Week 5 Flashcards
What are the three sections of the Visual Pathway?
- Image formation 2. Transduction 3. Visual processing
What is the visual pathway?
Eye, retina, thalamus, primary visual cortex (occipital lobe), extrastriate cortex (occipital lobe), extended cortex (temporal and parietal lobes)
What are the key concepts of the visual pathway?
Decussation, retinotopic organisation, cortical magnification and receptive fields
What is Decussation?
Left visual field to right cortex, right visual field to left cortex, 50% of optic nerves cross at the optic chiasm
Optic nerves
Bilateral visual fields
Optic traits
Unilateral visual fields
Retinotopic
Adjacent points in the visual field map into adjacent points on the retina and mapping is maintained through processing
Cortical magnification
More cortex dedicated to processing the central visual field than the periphery-converge
Receptive fields
Particular neutrons respond depending on how the retina is stimulated and refer to regions on the retina which stimulate or inhibit the cells
Characteristics of Receptive Fields (RF)
Gives cells clues about cell’s function and can be small (high spatial resolution) or large (low Sofia’s resolution. RFs typically have both exciting and inhibitory regions.
The function of the eye
Form an image, generate a neural signal, early neural processing of signal and transmit the visual signal to the brain.
Forming the image
Cornea, lens, iris and pupil
Transduction/ processing
Retina, fovea
Retina
Receptors which transducer the light signal to neural signal.
Early processing of signal.
Retinal Ganglion Cells (RGC) final layer - axons to the brain
Retina is brain - processing centre
Fovea
Small specialised high acuity central vision.
Solves the “backward wiring problem.
Transmit to brain
Optic disc, optic nerve
Blind spot
In each eye
Vision is constructed
Edges are continued
Surfaces are interpolated
5 different types of neutrons in the retina
- Receptors
- Horizontal cells
- Bipolar cells
- Amacrine cells
- Retinal ganglion cells
The process of the retina
Light -> receptors-> bipolar -> RGCs -> brain
Horizontal and amacrine cells
Lateral communication
Cone and rod receptors
Transduction
Amacrine, bipolar and horizontal cells
Early processing
Retinal ganglion cells
Transmission to the brain
Cones
Lower sensitivity
High positional acuity
Photopic vision (well lit) colour perception
Types of cones
Short, medium and long wavelength
Rods
High sensitivity
Low positional acuity - high convergence
Scotopic vision (low light)
Fovea
Solution to backward retina
Clearance of RCGs
Very high acuity-cones
Lateral inhibition
Match bands - contrast enhancement for edge detection
What happens in lateral inhibition
2 receptor responses
Firing rate is proportionate to intensity of the light
Each receptor inhibits it’s neighbour
Inhibition greater with intensity
Inhibition greater with closest neighbours
CNS
Central nervous system
PNS
Peripheral Nervous System
RGC
Retinal Ganglion Cells
SC
Spinal Cord
Transmission to the brain
RGC to the optic nerve CNS not PNS ODCs not Schwann cell’s Meninges First synapses at thalamus Lateral geniculate 10% to other areas
Optic Chiasm
50% decussation for humans but in prey animals more lateral (less binocular)
75% in rodents and 85% in horses
Birds almost complete decussation, but owls have good stereopis
Albinism
Disruption in melanin synthesis
Abnormal projection to thalamus
Stimulate eye and get larger and faster response in contralateral hemisphere
Receptive fields-RGC
Centre-surround RFs
“On” and “Off” cell’s where it refers to the centre of the cells
Small image elements
Contrast rather than simple light detection
Multiple inputs to the RGC
Inputs spread over space- small at fovea, large at periphery
Early processing determines excitatory vs inhibition effects
Visual system- visual thalamus (LGN)
6 layers Separation of the visual streams Lefts and right eyes P and M channels Same centre-surround RFs as RGCs Other inputs to LGN
Primary Visual Cortex-V1
Retina-geniculate-striate pathway
Axons from LGN project to lower layer 4
First neurons centre-surround RFs as per RGCs and LGN cells
V1- identify object boundaries
Most V1 cells are either “simple” or “complex”
Simple cortical cells
Centre-surround cells in layer 4 project to simple cells in layer 3
Simple cells detect line segments
Simple cells (LGN and RGCs) are monocular
Preference of simple cortical cells
- Type of edge- bars of light in dark field, dark bars in light field, straight edges between dark and light
- Orientation
- Location (retinotopic)
Best response is an appropriate bar leaving an OFF region and entering an ON region
Contour integration- Simple cortical cells
Contours of the outlines of objects- first step of shape perception
Gestalt principle of “good continuation”
Elements which are close together, with small changes, local direction close to global direction- salience
Contour integration
Lateral facilitation- Lil & Gilbert (2002)
Lateral connections between directionally similar and retinotopic ally adjacent simple cells
Simple cells and spatial scale- spatial frequency
Contrast changes in any image are a mix of different spatial frequencies
Low - texture info but low frequencies filtered out = edges
High- edge info but high frequencies filtered oh = texture
Simple cells and spatial scale- spatial scale (SF)
Low SF activates cells with wide subfields
High SF activates cells with narrow subfields
Complex cortical cells
Multiple overlapping simple cells project over to complex cells
No distinct on/off regions
Respond if any simple cell inputs region
Responds to straight edge stimulus anywhere in RF
Respond continuously as a line or edge transverses the RF perpendicular to the orientation
Complex cells and depth perception
Complex cells are binocular
Cells will increase firing if inputs arrive from either eye but more vigorous from both simultaneously
