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