SENSORY SYSTEMS: VISION Flashcards
What is the fovea?
Area of the eye that has the highest density of photoreceptors - so highest visual aucity (quality)
Why are there no photoreceptors in the blind spot?
Because there are blood vessels coming out
What are objects coming in the eye further than 6m saif to be?
PARALLEL
What is normal image formation known as (normal eye) ?
Emmetropia
In a normal eye, what does the lens do when far light (>6m ) is entering eye?
LENS FLATTENS
What does the lens do if there is a close light source?
LENS IS ROUNDER (due to ciliary muscles relaxing) so it has a high refractive power (bending the light)
What is myopia?
When the eyeball is too long/lens too strong.
- Light is focused further inside orbit and doesn’t reach the back of the eye
How is myopia corrected?
With a CONCAVE LENS
- diverges light rays before they enter the eye
How is hyperopia corrected?
With a convex lens, converges light rays before they hit the eye
What is presbyopia?
Far sightedness due to loss of elasticity in the lens (common with aging)
- inability to accomodate in the long term
- reading glasses or multifocal as treatment
What is the info flow once light reaches the retina?
Photoreceptors–> bipolar cells–> ganglion cells
Which direction does light go?
The opposite way to info flow
What is the pathway of light and info processing?
Light ENTERS retina–> passes through all ganglion and bipolar cells then reaches the photoreceptors__> then once here the photoreceptors (rods and cones) transduce light and info flows BACK form photoreceptors–> bipolar cells–> ganglion cells–> until reaching the optic nerve
What is astigmatism?
Blurry image-rotational assymetries in area
What are rods?
HIGH LIGHT SENSITIVITY
- So active in LOW LIGHT LEVELS
- have more pigment
What is scotopic vision?
In the nighttime light where ONLY THE RODS ARE ACTIVE (not cones)
What are cones?
Photoreceptors active in HIGH LIGHT LEVELS - Only snesitive to DIRECT LIGHT ( because of their shape)
How many types of cones are there?
3 types
What is photopic vision?
Daylight where only CONES ARE ACTIVATED
What is mesopic vision?
twilight vision (Rods and cones active)
What are disks in rods and cones filled with?
Opsin which is a protein that absorbs light
In the darkness, what occurs when the rod mempot is -30mV?
It is HIGHLY DEPOLARISED due to Na+ channels being kept open by cGMP which is continually produced in the PHOTORECEPTOR (this is very metabolically expensive)
What occurs when light reaches the eye in terms of pathways in the ROD cells?
Light activates a cascade–> light sensitive protein (photopigment rhodopsin in rods) undergoes conformational change–> this activates a G protein –> this then activates phosphodiesterase enzyme (reduces the cGMP levels)—> Results in CLOSING OF NA+CHANNELS AND HYPOERPOLARISES CELL
What determines which colours rods or cones respond to?
Light waves absorbed by opsin/rhodopsin
What does the rate at which glutamate (neurotransmitter) is released depen upon?
- mempot
- More POSITIVE mempot= HIGHER RATE of neurotransmitter release (means Na+ channels will be open)
What does light lead to?
Mempot becoming more negative (less glutamate release, less cGMP) so Na+ channels close
What are blue cones best activated by?
Short wavelength (opsin proteins in blue cones will most easily absorb short wavelengths (BUT in general, RODS have a higher sensitivity -so more likely to absorb photons)
What is colour perception based on?
RELATIVE ACTIVATION OF the 3 cone classes
Are dogs colour blind?
no! Just less sensitive to colours as they have 2 instead of 3 cones
What wavelengths do green cones absorb?
Medium wavelength
What do red cones absorb?
Long wavelength
What determines the size of the ganglion cells receptive field?
Spacial extent of horizontal cells axons and the ganglion cells dendrites
What is spacial aucity?
The smallest feature you can recognise/how well you can resolve 1 form 2 pts of light
What does the snellen chart test?
The aucity in the fovea
What does 20/40 (6/12) vision mean?
You are short sighted
- can only read down to the line that poeple who are 12m away can read
What is the central retina dominated by?
Cones about 0.5 microm in diameter
What is the peripheral retina dominated by?
Rods
(4 microm) - much larger as only needed for gross detail; not finer detail
What does spatial aucity depend on?
- Receptor density and convergence ratio( desity of cones and convergence of cones to ganglion cells-outputs)
- aucity is reduced if you have blurry vision
Where do ganlgion cells exit eyeball?
Blindspot
What is the receptive field?
Region of visual space in which light affects a cells mempot
What are OFF bipolar cells?
They are HYPERPOLARISED by light in RF centre and DEPOLARISED by light in RF surround
- Characterised by ionotropic glutamate gateed cation channels (excitatory)
What are ON bipolar cells?
They are DEPOLARISED by light in the RF centre and HYPERPOLARISED BY light in RF surround.
- invert the sign of photoreceptors
- Characterised by metabotropic G prtein coupled receptros
What does a inverted photorecpetro signal mean (in ON bipolar cells)
Increased i lumination= less glutamate so the cell is more depolarised-graded
What does having a surround strucucture in ON OFF bipolar cells help with?
Specifically locating where the stimulus is
What do on/off centre bipolar cells target?
Target the corresponding ON OFF ganglion cells
What happens when spinning your head to the left?
You get INCREASED snesory inputs from the LEFT HORIZONTAL CANAL.
