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’
Simple Cortical Cells
• Centre-surround cells in layer 4 project to simple cells in layer 3 • Simple cells are about detecting line segments • Simple cells (and LGN and RGCs) are monocular Preference 1. Type of edge • Bars of light in a dark field • Dark bars in a light field • Single straight edges between dark and light 2. Orientation 3. Location (retinotopic) Best response – an appropriate bar leaving an OFF region and entering an ON region Contour integration • Contours form the outlines of objects - first step in shape perception • Gestalt principle of ‘good continuation’ • Elements that are close together, with small changes, local direction close to global direction - salience
Contour Integration
Lateral Facilitation • Li & Gilbert (2002) • Lateral connections between directionally similar and retinotopically 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 High – edge info
Low frequencies
Low frequencies filtered out
EDGES
Low SF activates cells with wide subfields
High frequencies
High frequencies filtered out
TEXTURE
High SF activates cells with narrow subfields
Complex Cortical Cells
• Multiple overlapping simple cells project to complex cells • Larger RFs than simple cells • No distinct ON/OFF regions • Respond if any simple cell inputs respond • Responds to straight edge stimulus anywhere in RF • Respond continuously as a line or edge traverses the RF perpendicular to the orientation
Complex Cells and Depth Perception
• Many complex cells are binocular - they receive inputs
from both eyes
• The cell will increase firing if inputs arrive from either
the left or right eye
• More vigorous response if inputs arrive from both eyes
simultaneously
• Some cells favour one eye over the other and respond
more vigorously to one eye - ocular dominance
• Some cells respond if similar contours fall on nearly
the same positions in the two eyes - binocular
disparity
• Complex cells underlie stereoscopic depth perception
Columnar Organisation of V1
Functionally similar cells located in columns:
• RFs in same general area of visual field
• Same orientation preference
• Same eye (monocular neurons) or same eye
dominance (binocular neurons)
Across columns:
• Dominance alternates with columns
• Orientation slowly rotates with columns
• RF location slowly shifts with columns
Cross section through
primary visual cortex
Damage to V1
Scotoma • Damage to V1 can produce an area of blindness in contralateral field of both eyes • Often no conscious awareness of even extensive scotoma due to completion (recall blind spot) • Perimetry test to determine Blindsight • See but without any conscious awareness • Respond to visual stimuli in scotoma • Especially motion – throw something at them • Better than chance identification • Better than chance reaching • Maybe some intact V1 mediating some visual abilities without conscious awareness • Subcortical visual structures project up to secondary visual cortex (V2)
Extrastriate Cortex
• Visual areas beyond V1 in the occipital lobe
• Not sequential processing – extensive
interconnections – convergence, divergence and
reciprocal
• Each area is retinotopic and respond preferentially to
differing aspects of the visual stimulus
• Colour, movement, shape
• Not a hierarchy
• Distributed processing – many maps of visual space,
each representing different types of information
• Zeki et al. (1993) PET study
• Static versus moving squares
– bilateral activation near
TPO junction – V5
• Greyscale versus colour
rectangles – bilateral
activation anterior to V1/V2
on lateral cortex – V4
Dorsal and Ventral Streams
2 visual pathways through extrastriate cortex and into extended cortical areas Posterior parietal cortex Inferior temporal cortex
Dorsal Stream
• A.T. – occipitoparietal lesion interrupting dorsal stream • Recognises objects and can demonstrate size using fingers • Hand shape during object directed movement incorrect • Unimpaired for familiar objects where size is a fixed property (e.g. lipstick)
Ventral Stream
• D.F. – bilateral damage to ventral V2 interrupting ventral stream • Can’t describe size, shape or orientation of objects (but can if put in hand) • 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) 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
What does the dorsal stream do?
• 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 – could infer motion
from changed position
• CT – large bilateral lesions on posterior middle temporal cortex – V5
• Nefazodone (for depression) - reports of an effect on motion
perception
• Moving objects followed by a trail of freeze frame images which
disappeared when motion ceased; stationary objects looked
normal; normal vision returned with reduced dosage
• Suggests a selective impairment of motion processing
Akinetopsia – Motion Blindness
• 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
• Blocking MT activity with TMS produces motion blindness
• Electrical stimulation of MT induces the visual perception
of motion.
What does the ventral stream do?
• 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)
• More to come in tutorials
