Optically induced changes in the couplings between vergence and accommodation.

Vergence-induced accommodation (V-A) and accommodation-induced vergence (A-V) were measured in human subjects before and after they had worn various optical devices for 30 min. Laterally displacing periscopic spectacles, which increase the required change in vergence per unit change in accommodation, caused decreases and increases in the gain of V-A and A-V responses, respectively. These observations are consistent with the view that the gain of the neural cross-linkages between vergence and accommodation are subject to adaptive regulation. However, there were strong asymmetries: Medially displacing periscopes (cyclopean spectacles), which reduce the required change in vergence per unit change in accommodation to zero, were almost totally without effect. Base-out prisms, which increase the required convergence by an amount that is constant for all viewing distances, caused downward and upward shifts in the V-A and A-V response curves, respectively. Base-in prisms, which reduce the required convergence by a constant amount for all viewing distances, caused downward shifts of A-V curves but had no significant effect on V-A curves. These effects of prisms are in essential agreement with the work of others and confirm the existence of adaptive elements that regulate the bias in the vergence and/or accommodation control systems. Secondary effects of wearing periscopes and prisms indicated a certain lack of specificity in the sensing of gain and bias errors: vertical shifts of V-A and A-V curves (resembling those seen with base-out prisms) often occurred with the laterally displacing periscopes, and gain changes (generally resembling those seen with laterally displacing periscopes) often occurred with the base-out prisms.