Preferred vertical gaze direction and observation distance

Hill and Kroemer (1986) and Kroemer and Hill (1986) found that the preferred vertical direction of gaze is lower with a nearer binocular stimulus than with a more distant one. A model is proposed that relates this phenomenon to characteristics of the resting state of the oculomotor system. Three predictions of the model were tested, based on measurements of the preferred vertical gaze direction and dark vergence in the same subject sample. On average the effect of observation distance on the preferred vertical gaze direction served to reduce the discrepancy between the resting state of vergence, operationally defined as dark vergence, and actual convergence during inspection of the binocular stimulus. Second, dark vergence data from individual subjects could be successfully used to predict whether they raised or lowered their eyes on inspection of a binocular stimulus as compared with the preferred vertical gaze direction while viewing a monocular stimulus. Finally, predictions of the size of the change of the preferred vertical gaze direction on introduction of a binocular stimulus produced only small and non-significant correlations.