Vergence eye movements are not essential for stereoscopic depth

Abstract

The brain receives disparate retinal input owing to the separation of the eyes, yet we usually perceive a single fused world. This is because of complex interactions between sensory and oculomotor processes that quickly act to reduce excessive retinal disparity. This implies a strong link between depth perception and fusion, but it is well established that stereoscopic depth percepts are also obtained from stimuli that produce double images. Surprisingly, the nature of depth percepts from such diplopic stimuli remains poorly understood. Specifically, despite long-standing debate it is unclear whether depth under diplopia is owing to the retinal disparity (directly), or whether the brain interprets signals from fusional vergence responses to large disparities (indirectly). Here, we addressed this question using stereoscopic afterimages, for which fusional vergence cannot provide retinal feedback about depth. We showed that observers could reliably recover depth sign and magnitude from diplopic afterimages. In addition, measuring vergence responses to large disparity stimuli revealed that that the sign and magnitude of vergence responses are not systematically related to the target disparity, thus ruling out an indirect explanation of our results. Taken together, our research provides the first conclusive evidence that stereopsis is a direct process, even for diplopic targets.

The (In)Effectiveness of Simulated Blur for Depth Perception in Naturalistic Images

“…In the temporal 2AFC paradigm we would have expected improved performance in the stereoscopic conditions by added depth information from oculomotor vergence cues with respect to the spatial experiment. The fact that we did not observe differences between the paradigms supports previous evidence that stereopsis is a direct process [ 40 ], where depth is estimated from retinal disparity and not from fusional vergence responses.…”

Mice Discriminate Stereoscopic Surfaces Without Fixating in Depth

“…7I-K ). Nonetheless, humans can still perceive the sign and magnitude of disparities without any vergence eye movements (Lugtigheid et al, 2014). This suggests that humans may retain the more primitive form of stereopsis that we have demonstrated in mice, but have evolved to use their fovea combined with precise changes of vergence angle to fixate on an object or surface in depth.…”

Sensitivity of vergence responses of 5- to 10-week-old human infants

“…Masson, Busettini, and Miles (1997) demonstrated that anticorrelated disparity stimuli could drive a vergence response opposite in sign (i.e., crossed anticorrelated disparity drives divergence and uncrossed drives convergence) without subjects reporting a depth percept. Conversely, Lugtigheid, Wilcox, Allison, and Howard (2014) were able to drive a depth percept independently of vergence responses using the afterimages of disparate lines. These results suggest that disparity for vergence and stereopsis may be processed somewhat separately.…”