Abstract
1. 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-'), 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 -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. At low velocities the luminance mechanism was
strongly affected by L cone contrast, but at high velocities the L and M cones contributed more
equally. These changing cone weights were observed with both luminance flicker and motion.
7. Phase shifts between L and M cone signals within the two motion mechanisms were measured
by varying the relative temporal phase of the two flickering gratings. Only small phase shifts
were found in the spectrally opponent motion mechanism measured 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
lower frequencies; at 21 Hz there is little phase shift. These large phase shifts may reflect
properties of the phasic retinal ganglion cells. Such large phase shifts imply that moving
chromatic gratings, when suprathreshold, will directly stimulate the luminance mechanism.
Original language | American English |
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Journal | Journal of Physiology |
Volume | 485 |
DOIs | |
State | Published - May 15 1995 |
Externally published | Yes |
Keywords
- cone signals
- motion detection
- luminance mechanism
- vision
Disciplines
- Vision Science