When do I use the I-VT Attention filter?

Tobii Pro Lab Tobii Pro Glasses 2 Tobii Pro

The Attention Filter in Tobii Pro Lab is essentially the Tobii Pro IV-T Filter, with the velocity threshold parameter set to 100 degrees/second instead of the default 30 degrees/second.

The Attention Filter was created to handle eye tracking data from Tobii Pro Glasses 2 recordings, performed under dynamic situations, where either the subject is constantly moving, or the objects or targets are moving around the subject. In these situations, we use a large array of eye movements to help us keep our fovea aligned with objects and other visual features in the environment – fixations, saccades, smooth pursuits and vestibular ocular reflexes (VOR). (Read our article entitled Types of eye movements for more information).

We use the filter to separate the moments when we are trying to stabilize our fovea onto something (fixation, smooth pursuit and VOR), and thus potentially extracting information from that location or object, from the moment the eyes are moving too fast to extract information (saccades).

The threshold was set to 100 degrees/second using the following information from the literature:

  • Saccades over 1-2 deg in length typically have a velocity of about 70-110 deg/sec (Bahill, Clark & Stark 1975; Collewijn, Erkelens, Steinman 1988).
  • The portion of saccades under 2 degrees is about 10-15% (Bahill, Adler & Stark 1975).
  • Maximum smooth pursuit can be in excess of 100 deg/s in a professional athlete (Bahill & LaRitz 1984)
  • However, prior evidence suggest pursuit performance of gain becomes much worse between 50-100 deg/s (when head fixed in non -professionals)
  • Head may turn up to 120-180 deg/s and VOR can operate at up to about 120 deg/sec (Schubert et al 2006).

By setting the threshold to 100 degrees/second, we are classifying fixation, smooth pursuit and most VOR data as “Attention”. However, we will also classify 10-15% of saccades as “Attention”, thus we will slightly overestimate “Attention”.

Setting the IV-T Filter to the default setting of 30 degrees/second, will underestimate the periods of “Attention” or information gathering, since quite a large portion of data belonging to smooth pursuits and VOR periods will be classified as saccades.

Note: When using the “Attention” filter, any fixation based AOI metrics will become “Foveal stabilization” metrics instead, e.g. Time to First Fixation will become “time to the first moment the fovea is stabilized on an AOI”

References:

Bahill, A.T., Clark, M. R. & Stark, L., 1975, The main sequence, a tool for studying human eye movements. Mathematical Biosciences, 24, 191-204.

Collewijn, H., Erkelens, C.J. & Steinman, R.M., 1988, Binocular co-ordination of human vertical saccadic eye movements. The Journal of Physiology, 404, 183-197.

Bahill, A.T.,  Adler D. & Stark L., 1975. Most naturally occurring human saccades have magnitudes of 15 degrees or less. Investigative Ophthalmology & Visual Science, 14, 468-469.

Bahill, A. T., & LaRitz, T., 1984. Why can't batters keep their eyes on the ball? American Scientist, 72, 249-253.

Schubert, M.C., Migliaccio, A.A., & Della Santina, C.C., 2006. Dynamic Visual Acuity during Passive Head Thrusts in Canal Planes. Journal of the Association for Research in Otolaryngology, 7, 329-338.