Plasticity of the Near Response

Abstract

The near response is composed of cross-coupled interactions between convergence and other distance-related oculomotor responses including accommodation, vertical vergence, and cyclovergence. These couplings serve to guide involuntary motor responses during voluntary shifts of distance and direction of gaze without feedback from retinal image disparity. They function to optimize the disparity stimulus for stereoscopic depth perception, and they can be modified by optically induced sensory demands placed on binocular vision. In natural viewing conditions, the coupling of accommodation and convergence is modulated by the activity of adaptable tonic components of both motor responses. The binocular extension of Listing’s law could be achieved parsimoniously by a combination of passive orbital mechanics and an active gain control of the vertical ocular muscles that depended on convergence angle but were independent of gaze direction or elevation. The normal coupling of vertical vergence with convergence could be a by-product of passive orbital mechanics that would not require active convergence dependent neural gain control mechanism. Adapted changes of vertical vergence gain (K v ) in response to unequal ocular magnification (aniseikonia) and adapted changes in the orientation of Listing’s planes (K t ) in response to torsional disparities could be achieved by a combination of passive orbital mechanics and active gain control of the vertical ocular muscles that depended on convergence angle but were independent of gaze direction. However, several adaptation studies suggest that it is possible to achieve non-linear changes in coupling of both vertical vergence and cyclovergence with gaze direction (Schor, Maxwell and Graf, 2001) that could be achieved with changes in neural control that depend upon both convergence and direction of gaze (McCandless and Schor, 1997).