The Role of Inertial and Visual Mechanisms in the Stabilization of Gaze in Natural and Artificial Systems

Abstract

Vision is arguably our premier navigational aid, allowing us to map out and actively explore our surroundings. However, we view the world from a constantly shifting platform and some visual mechanisms function optimally only if the images on the retina are reasonably steady. As we go about our everyday activities, visual and vestibular mechanisms help to stabilize our gaze on particular objects of interest by generating eye movements to offset our head movements. The general picture that has emerged of gaze stabilization in primates during motion is of two vestibulo-ocular reflexes, the RVOR and TVOR, that compensate selectively for rotational and translational disturbances of the head, respectively, each with its own independent visual backup mechanisms. A major objective of this chapter is to review recent work on low-level, pre-attentive mechanisms that operate with ultra-short latencies and are largely independent of conscious perception. Recent advances in the field of robotics and, particularly, in the domain of active vision, provide a complementary view of the uses and associated problems of visuo-inertial integration for the stabilization of gaze. Much like biological systems, robots have to comply with physical constraints imposed by the environment and/or by the need to coordinate their sensori-motor components in an efficient way. In contrast with biological systems, however, the experimental variation of implementation parameters and control strategies allows, among other things, a comparison of the different hypotheses and implementations. The goal of this chapter is to draw parallels between the results from biology and from a robot which uses inertial and visual information to stabilize its cameras/eyes. The Chapter is organized as follows. Section 2 describes the peculiarities of the patterns of retinal motion (optic flow) experienced by an observer moving through the environment. The appropriate compensatory eye movements required to stabilize gaze are described in Section 3, introducing the distinction between vergence and version eye movements. Section 4 describes the main characteristics of the vestibular system from a biological and artificial perspective and the distinction between “rotational” and “translational” components of the vestibulo-ocular reflex (VOR). The magnitude of the eye movements required for complete compensation depends on various kinematic parameters such as the position of the eyes in the head, the inter-ocular distance, as well as the distance to the fixation point. Section 5 deals with the integration of visual and inertial information for gaze stabilization. This Section builds upon the concept of “translational” and “rotational” components of the VOR and highlights the differences between “version” and “vergence” control of compensatory eye movements. In particular the role of the radial component of optical flow in the feed-forward control of eye movements is compared with the feed-back loop mediated by binocular disparity. In Section 6, the contribution of inertial and visual information in gaze stabilization is discussed with reference to the different latencies and processing power required by the two modalities. The advantage of the integration of visual and inertial data is discussed from a biological and robotics perspective in the concluding Section.