Adaptation of reflexive fusional vergence is directionally biased.

Divergence is known to differ from convergence across a wide range of clinical parameters. We have postulated that a limited neural substrate results in reduced fusional divergence velocities and subsequently a reduced capacity to adapt tonic vergence to uncrossed disparities. We further investigated this hypothesis by characterizing the degree of plasticity in reflexive fusional vergence to repetitive end-point errors using a disparity-based double-step paradigm. 10 adults completed 4 study visits where reflexive fusional convergence or divergence was measured (250 Hz infrared oculography) to a 2° disparity step and then lengthened or shortened via a repeated double-step (2° ± 1.5°). Stimuli were presented dichoptically at 40 cm. Adaptive modification of vergence responses was similar between directions for the shortening conditions, suggesting a common neural mechanism responds to overshooting errors. In comparison, adaptive lengthening of convergence was slower, but of equal magnitude, suggesting a second neural mechanism with a longer time constant for undershooting errors. Divergence response velocities were slower at baseline and did not increase after adaptive lengthening. Instead, increases in divergence response amplitudes were a result of increased response duration, implying saturation of the reflexive, preprogrammed response. Adaptive responses serving to increase or decrease reflexive fusional vergence recruitment were asymmetric. Adaptive lengthening of convergence and divergence identified further directional asymmetries. The results support the hypothesis that the neural substrate underlying divergence is attenuated, resulting in reduced reflexive plasticity when compared to convergence. The clinical and technological implications of these results are discussed.