Is colour vision possible with only rods and blue-sensitive cones?

Abstract

At night all cats are grey, but with the approach of dawn they take on colour. By starlight, a single class of photoreceptors, the rods, function, whereas by daylight, three classes, the blue-, green- and red-sensitive cones, are active and provide colour vision. Only by comparing the rates of quantal absorption in more than one photoreceptor class is colour vision possible. Although the comparisons generally take place between the cones, they can involve the rods as well. Here we investigate the wavelength discrimination of an extremely rare group of individuals, blue-cone monochromats, who have only rods and one class of cones. We find that these individuals can distinguish wavelengths (440 to 500 nm) in the twilight region where the rods and blue-sensitive cones are simultaneously active.

Ambient illumination switches contrast preference of specific retinal processing streams

“…Basic features of visual scenes (e.g., contrast) are encoded in a distributed manner across RGC types, whereas more elaborate features (e.g., motion direction, movement of an object relative to its background) are selectively detected by a few RGC types. Luminance-dependent changes in chromatic, temporal, and spatial tuning of RGCs were identified early in the history of retinal physiology (Barlow et al 1957;Creutzfeldt and Sakmann 1969;Enroth-Cugell and Lennie 1975;Enroth-Cugell and Robson 1966;Ogawa et al 1966;Reitner et al 1991) and have been analyzed in detail since (Farrow et al 2013;Field et al 2009;Grimes et al 2014). By comparison, how contrast encoding and the detection of complex features depend on ambient illumination is not well studied.…”

Multifocal lenses in a monochromat: the harbour seal

“…This hypothesis is supported by the low brightness discrimination thresholds assessed in Bering sea spotted seals (Wartzok and McCormick, 1978) and in harbour seals (Scholtyssek et al, 2007). Mesopic colour vision, shown in owl monkeys (Jacobs et al, 1993) and human blue-cone monochromats (Reitner et al, 1991), is currently under investigation in our lab in harbour seals using the lower border of mesopic colour vision in humans as a reference luminance because mesopic light conditions are not clearly defined for harbour seals. If the pupil is wide under lighting conditions that are mesopic for seals (see Photorefractive measurements in live animals, above), the lens might be optimized for mesopic colour vision in the blue-green range of the spectrum where the rods and cones have their λ max .…”

Blue cone monochromatism: a phenotype and genotype assessment with evidence of progressive loss of cone function in older individuals

“…This contrasts with blue-cone monochromats who do have access to colour discrimination, though this does depend upon the luminance of the task: at mesopic levels, they have rudimentary dichromatic colour discrimination based upon a comparison of the quantum catches obtained by the rods and the S-cones, with their neutral point lying at about 460-470 nm. 16,17 Hue discrimination is reported to deteriorate with increasing luminance. 18 Blue-cone monochromats may be distinguished from rod monochromats by means of colour vision testing: bluecone monochromats are reported to display fewer errors along the vertical axis in the 100-Hue test (fewer tritan errors), and they may also display protan-like ordering patterns on the D-15.…”

“…This provides additional evidence that it is possible to derive colour discrimination at low photopic levels via a comparison of quantum catches in the S-cones and rods, as has been suggested previously. 16,18 It is proposed that the latter two tests, which are suitable for use in children, can be readily employed in probing the differential diagnosis of congenital achromatopsia.…”