Transcript Visual System

Sensation and Perception
Sensation and Perception
• The link between the outer physical world and
the inner psychological world:
– The means by which we understand the world around
us.
• Different forms of energy from the physical world
are transduced by specialized receptor neurons
into neural impulses.
– The process is called sensory coding.
• It is transmitted to the thalamus which then
relays it to the appropriate areas of the cortex.
• The cortex then interprets the sensory coding to
form our perceptions of the world around us.
The Sensory Systems
Sensory
System
Stimulus
Energy
Specialized
Receptors
Vision
Electromagnetic energy
(light waves)
Rods and cones in the back
of the eye.
Hearing
Sound waves
Hair cells in a special organ
in inner ear.
Touch
Pressure on the skin
Touch receptors embedded
in skin.
Pain
Intense stimuli such as
pressure, heat, cold.
Pain receptors embedded in
skin.
Taste
Water soluble chemical
energy.
Taste receptors in taste buds
in tongue.
Smell
Air soluble chemical
energy.
Olfactory receptors in mucus
membranes of nose.
Sensory Systems
• Designed to respond to change in
environmental stimulus.
– Become less sensitive to constant stimulation
over time.
– Called sensory adaptation.
– When a continuous stimulus stops a sensory
system will again react more strongly.
The Visual System
How Does it Work?
Electromagnetic Energy
www.ucar.edu/learn/images/ spectrum.gif
Light
• Visible spectrum ranges from 400 to 700
nanometers (1 millionth of a millimeter).
• Below 400 nm is ultraviolet, above 700 nm is
infra red. Not visible but affect our systems.
• Light can be emitted from a source, or
reflected. In either case it varies in intensity
(number of photons) and wavelength.
• Most light that reaches the eye has been
reflected off the objects we are viewing.
The Eye
Fovea
The Lens of the Eye
Lens
Incoming
light
Focal Point
• If the lens of the is working properly the image
will be focused directly on the retina. It
accommodates to varying sizes and distances
of objects to focus the image on the retina at the
back of the eye, ideally on the fovea.
Focusing on the Retina
Errors of Refraction
Myopia--Nearsightedness
Far objects
tend to be
blurry.
Errors of Refraction
Hyperopia--Farsightedness
Close objects
tend to be
blurry.
Errors of Refraction
Presbyopia
• As we age, the lens of the eye becomes
less flexible and therefore less able to
accommodate to variations in the light.
• We become increasingly farsighted—it
becomes difficult to focus clearly on objects
that are close up.
• Almost everyone over the age of 45
requires corrective lenses for presbyopia.
Eye Movements
• Saccades: Constant small jittery
movements . Eyes are never still.
• Conjugate movements: Eyes move
together.
• Pursuit movements: Eyes track moving
targets.
The Retina
•
Composed of layers of several types of
cells.
1) Light sensitive receptor cells, called
rods and cones.
2) Horizontal cells.
3) Bipolar and amacrine cells.
4) Ganglion cells.
• Light must pass through all these layers,
from 4) to 1) to stimulate the rods and
cones.
The Rods
• Contain a photopigment called rhodopsin that
bleaches (breaks down) when exposed to light.
• Degree of bleaching depends on the intensity of
the light.
• This starts neural activity that is passed from the
rods to the horizontal cells, bipolar and amacrine
cells, and finally to the ganglion cells.
• Rods transmit brightness information—black,
grays and white. Are not sensitive to
wavelength.
• Rhodopsin is constantly restored unless the
mammal is malnourished. Requires Vitamin A.
The Cones
• Four types, three of which have different
photopigments.
• Each of the three main ones is maximally
sensitive to a different wavelength of light.
They are responsible for our colour vision.
• The fourth type, reported in 2002, seems
to have something to do with our circadian
rhythms. Explains why blind people can
adjust their sleep/waking rhythms to
daylight savings time, and to jet lag.
Rods and Cones
Distribution of
Rods and Cones
Distribution Varies--1
Distribution Varies--2
Other Cells of the Retina
• The retina includes layers of horizontal, bipolar,
and amacrine cells that finally synapse with
ganglion cells.
• It is the axons of the ganglion cells that form the
optic nerve.
• The optic nerve leaves the back of the retina at
the area of the optic disk, making us blind to light
that falls on that small area of the retina.
• All these cells continue the sensory coding
process.
• Approximately 120 million rods and 6 million
cones converge on about 1 million ganglion cells
in each retina.
Gazzaniga and Heatherton, 2003
How Does the Varying
Sensitivity Affect Vision
Colour Vision:
Properties of Colour Sensation
• Hue: varies with wave length
• Brightness: sensation without hue, ranges
from black through greys to white.
• Saturation: depth, or purity of hue,
Colour Vision:
Properties of Colour Sensation
Hue: varies with wave length
• Most hues are composed of several wave
lengths.
• Three unique hues: blue (465 nm), green (465
nm), yellow (570 nm). Unique red is contrived.
Colour Vision:
Properties of Colour Sensation
Brightness: sensation without hue, ranges
from black through greys to white.
• Applies to both achromatic and chromatic
colour sensation.
• Achromatic—without hue, all greys
• Chromatic—with hue, colours can vary in
brightness.
Colour Vision:
Properties of Colour Sensation
Saturation: depth, or purity of hue
• Blue wavelengths of different saturations,
but same brightness. Darker indicates
greater saturation. The lighter are purer
(more saturated) blues than the darker.
Colour Vision:
Properties of Colour Sensation
Colour of certain hue, brightness, and saturation.
Different hues, same brightness and saturation.
