Manipulation of extraocular muscle afference has no effect on higher order perceptual judgments

Observers perceive targets as farther while performing the Jendrassik Maneuver (JM) suggesting that eye position is registered as more divergent. We examined the effects of the JM perturbation in three studies of perceptual judgment that rely on accurate registration of absolute distance: size constancy, stereoscopic depth, and the magnitude of the Pulfrich illusion. The data showed no significant differences between the JM and control conditions. The lack of an effect may be due to the fact that vergence is not a perfect cue to distance. Furthermore, the relative contribution of extraocular muscle afference to registered eye position may be less significant for higher order perceptual judgments.