Neural pathways of vision Flashcards
(51 cards)
What is its function?
Ganglion nerve fibres synapse here (in lateral geniculate nucleus) and go on in tract as 4th order neurons in optic radiation
What are the first, 2nd and 3rd order neurons in the optic pathway?
- st= photoreceptors
- nd bipolar cells
- rd= ganglionic cells
Explain the role and pathway of the ganglionic cells in the visual system
Come from retina, 53% cross at optic chiasm, continue further as optic tract to lateral geniculate nucleus
What is the receptive field of a photoreceptor?
It is the area of the retina where light infall causes fiering of the cell
What does convergence mean?
Convergence describes the number of low order neurons that synapse on to a high order neuron
Explain the convergence of cones and rods in the visual system
Cones have a low convergence (few photoreceptors are connected to higher order neuron) whereas rods have a higher convergence
Explain the convergence in the central part of the retina vs the peripheral part of the retina
In central part= low convergence (at macula almost a 1:1 matching of neurons) whereas there is a high convergence at peripheral retina
Explain the characteristics of low and high convergence within the visual system
Low convergence
- small receptive field
- fine vision
- low light sensitivity
High convergence
- larger receptive field
- coarse visual activity
- high light sensitivity
Explain the concept of on-centre and off centre ganglions
What is their function?
On centre
- when light falls on centre of ganglion–> ganlion fires
- when light falls off centre of ganglion–> inhibition of fiering
Off centre
- when light falls on centre of ganglion–> inhibiton of fiering
- when light falls off centre of gaglion–> ganglion fires
–> important in contrast vision
Which fibres cross at the optic chiasm?
Which part of the visual field to they percieve?
Crossing fibres= located nasally, responsible for temporal vision
Ipsilateral fibres=located temporaly, responsible for nasal vision
What does a lesion anterior to the optic chiasm lead to?
It leads to loss of one eye
Unilateral field loss
What does a lesion at the optic chiasm lead to?
Bitemporal hemianopia –> loss of temporal vision at both sides
What does a lesion posterior to the optic chiasm lead to?
What is the most common reason for it?
Homonymos hemianopia –> loss of vision on either right or left side on both eyes –> normally due to Stroke
When does a right nasal hemianopia occur
With a lesion in the right perichiasmal area
Where would a lesion be in a patient presenting with Quadrant anopia?
A lesion within the optic radiations would be the cause
Where would a lesion be in a patient presenting with a macula sparing visual field defect?
Why might the macula not be affected by visual field defects?
Normally in the visual cortex
–> Area representing the macula gets blood supply from different part of the brain: still supplied
Visual cortex organisation
Explain the location of the
- Macula
- Superior/inferior visual fields
- Right /left visual fields within the visual cortey
- Macula: has a disproportiaonal big representation lateral in the visual cortex
- Superior visual field information is located inferior to calcarine fissure
- Inferior visual field information is located superior to calcarine fissure
- Right visual fields are located in left visual cortex
- Left visual fields are located in left right cortex
Explain the organisation of information form the left and right eye in the primary visual cortex
Organized as columns with unique sensitivity to visual stimulus of a particular orientation
Right eye and left dominant columns intersperse each other
What is the extrastriate cortex?
What is its function?
- area around primary visual cortex
- converts primary information, orientation and position into complex information
