Week 5 Flashcards

1
Q

Visual Pathway Overview

A
  1. Image formation- eye
  2. Transduction- eye, retina
  3. Visual processing- retina, thalamus, primary visual cortex (occipital lobe), extrastriate cortex (occipital lobe), extended cortex (temporal and parietal).
    retina- superior colliculs 10%
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2
Q

Decussation (remember spilt

brain patients)

A
• 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
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3
Q

Retinotopic

A
• Adjacent points in the visual
field map onto adjacent points
on the retina
• This mapping is maintained
through the processing
hierarchy
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4
Q

Cortical Magnification

A

• More cortex dedicated to
processing the central visual
field than the periphery -
convergence

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5
Q

Receptive Fields

A
• 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
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6
Q

The Eye

A
  • Form an image
  • Generate a neural signal (transduction)
  • Early neural processing of the signal
  • Transmit the visual signal to brain
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7
Q

The Eye – Form an Image

A
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
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8
Q

The Eye – Transduction/Processing

A
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
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9
Q

The Eye – Transmit to Brain

A
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
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10
Q

The Eye – Blind Spot

A
• 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
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11
Q

The Retina

A

• 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

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12
Q

The retina- transduction

A

cone receptors and rod receptors

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13
Q

The retina- early processing

A

amacrine cells, bipolar cells and horizontal cells

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14
Q

The retina- transmission to the brain

A

retina ganglion cells

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15
Q

Transduction - Receptors

A
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
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16
Q

Fovea

A

Solution to backward retina
• Clearance of RGCs
• Very high acuity - cones

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17
Q

Acuity

A

sharpness of vision

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18
Q

Early Processing

A

• 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)

19
Q

Lateral Inhibition

A
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
20
Q

Transmission to Brain

A
• 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)
21
Q

Optic Chiasm

A
• 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
22
Q

Receptive Fields

A
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
23
Q

Visual Thalamus

A
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
24
Q

Primary Visual Cortex – V1

A

• 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’

25
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 ```
26
Contour Integration
``` Lateral Facilitation • Li & Gilbert (2002) • Lateral connections between directionally similar and retinotopically adjacent simple cells ```
27
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 ```
28
Low frequencies
Low frequencies filtered out EDGES Low SF activates cells with wide subfields
29
High frequencies
High frequencies filtered out TEXTURE High SF activates cells with narrow subfields
30
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 ```
31
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
32
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
33
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) ```
34
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
35
Dorsal and Ventral Streams
``` 2 visual pathways through extrastriate cortex and into extended cortical areas Posterior parietal cortex Inferior temporal cortex ```
36
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) ```
37
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 ```
38
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 ```
39
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
40
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
41
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.
42
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
43
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