Vision Flashcards
What is an interesting fact about human visual spectrum?
Mammalian vision was developed under water, therefore eyes only developed to perceive light that could penetrate the ocean
What is the difference between retinal arteries and veins?
Arteries have smooth muscle to allow them to constrict
In imaging they appear thicker
What is unusual about the organisation of the retinal cells?
Light must pass through the ganglion cells and bipolar cells before reaching the photoreceptors to then be transmitted through bipolar and ganglion cells
How how does light get to the photoreceptor cells?
Rather than passing through every other cell membrane the light will travel through the muller cells
What are the types of cones and what light do they specifically detect?
Short cones - blue light
Medium cones - green light
Long cones - red light
What is the distribution of rods and cones in the retina?
Centre (fovea) - higher proportion of cones
Periphery - higher proportion of rods
Give two examples of colour blindness
Red-green blindness (most common)
Blue-yellow (rare, thought to be related to bipolar dysfunction)
What are the four types of red-green blindness?
Deuteranomaly
Protanomaly
Protanopia
Deuteranopia
What is deuteranomaly?
The most common type of red-green colour blindness that makes green look more red
Mild and does not interfere with life too often
What is protanomaly?
A type of red-green colour blindness that makes red look more green and less bright
Mild type of colour blindness
What forms of colour blindness make it so that you cannot distinguish between red and green at all?
Protanopia and deuteranopia
Which gender is colour blindness more frequent in?
Men as it is an X linked condition and men only have one X chromosome
Describe the photoreceptor structure
Outer segment (contains membrane shelves lined with rhodopsin or iodopsin)
Inner segment (dense with mitochondria)
There will be an outer limiting membrane
Then the nucleus
Then the synaptic body
List the steps to the dark phase of phototransduction
Guanylate cyclase is constitutively active
cGMP is constantly formed in the outer segment
This opens cation channels in the OS membrane (called a dark current)
OS depolarises
Depolarisation spreads to IS
VGCC open on presynaptic membrane
Glutamate enters the synaptic cleft
mGLUR6 receptors on the bipolar cell activate
These receptors are GPCR which link to G alpha o (inhibitory)
This inhibits TRPM1 in ON bipolar cells
Hyperpolarisation of bipolar cell
Switches off
What are the steps of the light phase of phototransduction?
Light enters the retina
Causes opsin to isomerise and activate transducin
This activates phosphodiesterase (PDE)
PDE breaks down cGMP
CNGAs close - reduced dark current
OS hyperpolarises
This spreads to IS
VGCC close in presynaptic membrane
Glutamate release is decreased
MGLUR6 receptors inactivate
G alpha o stops inhibiting TRPM1
These channels open and Na2+ enters
On bipolar cells depolarise
What is a positive after image?
This can happen if you close your eyes or the lights go out suddenly after looking at something bright
It is when you can still see an image but in the absence of photoreceptor stimulation and it will be the same colour as the actual image you were looking at
What is happening when you see an after image?
The photoreceptive pigment is still being rebuild and replaced after having been broken down and so you continue to see the previous image
What do bipolar cell do in the dark?
ON bipolar cells hyperpolarise
OFF bipolar cells depolarise
What is the neurotransmitter responsible for bipolar cells depolarising or not?
Glutamate
ON - mGLuR6
OFF - iGLuR2
What do bipolar cells do in the light?
ON bipolars depolarise
OFF bipolars hyperpolarise
What is lateral inhibition?
When light falls directly onto a bipolar cell the adjacent bipolar cells will be inhibited (hyperpolarised)
This is important for image processing and to prevent things looking blurry
How are signals propagated on from bipolar cells?
Bipolars will send graded potentials to a neighbouring retinal ganglion cell (RGC) via glutamate release
These activate iGLuR1 receptors and when enough Na2+ is reached the action potential will happen
Are cells myelinated in the retina?
Cells don’t tend to be myelinated in the retina as this can lead to problems with vision
RGCs aren’t myelinated until they reach the optic nerve, if there is abnormal and early myelination then this can increases a persons blind spot
What are the two ways RGCs are categorised?
Based on axon diameter (morphology)
Based on thalamic projections and functions
What are the 3 morphology types of RGCs?
