Transcript Psy393: Cognitive Neuroscience

Psy393: Cognitive
Neuroscience
Prof. Anderson
Department of Psychology
Week 3
The Eye:
Proof for the existence of
God?
And then there was light


Optics
Perception


Eye is receiver
not sender



Absorption
Plato
Euclid
Send & receive


Echolocation
Sonar
Perception is relative


Perception not of environment (realism) but of our
interpretation (relativism)
• Butterflies see
Electromagnetic spectrum
ultraviolet markings
• Snakes see infrared waves

The first stage of transformation
Retinal sensitivity to “visible light”
 400-700 nanometers (nm is 1/100 millionth of a meter)

Accomodation

Start with getting things in
focus on the retina


Do it all the time unconsciously
Accomodation


Changes is lens curvature
though muscles
Far-sightedness in the elderly
More convex
Retina: Rods & Cones

Not just morphological differences

2 types of vision

Photopic




Scotopic




Diurnal
Colour
Bright light
Nocturnal
Monochromatic
Dim light
Predator vs prey
Distribution of rods & cones

5 million cones per retina


1% are in the fovea, 99% in
periphery
120 million rods per retina


Fovea
None in fovea
Rods:cones

20:1 in periphery
Periphery
Retina: Its all backwards

Pigment
epithelium at
back of eye


Epithelial layer
Back
Nourishment of
photoreceptors
Light must pass
through neural
machinery
Front


Why don’t we see the
cells/blood vessels in our
eyes?
Images stabilized on
retina disappear
How’s that for
perception!
The retinal “black hole”

The blind spot

Ganglion cells—>optic nerve exit eye
From fovea
Filling in the blind spot

“filling-in” the blind spot?


Ramachandran example
Inference or perception
Transduction: Light to energy

Visual pigment molecules


Retinal







Light reactive chemical
Absorbs a single photon!
Isomerization: Morphing
Change in shape of ion channels
Change in membrane potential
Electricity!
Amplification:


Opsin & retinal
1 pigment molecule —> cascade of million
others
Perception of light

Can perceive a single rod activation
Photoreceptor
Visual pigments

Not all pigments are created equal


Rods vs Cones
Dark adaptation
Changes in sensitivity to light related to
difference in time to pigment regeneration
 Timecourse parallels
light sensitivity in
dark adaptation curve

Light
Time
Dar
Dark adaptation:
Switching visual systems

Dark adaptation curve


Switch from photopic to scotopic vision
Max adapt

Cones
Test fovea
 3-5 min


Rods
Rod
monochromat
 25-30 min

“Racoon” vision?
In living colour:
Spectral sensitivity

“Monochromatic” light


1 wavelength
Method of adjustment


Fovea (cones)
Periphery (rods)


Threshold
Cones
After dark adaptation
Sensitivity = 1/threshold
Overall cone
sensitivity
Need less photons
Need more photons
Spectral sensitivity curve
1 rod, 3 types of cones

3 cone pigments types

Short (S)


Btwn S & M (green-blue)
S
M L
Not color specific



558
1 rod pigment


531 nm
Long (L)


419 nm
Medium (M)


Absorption spectra
E.g., blue, green, red
Maximally responsive to these colours
Spectral sensitivity associated with absorption spectra


Weighted towards long wavelength cones
Most prominent
Convergence:
Acuity vs Sensitivity



Tradeoffs: Power vs grace
Less light needed for rod receptors
Also, differential convergence on to
neurons



Rod:ganglion cell, 120:1
Cone:ganglion cell, 6:1
Decreases threshold for ganglion response
Foveal and peripheral vision



Differential convergence
Why periphery is blurry relative to fovea?
Fovea: All cones
Most acute
 But least sensitive


What the use of sensitivity if you cant tell
what it is?

Foveation
Z C H S K E T D K F L F G LAD N X
Neural transformation



Convergence is allows transformation of
information
Different forms of convergence allow diversity in
response
Up in the CNS circuits get more complex


Thousands of interconnected neurons
Electrical engineering
Neural circuit designs:
Excitation

“feature” detectors
Output of red neuron
Preferred response


No convergence
Convergence


Responsive to line length
But not unique to line
length
Neural circuit designs:
Excitation & inhibition

More complex response properties
Preferred response (cell likes
medium sized lines!
Transformation of information
in ganglion cells

Between
photoreceptors and
ganglion cells




Horizontal
Bipolar
Amacrine
Pattern of
convergence btwn
these cells
Receptive fields


Area of space (retina for vision) that when
stimulated influences a neurons firing rate
Receptive field properties

The features of a stimulus that increase a
neurons firing rate
Receptive fields


A neuron’s window onto the world
Classical definition:


Region of sensory surface (retina for vision) that when
stimulated influences a neurons firing rate
Receptive field properties

The features of a stimulus that increase a neurons firing rate
Simple: spot of light
 Complex: A friends
face

Journey through the visual
system

RF properties tell us about the
development of perception

Like the development of complex behaviour

Early versus later stages
Receptive field: Ganglion
cells

Restricted portion of space

Small receptive fields (RF)


RF properties




Convergence from photoreceptors
Center-surround antagonism
On-cells (on center, off surround)
Off-cells (off center, on surround)
What is it for?


Enhancing contrast
Goal: Detection of change
On-cell
Lateral inhibition

Center-surround

Amacrine & horizontal cells

Lateral network that allows
cross-talk

Transformation of information


Spots of light at photoreceptors
Center-surround at ganglion
cells
Ghosts in the machine
Lateral inhibition and perception


Experience of light
is diminished by
summation of
inhibitory
influences
Result: Illusory
Dark spots
Maximal inhibition
Reduced inhibition
Lateral inhibition and perception

Mach bands


Dark and light bands at contrast borders
Hyper-realism

Raphael’s Madonna
Perceptual contrast effects in renaissance drawings
Lateral inhibition and perception

Mach bands

Transformation from physical to perceptual
energies
Transformation takes place in ganglion cells

Objective
Physical
reality
Subjective
perception
Lateral inhibition and perception


Less inhibition from
lighter side
More inhibition from
darker side
Simultaneous contrast

Perception of lightness is influenced by more
than just lateral inhibition (LI)

Lightness perception is achieved by ganglion
cells alone
Other higher-order contrast
effects

LI at ganglion cell insufficient to explain
illusory perceptions of lightness




Your visual system “reasons”



Perceptual rationalization
Hypothesis testing
Dichoptic viewing
White’s illusion
Opposite of what
would be
predicted from LI
“belongingness”
Does understanding the
retina explain vision?

World projected on retina = vision?


No
Why?

Illusory lightness


Can’t be accounted for by retina alone
Retinal representations of world is local
Bits of lightness and darkness
 Need sharing of information

It takes a village …




Its all about sharing of
information
Retina has no global
“representation” of Brad
Pitt
Photoreceptor A doesn’t
talk with photoreceptor B
Respond to small spots of
light
A
B