An esophoric shift associated with sustained fixation.

Some subjects show a progressive increase in their near phoria, in the direction of more esophoria, when the test is repeated. Eighteen of 100 subjects showed 5 delta or more esophoric shift. This result suggests that similar phoria changes may occur during prolonged intensive nearwork. The results reported are not the same as prism adaptation because the esophoric shift was found with dissociated targets. Autonomic arousal with sympathetic activation associated with heightened attention and concentration is proposed as the mechanism.