Spatial constancy and the brain: Insights from neural networks

Introduction As you read this article, your eyes make a sequence of rapid movements (saccades) to direct your gaze to the words on this page. Although saccades move the eyes at speeds of up to 400° per second, you do not perceive that the visual world is moving rapidly. The characters, which are stationary on the page, do not appear to move, even though their projection onto the retina changes. The ability of the visual system to account for self-motion is called space constancy (Holst & Mittelstaedt, 1950; Helmholtz, 1962; Stark & Bridgeman, 1983; Bridgeman, 1995; Deubel et al., 1998) and is an important component of goal-directed behaviour: in order to interact with objects in our surroundings, we must account for motion of the eyes, head and body for accurate visually guided movements (e.g. saccades, reaching). Space constancy is particularly important when remembering the locations of objects that become occluded from view. In this case, compensation for self-motion is critical for maintaining an accurate representation of an object's location while it is being remembered. Both humans and monkeys are capable of performing such compensation, which is also known as spatial updating. To update accurately, the brain must synthesise information regarding movements of various types (eye-in-head, head-on-body, body-in-world) with remembered visuospatial information. However, multiple possible solutions exist. It has been suggested that spatial locations could be stored with respect to an allocentric, world-fixed coordinate system that does not vary with the motion of the observer.