The visual system Flashcards
The visual system is tuned to recognise what three things?
Food
Predators
Mates
How does the optic nerve respond to light?
It increases or decreases the spike rates
The right hemifield activates what?
The left brain
What is the main function of the retina?
Image acquisition
What is the function of the geniculate nucleus?
It preprocesses the visual information and projects to the primary visual cortex
What is the function of the superior colliculosis?
Responsible for focusing the eye eg turning the head to see better
Where does the main processing of visual information occur?
The visual cortex
What are the two main pathways in the cortex and what do they do?
Ventral - ‘What?’ eg object recognition occurs in the inferior temporal part of the brain
Dorsal - ‘Where?’ spatial location occurs in the posterior parietal part
What is the role of the pupil?
It regulates the amount of light to fall on the retina
What is the role of the lens?
To focus images on the fovea
What is the fovea?
The part of the retina with the highest visual activity
The rest of the retina has smaller acuity and contains primarily rods
What does light travel through in the retina?
Muller cells - these span the whole of the retina
What is the basic layout of the retina?
There are three layers of neurons and 2 layers of synapses
Feedforward neurons include; photoreceptors, bipolar cells and ganglian cells
Feedback neurons are the horizontal and amacrine cells
What is the role of the role of the feedback neurons?
Horizontal and amacrine cells inhibit and feedback to the bipolar cells, they are GABA-ergic
When are rods active?
At dim light (cones are active in bright light)
Where does phototransduction occur in rod and cone cells?
The outer segment
How do photoreceptors respond to light flashes?
They hyperpolarise eg the membrane potential decreases
Describe the cascade that occurs in photoreceptors
At the end of the cascade channels are selctive for calcium and sodium - when these are open the membrane depolarises
These channels are open in DARKNESS
They are activated by cyclic GMP in the cytoplasm
Ligand gated g protein coupled receptors activate phosphodiesterase which decreases cGMP so the channels close and cells hyperpolarise in the presence of light
What are the properties of the photoreceptors synapses?
They have special ribbon synapses which are able to keep vesicle release permanently
They do not spike- they use graded potentials
They release glutamate = hyperpolarisation
When light increases the amount of glutamate released from the photoreceptors decreases
Where do bipolar and horizontal cells receive input from photoreceptors?
The outer plexiform layer
Where do gangloin cells and amacrine cells receive input from bipolar cells?
The inner plexiform layer
When do ON cells depolarise?
When light increases
OFF cells express which receptor?
Ionotropic glutamate receptor (excitatory)
ON cells express which receptor?
Metabotropic glutamate receptor (inhibitory)
What happens in the receptive field?
An area in the retina which when illuminated activates a visual neuron
Many visual neurons have centre surround organisation of the receptive field, meaning the illumination of the center and the surround leads to responses in opposite polarities
What are the main methods to study vision?
Psychophysical methods and illusions Lesions/ other ways to silence neurons Anatomical studies and morphology fMRI, electrophysiological recordings and imaging Modeling and theoretical stimulations
What are the advantages and disadvantages of human models?
Good as the can give feedback
Bad as you need to use electrodes, unethical, long life span
Why are zebrafish brains good to use?
You can record the entire neurons of the whole brain
How can lesions occur?
They can be directed eg in animal models
They can occur after strokes or other neurodegenerate diseases
What is akinetopsia?
The inability to perceive motion
What is fMRI?
A way to study what areas of the brain are involved in visual processing
The parts being used are detected as they use a lot of glucose and oxygen due to their high activity
The subject looks at particular stimuli eg moving black dots to see what area of the brain responds
What are two ways of carrying out morphological studies?
Golgi staining for neurons - shows morphology of the ganglion cells
Fluorescent proteins labelled sparsely that express GFP - advantages is that it drives expression
What did Hubel and Weisel do?
They recorded from the visual area of the cat brain and gave it a visual stimulus like bars in different sizes and orientations
They used a sharp electrode to record spikes
What is GCaMP3 and how does it work?
Reports changes in neuronal calcium concentration, and therefore, neuronal activity
2 proteins CAM and MI3 fuse in the presence of calcium
If calcium is present in the cell they fuse and interact with GFP giving a ‘bright conformation’
Increase in the activity of neuron increases the brightness
What are the benefits of GCaMP3?
Many neurons can be recorded from
Simultaneously you can record an input and output
4 frames per second recorded
Particular neuron types can be recorded
What is channelrhodopsin?
Light activated channels causing sodium to move into the cell = depolarisation = spiking
Blue light = depolarisation
What is halorhodopsin?
Cause hyperpolarisation when activated by light (yellow light)
What is retinitis pigmentosa?
Most common cause of blindness
Currently there is no cure but progression can be slowed down
It causes major structural differences in the retina - photoreceptors are not present which causes the rest of the retina to degenerate and appear messy
Which parts of the brain can be stimulated to treat blindness?
Retina (in retinitis pigmentosa) Visual cortex (when the optic nerve is destroyed)
What are the problems with artificial retinal implants?
They stimulate retinal ganglion cells and not photoreceptors or bipolar cells
What is a receptive field?
An area in the retina (or space) which when illuminated activates a visual neuron
What is centre surround organisation?
Illumination of the centre and the surround leads to responses in opposite polarities
Where do bipolar cells receive their input from?
Photoreceptors
What is the effect of shining light on to the centre of an OFF bipolar cell’s receptive field?
The photoreceptors hyperpolarise which causes the bipolar cells to hyperpolarise
What is the effect of shining light on to the surrounding of an OFF bipolar cell’s receptive field?
Photoreceptors hyperpolarise causing the horizontal cells to also hyperpolarise
What do ganglion cell receptive fields have?
Center-surround organisation
What is the difference between bipolar and ganglion cell receptive fields?
