Principles of eye-head coordination

Abstract

To explore the visible world, humans and other animals rapidly shift their line of sight to potential targets. These voluntary movements are executed by the eyes either alone, in which case they are called saccades, or in coordination with the head in which case they are called gaze shifts. Both of these movements serve the same goal reliably, accurately and fast while displaying highly stereotypical metrics and kinematics during movement execution. The strategy and mechanisms that are employed by the nervous system when planning gaze shifts are gleaned from two computational models that were formulated in this thesis and address the phenomenon of eyehead coordination at two different levels of abstraction.The first model used optimal control theory to discover the crucial variables and constraints that are important during motor planning of gaze shifts. This model accounts for the stereotypy that is observed in the psychophysics of headfixed saccades and headfree gaze shifts. Results from these simulations suggest that the organization principle that guides these movements exploits a balance between effort and movement duration, thus implying that extraocular muscle tension and energy dissipation due to inertial effects of the head and movement duration are parameters that are collectively taken into consideration and minimized at the neural level. The second computational model simulates the neural circuitry that generates eyehead gaze shifts. The goal of these neural network simulations is to model the relevant neural processes starting from the spatial representation of the target (a neural map found in the superior colliculus) and ending with the set of requisite neuromuscular commands that drive the eyes and head. A controltheoretic systems approach is used to explore the computational repertory of the connectivity established by the model units and to explore if it generates results consistent with the experimental data obtained from neurophysiology, electrical stimulations, neuroanatomy, neurology and psychophysics. We conclude that minimumeffort is the movement organization principle of eyehead coordination which is implemented by the brain using a crosstalk mechanism between independently controlled head and eye motor pattern generators.