location that exits the screen boundaries, followed by an upward saccade (as the eye is uncovered) back to the fixated location - see the right-side of the video clip below.
The upward/downward saccade is the blink-saccade event. Note that this blink-saccade characteristic is typical only for blinks during a fixation and cannot, alone, be used to distinguish all physiological blinks from other data loss.