Vision and visual pathways Flashcards
What is the pathway of light reaching the optic nerve?
Light
Cornea (shape helps to focus light onto pupil)
Pupil
Lens (focuses light onto retina at back of eye)
Retina
Optic nerve
What happens if the cornea gets damaged?
Hypersensitivity to light
What are the anterior and posterior chamber?
ANTERIOR = Between cornea and iris, filled with aqueous humour which nourishes cornea and lens POSTERIOR = Between iris and lens, filled with aqueous humour produced by structure alongside the lens called the ciliary body, the aqueous passes into the posterior chamber and then flows forward through the pupil into the anterior chamber of the eye
What is meant by accommodation of the lens?
Lens changes shape to adjust the focus of the eye for stimuli at different distances
What are “near points”?
Points at which you can no longer focus clearly
Shift further away with increasing age (have to read from further away)
Lens hardens and ciliary muscles weaken (less elastic)
Compounded by neural degradation with age too
What is “presbyopia with age”?
Hardening of the lens causes light to be focused behind rather than on retina when looking at close object –> blurred vision
What is the fovea?
Where the centre of an image falls
Highest visual acuity due to highest density of photoreceptors and also lower density of ganglion cells so the light can reach photoreceptors more efficiently
What is the posterior cavity?
Behind the lens containing vitreous which is important for holding eye shape and keeping retina in place at back of eye
What are the layers of the retina?
Ganglion cells which connect to optic nerve fibres
INNER RETINA of horizontal and bipolar cells
OUTER RETINA of photoreceptors
What do photoreceptors do?
Photopigments detect light in retinal image and transform photons into electrical energy via PHOTOTRANSDUCTION
What are the properties of rods?
~120 million
Rod shaped
Permit vision at low light levels (sensitive to a single photon)
Medium wavelengths
Coarse spatial structures, poor acuity partly to do with fact that not packed so tightly on retina
Majority found in peripheral vision
What are the properties of cones?
~6 million (far fewer) Vision at high light levels Can detect different wavelengths i.e. colour Visual acuity One photoreceptor present at the fovea
What happens during the phototransduction process?
Photopigments contain opsin and retinal
Retinal absorbs light and changes shape which then activates opsin and alters photoreceptor membrane state
What is the blind spot?
Point on retina where optic nerve leaves brain - there are no photoreceptors here
We don’t notice it because it is in the poor acuity region of peripheral vision - the brain “fills the gap”
What are the 4 synapses along the visual pathway?
2 in eye - receptors to bipolars, bipolars to ganglion cells
Ganglion cells synapse at thalamus
A small number also synapse at midbrain here for direction of attention, regulating “body clock” and controlling pupil diameter
What is convergence in the retina?
FOVEA - each ganglion cell connects to 7 photoreceptors
PERIPHERY - connection to 10s of rods
So ganglia in central vision carry precise info, small receptive field
In periphery the receptive field is large
RECAP: What is a receptive field?
Region that elicits a response from a given neuron
How do bipolar cells in the retina behave?
In one of two ways:
1) ON - Detecting light areas on dark background
2) OFF - Detecting dark areas on light backgrounds
What do retinal ganglion cells do?
Perform even higher level processing, with more complex on/off properties - CENTRE SURROUND RECEPTIVE FIELDS
Don’t respond to light but rather to DIFFERENCES in light across the receptive field
What is the receptive field of retinal ganglion cells defined by?
Area of retina covered by photoreceptors connecting to it
Why are centre-surround cells important?
Form building blocks for more advanced processing by the brain e.g. enable detection of edges and orientations
How do central and peripheral ganglion cells differ with regards to colour detection?
CENTRE - fed by cones so sensitive to colour, some will have red centre and green surround or vice versa
PERIPHERY - mixture of cone types in both the centre and surround so don’t show sensitivity to colour
So some ganglion cells can signal wavelength variation while others simply indicate illumination variation
What has happened by the time light signals leave the eye?
The visual system has already started processing and analysing the signals into useful info - only info about variation is preserved, unchanging illumination is largely ignored
What are the thalamic cells in the visual pathway like?
Have same general receptive field layout as connecting ganglion cells i.e. circularly symmetrical, antagonistic centre-surround organisation
What are the properties of light?
Travels in waves
Amplitude = light intensity i.e. peak
Wavelength = distance between peaks
Frequency (Hz) - number of waves passing given point per second
What is the visible spectrum?
