Gaze position effects and
position-dependent motor tuning in primate
superior colliculus (SC) neurons during head-unrestrained visually
guided movements
J. F. DESOUZA, *X. YAN, G. BLOHM, H. WANG, J. D. CRAWFORD
Ctr Vision Res, York Univ, Toronto, ON, CANADA.
Neural activity in the SC is highly correlated with gaze shifts
composed of both eye and head movements (Freedman and Sparks, 1996;
Munoz et al. 1991). Further, SC stimulation evokes eye and head gaze
shifts that converge as a function of amplitude and initial gaze
position, consistent with an eye-fixed motor code for gaze (Klier et
al. 2001). We hypothesize that SC neural activity correlates best with
gaze target location in retinal coordinates. This predicts that the
optimal directional tuning of SC neurons will change as a specific
function of amplitude tuning and initial gaze position (Smith and
Crawford 2005). Electrical stimulation and/or visual receptive field
examination are being used to estimate the optimal gaze amplitude and
direction for each recording location. Monkeys randomly fixate one of
three different initial gaze directions each separated by 20+/-10
degrees for 500 ms and then make their head-free gaze shift to one of
five visual targets placed along a semi-circle of iso-amplitude targets
(centered around the position of the receptive field maximum for
straight-ahead gaze). To date, we have recorded from thirty-four SC
neurons that have been fully tested in the head-unrestrained paradigm.
Analysis of these neurons show that SC neurons do indeed show strong
initial gaze position dependent firing changes during head-unrestrained
gaze shifts. Furthermore, a large majority of our SC neurons show a
position-dependent modulation of their firing rates as a function of
the initial gaze position, as required for a non-linear transformation
of retinal coordinates into gaze motor coordinates (Smith and Crawford
2005).