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.