We have studied the influence of chromatic adaptation upon the
perceived visual position of a test stimulus using a Vernier alignment
task. Maximum and minimum offsets in spatial position are generated
when the adapting and test stimuli lie on the same and orthogonal axes
in MBDKL color space, respectively. When the test stimuli lie on
intermediate color axes, the measured positional shifts decrease as a
function of the angular separation in color space (φ) from the
adapting stimulus. At low stimulus contrasts, these shifts follow a
sinusoidal function of φ and exhibit broad chromatic tuning and
can be accounted for by a model in which the centroid is extracted from
the linear combination of after-image, formed by the adapting stimulus,
and the test stimulus. Such linear, broadband behavior is consistent
with the response properties of chromatic neurons in the precortical
visual pathway. At high contrast, and when adaptation gets closer to
the S/(L+M) axis, the tuning functions become narrower and require
sinusoids raised to increasingly higher exponents in order to describe
the data. This narrowing of chromatic tuning is consistent with the
tuning properties of chromatic neurons in the striate cortex, and
implies the operation of a nonlinear mechanism in the combination of
cone outputs.