Dry dissection of disparity divergence eye movements using independent component analysis

Dry dissection, a concept developed by Lawrence Stark, includes a variety of techniques designed to isolate internal neural control components by using cleverly designed stimulus or measurement protocols. As envisioned by Stark, the concept applies only to motor systems that have multiple stimulus inputs and/or response behaviors. A new application of independent component analysis (ICA) can be used to extend the dry dissection concept to identify motor components from a single, isolated response. It is only necessary that multiple responses can be obtained to the same stimulus. This "ensemble ICA" technique is well suited to analyze various eye movement behaviors as even isolated motor systems often include multiple control processes. Here we apply ensemble ICA to vergence eye movements: the inward (convergence) or outward (divergence) turning of the eyes that allows us to view images at various distances. Previous studies concerning the dynamics of convergence and divergence eye movements have produced varied, sometimes contradictory, results: most studies report that convergence is considerably faster than divergence, but opposite results have also been reported. Experimental results have shown that the dynamics of divergence movements depend on the initial vergence position while those of convergence do not: divergence eye movements in response to targets initially near to the subject can attain peak velocities twice that of those driven by more distant targets. To determine the underlying cause of this behavior, ensemble ICA was applied to divergence responses. Results show that both convergence and divergence contain a sustained (step-like) and a transient (pulse-like) control component, but the amplitude of the transient component in divergence is dependent on initial stimulus position.