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The visual system uses several signals to deduce the three-dimensional structure of the environment, including binocular disparity, texture gradients, shading and motion parallax. Although each of these sources of information is independently insufficient to yield reliable three-dimensional structure from everyday scenes, the visual system combines them by weighting the available information; altering the weights would therefore change the perceived structure. We report that haptic feedback (active touch) increases the weight of a consistent surface-slant signal relative to inconsistent signals. Thus, appearance of a subsequently viewed surface is changed: the surface appears slanted in the direction specified by the haptically reinforced signal.

Extra-retinal information is critical in the interpretation of visual input during self-motion. Turning our eyes and head to track objects displaces the retinal image but does not affect our ability to navigate because we use extra-retinal information to compensate for these displacements. We showed observers animated displays depicting their forward motion through a scene. They perceived the simulated self-motion accurately while smoothly shifting the gaze by turning the head, but not when the same gaze shift was simulated in the display; this indicates that the visual system also uses extra-retinal information during head turns. Additional experiments compared self-motion judgments during active and passive head turns, passive rotations of the body and rotations of the body with head fixed in space. We found that accurate perception during active head turns is mediated by contributions from three extra-retinal cues: vestibular canal stimulation, neck proprioception and an efference copy of the motor command to turn the head.

When a person walks through a rigid environment while holding eyes and head fixed, the pattern of retinal motion flows radially away from a point, the focus of expansion (Fig. 1a). Under such conditions of translation, heading corresponds to the focus of expansion and people identify it readily. But when making an eye/head movement to track an object off to the side, retinal motion is no longer radial (Fig. 1b). Heading perception in such situations has been modelled in two ways. Extra-retinal models monitor the velocity of rotational movements through proprioceptive or efference information from the extraocular and neck muscles and use that information to discount rotation effects. Retinal-image models determine (and eliminate) rotational components from the retinal image alone. These models have been tested by measuring heading perception under two conditions. First, observers judged heading while tracking a point on a simulated ground plane. Second, they fixated a stationary point and the flow field simulated the effects of a tracking eye movement. Extra-retinal models predict poorer performance in the simulated condition because the eyes do not move. Retinal-image models predict no difference in performance because the two conditions produce identical patterns of retinal motion. Warren and Hannon observed similar performance and concluded that people do not require extra-retinal information to judge heading with eye/head movements present, but they used extremely slow tracking eye movements of 0.2-1.2 deg s-1; a moving observer frequently tracks objects at much higher rates (L. Stark, personal communication). Here we examine heading judgements at higher, more typical eye movement velocities and find that people require extra-retinal information about eye position to perceive heading accurately under many viewing conditions.

The ability to detect, discriminate and identify spatial stimuli is much poorer in the peripheral than in the central visual field. Some deficits are eliminated by scaling stimulus size. For example, grating detectibility is roughly constant across the visual field when spatial frequency and target extent are scaled appropriately. Other deficits persist despite scaling. For instance, some readily detectable patterns are more difficult to identify peripherally than in the fovea. This deficit is caused, at least partially, by a reduced ability to encode spatial phase (or relative position). To specify the properties of foveal and peripheral phase-encoding mechanisms, we measured discrimination thresholds for compound gratings at several eccentricities. Our observations are consistent with a two-channel model of phase encoding based on even- and odd-symmetric mechanisms (see Fig. 1), but the sensitivity of the odd-symmetric mechanisms decreases dramatically with eccentricity. Thus, the loss of sensitivity in one type of mechanism may underlie the reduced ability to encode spatial phase peripherally.