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Richard Arsdel, Michael Loop; Color thresholds in normal dichromats. Journal of Vision 2002;2(7):138. doi: 10.1167/2.7.138.
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© ARVO (1962-2015); The Authors (2016-present)
The shape of spectral sensitivity functions indicates that normal and dichromatic humans detect spectral flashes with wavelength opponent mechanisms.As wavelength opponent mechanisms signal color, and normal humans detect the color of spectral flashes at threshold, we presumed that dichromatic humans would too. They do not. To see color, dichromatic humans require a flash around 0.4 log units above detection intensity. This suggests that dichromatic humans may have a defect in postreceptoral color processing, perhaps because dichromacy is an abnormal condition in humans. To test this, we determined color discrimination thresholds in normal dichromats: chipmunk, ground squirrel, and tree shrew. Animals were trained with food in a spatial two-choice discrimination. On 130 and 46 cd/m2 white backgrounds, detection thresholds were determined for increments that were white, 460nm, 540nm, 560nm, 580nm, 500nm long pass, and 500nm short pass. Animals were then trained to respond to the colored increments paired with the white when both were 0.5 log units above each animal's detection threshold intensity. Color discrimination thresholds were then determined by dimming stimulus pairs (colored vs. white). Across all stimulus pairs and animals these normally dichromatic species could discriminate the color vs the white at 0.11 (± 0.10) log units above detection threshold intensity. The ability of normally dichromatic species to discriminate color near detection threshold intensity is in keeping with spectral sensitivity results indicating detection by wavelength opponent mechanisms but suggests that the low color vision sensitivity of dichromatic humans is not due to dichromacy per se. USAF (RV); NSF (76972); NEI (07084).
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