- Signal sent to medial vestibular nucleus
- Here it can either be inhibitroy projection (ipsilateral side- inhibitory allows opposite motor commands from eyes) or a excitatory projection (contralateral side)
What are the two main functional visual streams?
Parietal and Temporal
What does the parietal visual stream involve?
The ‘where are things going’ - so action
What does the temporal visual stream involve?
The “what” - so perception- Identification of objects
What does the MT area process?
MT-motion
What does the V 4 cortical area process?
Shape and colour (eg. some neurons could respond more to concave/convex curvature)
What does the TE cortical area process?
TE- object recognition (faces and images of hands)
How do V4 and MT act?
In PARALLEL with MT extracting motion information and V4 extracting shape and colour
How do photoreceptors work?
In parallel
What is serial processing?
Each information is processed one at a time. For example the ventral (temporal stream):
V1 extracts orientation info
V4 extracts simple shapes
TE contains neurons that encode objects and faces
Is there divergence from retinal cells to ganglion cells?
Yes , ganglion cells have overlapping receptive fields
What is the receptive field of one neuron?
Region of visual space in which flash of light will change mempot
How many parallel circuits are there for ganglion cells?
More than 12 with unique classes of ganglion cells
- All have different function and if you organise them based on function, RFs do not overlap
What are parasol cells and which pathway are they involved in?
Also known as M type cells (type of ganglion cell)
- Part of the parietal pathway
- Have LARGE CELL bodies, dendritic arbours, receptive fields
- Sensitive to rapidly chanfging stimuli
- NOT COLOUR SENSITIVE (only care how BRIGHT it is (luminance contrast- also receive more inputs from rod cells) lower contrasts, larger region of space that they collect from )
(Layers 1,2 of LGN)
What are midget cells and what pathway are they involved in?
A type of ganglion cell (Parvocellular–> Layers 3-6 upper)
- AKA midget cells
- Part of the temporal (ventral) pathway-distinguishes colours
- Have SMALL cell bodies dneritic arbours RFs
- sensitive to FINE stimulus features (don’t integrate scross as many photoreceptors)
Why are parasol cells also known as M cells?
Because they project into the Mgnocellular layers of the Lateral Geniculate Nucleus
What types of inputs do parasol cells recieve?
Excitatory synapses from diffuse bipolar cells
Where to Midget cells project to?
The Parvivocellar LGN layers (parvo=object recognition)
- Also known as P cells
Where are midget cells primarily found?
In the fovea
Where does information go from the retina?
Can go to the Pretectum (reflex control of pupil and lens)
OR can go to the Lateral Geniculate Nucleus
What is the LGN for?
Gateway to cortex and CONSCIOUS vision
- 90% of retinal projections are to LGN and then projections are sent to the cortex)
- It is for PARALLEL processing
- contains 6 layers (6 parallel representations of the contralateral visual field)
What is the pretectum for?
Reflexive eye movements and pupil size
What is the superior colliculous for?
Controls eye and head orientating responses
What is the suprachiasmatic nucleus?
Circadian rhythms
What is binocular vision?
Where you can see both visual fields (stereoscopic vision)
- you make judgements about depth
What is decussation and what does it allow us to do?
Axons from NASAL retina cross overand axons from TEMPORL retian DO NOT CROSS OVER
-Allows both eyes to see a part of visual field from other eye (evolved to be like that)
What does the left visual cortex represent?
The RIGHT visual field
What does the right visual cortex represent?
The LEFT visual field
What happens when you transect an optic nerve/
Visual field is the equivalent of closing one eye
What happens when there is a transection of the optic tract, or the LGN or V1?
- You only see things in one hemifield
- but both eyes are still functional
What happens when the there is a transection of the optic chiasm?
Both eyes will still function but you lose stereoscopic vision (it will be monocular)
- Only crossing fibres affected
- central left visual field only being processed by the right eye
What is a bitemporal hemianopia?
Half vision in each eye due to a pituitary tumor
What are layers 1 &2 of the LGN for?
Magnocellular layers - (poor spacial aucity, low contrast, monochromatic, high temporal aucity )
- Inputs from parasol retinal ganglon cells (motion)
What are layers 3-6 of LGN?
- Parvocellualr layers
- Inputs from midget retinal ganlion cells
- Colour, fine spatial aucity
Where do layers 1,4 and 6 recieve inputs from?
Contralateral eye
Where do layers 2,3,5 receive inputs from?
Ipsilareral eye (temporal retina)
What are some other names for the primary visual cortex?
V1, Brodmanns area 17, or striate cortex
What K cells target?
Layers II and III
What is a LGN ON cell similar to?
A retinal cell
Simple and complex cells..
RESPOND BEST TO ELONGATED BARS/EDGES
What are simple cells?
- Have spatially segregated on/off subregions
- Position of bar within RF is important
- Are often monocular, respond to inputs coming from only one eye(projections from retina–>LGN–> cortex) only coming from one eye
- moving positions of the bar slightly changes response
What are complex cells?
- Almost all are binocular
- spatially HOMOGENOUS receptive fields ( no segregation of on/off subregions)
- POSITION OF LIGHT BAR WITHIN RF NOT IMPORTANT
What do adjacent neurons in V1 respond to ..
Stimuli on adjacent regions of the visual field
What is the concetration of rods like as you move further from the fovea (eccentrictiy in degrees)?
Concentration of rods is progressively less!
What do the superior colliculi do?
Play a role in generating orienting head and eye movements to sudden visual (and other sensory) stimuli
- superficial layers soley process visual information
