APPENDIX AVI212 Manual do Utilizador descarregar pdf (Página 199)

Event Detection

192

The algorithm identifies fixations as groups of consecutive points within a

particular dispersion, or maximum separation. It uses a moving window

that spans consecutive data points checking for potential fixations. The

moving window begins at the start of the protocol and initially spans a

minimum number of points, determined by the given Minimum Fixation

Duration and sampling frequency.

The algorithm then checks the dispersion of the points in the window by

summing the differences between the points' maximum and minimum x

and y values; in other words, dispersion D = [max(x) - min(x)] + [max(y) -

min(y)]. If the dispersion is above the Maximum Dispersion Value, the

window does not represent a fixation, and the window moves one point to

the right. If the dispersion is below the Maximum Dispersion Value, the

window represents a fixation. In this case, the window is expanded to the

right until the window's dispersion is above threshold. The final window is

registered as a fixation at the centroid of the window points with the given

onset time and duration.

Saccade Detection

At the end a saccade event is created between the newly and the

previously created blink or fixation.

Parameters

The parameters can be changed in the Adjust Event Detection dialog.

Min. duration: minimum fixation duration in [ms]

Max. dispersion: maximum dispersion value. The unit depends on the

experiment type :

Unit

standard data

pixels

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APPENDIX AVI212 Manual do Utilizador Página 199