Types
Saccades can be categorized by intended goal in four ways:
- In a visually guided saccade, the eyes move towards a visual transient, or stimulus. The parameters of visually guided saccades (Amplitude, Latency, Peak Velocity, Duration) are frequently measured as a baseline when measuring other types of saccades. Visually guided saccades can be further subcategorized:
- A reflexive saccade is triggered exogenously by the appearance of a peripheral stimulus, or by the disappearance of a fixation stimulus.
- A scanning saccade is triggered endogenously for the purpose of exploring the visual environment.
- In an antisaccade, the eyes move away from the visual onset. They are more delayed than visually guided saccades, and observers often make erroneous saccades in the wrong direction. A successful antisaccade requires inhibiting a reflexive saccade to the onset location, and voluntarily moving the eye in the other direction.
- In a memory guided saccade, the eyes move towards a remembered point, with no visual stimulus.
- In a sequence of predictive saccades, the eyes are kept on an object moving in a temporally and/or spatially predictive manner. In this instance, saccades often coincide (or anticipate) the regularly moving object.
As alluded to above, it is also useful to categorize saccades by latency (time between go-signal and movement onset). In this case the categorization is binary: a given saccade is either an express saccade or it is not. The latency cut-off is approximately ~100 ms, any longer than this is outside the express saccade range.
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