CONTRIBUTIONS OF HUMAN LONG-WAVE AND MIDDLE-WAVE CONES TO MOTION DETECTION

1. It has been suggested that motion may be best detected by the lumin ance mechanism. If this is the most sensitive mechanism, motion thresh olds mag he used to isolate the luminance mechanism and study its prop erties. 2. A moving (1 cycle deg(-1)), vertical, heterochromatic (red- plus-green), foveal grating was presented on a bright yellow (577 nm w avelength) 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 antip hase. 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 t o a weighted difference. 3. Detection and motion thresholds, measured at 1-4 Hz, were identical for luminance gratings, having equal cone co ntrasts, L' and M' of the same sign. For chromatic gratings, with L' a nd M' of opposite sign, motion thresholds were higher than detection t hresholds. 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 moti on identification, L' and M' contributed equally at 1 Hz, but the M' c ontribution 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 similar to 4 times more sensitive than the luminance mec hanism. 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 grating s, counterphase flickering in spatial and temporal quadrature phase wi th respect to each other. One grating was set slightly suprathreshold and oriented in cone contrast (L',M') so as to potentiate a single mot ion mechanism, the sensitivity of which was probed with the second gra ting, which was varied in (L',M'). This allowed us to measure the moti on detection contour of the less sensitive luminance mechanism at low velocities. At low velocities the luminance mechanism was strongly aff ected by L cone contrast, but at high velocities the L and M cones con tributed more equally. These changing cone weights were observed with both luminance flicker and motion. 7. Phase shifts between L and M con e signals within the two motion mechanisms were measured by varying th e relative temporal phase of the two flickering gratings. Only small p hase shifts were found in the spectrally opponent motion mechanism mea sured at 4 and 6 Hz (essentially no phase shifts were observed in the red-green hue mechanism). Within the luminance mechanism, the L signal lags M by as much as 30 deg at 4-9 Hz, and by a lesser amount at lowe r frequencies; at 21 Hz there is little phase shift. These large phase shifts may reflect properties of the phasic retinal ganglion cells. S uch large phase shifts imply that moving chromatic gratings, when supr athreshold, will directly stimulate the luminance mechanism.