evolution and genetics of colour vision Flashcards
if a visual cortical area was specialized for color where would you expect to find it?
ventral or dorsal stream
ventral stream
can you name a cortical area considered as a possible colour center and what staining they use which may have caused this assumption, however, why is it proven wrong
V4
cytochrome oxidase staining in V1, which formed blobs appearing darker in the upper cortical layers and got fainter towards the bottom. These blob areas were thought to be receptive to colour but later they found that areas outside of this were also responding to colour information.
what is the parallel distinct streams functions and include the onward connections from early visual processing centers (V1)
ventral stream: colour, shape (visual/object recognition)
From V1, the ventral stream involves area V2 and then proceeds to area V4, which is associated with color and shape processing. The pathway continues to the inferior temporal (IT) cortex, including areas such as the fusiform face area (FFA), which is crucial for face recognition, and the then firther projections to prefrontal cortex and temporal lobe.
dorsal stream: motion, orientation, depth, direction (spatial localisation)
After originating in V1, the dorsal stream passes through visual area V2 and then to the dorsal areas V5/MT (middle temporal area) and MST (medial superior temporal area) which are involved in motion perception. From there, it projects into the parietal regions, such as the prefrontal cortex, which processes spatial information for navigation, and directing attention.
what is the difference between colour coding cells and wavelength coding cells and where are they found- Zeki experiment (how is this proof that colour is processed in V4 and what are the against evidence using V4 lesions)
WC- respond to wavelength composition of objects
CC- respond to perceived colour appearance independent of their wavelength composition
WC only found in V1 where as significant portion color sensitive found in V4
this is evidence that regions in V4 process colour information from visual stimuli
However, V4 lesions had little effect on colour vision and greater effect on shape
discrimination
* V4 may be involved in colour processing, but it’s not ‘the cortical colour centre’
what is cerebral achromatopsia? and why can this be evidence for V4 being involved in colour processing
It is a condition associated with damage in ventral stream regions of the brain, not including V1, but including areas such as V4. this results in complete loss of colour sensation or perception and deficits in object perception, but patients will have intact wavelength discrimination
whats the difference between single and double opponent cells
They allow for the perception of fine details, including borders and contrast. involved a center-surrounded receptive field organisation with either an inhibitory or excitatory centre and an inhibitory or excitatory surround, leading to signal propagation to the next neuron (bipolar cells, then ganglion cells).
So, when red light stimulates the center and green light stimulates the surround, the cell will respond strongly. However, if green light stimulates the center and red light stimulates the surround, the cell will be inhibited and respond weakly.
DO- they allow for colour discrimination and perception of colour and contribute to colour constancy. They combine inputs from both red-green and blue-yellow opponent processes. Each double opponent cell has two parts: a center, which is sensitive to the difference between L and M cones, and a surround, which is sensitive to the difference between S cones and the combination of L and M cones.
When light stimulates the center of a double opponent cell, it compares the activity of the L and M cones. If the L cone response is higher than the M cone response, the cell will be excited. Conversely, if the M cone response is higher than the L cone response, the cell will be inhibited. Similarly, when light stimulates the surround, it compares the activity of the S cones with the combined activity of the L and M cones. If the L/M response is higher than the S cone response, the cell will be an overall inhibition due to presence of yellow light, and if the S cone response is higher, the cell will be excited due to abscence of yellow light .
what makes each cone unique
the photopigment, as its has its own sensitivity range with regards to the wavelength it responds best to
what is the visual pigment made out of
chromophore (11-cis retinal) and opsin
what part of the photopigment absorbs photon
the chromophore converting it from 11-cis retinal to all trans retinal upon conformational change
what part of the photopigment confers spectral sensitivity
the opsin AA chain or sequence
what are the four families of cone opsins and 1 family of rhodopsin
and which which ones do we have?
cone opsins: SWS1, SWS2, RH2, LWS
rhosopin: RH1
we have LWS and SWS1 and RH1
which gene underwent duplication and divergence and what did this result in
LWS gene which split into the L and the M cone providing trichromacy
what chromosome is LWS found on
x-chromosome
what is LCR and its function?
locus control region that will randomly activate the promoter of the L or M opsin, allowing for only one opsin to be expressed in a single cone
which substitutions allow the greatest difference in spectral sensitivity between the M and L opsin
and mention how allelic diversity can be different between individuals by a few nm how this can cause a shift in their peak sensitivities
2 AA substitutions on exon 5
Allelic diversity is caused by substitutions in other places, so the peaksensitivity of M and L can be a few nm different between people. ie my M cone could have a peak sensitivity of 480nm and Aisha’s may have it at 490nm.
