Visual Cortex and Perception Flashcards
What happens at the optic chiasm?
fibres that process the right hemifield cross over to the left side of the brain
temporal side of the right eye and nasal side of left eye
Fibres that process the left hemifield cross over to the right side of the brain
temporal side of the left eye and nasal side of the right eye
what is the LGN
relay in the thalamus for the visual pathway
parvocellular, magnocellular and koniocellular all project to separate layers within the LGN
similar electrophysical properties to ganglion cells
on-off cells remain independent
precise retinotopic mapping in all layers
Stellate cells receive information from LGN and pass it on to the dendrites of pyramidal cells
Pyramidal cells are the only ones that can send information out of the cortex
what happens if you lesion the LGN?
Damage to magnocellular LGN causes sharp reduction in ability to perceive rapidly changing stimuli but no effect on visual acuity or colour vision
Damage to parvocellular LGN severely impairs visual acuity and colour perception but no effect on motion perception
Where doe the LGN project to?
V1 - primary visual cortex
V1 is split into 9 layers in the cortex
(SC - superior colliculus)
What is cortical magnification?
the fovea is smaller than the periphery yet has larger cortical representation
this reflects the greater number of ganglion cells associated with the fovea due to its specialisation for high visual acuity
there is greater convergence in the periphery
What layers do the respective pathways dock at?
Magnocellular information docks in layer 4C-alpha
Parvocellular information docks in 4C_ß
Koniocellular information docks in layer 3B
what are ocular dominance columns?
The processing of information changes RFs, making them more complex - the analysis of the visual scene
Provided basis for understanding visual physiology and how the brain interprets edges, motion, depth and colour.
V1 contains neurons with a range of RF properties: binocularity, orientation selectivity, direction sensitivity and colour processing
Columnar organisation of information from each of the eyes (prolene tagging experiment)
why is vision ‘binocular’?
Requires both eyes to be correctly functioning and the circuitry to be properly formed
Each eye receives a slightly different image when looking at the same point in visual space - retinal disparity
the neurons in layer 4 respond to input from one eye only
Projections from layer 4 to layers above or below show convergence of information and these neurons repons electrophysiologically to stimuli in either eye - binocular.
Merging of the images from both eyes results in a single stereoscopic perception of depth and distance
Improper function results in double vision
To prevent this the brain can ignore the information from the weaker eye
Combining input from both eyes enables topographical mapping
What is amblyopia
Loss of binocular vision in children
poor or indistinct vision in an otherwise normal eye
Caused by the eye not pointing in the right direction
strabismus/ congenital cataract/ refractive errors
Children learn to ignore the vision from the weaker eye and the ocular dominance columns are not formed properly
Can be treated up to the age of 8 (most effective in earlier ages) after this you can’t undo this because the circuitry within the brain is not there and the other eye becomes blind.
Force stimuli to be perceived in the the weaker eye by at the expense of the normal eye which is patched for part of the day
Can also use atropine or contact lenses in good eye
What are simple cells?
Respond to bars of light that must be in the correct orientation
Linear RFs formed from converging cumulative input from 3 LGN neurons with RFs aligned along one axis. There are obvious on and off region
what are complex cells?
no distinct excitatory or inhibitory regions, they have larger more complex RFs and give responses to stimuli through the RF in the correct orientation.
Most abundant in layers 2,3 and 4
Composed of several like oriented simple cells but no obvious on and off regions
How is orientation selectivity mapped in the brain?
Orientation selectivity of the neurons that were response changes as they went across the cortex such that they has a systematic change in the direction.
Electrode across one millimeter of cortex they found every possible orientation
Electrode vertically found the same orientations
Cortical column of organisation
what are the cells in layers 2 and 3
blobs and interblobs
Found different response properties between blobs and interblobs
Each blob is centred on an ocular dominance column
key roles in visual processing providing most of the cortico-cortical projections from the koniocellular and parvocellular pathways.
What is the parvocellular interblob pathway
responsible for discriminative form or shape processing cells of V1
Stellate cells in layer 4 receive input front eh parvocellular layers of the LGN to form simple cell RFs
Projections from layer 4 to the interblob areas in layers 2 and 3 exhibit complex cell RFs
Properties of the neurons found in the interblob areas:
Found around the clusters of colour-sensitive blobs
Complex cell RFs are orientation selective necessary for fine detail processing
Wavelength insensitive thus unable to discriminate colours
Responds to achromatic contrast
Binocular
parvolcellular blob pathway
Cells in cytochrome oxidase positive blobs in V1 Respond selectively to colour contrast
Such a double opponent cell is excited by green and inhibited by red in RF centre etc
Does not respond strongly either to uniform illumination or to achromatic contrast
Blob pyramidal cells n layers 2 and 3 receive input from the parvocellular pathways in layer 4 and directly from the single opponent koniocellular layers of the LGN
Properties
Neurons with similar properties found in clusters known as blobs
Each blob is centered on an ocular dominance column
Wavelength sensitive - double colour opponent circular RF for detection of colour contrast. Used in the perception of colour and colour discrimination.
4 classes of double opponent cells characterised by their preferred stimuli
Insensitive to achromatic contrast
Monocular
Insensitive to orientation or direction
