Vergence eye movements signifying 3D depth perception from 2D movies

Abstract

A sensation of depth can arise from two-dimensional (2D) movies without any stereoscopic depth cue. Depth perception in three-dimensional (3D) space depends on the stability of stereoscopic gaze by vergence – coordinated movement of the two eyes in opposite directions – compensating the misalignment of the retinal images from the two eyes (i.e. binocular disparity) [1]. On the other hand, the oculomotor mechanisms that stabilize stereoscopic gaze and depth perception in 2D movie space remain unclear [2]. Here, we propose a hypothesis that vergence eye movements signifying 3D depth perception persist during prolonged 2D movie presentation without binocular disparity cues. By measuring eye positions while the subject viewed moving random-dot video stimuli, we show that sustained vergence is induced during 30-s exposure to radially expanding 2D optic flow. Moreover, a 2D video movie showing a passenger’s view of a roller coaster induces continuously changing vergence. In the absence of binocular disparity cues, the pictorial depth information within a 5° × 5° gaze window and optic flow in the movie simultaneously and independently influence vergence. The pictorial gaze-area depth information affects vergence mainly in the virtual far space, whereas optic flow robustly affects vergence irrespective of the nearness. These findings demonstrate that vergence serves as a reliable marker signifying 3D depth perception from 2D movies, imposing critical constraints on creation of an effective and safe virtual reality.

Introduction

Virtual reality, a key technology for 3D entertainments and clinical, educational, or industrial innovations, creates an environment with a dynamic and strong sensation of depth based on 3D stereoscopic vision [3]. However, even in the absence of stereoscopic depth cues, a sensation of depth arises from 2D movies with proximal cues such as object size changes or optic flows [2]. Depth perception in real 3D space depends on the stability of stereoscopic gaze maintained by vergence eye movements [1], [2], [4]. The vergence angle between the axes of the two eyes increases (convergence) or decreases (divergence) to adjust the misalignment of the retinal images from the two eyes (i.e. binocular disparity) [5], [6]. Vergence eye movement has also been implicated in depth perception in virtual 3D environments [7], [8]. On the other hand, the oculomotor mechanisms that enable depth perception in 2D conditions have not been fully clarified [9], [10]. A key factor to elicit vergence in 2D conditions is optic flow [11], [12]. Ultra-short latency vergence was triggered, as a rapid ocular response, by transient 2D visual stimulation with radial optic flows [13]. Transient convergence was induced by an expanding optic flow, whereas divergence was induced by a contracting flow. Nonetheless, it remains unclear whether prolonged 2D visual stimulation can sustain vergence after elaboration of depth perception through higher-order cortical interactions [14], [15]. Here, we propose a hypothesis that vergence signifying 3D depth perception should persist during prolonged 2D movie presentation in the absence of binocular disparity. To test the hypothesis, we firstly examined whether vergence in 2D condition would sustain or not during the prolonged presentation of expanding random dot stimuli with constant velocity. Second, we examined how sustained vergence eye movements, if any, would be affected by multiple depth cues such as binocular disparity, optic flow, and other pictorial cues in 2D and 3D realistic movies.