Implemented Methods

Engbert and Kliegl (2003)

The algorithm labels a sample as a saccade if velocity, measured in units of its standard deviation (see below), exceeds a predefined threshold (defaults to 6) and the duration of a super-threshold period exceeds a minimal duration (defaults to 12 milliseconds). This method makes no assumptions about velocity in the units of degrees per second and can be used on any data (e.g., where samples encode gaze in screen position units).

The standard deviation is computed following formula #2 in Engbert and Kliegl (2003) as

sqrt(median(x^2) - median(x)^2)

However, if the value is smaller than .Machine$double.eps, it is recomputed via a mean estimator

sqrt(mean(x^2) - mean(x)^2)

Only a monocular version of the algorithm is implemented with binocular saccades computed using overlapping votes from two eyes. For method details and rationale for default parameter values refer to Engbert and Kliegl (2003).

Parameters:

• ek_velocity_threshold Velocity threshold for saccade detection in standard deviations. Defaults to 6.
• ek_sd_fun Function used to compute standard deviation for velocities. Defaults to sd_via_median_estimator() that implements formula #2 in Engbert and Kliegl (2003). Can be replaced with sd(), mad(), or a custom function.
• ek_minimal_duration_ms Minimal duration of a saccade in milliseconds. Defaults to 12.
• ek_minimal_separation_ms A minimal required time gap between saccades. Defaults to 12.

Otero-Millan et al. (2014)

The algorithm:

1. Detect local velocity peaks.
2. Filter them based on a minimal inter-peak interval (defaults to 30 ms) and maximal allowed saccade rate (defaults to 5 Hz).
3. Identify the onset and offset of a potential saccade using an absolute velocity threshold in degrees per second (defaults to 3).
4. Compute peak velocity and peak acceleration before and after the peak velocity was reached.
5. Rotate velocity and peak acceleration via PCA and retain components above a predefined threshold for explained variance (defaults to 0.05).
6. Perform a cluster analysis on the remaining components for 2, 3, and 4 picking the solution with the smallest silhouette.
7. Saccades are identified as a cluster with the highest average peak velocity.

For method details and rationale for default parameter values refer to Otero-Millan et al. (2014).

Parameters:

• om_minimal_inter_peak_time_ms Minimal inter-peak interval in milliseconds. Defaults to 30
• om_maximal_peaks_per_second Maximal allowed number of peaks per second. Defaults to 5.
• om_velocity_threshold_deg_per_sec Threshold saccade velocity in °/s. Defaults to 3.
• om_pca_variance_threshold Minimal variance explained by retained rotated components. Defaults to 0.05.

Nyström and Holmqvist (2010)

The algorithm:

1. Identify physiologically implausible velocity and acceleration peaks (defaults to 1000deg /s and 100000deg /s²).
2. Identify noise onset/offset as samples around the implausible velocity/acceleration that are above median velocity. Exclude these samples from analysis.
3. Identify velocity threshold PT = mean(V_subthreshold) + 6 ⋅ std(V_subthreshold) via an iterative method starting at arbitrary PT value (defaults to 100deg /s). Stop when threshold change is below 1deg /s.
4. Identify saccades as periods with peaks peaks above threshold PT and adjacent samples that are above PT_onset/offset = mean(V_subthreshold) + 3 ⋅ std(V_subthreshold).

For method details and rationale for default parameter values refer to Nyström and Holmqvist (2010).

Parameters:

• nh_max_velocity Maximal physiologically plausible velocity in °/s. Defaults to 1000.
• nh_max_acceleration Maximal physiologically plausible acceleration in °/s². Defaults to 100000.
• nh_initial_velocity_threshold Initial velocity threshold in °/s. Defaults to 100.