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1. Increment threshold illumination levels, It, for visual detection of a one-dimensional, spatially periodic test pattern (i.e. a grating) presented to the right eye have been measured following a period of adaptation to high contrast patterns. The right (test) eye was adapted to a grating, matched in bar-width and orientation to the test grating, and the left eye was adapted to a two-dimensional matrix array of circular spots. It is shown that the value of It is dependent on the spatial and spectral parameters of the spot matrix. 2. The variation of It with change in matrix wavelength, lambda, is greatest for matrix spots of some 10-15 min of arc diameter. 3. Values of It were determined as a function of the matrix wavelength, lambda. Three kinds of spectral response have been observed, each associated with a different stimulus geometry, and each characterized by the wavelength, lambda, for which It is minimum. With an adaptation matrix consisting of coloured spots on a dark surround, It is minimum for values of lambda around 600 nm, whereas with a matrix of dark spots on a coloured surround, the minimum occurs for lambda around 520 nm. The third spectral response is found when a blue test grating is superimposed on a yellow background, conditions under which detection is mediated by the blue-sensitive, pi 1-increment threshold mechanism. In this case, It is minimum for lambda equal to about 450 nm. 4. Three subjects with normal colour vision each give similar spectral responses, but the data for two deuteranopic red-green dichromats deviate significantly from the normal response pattern. 5. The binocular interaction mechanism through which the spot matrix influences threshold detection of the test grating is discussed in relation to electrophysiological and other psychophysical data.