W-ganglion
X-ganglion
Y-ganglion
Describe the W-ganglion cells
Small
40% of total
Broad dendritic fields in retina
Excitation from rods
Detect direction movement anywhere in the field
Describe the X-ganglion cells
Medium diameter
55% of total
Small dendritic field
Colour vision
Sustained response
Describe the Y-ganglion cells
Largest
5% of total
Very broad dendritic field
Respond to rapid eye movement
Or rapid changes in light intensity
Which cells fit into the thalamic projections type of ganglion cell?
Midget cells (M RGCs)
Parasol cells (P RGCs)
Bistratified (K RGCs)
Intrinsic photosensitive ganglion cells (ipRGCs)
Describe midget cells
Magnocellular pathway
For rods
Movement and contrast
Describe parasol cells
Parvocellular pathways
For M&L cones
Green and red light
Describe bistratified cells
Koniocellular pathway
For S cones
Blue light
Describe intrinsic photosensitive cells
Control pupil diameter
Involved in circadian rhythm
Where does thalamic processing happen?
In the lateral geniculate nucleus
Where do the M, P and K RGCs project to?
Into the thalamus
Specifically the lateral geniculate nucleus
Where might the other RGCs project to
Hypothalamus (reflex orientation to visual stimuli)
Superior colliculus (stabilisation of gaze)
Pretectum (circadian rhythm and control of pupil diameter)
What areas of the lateral geniculate nucleus LGN do RGCs synapse in?
Magnocellular layer
Parasol layer
Koniocellular layer
How are the M P and K layers arranged in the LGN?
There are Koniocellular layers between the individual M and P
k1, M1, K2, M2, K3, P3 - K6, P6
What is an interesting fact about dyslexia and the eyes?
It is thought that dyslexia may occur due to the brain inability to determined which eye is dominant
What areas are found in V1-v5 of the occipital lobe?
V1 - primary visual cortex
V2-5 - extrastriate areas
What are the two complementary theories of colour vision?
The trichromatic theory
The opponent process theory
What is the trichromatic theory of colour vision?
Proposed in the 19th century by Thomas young and Hermann von Helmholtz
The retinas three types of cones are preferential to blue, green and red light respectively
What is the opponent theory of colour vision?
Proposed by Ewald Hering in 1872
Visual system interprets colour vision in an antagonistic way
Red vs green, blue vs yellow, black vs white
What happens when (for example) you are looking at a field of grass?
Green (M) photoreceptors are activated
GvR bipolars are activated
RvG bipolars remain inactive
GvR ganglion cells are activated
Transmit info to the parvocellular layers of the LGN
Moves on to layers 4C-6 of the visual cortex
What happens when perceiving blue and yellow light?
Blue - BvY bipolars are activated, transmit to k layers of LGN, layers 1-4A of the visual cortex
Yellow - RvG bipolars are activated, transmit to P layers of LGN, layers 4C-6 of visual cortex
What things factor into visual perception?
Binocular vision
Depth perception (stereopsis)
Dark/ light sensitivity
Orientation
Recognition of objects
Motion
What happens in the dorsal and the ventral pathways of the visual cortex?
Dorsal - Where = Motion, attention, localisation
Ventral - What = Object recognition, basic processing of form and colour
List some visual disorders of the ventral pathway
Cerebral achromatopsia
Cerebral dyschromatopsia
Colour anomia or agnosia
Prosopagnosia
Alexia
What is cerebral achromatopsia?
Complete absence of colour perception
What is cerebral dyschromatopsia?
Impaired colour perception
(Hues and saturation)
What is colour anomia or agnosia?
Can discriminate colours accurately but cannot name them
What is visual agnosia?
No longer recognise previously familiar objects and cannot learn to identify new objects
What is prosopagnosia?
Impaired ability to recognise familiar faces or to learn to recognise new faces
What is alexia?
The ability to write but not read well
Disconnection of the visual input in both hemifields from language areas in the left hemisphere
List some visual disorders of the dorsal pathway
Cerebral akinetopsia
Balints syndrome
Blindsight
What is cerebral akinetopsia?
A selective impairment in motion perception
What is balints syndrome?
A deficit in attention
A rare disorder associated with difficulties in visual and spatial coordination and is characterized by the three cardinal features: Optic ataxia. Oculomotor apraxia. Simultagnosia.
What is blindsight?
An impaired conscious awareness of visual stimuli
Is aware of their surrounding but unable to physically see them to name what they are
