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Cambridge, United Kingdom

Danilova M.V.,RAS Pavlov Institute of Physiology | Mollon J.D.,Downing St.
Journal of the Optical Society of America A: Optics and Image Science, and Vision | Year: 2014

Under conditions of adaptation to a steady neutral field (metameric to Daylight Illuminant D65), forced-choice thresholds for color discrimination were measured for brief targets presented to the human fovea. Measurements were made along -45° and ?45° lines in a MacLeod-Boynton chromaticity space scaled so that the locus of unique yellow and unique blue lay at ?45°. The lines were symmetrical relative to the tritan line passing through the chromaticity of D65. Thresholds increased with distance of the probe chromaticity from D65. Thresholds were higher for saturation discrimination than for hue discrimination. A region of enhanced discrimination was found for thresholds measured orthogonally to the locus of unique blue and unique yellow. There may be an analogous enhancement near the loci of unique red and unique green. © 2014 Optical Society of America.

Danilova M.V.,RAS Pavlov Institute of Physiology | Mollon J.D.,Downing St.
Vision Research | Year: 2012

Human color vision depends on the relative rates at which photons are absorbed by the three classes of retinal cone cell. The ratios of these cone absorptions can be represented in a continuous two-dimensional space, but human perception imposes discrete hue categories on this space. We ask whether discrimination is enhanced at the boundary between color categories, as it is at the boundary between speech sounds. Measuring foveal color discrimination under neutral conditions of adaptation, we find a region of enhanced discrimination in color space that corresponds approximately to the subjective category boundary between reddish and greenish hues. We suggest that these chromaticities are ones at which an opponent neural channel is in equilibrium. This channel would be 'non-cardinal', in that its signals would not correspond to either axis of the MacLeod-Boynton chromaticity diagram. © 2012 Elsevier Ltd.

Danilova M.V.,RAS Pavlov Institute of Physiology | Chan C.H.,Downing St. | Mollon J.D.,Downing St.
Vision Research | Year: 2013

We measured spatial resolution in the parafovea for targets designed to isolate either the long-wavelength (L) or the middle-wavelength (M) cones. Landolt C optotypes were presented for 100. ms on a calibrated monitor at an eccentricity of 5° to the left or right of fixation. There were large individual differences in the ratio of the resolution obtained with L targets to that obtained with M targets, and we suggest that these differences reflect variations in the relative sampling densities of L and M cones in the parafovea. In Experiment 1, we measured contrast thresholds for targets of varying size. Among 10 unselected observers, there was a threefold variation in the ratio of the contrast thresholds for the smallest targets. In Experiments 2 and 3, we held contrast constant and we varied size, in order to establish the minimal target that could be discriminated for each of the two classes of cone. In Experiment 2, two groups of observers, selected on the basis of their settings on a flicker-photometric test, showed a highly significant difference in the ratio of the M and L acuities on the spatial task. In Experiment 3, female carriers of protan or deutan deficiencies, classified only on the basis of their sons' phenotypes, also showed a large difference in the ratio of their acuities for M and L targets. In all three experiments, there was a strong correlation between the ratio of M and L spatial acuities and a flicker-photometric measure of relative sensitivity to long- and middle-wavelength light. © 2012 Elsevier Ltd.

Repiso A.,Downing St. | Repiso A.,University of Barcelona | Saavedra P.,Downing St. | Casal J.,Downing St. | Lawrence P.A.,Downing St.
Development | Year: 2010

The larval ventral belts of Drosophila consist of six to seven rows of denticles that are oriented, some pointing forwards, some backwards. We present evidence that denticle orientation is determined almost entirely by Dachsous and Fat, one of two planar cell polarity systems. If we change the distribution of Dachsous we can alter the polarity of denticles. We suggest that the orientation of the individual denticle rows, in both the anterior compartment (which mostly point backwards) and the posterior compartment (which point forwards), is determined by the opposing slopes of a Dachsous/Fat gradient. We show, by altering the concentration gradients of Dachsous during development, that we can change the polarity of the denticles made by larval cells as they progress between the first and third larval instars without mitosis.

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