Concomitant direction and distance aftereffects of sustained convergence: A muscle potentiation explanation for eye-specific adaptation

Abstract

Fixating a target for 6 min was shown to produce distance aftereffects that varied in direction and magnitude as a linear function of the convergence angle. Eye-specific direction aftereffects also were obtained in a nasal direction under conditions that produce increased perceived dtstance and in a temporal direction under conditions that produce decreased perceived distance. These aftereffects were shown to be sensitive to the range of horizontal versional eye movements that accompany the near or far convergence positions maintained during exposure. The results provide a logical alter­ native to perceptual learning accounts of eye-specific adaptation. Maintaining the eyes in a fixed or restricted range of convergence positions may induce a muscle potentiation aftereffect in the subset of extraocular muscles innervated to achieve and sustain those postures. Muscle potentiation refers to an involun­ tary component of continued innervation in the direction of previous muscular stimulation. Since the increase in muscle tension is involuntary, registra­ tion of eye-position information based on the monitoring of voluntary motor signals should be in error to the degree that the potentiation effect must be overcome. Assuming that the potentiation effect itself is not registered via an inflow from proprio­ ceptive units in the extraocular muscles, it follows that any perceived spatial dimension that requires compensation for or registration of ocular rotation should undergo changes consistent with the felt position of the eyes. Accordingly, changes in per­ ceived distance should occur whenever the eyes have been maintained predominantly in near or far con­ vergence positions. For example, if the medial recti of each eye are potentiated by maintaining the eyes in a near convergence position, then it would be as if each eye was being involuntarily pulled nasally. Therefore, in order to fixate a visual target, the observer would have to overcome the additional innervation due to potentiation by the same amount of voluntary innervation in the opposite direction, i.e., in a temporal direction that would normally result in a smaller convergence angle. This change in