Same hue, same brightness, different saturations.
Spectral Sensitivity
of Rods and Cones
Colour Vision:
Theories
• Young-Helmholtz Trichromatic Theory
• Opponent Process Theory
• Both are necessary to explain all the facts.
Colour Vision:
Mixing Colours
• Subtractive colour mixtures: using filters to
permit only certain wavelengths to pass through,
or mixing pigments. We experience a narrow
band of wavelengths.
• Additive colour mixtures: Two ranges of
wavelength are reflected back to the eye at the
same time. Receptors respond to a wider band
of wavelengths and the sensation will be that
produced by the wider band of wavelengths.
Colour Vision:
Trichromatic Theory
• Support: It is possible to create any hue
experience in our visual system by
combining only three wavelengthts of light.
• We have three types of cones, each
maximally responsive to either long
wavelengths (640 nm), green wavelengths
(500 nm), or blue wavelengths (460 nm).
• The combination of responses in the three
types of cones can explain our sensitivity to
various wavelengths.
Trichromatic Colour Theory
at The Level of The Cones
R
Red cones
fire at 50
G
B
Green cones
fire at 200
Blue cones
fire at 175
Transmitting the colour experience of
Colour Vision:
Trichromatic Theory
• However, trichromatic colour theory cannot
explain why we can see colours for wave
lengths that are not stimulating our
receptors.
• These are called after images.
Colour Vision:
After Images
• After images suggest some kind of
opponent process system.
• When the ‘red’ receptors tire, we then
have the experience of seeing green.
• After image research suggests the system
is wired in opponent pairs, red-green,
blue-yellow, and black-white.
Colour Vision:
Opponent Process Theory
 Need something to explain:
– Reducing numbers of neurons in the system.
– We don’t see reddish-green or bluish-yellow.
– After images.
 Beyond the level of the cone cells, there are
ganglion cells that become excited in response
to one set of wave lengths and inhibited for the
colour opposite.
 Some are in blue-yellow pairs, some in redgreen pairs and some in black-white pairs.
Colour Vision:
Opponent Process Theory
 If the receptor cells signal a green tone, some of
the ganglion cells will increase their level of
firing—excitatory impulse.
 If the signal is a red tone, they will decrease their
level of firing-inhibitory impulse.
 Similarly there are ganglion cells that are excited
by yellow tones and inhibited by blue tones.
 The degree of excitation or inhibition will depend
on the intensity and purity of the wavelengths
that originally excited the cone cells.
Trichromatic Theory:
At the Level of the Cone Cells
This could be one pure wave length
but is more likely a mixture of long,
intermediate, and short wave lengths.
R
G
B
+70
+200
+175
The red, green, and blue cones
will signal this wave length with
different rates of firing
Hypothetical firing levels.
The horizontal cells will combine the effect of the cones’ firings and transmit
it to the bipolar and amacrine cells, that will, in turn pass on the combined
effect to the ganglion cells.
Opponent Process Theory:
At the Level of the Ganglion Cells
+
--
--
++
+
-
--
The intermediate bipolar and
amacrine cells signal the
ganglion cells that this was
the light stimulus that started
the process.
+++
Hypothetical ganglion
cells receiving input from
many intermediate cells
Input from long (red) wave lengths and from short intermediate (green) wave
lengths will have opposing effects on the firing of an opponent process redgreen ganglion cell. Similarly, short (blue) and long intermediate (yellow)
wave lengths will have opposing effects on the blue-yellow ganglion cells.
The firing of an individual cell will depend on the combined effects.
The Sensation of Colour
Beyond the Ganglion Cells
Optic nerve formed from the
axons of the ganglion neurons
The axons do not synapse with other neurons until they reach the lateral
geniculate nucleus of the thalamus. The opponent process effect is continued
here. It is from the thalamus that the combined effect of their many firings is
transmitted to the cortex.
Optic Tract
(once more)
Colour Blindness
 If one subset of cone cells is inactive,
nonexistent, or has the wrong pigment, this will
affect colour vision.
 If a subset of the ganglion cells is inactive or
nonexistent, this will affect colour vision.
 Similarly, problems with any of the later neurons
specialized for transmitting wave length
information, or in the receiving areas of the
cortex may affect colour vision.
Colour Deficiencies
Some Colours Are Missing
Image from: http://www.psychology.psych.ndsu.nodak.edu/mccourt/
website/htdocs/HomePage/Psy460/Color%20Vision/Color%20Vision.html
Colour Blindness
• There are many different kinds of defective
colour vision:
– Sex-linked red-green colour blindness results from a
defect in either the pigment for the red cones, or that
for the green cones.
– These defects alter the spectral sensitivity curves for
the particular cones involved so that the cones are
sensitive to different wave lengths of light that normal.
– It results in four different kinds of red-green colour
deficiency
Colour Blindness:
One Type of Red-Green Deficiency
Normal spectral sensitivity
Green cones now have spectral
sensitivity shifted much closer to that
of the red cones.
Red-Green Deficiency
Simulation:
With only
red
sensitive
cones
Picture reflecting primarily red
and green wave lengths.
Simulation:
With only
green
sensitive
cones
Probably all of these pictures would be indistinguishable for someone
with any red-green deficiency.
Images from http://www.firelily.com/opinions/color.html
Colour Blindness
As seen by someone
who is not sensitive to
blue wave lengths.
Picture of poppies and
cyclamen as seen with
someone with normal
colour vision.
As seen by someone
who is not sensitive
to red wave lengths.
Images from http://vischeck.com/examples
Different Colour Vision
Across Species
Monkey sees
Owl sees
Human sees