Neural control strategies of the human *focusing mechanism

Ocular accommodation, the ability of human eye to change its optical power, operates by changing the curvature of the crystalline lens in response to constriction of the ciliary muscle. The past century has seen important advances concerning the biomechanical properties of the structures involved in accommodation and the changes in these structures that contribute to presbyopia (an age-related diminution in the ability to accommodate). However, there have been limited advances in our knowledge of the neural control of accommodation. For example, we lack information on how neural signals generated in the cortex and midbrain control the dynamics of accommodation or how these neural signals respond to age-related changes in biomechanics of the accommodative structures. Hence, the present studies investigated the neural control of accommodation (far-to-near focusing) and disaccommodation (near-to-far focusing) step responses and how the neural control altered with the progression of presbyopia. These experiments employed behavioral changes in the first- (velocity) and second-order (acceleration) dynamics as tools to study the neural control of accommodation and disaccommodation. The results showed distinctive differences in the dynamics of accommodation and disaccommodation. For accommodation, irrespective of the response starting position, peak velocity increased with response magnitude while peak acceleration remained invariant with response magnitude. When analyzed as a function of age, the peak velocity did not change with age while the peak acceleration decreased with age. For disaccommodation, irrespective of the response magnitude, both peak velocity and peak acceleration increased with the proximity of starting position. These results illustrate that accommodation and disaccommodation step responses utilize different neural control strategies. Accommodation responses are initiated towards their desired final destination with the dynamics determined by magnitude of defocus-step stimuli whereas disaccommodation responses are initiated towards an initial (default) destination with the dynamics determined by dioptric separation between the starting position and initial destination. These neural control strategies could be modeled using independent acceleration-pulse and velocity-step innervations that control the dynamics and magnitude of the step response respectively. Overall, these studies demonstrate that accommodation and disaccommodation step responses do not operate with machine-like properties, but rather they utilize different neural control strategies to optimize the response characteristics.