Week 5 Flashcards
Visual Pathway Overview
- Image formation- eye
- Transduction- eye, retina
- Visual processing- retina, thalamus, primary visual cortex (occipital lobe), extrastriate cortex (occipital lobe), extended cortex (temporal and parietal).
retina- superior colliculs 10%
Decussation (remember spilt
brain patients)
• Partial decussation • Left visual field to right cortex • Right visual field to left cortex • 50% of optic nerve fibres cross at the optic chiasm • Optic nerves – bilateral visual fields • Optic tracts – unilateral visual fields
Retinotopic
• Adjacent points in the visual field map onto adjacent points on the retina • This mapping is maintained through the processing hierarchy
Cortical Magnification
• More cortex dedicated to
processing the central visual
field than the periphery -
convergence
Receptive Fields
• Particular neurons respond depending on how the retina is stimulated • RFs refer to regions on the retina and the features that excite or inhibit the cell • The nature of the RF of a cell gives clues about the cell’s function • RFs may be small (high spatial resolution) or large (low spatial resolution • RFs typically have both excitatory and inhibitory regions
The Eye
- Form an image
- Generate a neural signal (transduction)
- Early neural processing of the signal
- Transmit the visual signal to brain
The Eye – Form an Image
Cornea • Transparent outer layer • Most light bending (refraction) occurs here Lens • Fine tunes image formation • Adjustable • Accommodation reflex • Stiffens with age Iris and Pupil • Size of the opening (pupil) regulated by contractile tissue (iris) • Varies light, but more importantly focal length • Reflex
The Eye – Transduction/Processing
Retina • Receptors to transduce light signal to neural signal • Layers of neurons for early processing of the signal • Retinal ganglion cells (RGCs) final layer - axons to the brain Fovea • Small specialised high acuity central vision • Solves the “backward wiring” problem
The Eye – Transmit to Brain
Optic disc • Point on the retina where RGC axons leave to become the optic nerve • Blind spot – no receptors Optic nerve • Neural transmission to thalamus • Partial decussation at the optic chiasm • Optic tract beyond the optic chiasm
The Eye – Blind Spot
• Each eye has a blind spot but there is no black hole in vision • VISION IS CONSTRUCTED!! • Completion • Receptors around the blind spot provide information to fill in the gaps • Edges are continued • Surfaces are interpolated • Best guess at what is in the blind spot based on what is around it
The Retina
• 5 layers of different types of neurons (many
subtypes)
1. Receptors
2. Horizontal cells
3. Bipolar cells
4. Amacrine cells
5. Retinal ganglion cells
• Light -> receptors -> bipolar -> RGCs -> brain
• Horizontal and amacrine cells – lateral
communication
The retina- transduction
cone receptors and rod receptors
The retina- early processing
amacrine cells, bipolar cells and horizontal cells
The retina- transmission to the brain
retina ganglion cells
Transduction - Receptors
Cones • Lower sensitivity • High positional acuity due to low convergence • 3 types – short (S), medium (M), and long (L) wavelength • Photopic vision (well lit) • Colour perception • 6-7 million per retina Rods • High sensitivity • Low positional acuity due to high convergence • Scotopic vision (low light) • 120 million per retina
Fovea
Solution to backward retina
• Clearance of RGCs
• Very high acuity - cones
Acuity
sharpness of vision
Early Processing
• Retina is more than a sensory organ
• Retina is brain – processing centre
• Convergence is simple early processing – reduce
axons to brain
• 130 million photoreceptors per retina and only about 1
million axons in each optic nerve
• More low level processing – detection of simple
important features (fast)
• Edge detection
• Motion detection (directional selectivity)
Lateral Inhibition
Mach Bands • Edges are important • Contrast enhancement for edge detection • Perception of edges better than actual light difference Mach Bands • Horseshoe crab • Firing rate proportional to intensity of light • Each receptor inhibits its neighbours • Inhibition greater with more intensity • Greater inhibition for closest neighbours
Transmission to Brain
• RGC axons form the optic nerve • CNS not PNS • ODCs not Schwann cells • Meninges • First synapse at thalamus • Lateral geniculate nucleus • 10% to other areas (esp SC)
Optic Chiasm
• 50% decussation in humans but in prey animals – more lateral eyes, more complete decussation (less binocular vision) • 75% in rodents, 85% in horses • Birds almost complete decussation, but owls have good stereopsis Albinism • Disruption of melanin synthesis • Abnormal projection to thalamus • Stimulate eye and get larger and faster response in contralateral hemisphere
Receptive Fields
Retinal Ganglion Cells • Centre-surround RFs • ‘ON’ cells and ‘OFF’ cells • ‘ON’ or ‘OFF’ refers to the centre of the RF – whether the cell fires to light on dark in the centre • Small image elements • Contrast rather than simple light detection Retinal Ganglion Cells • Multiple receptor inputs to the RGC • Inputs spread over space – small at fovea, large at periphery • Early processing determines excitatory versus inhibitory effects
Visual Thalamus
LGN • 6 layers • Separation of visual streams • Left and right eyes • P channel and M channel • Same centresurround RFs as RGCs • Other inputs to LGN
Primary Visual Cortex – V1
• Retino-geniculate-striate pathway
• Axons from LGN project to lower layer 4
• Lots of processing before reaching the cortex
• First neurons centre-surround RFs as per RGCs and LGN
cells
• Key function of V1 – identify object boundaries
• Need to start integrating basic contrast (and motion)
information
• First – line segments and spatial scale
• Most V1 cells are either ‘simple’ or ‘complex’