Visual Cortex Flashcards
Where do Ganglion fibres leave the retina?
along the optic nerve – each ganglion cell has a nerve fibre that come together to form the optic nerve
Where does the Optic nerve leave the eye from?
The blind sport
What is the optic chiasm?
– cross over point – some of the left eye and right eye fibres cross over (not all of them)
Beyond the optic chiasm what does the optic nerve become?
The optic tract
How is information separated in the optic tract?
- Information now separated by visual field rather than by eye
- Left and right optic nerve carry information about left and right side of the world
- In optic tract information about right side of world crosses over to the left and vice versa:
- Information from right visual field represented by left hemisphere and vice versa
What is the Lateral Geniculate Nucleus (LGN)?
- The optic tract feeds into LGN
- LGN = bilateral structure (one in left hemisphere and one in right)
- Each LGN receives input from left and right eyes but keeps these inputs separate
What are the LGN receptive fields?
- LGN cells have the same receptive field organisation as retinal ganglion cells: centre-surround antagonism
- Ideal for detecting spots of light and edges
- But not able to detect orientation
What is the V1?
the primary visual cortex in the occipital lobe
Where does the V1 receive it’s input from?
the LGN
What is retinotopic mapping in V1?
Objects close together in the visual scene are analysed by neighbouring parts of V1
What is cortical magnification in the V1?
- Amount of cortex devoted to representing each part of the retinal field is distorted
- Fovea represented by large area of cortex – explains why we have such good acuity when an image falls on the fovea
- Periphery is represented by a much smaller area of cortex
How does single cell recording take place in V1?
- Animal presented with stimuli
- An electrode, inserted into a V1 neuron measures electrical activity
- Activity is that of a single neuron
What is the V1 cell response and how was this discovered?
- V1 cells have a level of baseline activity when no stimulus is presented
- In 1950s, Hubel and Wiesel could not find a stimulus to excite the V1 cell (tried presenting lots of spots of light on the retina but didn’t work)
- Until they found they got a big response when the edge of the glass slide moves across the receptive field: realised V1 cells will only respond to lines instead of spots
What are the three different types of cells in V1?
Simple cells
Complex cells
Hypercomplex cells
Give features of simple cell receptive fields
- Simple cells respond to oriented bars and edges
- The receptive field has excitatory and inhibitory regions, but they are elongated
- A vertical bar covers only the excitatory region causing a big excitatory response
- A bar tilted slightly away from vertical, covers some of the excitatory region but also some inhibitory region causing a weaker excitatory response
- A horizontal bar only covers a small part of the excitatory region but a larger part of the inhibitory region causing an inhibitory response
- We say simple cells have orientation selectivity – this differs from ganglion cells
- Orientation tuning
- Orientation tuned neurons respond best to their preferred orientation but also respond to other similar orientations – this is what created the orientation response curve
- Some simple cells have On-centre RFs and some have Off-centre RFs, but all have a preferred orientation
- edge detectors and bar detectors
What are edge detectors?
simple cells with only one excitatory and one inhibitory region
What are bar detectors?
Cells with 3 regions
How are simple cells different from ganglion cells?
They have orientation selectivity
Give features of Complex cell receptive fields
- Respond to oriented lines but no discrete On and OFF regions
- It will always give an excitatory response to a vertical bar of light no matter where it falls on the cell
- Respond to moving oriented bars and edges
- Respond best to a particular direction of movement
Give features of hypercomplex cell receptive fields
- Also called End-stopped cells
- Respond to bars of:
Particular orientation And
Moving in a particular direction AND
Particular length
How do the receptive fields increase in complexity as we move through visual processing?
- Ganglion cells respond as long as there are some kinds of light and dark
- Simple cells only respond if it is a line
- Complex cells only respond if it is a line moving in a particular orientation and direction
- Hypercomplex cells only respond to a line of a particular orientation, direction and length
How many visual areas are there beyond V1?
Over 30
What are areas V3, V4 and V5 specialised in?
- V3 - form of objects
- V4 - colour information in stimulus
- V5 - motion information in stimulus
Is there separation of function in the visual cortex?
No - all areas are interconnected
What are the two processing streams in the ventral cortex?
What (ventral) and Where (dorsal)
What is the Ventral (what) stream?
- Travels ventrally to inferotemporal cortex
- Important for recognising and discriminating objects
What is the Dorsal (where) stream?
- Travels dorsally to posterior parietal cortex
- Important for determining where an object is and how to act upon it
- Sometimes referred to as the ‘How’ stream
What is the monkey lesion study?
- Task 1: object discrimination. Food is always under the triangular prism
- Task 2: Landmark discrimination. Food is always close to the cylinder
- Lesion to inferotemporal cortex (what pathway) causes problems for object discrimination task but not landmark discrimination task
- Lesion to posterior parietal cortex (where pathway) causes problems for landmark discrimination task but not object discrimination task
What was Milner and Goodale’s experiment on patient DF with damage to his ventral pathway?
- Presented oriented slot to patient DF and asked patient to match the orientation of the block they were holding to the slot
- Patient DF couldn’t do this – controls could
- Milner and Goodale changed it so that DF needed to post the block through the hole – now he could do it
- Shows that there are two different processing mechanisms responsible for the two different paths – suggesting in the dorsal pathway the ‘where’ stream is still intact
How does Optic ataxia provide evidence to the two difference processing streams?
- Optic ataxia
- Damage to dorsal pathway (‘where/how’ stream):
- Cannot reach to grasp objects, but can recognise and describe them
- Opposite deficits to those shown patients with visual form agnosia
