An association matrix model of context-specific vertical vergence adaptation

The plasticity of the oculomotor system can be demonstrated experimentally through vertical vergence adaptation, in which an open-loop (non-binocular feedback) vertical vergence response is trained to vary with eye position along a single meridian (such as horizontal conjugate eye position). During context-specific adaptation, vertical vergence is trained to vary along two eye position meridians. In this condition, a unique combination of information from two separate pathways is necessary to produce the correct vertical vergence response. Context-specific adaptation experiments were conducted in three adapting planes (midsaggital, frontoparallel, and transverse). Midsaggital plane stimuli were associated with horizontal vergence and vertical conjugate eye position, frontoparallel plane stimuli were associated with horizontal and vertical conjugate eye position, and transverse plane stimuli were associated with horizontal vergence and horizontal conjugate eye position. As a means of investigating whether brainstem inputs are a feasible source of information driving the vertical vergence response, an association matrix model was developed which combined two input sources via AND gates. The inputs to the model reflect units associated with vertical conjugate eye position, horizontal conjugate eye position, and horizontal vergence. The AND gate outputs were summed and weighted to form the vertical vergence response at different eye positions. This model illustrates a biologically plausible means of coupling the adapted vertical vergence response with other oculomotor behaviour.