Second-site adaptation in the red-green detection pathway: only elicited by low-spatial-frequency test stimuli

The red-green (RG) detection mechanism was revealed by measuring threshold detection contours in the L and M cone contrast plane for sine-wave lest gr atings of 0.8-6 c deg(-1) on bright adapting fields of yellow or red. The s lope of the RG detection contours was unity, indicating that the L and M co ntrast signals contribute equally (with opposite signs) on both the yellow and the red fields. this reflects rat-site, cone-selective adaptation. Seco nd-site adaptation, which may reflect saturation at a color-opponent site, was evidenced by the RG detection contours being further out from the origi n of the cone contrast plane on the red field than on the yellow field. Sec ond-site adaptation was strong (3-fold) for low spatial frequency test grat ings but greatly diminished by 6 c deg(-1). The disappearance of second-sir e adaptation with increasing spatial frequency can be explained by spatial frequency channels. The most sensitive detectors may comprise a low spatial frequency channel which is susceptible to masking by the chromatic, spatia l DC component of the red field. The 6 c deg(-1) patterns may be detected b y a less sensitive, higher frequency channel which is less affected by the uniform red field. The RG spatial frequency channels likely arise in the co rtex. implicating a partially central site for the second-site effect. (C) 1999 Elsevier Science Ltd. All rights reserved.