Perceptual learning for second-order cues in a shape-from-shading task

When a corrugated, patterned surface is illuminated this results in first-order modulations of luminance (LM) and correlated second-order modulations of the local luminance amplitude of the pattern (AM). Observers see LM and AM gratings presented in-phase (LM+AM) as shaded corrugations. When an additional anti-phase (LM-AM) component orthogonal to the first component is added to form a plaid, the LM-AM component is seen as a flat reflectance change (Schofield et al, 2006 Vision Research 46 3462-3482). However, for brief presentations (250 ms) naive observers are unable to differentiate the cues. We trained 6 observers over a period of 5 days to discriminate the components of orthogonal LM+AM/LM-AM plaids on the basis of which appeared more corrugated. After training, the AM threshold for discriminating the cues reduced dramatically. This benefit transferred to orthogonal plaids with a different overall orientation but not to the same cues with higher spatial frequency. We then varied the angle between the two members of the plaid. Training transferred when this angle was close to 90° but not for more acute combinations. We conclude that the role of AM in differentiating shading from reflectance changes can be learnt at a perceptual level resulting in cue specific performance benefits.