Download Vision I

Introduction to
Physiological Psychology
Vision
[email protected]
cogsci.ucsd.edu/~ksweeney/psy260.html
This class…
n  Sensation
vs. Perception
n  How light is translated into what we see
n  Structure and anatomy of the eye
n  Photoreceptors: rods and cones
1
Imagine…
Lighta part of the
spectrum of
Electromagnetic
Energy
(the part that’s
visible to us!)
2
Different ways for eyes and brains to do vision
Different ways for eyes and brains to do vision
3
Take home point:
n  What
you ‘see’ is NOT what is “out
there”, but just the impression created
for you by your brain “in here”!!
n  How
does your brain do the creating?
n  For
a human to 'see' stimuli in their
external environment, several processes
must occur: –  Light rays are collected and focused on the
retina
–  Visual signals are transduced (converted to
neural signals)
–  The brain integrates the visual information
and provides a perception
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Sensation vs. perception
n  Sensation
– The process in which the
sense organs receptor cells are
stimulated and bring information to the
brain.
n  Perception – The process in which an
organism selects and interprets sensory
input so that it acquires meaning
n  Sensation – Detection of stimuli
n  Perception – Comprehension of stimuli
Sensory Transduction
n  To
transduce is to convert energy from
one form to another.
–  Sensory transduction is the process by which
sensory stimuli are “translated” into changes
in the cell membrane potential.
n  In
vision: what sensory stimuli?
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Light!
n  When
light passes
from one medium
to another it
changes speed- it
becomes
‘refracted’.
n  That is, the angle
changes.
Light!
n 
n 
White light is made up of
lights of all the different
colors of the rainboweach of which gets
refracted differently
So, using a prism, white
light can be separated
into the spectral colors
that make it up… the
colors of the rainbow!
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Light travels at a constant speed of ~186,000 miles/
sec. So if the frequency of the oscillation varies, the
distance between peaks (or wavelength) also varies.
What is color ?
n 
Hue - this is the quality that
distinguishes red from blue
(i.e. the hues of a rainbow.)
n 
Brightness - describes the
perceived intensity of light.
n 
Saturation – refers to
‘purity’ of light- all one
wavelength?
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What is color ?
n 
Hue – determined by wave
length (red= longer, violet =
shorter)
n 
Brightness – determined by
amplitude
n 
Saturation – determined by
purity
Anatomy of an eye
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Anatomy of an eye
n 
The image projected on the retina is upside down and backwards!
n  Fovea
§  The region of the retina that mediates the most
acute vision of birds and higher mammals. Only
color-sensitive cones are found in the fovea.
n  Optic
disk
§  The location of the exit point from the retina of
the fibers of the ganglion cells that form the
optic nerve; responsible for the blind spot.
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n 
An octopus doesn’t have a blind spot!
n 
Visual receptors have been selected for in a
way that facilitates survival.
–  Dogs and cats see better in low-light than we do thanks
to the tapetum lucidum, but their color vision isn t as
rich as ours.
–  Hawks have greater density of receptors on top half of
retina than on the bottom half
–  Rats have greater density on the bottom half
Anatomy of an eye
n 
The image projected on the retina is upside down and backwards!
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Finding your blind spot
Image courtesy of Michael Bach
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Eye Movement
n  We
continually scan the world with small
and quick eye movements – saccades
n  The various bits of information are then
integrated in higher visual areas.
n  If we stabilize retinal image…
n  Visual
system (and the brain in general) is
sensitive to change!
n 
When performing a pursuit movement you can suppress
saccades.
Why have two of them?
n  Binocular
disparity
–  What does it get us?
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Why have two of them?
n  Convergence
:
–  eyes must turn slightly inward to focus when
objects are close
n  Binocular
disparity:
–  difference between the images on the two
retinas
n  Both
are greater when objects are close –
provides brain with a 3-D image and
distance information
Accomodation
n  A
change in thickness of the lens,
accomplished by the ciliary muscles.
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The case of farsightedness
Images: 1999 Joy Wagon
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n  To
see something
in fine detail, you
need to get the
image of it on the
fovea!
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We talked about bipolar sensory neurons….
(Somato)sensory Neurons
Sensory Neurons
Interneurons/ Motor Neurons
Communicate sensory
Communicate sensory Interneurons communicate
information (temperature,
information (visual, from neuron to neuron; Motor
touch, position) from
auditory) from
neurons communicate
environment or joints and
environment to the
information to muscles and
muscles to the CNS
CNS
glands in the body
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–  Sensory neurons
§  detect changes in the external or internal
environment and sends information about these
changes to the central nervous system.
