Associative learning of shape as a cue to appearance: a new demonstration of cue recruitment.

The perceived rotation direction of a wire-frame Necker cube at stimulus onset can be conditioned to be dependent on retinal location (B. T. Backus & Q. Haijiang, 2007; S. J. Harrison & B. T. Backus, 2010a). This phenomenon was proposed to be an example of the visual system learning new cues to visual appearance, by adaptation in response to new experiences. Here, we demonstrate recruitment of a new cue, object shape, for the appearance of rotating 3D objects. The cue was established by interleaving ambiguous and disambiguated instances of two shapes, cubes and spheres, at the same retinal location. Disambiguated cubes and spheres rotated in opposite directions. A significant bias was consequently introduced in the resolution of ambiguity, whereby the proportions of ambiguous shapes perceived as rotating clockwise differed, in the direction predicted by their disambiguated counterparts. This finding suggests that training led the visual system to distinguish between the two shapes. The association of rotation direction and shape was only achieved when monocular depth cues were used to depict rotation in depth; shapes disambiguated by binocular disparity did not lead to recruitment of the shape cue. We speculate that this difference may be the consequence of a difference in the neural pathways by which the disambiguating cues act. This new instance of the cue recruitment effect opens possibilities for further generalization of the phenomenon.