400-700nm
How are cones subdivided?
Respond over different wavelength ranges within visible spectrum:
RED - Long wavelengths
BLUE - Short wavelengths
GREEN - Medium wavelengths
What causes colour blindness?
Abnormality in wavelength response of cones thus impairing ability to discriminate between colours
Most commonly red and green sensitivities shift closer together
More rarely one cone type can be completely absent
Summarise the pathway once signals leave the back of the eye
At optic chiasm optic nerve fibres cross - RHS fibres entirely onto left side, LHS entirely onto right
Synapse at LATERAL GENICULATE NUCLEUS
Largest projection from here goes to visual cortex in occipital lobe
Branch before LGN goes to SUPERIOR COLLICULUS (10% fibres) and then pulvinar nucleus
What is cortical magnification?
Area of cortex devoted to visual field varies with eccentricity i.e. fovea has largest area
Where are signals sent upon reaching the primary visual cortex?
Temporal lobe (what pathway) Parietal lobe (where pathway) Signals also reach frontal lobe
What happens when damage occurs at the primary visual cortex?
Symptoms range from loss of specific visual abilities e.g. depth perception to complete loss of conscious awareness of stimuli
Why is the superior colliculus pathway important?
Involved with eye movement programming/execution, largely unconscious visual processing (fast and unseen fear responses (Morris et al, masking of angry faces; and blindsight)
Ancient evolutionary brain area key to survival
When can blindsight occur?
When damage to visual cortex or pathways leading there –> no visual experience in the damaged areas of visual field but residual function remains due to function of intact subcortical areas
What experimental evidence supports blindsight?
Patient GY
Cortical damage for parts of visual field
In forced choice task he was asked whether a stimulus was presented at a first or subsequent beep
Maintained eye fixation throughout
Showed high accuracy of guessing although little or no conscious awareness of the stimuli
What is a second example of an evolutionarily older visual pathway?
Route to suprachiasmatic nucleus in hypothalamus responsible for info about night and day i.e. setting biological clock
What is the difference between loss of an eye and hemianopia?
Loss of eye –> remaining eye still sends info for both sides of space
Hemianopia = cortical blindness, damage to one whole side of visual cortex which will be evident regardless of which and how many eyes used
What is quadrantanopia?
Partial damage to primary visual cortex, the effects of which depend on the CALCARINE FISSURE
Damage above here affects perception of bottom of space
Damage below affects above perception
Get cortical blindness in one quarter of visual field
What is blindsight?
Impaired vision (subcortical routes don’t permit the same specificity of processing, because LGN receives more cone-rich info but superior colliculus is more rods) plus no conscious awareness of vision
What would result from an optic chiasm lesion?
Bipolar hemianopia
What is LGN processing like?
LGN actually receives most input from the cortex, a little less from the retina, and then the least amount gets sent back to cortex
Important for regulation of info flow and also MAPS the info based on position in eye, the receptors that generated the info etc (retinotopic map)
LGN neurons also have same centre-surround configuration as the retinal ganglion cells i.e. respond best to small spots of light on retina
What is the LGN like structurally?
Layered - layers 2,3 and 5 receive signals from the ipsilateral eye, while layers 1, 4 and 6 receive from contralateral eye
i.e. each eye sends half neurons to LGN in each hemisphere
Each layer has a retinotopic map and maps in each layer line up with each other i.e. a perpendicular electrode would hit electrons which all have receptive fields at the same location on the retina
What is the importance of PARALLEL PROCESSING?
Processing same visual information but for different functioning Dorsal pathway (posterior parietal) --> action Ventral (occipitotemporal) --> "what" i.e. perception Helpful to have pathways in different areas - if one damaged, still have function in the other
What causes visual form agnosia and what is the presentation?
Damage to ventral pathway (as in the case of DF)
Can put hand into an oriented slot, but cannot show the orientation of the slot using a card
i.e. cannot recognise objects but can perform actions on them
Cannot copy drawings but can draw from memory
What happens in optic ataxia?
Unimpaired recognition/perceptual abilities but make errors in hand rotation
How are the properties of these processing streams established?
By 2 different types of ganglion cells in retina, which transmit signals to different layers of LGN
Pathways not fully separate - connections exist between them to allow cross-talk and info also feeds back to V1 to allow coordination
Where is the parafovea?
2-4 degrees from the fovea
Periphery is anything more eccentric than this
Who outlined the parallel processing pathways?
Milner and Goodale
