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Matthew Schneps, L. Todd Rose, Susana Martinez-Conde, Marc Pomplun; Covert orienting reflex: Involuntary pupil response predicts microsaccade production. Journal of Vision 2009;9(8):399. doi: https://doi.org/10.1167/9.8.399.
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Vision is suppressed during gaze shifts, and so visual perception is largely shaped during moments of fixation. However, if during fixation an unexpected event draws attention, miniature eye movements, known as microsaccades, are often produced (Hafed and Clark, 2002; Engbert and Kliegl, 2003). Microsaccades are known to play a role in enhancing perception (Martinez-Conde, et al., 2006), but the mechanism by which attention modulates the production of these movements is not known.
Here we report that when an unexpected peripheral stimulus is briefly flashed during fixation, the stimulus triggers a reflexive pupillary response that predicts the onset of microsaccades. In the case of a bright flash against a dark background, the pupil will constrict beginning ∼180ms after the flash, coinciding with the time when microsaccade production is inhibited.The rate of pupil change peaks ∼380 ms after stimulus onset, at a time that coincides with an observed overshoot in the microsaccade production rates. A pupillary response is also observed when the stimulus is purely auditory or visual equiluminant, but the dynamics of the response differs in these cases.
The pupil shares pre-motor circuitry with systems for vergence and accommodation (Mays and Gamlin, 1995). Therefore, the observed pupillary reflex may signpost a general release of tension in the extraocular muscles used to maintain fixation. If so, the observed overproduction of microsaccades following an exogenous stimulus may be the result of an orienting reflex (Sokolov, 1963) triggered by the stimulus that is imperfectly inhibited during this period of relaxation.
These findings are potentially important because microsaccades (as well as pupil size changes) are known to affect perception (Martinez-Conde, et al., 2006). Thus, interactions between a reflexive ocular response and the biomechanical constraints of fixation may underpin the earliest stages of exogenous attention.
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