Senses 4: Colour vision and hearing Flashcards
what is the M pathway (magnocellular)
motion
what is the P pathway (parvocellular)
colour
what does white sun do to objects
illuminates them
what happens to non-absorbed light?
reflected and picked up by eye of observer
eye media and water (in the aquatic environment) often absorb some of the reflected light within specific wavelength bands
the light stimulus that reaches the photoreceptors in the retina will differ depending on the object surface and the transmitting media
how can spectral reflectance of diff coloured object surfaces me measured?
with a photospectrometer
absorption of light by ocular media
lens of the human eye absorbs UV and with increasing age also short wavelengths (violet, blue)
vision in bright and dim light
see notes
what are the proven dimensions of colour vision in animals?
dichromacy
trichromacy - discriminate more colours that dichromats
tetrachromacy
colour vision defects
S-cone opsin gene on chromosome 7
M- and L-cone opsin genes on X chromosome
severe colour-deficiency (only two cone opsins expressed in retina)
Lacking cones
mild colour deficiency (one cone opsin is anomalous)
Mutated form of pigment – small shifts in the spectrum
protoanomaly
reduced sensitivity to red light
deuteranomaly
reduced sensitivity to green light - most common
tritanomaly
reduced sensitivity to blue light - rare
tritanopia
lacking s-wavelength cones
x chromosome recombination causes colour vision defects
normal colour vision requires at least one L, one M and one S pigment gene.
because the nucleotide sequences of OPN1LW and OPN1MW are nearly identical and they are arranged in tandem, the L and M genes are prone to unequal homologous recombination, producing a huge amount of diversity in the gene sequences, the number and arrangement of genes in the array and in colour vision phenotype among modern humans
unequal recombination between two such arrays produces one array with three genes, and another with one gene
a male who inherits an array with one gene is an obligate dichromat, and will be a protanope if the gene encodes an M‐class pigment or a deuteranope if it encodes an L‐class pigment
co-existing tri- and dichromats in polymorphic populations of new world monkeys - which selective pressures have mediated the switch to trichromatic vision in primates
this condition results from the sorting of allelic versions of X-chromosome cone opsin genes at a single gene site, yielding a mixture of dichromatic and trichromatic phenotypes in the population.
in the context of primate colour vision, the idea is that most primates include some fruits in their diets and that trichromatic animals may be particularly advantaged in the detection of yellow and orange fruits embedded in a sea of green foliage.
one implication is that the polymorphic colour vision of most platyrrhine species is a less than optimal arrangement for a frugivorous life style.
according to this idea trichromatic females would be well adapted for fruit detection, but the remainder of the animals (up to two-thirds of all monkeys and every male) would be relatively poorer at distinguishing fruit from foliage on the basis of chromaticity cues alone.
if this is a correct conclusion, one might expect to see some consistent individual variations in diet choice or harvesting style in these polymorphic species that reflect these variations in color vision capacity.
so far none have been noted.
how do discriminable colours differ?
in their S:M:L-cone response ratios
why are cone receptor signals subtracted/added?
most ganglion cells in the LGN fire in response to some wavelengths and are inhibited by other wavelengths
chromatic pathways:
achromatic (brightness) pathway:
when does the response of an LGN cell change?
depends on the wavelength of the stimulus
spectrally opponent cell
a visual receptor cell that has opposite firing responses to different regions of the spectrum
how red is red?
difficult to measure colour precisely using language
colour names differ between languages
the spectral composition
Isaac Newton discovered the spectral composition of daylight (1672) and proposed in his book Opticks (1704) that light is composed of particles or corpuscles
colours can be primary or mixed
Thomas Young (1773-1829) demonstrated interference patterns when passing light through slits. Light behaves as a wave.
Young’s theory of colour based on three primaries – blue, green and red (1802)
Young-Helmholtz theory (1850) of trichromatic colour vision in humans
additive colour mixing
any colour visible to humans can be created by mixing the three wavelengths that correspond to the peak sensitivities of the three types of cone receptors
how TV and photography work
