Vergence can be controlled by audio feedback, and induces downward ocular deviation

We measured horizontal and vertical eye positions, using binocular search coils, in three humans. Subjects could maintain vergence by means of audio biofeedback. Feedback consisted of a pair of audio tones, one variable and one fixed at a reference frequency. The variable tone was controlled by instantaneous vergence and provided immediate feedback on the vergence state. The reference frequency, which they attempted to match, was set to correspond to a target distance of either 0.34 m or 0.14 m. Subjects could maintain vergence consistently, even while undergoing lateral motions at 0.5 Hz and 0.2 g peak acceleration in darkness. There was also a consistent tendency for the eyes to deviate downward during near vergence. The results may be useful in experiments in which one wishes to control vergence without providing a visual reference which might inhibit conjugate eye movements.