–  Motor neurons
§  control the contraction of a muscle or the
secretion of a gland.
–  Interneurons
§  located entirely within the central nervous
system, between sensory and motor neurons
The Retina
n 
The retina is in a sense
inside-out :
–  Light passes through
several cell layers
before reaching the
photoreceptors
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The Vertical and Lateral Pathways
n 
Vertical pathway:
photoreceptors >
bipolar cells >
retinal ganglion cells
n 
Lateral communication
–  Horizontal cells
–  Amacrine cells
Photoreceptors: Rods and Cones
n  RODS:
–  ~120 million rods
–  Scotopic Vision (skotos=darkness)
–  Sensitive to brightness, but
not color (shades of gray)
–  Many rods converge onto one
retinal ganglion cell
–  Responsible for low-light
vision
–  Not present at all in fovea
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Photoreceptors: Rods and Cones
n  CONES:
– 
– 
– 
– 
~6 million cones
Photopic Vision (photos=light)
Sensitive to color
A single retinal ganglion cell
receives signals from one (or
few) cones.
–  Responsible for high acuity
vision (fine detail)
–  Fovea contains only cones
The Vertical Pathway
n  Rods:
Photoreceptor cells of the retina, sensitive to light of low
intensity.
n  Cones:
Photoreceptor cells of the retina; maximally sensitive to one of
three different wavelengths of light and hence encodes color vision.
n  Bipolar
cells:
n  Ganglion
bring info from photoreceptors to the …
cells:
Receive information from bipolar cells, their
axons form the optic nerve.
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Lateral communication
n  Horizontal
cells:
neurons in the retina that interconnect
adjacent photoreceptors and the outer processes of the bipolar
cells.
n  Amacrine
cells:
neurons in the retina that interconnect
adjacent ganglion cells and the inner processes of the bipolar
cells.
Back
of eye
Front
of eye
The Retina
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Rods and Cones
n  The
outer segment of a photoreceptor
contains hundreds of lamellae.
n  Within the lamellae you find
photopigments- molecules that contain
an opsin and a retinal. E.g. rhodopsin
n  Rhodopsin is a receptor that responds to
light rather than to neurotransmitters
(photons bind to it)
n  When
rhodopsin is exposed to light, it
breaks down and the opsin bleaches.
The effect of a bleached
photopigment…
n  …
is that a the photoreceptor s
membrane potential changes.
n  Receptor s membrane potential affects
release of NT onto bipolar cells.
n  Bipolar cells speak to ganglion cells,
which bring information to the brain.
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Transduction: how light becomes
neural signals
n 
A cone or rod actually releases
LESS neurotransmitter when
stimulated by light!
–  Rhodopsin molecules are bleached
by light, causing hyperpolarization
of rods.
–  Thus, inhibition: less release
of neurotransmitter (glutamate)
–  Result is: depolarization of
bipolar cell (= more release of
neurotransmitter)
–  Ganglion cell is more likely to
fire (generally)
Cone and Rod Vision
Distribution of rods and cones
n 
Only cones are
found at the
fovea!!
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Cone and Rod Vision
n 
Less convergence
in cones,
increasing acuity
while decreasing
sensitivity
n 
More convergence
in rod system,
increasing
sensitivity while
decreasing acuity
So we have a response from a
ganglion cell… now what?
n  Bundle
of ganglion cell axons exiting the
eye: blind spot
n  No receptors where
information exits
the eye:
–  Visual system uses
information from cells
around the blind spot for
completion, filling in
the blind spot
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From the Eyes to the Visual Cortex
Lateral Geniculate Nucleus
n  A
nucleus within the thalamus (
relay center )
–  receives information from the retina and
projects to primary visual cortex.
n  Contains
six layers of neurons
–  each layer receives information from only 1
eye.
n  First
two layers: magnocellular
n  Next four layers: parvocellular
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M and P channels
n  Magnocellular
–  Larger cell bodies
–  Responsive to movement
–  Input primarily from rods
n  Parvocellular
–  Small cell bodies
–  Responsive to color, fine
details
–  Input primarily from cones
M and P channels
n  Layers
1, 4, 6- contra
n  Layers 2, 3, 5- ipsi
n  In
between magno and
parvo layers are
koniocellular sublayers
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From the Eyes to the Visual Cortex
n  The
visual system is
organized
retinotopically:
–  The left hemiretina of each
eye (right visual field)
connects to the right lateral
geniculate nucleus (LGN)
–  the right hemiretina (left
visual field) connects to the
left LGN
Primary Visual Cortex (V1, Striate Cortex)
~140 million neurons just in V1!
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