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
