It has been suggested that motion may be best detected by the luminance mechanism. If this is the most sensitive mechanism, motion thresholds may be used to isolate the luminance mechanism and study its properties. 2. A moving (1 cycle deg-1), vertical, heterochromatic (red-plus-green), foveal grating was presented on a bright yellow (577 nm wavelength) field. Detection and motion (direction identification: left versus right) thresholds were measured for different amplitude ratios of the red and green components spatially summed in phase or in antiphase. Threshold contours plotted in cone-contrast co-ordinates (L',M') for the long-wave (L) and middle-wave (M) cones, revealed two motion mechanisms: a luminance mechanism that responds to a weighted sum of L and M contrasts, and a spectrally opponent mechanism that responds to a weighted difference. 3. Detection and motion thresholds, measured at 1-4 Hz, were identical for luminance gratings, having equal cone contrasts, L' and M', of the same sign. For chromatic gratings, with L' and M' of opposite sign, motion thresholds were higher than detection thresholds. A red-green hue mechanism may mediate chromatic detection, and a separate spectrally opponent motion mechanism may mediate motion. 4. The red-green hue mechanism was assessed from 1 to 15 Hz with an explicit hue criterion. The detection contour had a constant slope of one, implying equal L' and M' contributions of opposite sign. For motion identification, L' and M' contributed equally at 1 Hz, but the M' contribution was attenuated at higher velocities. 5. The cone-contrast metric provides a physiologically relevant comparison of sensitivities of the two motion mechanisms. At 1 Hz, the spectrally opponent motion mechanism is approximately 4 times more sensitive than the luminance mechanism. As temporal frequency is increased, the relative sensitivities change so that the luminance mechanism is more sensitive above 9 Hz. 6. The less sensitive motion mechanism was isolated with a quadrature phase protocol, using a pair of heterochromatic red-plus-green gratings, counterphase flickering in spatial and temporal quadrature phase with respect to each other. One grating was set slightly suprathreshold and oriented in cone contrast (L',M') so as to potentiate a single motion mechanism, the sensitivity of which was probed with the second grating, which was varied in (L',M'). This allowed us to measure the motion detection contour of the less sensitive luminance mechanism at low velocities.(ABSTRACT TRUNCATED AT 400 WORDS)
