Are Vergence and Accommodation Effective Cues to Distance?

Are Vergence and Accommodation Effective Cues to Distance?

Abstract

Vergence (the rotation of the eyes as measured by the tension in the extraocular muscles) has long been regarded as a preeminent cue to distance (Descartes, 1637), especially in the context of peripersonal (or reachable) space (Mon-Williams & Tresilian, 1999; Viguier, ClĂ©ment, & Trotter, 2001). However, studies confirming vergence as a distance cue have not controlled for either (a) initial diplopia in the stimulus, which is also considered a distance cue (Morrison & Whiteside, 1984), nor (b) changes in the retinal image during convergence. To better evaluate the extra-retinal contributions to distance perception we asked subjects to fixate on a luminous surface for 30 seconds in otherwise dark surroundings before pointing at the distance of a subsequently presented dot. Unbeknownst to the subjects we varied their vergence during the fixation period (between 20 and 50cm in Experiment 1 and between 23 and 46cm in Experiment 2). Our current results suggest that this manipulation of vergence had only a minor effect on perceived distance, even when (as in Experiment 2) accommodation cues were also present. This observation not only questions the contribution of vergence to distance estimation, but also, given vergence’s pre-eminence as a distance cue, the importance of absolute distance to vision itself.

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Paul Linton
Centre for Applied Vision Research

Vision scientist working on 3D vision. Author of The Perception and Cognition of Visual Space (Palgrave, 2017).