Spectral-sensitivity functions for large, long-duration increments
presented on a photopic white background indicate that
wavelength-opponent mechanisms mediate detection in both normal and
dichromatic humans. Normal humans exhibit high color-vision sensitivity
as they discriminate the color of spectral flashes at
detection-threshold intensities. However, dichromatic humans require
stimuli up to about 0.4 log units above detection intensity to see
certain colors. This low color-vision sensitivity in human dichromats
may be an abnormal condition involving a defect in postreceptoral color
processing. To test this hypothesis, we determined color-discrimination
thresholds in normally dichromatic species: chipmunk, 13-lined ground
squirrel, and tree shrew. For comparison, we also tested humans with
normal and abnormal (deutan) color vision with the same apparatus and
methods. Animals were trained to perform spatial two-choice
discrimination tasks for food reward. Detection thresholds were
determined for increments of white, 460 nm, 540 nm, 560 nm, 580 nm, 500
nm/long-pass, and 500 nm/short-pass on white backgrounds of
1.25 cd/m2, 46 cd/m2, and 130
cd/m2. Animals were also trained to respond to the
colored increments when paired with the white increment when both were
at equally detectable intensities Color-discrimination thresholds were
determined by dimming stimulus pairs (colored vs. white) until
the subjects could no longer make the discriminations. Results
indicated that the normally dichromatic species could discriminate
colored stimuli from white at a mean intensity of 0.1 (±0.1) log
units above detection threshold. The ability of normally dichromatic
species to discriminate color near detection-threshold intensity is
consistent with increment spectral-sensitivity functions that indicate
detection by wavelength-opponent mechanisms. In keeping with previous
studies, normal human trichromats discriminated color near
detection-threshold intensities but humans with deutan color vision
required suprathreshold intensities to discriminate the color of middle
and long wavelengths. This high color-vision sensitivity of normally
dichromatic species suggests that the low color-vision sensitivity in
dichromatic humans is an abnormal condition and indicates a possible
defect in their postreceptoral color-vision processing.