Aftereffects of Sustained Convergence: Some Implications of the Eye Muscle Potentiation Hypothesis

Aftereffects of sustained convergence are generally explained as the result of eye muscle potentiation (EMP). Three implications of this hypothesis for aftereffects in distance perception were tested: (i) According to the EMP hypothesis the aftereffects are based on oculomotor distance cues only. The addition of secondary cues during the test period should therefore reduce or eliminate the aftereffects. This expectation was confirmed. (ii) According to the EMP hypothesis the critical variable determining the aftereffect should be the distance of the inspection stimulus from the observer but not from the test stimulus. In confirmation of the expectation the aftereffects of certain inspection stimuli were of the same direction for test stimuli which bounded the range of inspection distances on both sides. (iii) When the aftereffects are based on oculomotor distance cues only, the effect of secondary cues during the test period should remain unchanged when they are present during the inspection period as well. Contrary to this expectation the aftereffects did not depend on the cues available during the test. This suggests that they are based on secondary cues, too, provided they are present during the inspection period.