An analysis of factors that contribute to the magnitude of placebo analgesia in an experimental paradigm.

Chromatic mechanisms have been studied extensively with psychophysical techniques in humans, but the number and nature of the mechanisms are still controversial. Appeals to monkey neurophysiology are often used to sort out the competing claims and to test hypotheses arising from the experiments in humans, but psychophysical chromatic mechanisms have never been assessed in monkeys. Here we address this issue by measuring color-detection thresholds in monkeys before and after chromatic adaptation, employing a standard approach used to determine chromatic mechanisms in humans. We conducted separate experiments using adaptation configured as either flickering full-field colors or heterochromatic gratings. Full-field colors would favor activity within the visual system at or before the arrival of retinal signals to V1, before the spatialtransformationofcolorsignalsbythecortex.Conversely,gratingswouldfavoractivitywithinthecortexwhereneuronsareoftensensitive tospatialchromaticstructure.Detectionthresholdswereselectivelyelevatedforthecolorsoffull-fieldadaptationwhenitmodulatedalongeither of the two cardinal chromatic axes that define cone-opponent color space [L vs M or S vs (L ϩ M)], providing evidence for two privileged cardinal chromatic mechanisms implemented early in the visual-processing hierarchy. Adaptation with gratings produced elevated thresholds for colors of the adaptation regardless of its chromatic makeup, suggesting a cortical representation comprised of multiple higher-order mechanisms each selective for a different direction in color space. The results suggest that color is represented by two cardinal channels early in the processing hierarchy and many chromatic channels in brain regions closer to perceptual readout.

Macaque monkeys are a model of human color vision. To facilitate linking physiology in monkeys with psychophysics in humans, we directly compared colordetection thresholds in humans and rhesus monkeys. Colors were defined by an equiluminant plane of coneopponent color space. All subjects were tested on an identical apparatus with a four-alternative forced-choice task. Targets were 28 square, centered 28 from fixation, embedded in luminance noise. Across all subjects, the change in detection thresholds from initial testing to plateau performance (''learning'') was similar for þL À M (red) colors and þM À L (bluish-green) colors. But the extent of learning was higher for þS (lavender) than for ÀS (yellow-lime); moreover, at plateau performance, the cone contrast at the detection threshold was higher for þS than for ÀS. These asymmetries may reflect differences in retinal circuitry for S-ON and S-OFF. At plateau performance, the two species also had similar detection thresholds for all colors, although monkeys had shorter reaction times than humans and slightly lower thresholds for colors that modulated L/M cones. We discuss whether these observations, together with previous work showing that monkeys have lower spatial acuity than humans, could be accounted for by selective pressures driving higher chromatic sensitivity at the cost of spatial acuity amongst monkeys, specifically for the more recently evolved L À M mechanism.

No abstract

No abstract