The ganglion cells respond with spikes
How do OFF and ON center ganglion cells respond to light?
Off centre - respond with spikes if the surrounding receptive field is stimulated
On centre - respond with spikes if the centre is stimulated
What happens if the whole of the ganglion cell receptor field is stimulated?
Cells do not usually respond as they receive inhbitory input from amacrine cells and excitatory input from the bipolar cells
What are the two main classes of ganglion cells and what are their relevant percentages?
Parvocellular (80%) and magnocellular (10%)
What are the differences between p and m ganglion cells?
p - midget (small field of dendrites), Sustained response, slow conduction velocity, low gain and sensitivity, function is for form and colour
m - parasol (large dendritic field), transient response, fast conduction velocity, high gain and sensitivity, function is for motion detection
How many different types of bipolar cells are there?
15
The position of bipolar cell stratification in the inner plexiform layer affects what?
The bipolar cells function
Projection to the top of the IPL = OFF cells
Projection to the bottom of the IPL = ON cells
It also affects if they are transient or sustained
Top part of IPL = transient
Bottom part of IPL = sustained
Where do ON ganglion cells project to?
The bottom part of the inner plexiform layer
How many types of amacrine cells are there?
50
What is a flat mount retina?
Where a retina is taken and is cut so it is flat with the ganglion cells on top
How can the morphology of the ganglion cell dendritic trees differ?
Size, density, diffuse, asymetric
What are the key functions?
Contrast adaptation and other forms of adaptation Processing of colour Processing of motion Orientation reflex Object recognition
How is adaptation to temporal contrast measured?
Record from ganglion cells in flat mount retina
Patch clamp to measure spike rate
Increase in contrast causes an increase in the spiking rate which then decreases as a result of adaptation
What are the two types of adaptation?
Depression
Facilitation
What is depression?
Decrease in sensitivity
What is facilitation?
Increase in sensitivity
What should facilitation not be confused with and why?
Should not be confused with recovery
Facilitation = increase in sensitivity when stimulus increases
Recovery = increase in sensitivity when stimulus decreases
Why do we need colour vision?
Helps with object recognition
How many types of cones are there?
Blue, red and green
Describe the distribution of cones in the retina and why they are distributed like this
The majority of green top of the retina because the grass is green and the image is flipped by the lens
The majority of the bottom has blue cones because the sky is blue
Which type of ganglion cells have colour opponent properties
P cells
What are colour opponent ganglion cells?
Cells that are tuned to respond to colour contrast eg if there is just green there is a small response but red and green gives a big response
Damage to what part of the brain removes the ability to perceive motion?
The dorsal stream
Where is direction selectivity prominent?
The retina
How can the preferred direction of a direction selectivity cell be guessed?
From the morphology of the cells dendritic tree ie they are asymetric
Which cells inhibit ganglion cells?
Amacrine cells
What is the role of the fovea?
High visual acuity
What is the orienting reflex?
Turning of the head and eyes so the stimulus is projected on the fovea
What is smooth persuit?
Following a moving object
What is a saccadic movement? Give an example
Eye movements as a result of inspection so the features project onto the fovea
The edges of the face, eyes, nose and mouth are the key features for recognition
Ablation of what causes the loss of the orientation reflex in fish?
The optic tectum
What is the optic tectum called in a lower verterbrate?
The superior colliculus
Where does the superior colliculus receive its input from?
Ganglion cells, auditory system and somatosensory system
What is the main function of the superior colliculus?
Saccadic movements
What is a retinotopic map?
Organisation whereby neighboring cells in the retina feed information to the neighboring places in their target structures (LGN, SC, cortex)
What does the foveation hypothesis state?
That interaction between these maps initiates orienting reflex
Is the foveation map valid?
No because interaction between the maps seems to be indirect
How many layers are there in the LGN?
6
What are the contralateral layers of the LGN?
1, 4 and 6
What are the ipsilateral layers if the LGN?
2, 3 and 5
Which layers of the LGN contain p cells?
Layers 3 to 6
How many ganglion cells connect with a LGN projection neuron
One
What are the two visual pathways in the cortex?
The ventral pathway and the dorsal pathway
What is the role of the dorsal pathway?
Motion detection
What is the role of the ventral pathway?
Object recognition and colour
What are the components of the ventral pathway?
V1 - V2 - V3 - medial temporial cortex
What are the components of the dorsal pathway?
V1 - V2 - V4 - IT
What is object invariance?
Large numbers of objects can be recognised in any orientation even if we haven’t seen that orientation before
What is scale invariance?
Objects recognised independently of their size
What is the heirarchal model of object recognition?
Detection of edges
Detection of combination of edges and contours
Detection of object parts
Detection of objects from one point of view
View invariant object recognition
Categorisation
Where does the detection of edges occur?
V1 cortex
Where does the detection of object parts occur?
The inferior temporal cortex
What are the two structural features of the cortex?
Layering and columns
What are the types of columns in the cortex?
Ocular dominance, orientation columns and blobs
How can ocular dominance be seen in the visual cortex?
Injection of radioactive proline into one eye diffuses along the optic nerve, through the LGN and into the visual cortex - shows a stripe pattern
Or inject radioactive glucose and stimulate one eye with light
What is the role of ocular dominance?
Helps for 3d vision
What is the hypercolumn?
‘Blobs’
Columns which when labelled look like blobs
Contain neurons specifically tuned to process object and column information
What are the three types of cell in the V1 cortex?
Simple, complex and hypercomplex
When do simple cells spike?
When the bar is in a certain orientation in the centre of the visual field
What is the simple cells receptive field like?
It is elongated
How do complex cells differ from simple cells?
They respond to a bar of light in any orientation but anywhere in the receptive field
