July 2013
Volume 13, Issue 9
Free
Vision Sciences Society Annual Meeting Abstract  |   July 2013
Population averaging in the distorted map of the superior colliculus: A new and simple account of systematic saccadic undershoot.
Author Affiliations
  • Françoise Vitu
    Laboratoire de Psychologie Cognitive, CNRS, Aix-Marseille Université
  • Soazig Casteau
    Laboratoire de Psychologie Cognitive, CNRS, Aix-Marseille Université
Journal of Vision July 2013, Vol.13, 1220. doi:10.1167/13.9.1220
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      Françoise Vitu, Soazig Casteau; Population averaging in the distorted map of the superior colliculus: A new and simple account of systematic saccadic undershoot.. Journal of Vision 2013;13(9):1220. doi: 10.1167/13.9.1220.

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      © ARVO (1962-2015); The Authors (2016-present)

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Abstract

It is well known that saccadic eye movements tend to systematically undershoot the center of a single target stimulus displayed in the periphery, with the undershoot being proportional to target eccentricity (~10%). This phenomenon investigated for many years, was attributed to visual-acuity constraints, insufficient saccadic programming time as well as adaptive visuo-motor strategies. Here, we show, based on human behavioral data, that systematic saccadic undershoot more simply results from the distortion of visual space (or foveal magnification) in the motor map of the Superior Colliculus (SC), where saccade metrics are computed as a result of population averaging. In our study, eight participants were asked to move their eyes as quickly and as accurately as possible to a single peripheral target, while their eye movements were recorded. The target (a 0.25° triangle), was displayed at variable eccentricities (1-15°), and in variable directions (0, 22.5, 45, 67.5, 80 and 90°). The landing position of the initial saccade was first measured in degrees of visual angle. It was then converted in millimeters of collicular space, using Ottes et al.'s (1986) logarithmic mapping function of the SC in monkeys, in order to estimate the underlying distribution of neuronal activity. Results first confirmed that saccades were overall hypometric; irrespective of target direction, the angular landing position error became gradually greater and more variable as target eccentricity increased. The second, more surprising finding was that initial landing positions, when expressed in millimeters of collicular space, were all centered on the presumed target location in the motor map. Thus, neuronal activity likely builds up almost perfectly at the center of the stimulus pattern in collicular space, but due to non-homogenous afferent/efferent mapping, this inevitably results in the execution of hypometric saccades. Whether this reflects limited adaptability of the oculomotor system will have to be determined.

Meeting abstract presented at VSS 2013

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