| Does
the visual system allocate discriminative ability to different regions
in colour space in a way that optimizes discrimination among natural colours?
If so, discrimination should satisfy "a cube root rule": in an optimized
system differential sensitivity will be greatest for the most commonly
encountered conditions, dropping to half its maximum under conditions
of relative frequency 1/8. Quantitatively, this principle is only very
roughly consistent with psychophysical data, but it does account for some
of the salient findings, such as the relative sensitivity for different
directions in colour space; the fit between theory and psychophysical
observation is improved by considering the stimulus to be the local contrast
between test field and background, rather than absolute luminance and
chromatic values of individual pixels. Comparison with physiological data
shows less satisfactory agreement: M cells appear to be too nonlinear,
and P cells too linear, for optimal metric representations of luminance
and colour respectively. The good colour discrimination of some strongly
anomalous trichromats may result from an optimization, during development,
of postreceptoral nonlinearity to match the limited range of inputs delivered
by the anomalous photoreceptors. For natural colours under natural illuminants,
the cone excitations for all surfaces in an image are scaled by approximately
the same factor with a change of illumination. This allows the effect
of varying illumination to be simply corrected by reciprocal adjustments
of sensitivity in the different cone types. The resulting representation
is illumination-invariant, but also fails to preserve information about
the overall chromatic cast of a scene. Experimentally, colouration of
the image is perceptually attributed in part to the illuminant and in
part to the viewed surfaces, resulting in "underconstancy". When the statistical
variation among natural illuminants and scenes is considered, underconstancy
can be viewed not as a failure of constancy, but as a best guess about
illuminant colour appropriately based on knowledge of relevant environmental
statistics. Natural images do generate small deviations from the scaling
principle. These can provide useful cues to the illuminant: statistics
(other than the mean) of the distribution of an image's elements in cone
excitation space can in principle resolve the ambiguity inherent in the
mean alone. Experiment suggests that vision does exploit these cues.and
gives them statistically justifiable weight. |
