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
what makes white light
equal mixture of green, red and blue
what is colour constancy?
ability to recognise colours under different illuminations
sun light is slightly coloured during dusk and dawn
visual system compensates for such slow changes by global adaptation
during visual search in a natural scene, the visual system can also adapt quickly but within the spatially restricted area(s) of visual search
is seeing and hearing believing?
interactions between vision and hearing: the McGurk effect
auditory perception is important language learning, communication and analysis of sound scenes
composers and musicians exploit the features of auditory perception to create a diversity of sensations
what are sound and light?
waves
sound – pressure waves, movement of air particles set in motion by vibrating structure
propagates in three dimensions, alternating compression and rarefaction of air, molecules move back and forth from regions of high pressure to low pressure
measures of sound – frequency (reciprocal of wavelength, perceived as pitch) and amplitude (loudness), phase and waveform
where do all sound waves hit?
the tympanum
the ear does not preserve the spatial arrangement of sounds across the hearing field
vibrations travel from the tympanum to the middle ear where they are amplified
what is the inner ear?
the cochlea
filled with fluid that contains ions
movement of the ossicles pushes the oval window which moves the fluid inside the scala tympani
one the other end of the cochlea the round window bulges outward
round window
a membrane separating the cochlear duct from the middle-ear cavity
what are the auditory receptors
inner and outer (IHC/OHC) hair cells
hair cells are non-spiking receptor cells
respond to mechanical stimulation (stretch ion channels) with depolarisation
stereocilium
relatively stiff hair that protrudes from a hair cell in the auditory or vestibular system
stereocilia
(stiff hair) help to stretch open the ion channels
bending of the stereocilia (right) opens large, nonselective ion channels, allowing K+ and Ca2+ to enter the stereocilia.
the resulting depolarization opens Ca2+ channels in the cell’s base, causing the release of neurotransmitter to excite afferent nerves
what is the auditory nerve?
axons of spiking auditory interneurons that innervate hair cells
auditory nerve fibers contact the hair cells at the base
the organ of Corti
auditory pathways
from receptor to primary sensory cortex
most projections from the cochlear project to the contralateral cortex
each superior olivary nuclei of the brainstem receives inputs from both cochlear nuclei for first stage of binaural analysis of sound-source location
inferior colliculi are located in the dorsal midbrain
medial geniculate nuclei of the thalamus
what does precise sound location require?
input from both ears
sound source location is computed from the differences in delay and intensity between two ears
Jeffress model of how the brain codes latency differences between sound heard by right and left ear by coincidence detection
what do many new world monkeys have?
polymorphic colour vision
what does IHC give rise to?
sound perception
what can OHC do?
change their length to fine tune the organ of Corti
where do projections from the cochlear go to?
contralateral cortex
auditory pathways (further information)
each auditory nerve fiber divides into two main branches as it enters the brainstem.
each branch then goes to one of two groups of neurons, one group in the dorsal cochlear nucleus and the other group in the ventral cochlear nucleus.
the output of neurons in the cochlear nuclei also travels via multiple paths.
one path from each cochlear nucleus goes to both superior olivary nuclei, so they both receive inputs from both right and left cochlear nuclei.
this bilateral input is the first stage in the CNS at which binaural (two-ear) effects are processed; as you might expect, this mechanism plays a key role in localizing sounds by comparing the two ears.
several other parallel paths converge on the inferior colliculi, which are the primary auditory centers of the midbrain.
outputs of the inferior colliculi go to the medial geniculate nuclei of the thalamus.
at least two different pathways from the medial geniculate extend to several auditory cortical areas.
receptors of the photopic system
cones
number of cones per eye
4 million
cone photopigments
3 classes of opsins - basis of colour vision
cone sensitivity
low
needs relatively strong stimulation
used for day vision
cone location in retina
concentrated in and near fovea
present less densely throughout retina
cone receptive field size and visual acuity
small in fovea, so acuity is high
larger outside fovea
temporal responses of cones
rapid
receptors of the scotopic system
rods
number of rods per eye
100 million
rod photopigments
rhodopsin
rod sensitivity
high
can be stimulated by weak light intensity
used for night vision
rod location in retina
outside fovea
rod receptive field size and visual acuity
larger, so acuity is lower
rod temporal responses
slow
chromatic pathways
S/(L+M) (old, mammalian)
L/M (new, trichromatic primates)
achromatic pathways
L+M
the organ of Corti
has four kinds of synapses and nerve fibers.
two of these are afferents that convey messages from the hair cells to the brain; the other two are efferents that convey messages from the brain to the hair cells
several different synaptic transmitters—especially glutamate and acetylcholine, but also GABA and dopamine—appear to be involved in activity at the various hair cell synapses