Ocular dominance
Binocular disparity
Complex cells underlie stereoscopic depth perception
Ocular dominance
Some cells favour one over the other and respond more vigorously to one eye
Binocular disparity
Some cells respond if similar contours fall on nearly the same positions in the two eyes
Columned organisation of V1
Functionally similar cell located in columns RFs in same general area of visual field Same orientation preference Same eye(monocular neurons) or same eye dominance (binocular neirons) Across columns Dominance alters with columns Orientation slowly rotates with columns RF location slowly shifts columns
Damage to V1- scotoma
Scotoma
Can produce an area of blindness in contralateral field
No conscious awareness of even extensive scotoma due to completion (recall blind spot)
Perimetry year to determine
Damage to V1- blindsight
See but no conscious awareness
Respond to visual stimuli in scotoma
Especially motion
Better than chance identification and reaching
Maybe some intact V1 mediating some visual abilities
Subcortical visual structures project up to secondary visual cortex (V2)
Extrastriate Cortex
Visual areas beyond V1 in the occipital lobe
Not sequential processing
Each area is retinotopic and respond preferentially to differing aspects of visual stimulus
Colour, movement, shape
Not a hierarchy
Extrastriate cortex - zeki study
PET study using subtraction logic
Static vs moving squares- bilateral activation near TPO junction V5
Greyscale vs colour rectangles- bilateral activation anterior to V1/V2 on lateral cortex- V4
Dorsal and central streams
2 visual pathways through extrastriate cortex and into extended cortical areas - posterior parietal and inferior temporal cortex (dorsal stream, primary visual cortex and central stream)
Dorsal stream
AT- occipitoparietalblesion interrupring dorsal stream
Recognised objects and can demonstrate size using fingers
Hand shape during object directed movement incorrect
Unimpaired for familiar objects where size is fixed
Ventral stream
DF- bilateral damage to ventral V2 interrupting ventral stream Can’t describe size, shape and orientation of objects- can if put in hands Incorrect size estimate using fingers Can reach out and grasp objects with grip accurately scaling with object size
Extended Cortical Processing
2 visual pathways through extrastriate cortex and into
extended cortical areas (lots of interconnection)
Dosal and ventral stream
dorsal stream
• Respond to spatial stimuli • Object location or direction of motion • Superior longitudinal fasciculus • Large RFs, mostly (60%) outside fovea
ventral stream
• Respond to characteristics of objects • Colour and shape • Inferior longitudinal fasciculus • Large RFs, all include fovea
Dorsal and Ventral Theories
What vs Where (Ungerleider & Mishkin, 1982)
• Dorsal specialises in visual spatial perception
• Ventral specialises in visual pattern recognition
• Difference in kind of information
Action vs Perception (Goodale & Milner, 1992)
• Dorsal specialises in visually guided behaviour
• Ventral specialises in conscious visual perception
• Difference in how the information is used - functional
Dorsal Stream
Key job of vision is to enable interaction with the
environment
• Parietal cortex central to spatial attention
• Parietal also central to selective attention – enhanced
processing at some locations to select objects for
further examination
• Highly connected to posterior frontal cortex – motor
areas
• Drives interaction with environment
• Drives fixations – saccades – explore environment
Dorsal Stream Dysfunction
Akinetopsia – Motion Blindness
• 1983 – Max Planck Institute – female patient with loss of motion
perception
• Perception like a series of snapshots
• Colour and form perception intact but ability to judge direction and
speed of moving objects severely impaired
CT – large bilateral lesions on posterior middle temporal cortex – V5
Nefazodone (for depression) - reports of an effect on motion
perception
MT/V5 is thought to be responsible for motion perception
• It has large receptive fields
• 95% of its neurons respond to specific directions of
motion.
• Patients with akinetopsia tend to have damage to MT in
one or both hemispheres.
• fMRI studies show enhanced activity in MT when humans
view movement
Ventral Stream
Visual experience is object centred
• Visual primitive (contours, surfaces, fields of motion)
need to be assembled into objects
• Also need to attach semantic significance to objects –
recognise what they are, what they are for, etc
• Ventral stream – inferior temporal cortex has 2
functional subdivisions – 2 stages of object
recognition
• Posterior – integration of visual features into objects
• Anterior – association of object with knowledge of
object
Ventral Steam Dysfunction
2 basic types of visual agnosia – apperceptive and associative –
depending on where the ventral stream is disrupted.
Show patients an object and ask them to draw it and name it.
Apperceptive Agnosia
• Loss of visual perception
• Impaired drawing; unimpaired naming
Associative Agnosia
• Loss of visual meaning
• Unimpaired drawing; impaired naming
Prosopagnosia
• Category specific agnosia: Face blindness
• Can recognise an object as a face but impaired at
recognising which face
• May even fail to recognise a photo of themselves
• Damage to right inferior temporal lobe (Fusiform
Face Area: FFA)
Key Learnings
Vision is constructed
• Pathway overview -mostly retina-geniculate-striate
• Decussation; retinotopic; magnification; receptive
fields
• Eye: image formation; transduction; processing;
transmission
• Retina: receptors; RGCs; blind spot; fovea; lateral
inhibition
V1: cells based on RFs: centre-surround; simple;
complex
• V1: Contour integration and lateral facilitation;
spatial scale
• V1: columnar organisation
• V1: scotomas and blindsight
• Extrastriate: distributed processing of visual features
• Dorsal and ventral streams
