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PSCY2020 2019 > Week 5 > Flashcards

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
Q

Simple Cortical Cells

A 
• 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
Q

Contour Integration

A 
Lateral Facilitation
• Li & Gilbert (2002)
• Lateral connections between
directionally similar and
retinotopically adjacent
simple cells
27
Q

Simple Cells and Spatial Scale

A 
Spatial Frequency
Contrast changes
in any image are a
mix of different
spatial
frequencies
Low – texture info
High – edge info
28
Q

Low frequencies

A

Low frequencies filtered out
EDGES
Low SF activates cells with wide subfields

29
Q

High frequencies

A

High frequencies filtered out
TEXTURE
High SF activates cells with narrow subfields

30
Q

Complex Cortical Cells

A 
• 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
Q

Complex Cells and Depth Perception

A

• 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
Q

Columnar Organisation of V1

A

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
Q

Damage to V1

A 
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
Q

Extrastriate Cortex

A

• 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
Q

Dorsal and Ventral Streams

A 
2 visual pathways
through extrastriate
cortex and into
extended cortical
areas
Posterior parietal cortex
Inferior temporal cortex
36
Q

Dorsal Stream

A 
• 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
Q

Ventral Stream

A 
• 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
Q

Extended Cortical Processing

A 
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
Q

Dorsal and Ventral Theories

A

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
Q

What does the dorsal stream do?

A

• 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
Q

Dorsal Stream Dysfunction

A

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
Q

What does the ventral stream do?

A

• 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
Q

Ventral Steam Dysfunction

A

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

PSCY2020 2019 (18 decks)

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