Summary: 1 Introduction
One of the oldest and most fundamental problems of vision involves understanding how
the luminance distributions on the retinae are transformed into a perception of bright-
ness or lightness. The brightness of a region refers to the amount of light it appears
to emanate. Lightness refers to the proportion of light a surface appears to reflect.
Although these terms refer to distinct physical dimensions, it is not always clear how
this distinction can or should be made psychologically. In this paper, I will therefore
use the term lightness to refer to the perceived gray of an (achromatic) target.
Two general theoretical frameworks have been invoked to explain departures from
veridicality in perceived lightness: those that emphasize the role of low-level mecha-
nisms that operate on the 2-D array of luminance values; and those that emphasize the
role of perceptual organization and/or surface-level computations.
One of the best known examples of an illusion that has been traditionally explained as
the consequence of low-level mechanisms is the phenomenon of simultaneous contrast.
A homogeneous gray patch surrounded by a homogeneous light surround appears
darker than an identical gray patch surrounded by a dark surround (see figure 1a). The
traditional explanation of this illusion is that antagonistic receptive fields sense the con-
trastive borders surrounding the central targets, and brightness signals are propagated
from these borders into the homogeneous interiors of the gray targets (Cornsweet and
Teller 1965; Shapley and Enroth-Cugell 1984; Grossberg and Todorovic¨ 1988; Paradiso