The human visual system adapts to the changing statistics of its environment. For example, the light-from-above prior, an assumption that aids the interpretation of ambiguous shading information, can be modified by haptic (touch) feedback. Here we investigate the mechanisms that drive this adaptive learning. In particular, we ask whether visual information can be as effective as haptics in driving visual recalibration and whether increased information (feedback from multiple modalities) induces faster learning.During several hours' training, feedback encouraged observers to modify their existing light-from-above assumption. Feedback was one of the following: (1) haptic only, (2) haptic and stereoscopic (providing binocular shape information), or (3) stereoscopic only. Haptic-only feedback resulted in substantial learning; the perceived shape of shaded objects was modified in accordance with observers' new light priors. However, the addition of continuous visual feedback (condition 2) substantially reduced learning. When visual-only feedback was provided intermittently (condition 3), mimicking the time course of the haptic feedback of conditions 1 and 2, substantial learning returned.The intermittent nature of conflict information, or feedback, appears critical for learning. It causes an initial, erroneous percept to be corrected. Contrary to previous proposals, we found no particular advantage for cross-modal feedback. Instead, we suggest that an "oops" factor drives efficient learning; recalibration is prioritized when a mismatch exists between sequential representations of an object property. This "oops" factor appears important both across and within sensory modalities, suggesting a general principle for perceptual learning and recalibration.

The pattern of shading across an image can provide a rich sense of object shape. Our ability to use shading information is remarkable given the infinite possible combinations of illumination, shape and reflectance that could have produced any given image. Illumination can change dramatically across environments (e.g., indoor vs. outdoor) and times of day (e.g., mid-day vs. sunset). Here we show that people can learn to associate particular illumination conditions with particular contexts, to aid shape-from-shading. Following a few hours of visual-haptic training, observers modified their shape estimates according to the illumination expected in the prevailing context. Our observers learned that red lighting was roughly overhead (consistent with their previous assumption of lighting direction), whereas green lighting was shifted by 10°. Greater learning occurred when training for the two contexts (red or green light) was intermingled rather than when it was sequentially blocked.

Perception of shaded three-dimensional figures is inherently ambiguous, but this ambiguity can be resolved if the brain assumes that figures are lit from a specific direction. Under the Bayesian framework, the visual system assigns a weighting to each possible direction, and these weightings define a prior probability distribution for light-source direction. Here, we describe a non-parametric maximum-likelihood estimation method for finding the prior distribution for lighting direction. Our results suggest that each observer has a distinct prior distribution, with non-zero values in all directions, but with a peak which indicates observers are biased to expect light to come from above left. The implications of these results for estimating general perceptual priors are discussed.

Identifying an object’s material properties supports recognition and action planning: we grasp objects according to how heavy, hard or slippery we expect them to be. Visual cues to material qualities such as gloss have recently received attention, but how they interact with haptic (touch) information has been largely overlooked. Here, we show that touch modulates gloss perception: objects that feel slippery are perceived as glossier (more shiny).Participants explored virtual objects that varied in look and feel. A discrimination paradigm (Experiment 1) revealed that observers integrate visual gloss with haptic information. Observers could easily detect an increase in glossiness when it was paired with a decrease in friction. In contrast, increased glossiness coupled with decreased slipperiness produced a small perceptual change: the visual and haptic changes counteracted each other. Subjective ratings (Experiment 2) reflected a similar interaction – slippery objects were rated as glossier and vice versa. The sensory system treats visual gloss and haptic friction as correlated cues to surface material. Although friction is not a perfect predictor of gloss, the visual system appears to know and use a probabilistic relationship between these variables to bias perception – a sensible strategy given the ambiguity of visual clues to gloss.

Glossy and matte objects can be differentiated using specular highlights: bright patches in the retinal image produced when light rays are reflected regularly from smooth surfaces. However, bright patches also occur on matte objects, due to local illumination or reflectance changes. Binocular vision provides information that could distinguish specular highlights from other luminance discontinuities; unlike surface markings, specular highlights lie not at the surface depth, but "float" in front of concave surfaces and behind convex ones. We ask whether observers implicitly understand and exploit the peculiarities of specular geometry for gloss and shape perception. Our participants judged the glossiness and shape of curved surfaces that included specular highlights at various depths. Observers demonstrated substantial deviations from a full geometric model of specular reflection. Concave surfaces appeared glossy both when highlights lay in front of and (incorrectly) behind the surface. Failings in the interpretation of monocular highlights were also apparent. Highlight disparity had no effect on shape perception. However, the perceived gloss of convex surfaces did follow geometric constraints: only highlights at appropriate depths produced high gloss ratings. We suggest, in contrast with previous work, that the visual system invokes simple heuristics as gloss indicators to accommodate complex reflections and inter-reflections that occur particularly inside concavities.