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"Optimal Control of Saccades by Spatial-Temporal Activity Patterns in the Monkey Superior Colliculus"
John Van Opstal
Radboud University Nijmegen, The Netherlands,
Donders Institute for Brain, Cognition and Behaviour
Department of Biophysics
"Optimal Control of Saccades by Spatial-Temporal Activity Patterns in the Monkey Superior Colliculus"
A major challenge in neurobiology is to understand how populations of noisy, broadly-tuned neurons in the brain can produce accurate goal-directed actions such as saccades. Saccades are
high-velocity eye movements that have stereotyped, nonlinear kinematics; their amplitude-duration relation follows a straight line, while peak eye velocity saturates for large saccades. Recent theories
(e.g. Wolpert and colleagues) suggest that these characteristics reflect a deliberate strategy that optimizes speed-accuracy tradeoff.
Here we argue that the midbrain superior colliculus (SC), a key sensorimotor interface in orienting behavior, is in an ideal position to implement such an optimization principle. Most previous models
attribute the nonlinear saccade kinematics to saturation in the brainstem pulse generator. However, there is little data to support this assumption. We here present new evidence for an alternative scheme,
which proposes that these properties reside in the spatial-temporal dynamics of SC activity. We demonstrate a clear spatial gradient in the burst properties of saccade-related cells along the rostral-to-
caudal dimension of the SC motor map: peak firing-rates systematically decrease, while burst durations and skewness show a systematic increase. Moreover, we show that all cells in the recruited
population synchronize their burst profiles, indicating that the burst-timing of each cell is determined by the planned saccade vector in which it participates, rather than by its anatomical location.
Simulations with our linear spike-vector summation model (Goossens & Van Opstal, J Neurophysiol 95: 2326, 2006) show that these burst properties can fully account for the kinematic nonlinearities of saccades.
We propose that the SC acts as the nonlinear, vectorial pulse generator that specifies an optimal straight eye-movement trajectory.
