Aftereffects of sustained vertical divergence: induced vertical phoria and illusory target height.

Maintaining vertical divergence of about 2 diopters for 6, 8, or 10 min was shown to yield an induced vertical phoria (IVP). IVP increased with the duration of the inducing period and decayed with time in the dark, though not completely. Decay of IVP decreased with duration of induction period and increased as a result of maintaining the normal fusional response for 30 s. Vertical eye movements in the dark, however, were ineffective in reducing IVP. Some evidence also was provided for eye-specific errors in the perception of elevation of a visible target that were appropriate to the direction of the induced phoria. It was proposed that IVP may be modulated by resetting the output of the system used to induce the original effect.