Perceptual Science Series

Lightness constancy at a slant: high-level, mid-level, and low-level

Alan Gilchrist

Monday, March 19, 2007, 01:00pm - 02:00pm

Rutgers University, FASN-Department of Psychology

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When a flat gray surface is rotated in space relative to the light
source, it gets brighter and darker, but continues to appear
approximately the same shade of gray, an example of lightness
constancy. High-level theories associated with Maloney (Boyaci et al,
2004) and Brainard (Ripamonti et al, 2004) claim that the visual
system achieves this by taking into account the direction of the
light source and the slant of the surface. Mid-level theories
(Gilchrist et al, 1999) claim that the process is much simpler and is
based on luminance ratios between surfaces that face in the same
direction. I will review several lines of research in which the mid-
level account is tested against both high-level and low-level
(spatial filtering) accounts. The results strongly support the mid-
level account.

For example, in one critical experiment we used an actual scene
containing blocks resting on the floor, spheres floating in midair,
attached shadows, cast shadows, glossy highlights and luminance
gradients.  The target square, always attached to the middle of the
right-facing side of a large cube, was presented in different
orientations.  The general light source was located to the upper
left, except that the cube was shadowed from this light source and
illuminated by a hidden projector located off to the right.  This
gave the right face of the cube a higher luminance than the left
face.  Thus, as a target is rotated successively away from the
general light source but its luminance is held constant, the high-
level taking into account theory predicts lower lightness values,
while the relaxed coplanar ratio principle predicts darker values.
We found that the target appeared about two Munsell steps darker in
the extreme rotated position, compared to a position normal to the
general light source.

Alan Gilchrist