Transcript The Visual System: Essentials of sight



Light- a form of electromagnetic radiation
that travels like a wave
Amplitude- the height within the wavelength
◦ Mainly affects perception of brightness

Wavelength- distance between peaks
◦ Affects the perception of color

Purity- how varied the mix is between the two
◦ Influences the perception of the saturation
(richness) of colors



Refers to the whiteness that is in the color
Whiteness declines=saturation increases
Most objects don’t omit light; the reflect it


Only including wavelengths that people can
see
There are so many more, visual spectrum is
Insectsonly a small portion
ultraviolet
Fish/reptilesinfrared

Two main purposes:
◦ Channel light into neural tissue that receives it
(retina)
◦ Houses the retina


Cornea- transparent “window” and the lens
behind it form upside down image of objects
then image is adjusted
Lens- transparent eye structure that focuses
the light rays falling on the retina

Accommodation
◦ Lens making adjustments that facilitate is called
accommodation
◦ Occurs when curvature of lens adjusts to alter
visual focus

Nearsightedness
◦ Close objects are clear;
far objects blurry

Farsightedness
◦ Far objects are seen
clearly; close objects are
blurry


Iris- the colored ring of muscle surrounding
the pupil
Pupil- opening in the center of the iris that
helps regulate the amount of light that
passes through to the rear chamber of the
eye



Retina- neural tissue lining the inside back
surface of the eye- absorbs light- processes
images- sends visual information to the brain
optic disk- fibers (axons) diving into a hole
behind the retina
Blindspot- the hole that the optic disks
creates in the retina




Cones- specialized visual receptors that play
a key role in daylight vision and color vision
Visual acuity- sharpness and precise detail
Fovea- tiny spot in the center of the retina
that contains only cones- greatest visual
acuity
Rods- specialized visual receptors that play a
key role in night vision and peripheral visionmore sensitive

Dark adaptation
◦ processes in which the eyes become more sensitive
to light in low illumination
◦ 30 minutes

Light adaptation
◦ process in which light
becomes less sensitive
to light in high
illuminations


Optic nerve- collection of axons that connect
the eye to the brain
Receptive field- retinal area that when it is
stimulated affects the firing of that cell


Light falls on the eye, but you see with your
brain.
The retina does a lot of information
processing for a sensory organ, visual input is
meaningless until it is processed in the brain.




Optic Chiasm- The point at which the axons from the
inside half of each eye cross over then project to the
opposite half of the brain.
Visual information gets to the brain by axons leaving
the back of each eye form the optic nerves, then
traveling to the optic chiasm. This insures that signals
from both eyes go to both hemispheres of the brain.
After reaching the optic chiasm. They split into two
pathways one leading to the thalamus. This is
considered the main pathway where about 90% of
axons from the retinas synapse in the lateral
Genicualte nucleus (LGN)
Visuals signals are processed in the LGN and then
distributed to areas in the occipital lobe making up the
primary visual cortex.





Visual input ultimately arrives in the primary
visual cortex located in the occipital lobe.
David Hubel and Torsten Wiesel identified three
major types of visual cells in the cortex which
were called simple cells, complex cells, an
hypercomplex cells.
THe key point of all this is that the cells in the
viual cortex seem to be highly specialized. They
have been characterized as Feature Detectors.
Feature Detectors- Neurons that respond
selectively to very specific fetures of more
complex stimuli.
After visual input is processed in the primary
visual cortex, it is often routed to other cortical
areas.




After this they travel to one of two streams
The ventral stream (what) or dorsal stream
(where).
These are known as the what, where streams.
As signals move farther along in the visual
processing system, neurons become even
more specialized or fussy about what turns
them on, and the stimuli that activate them
become more and more complex.
“Grandmother cell” was to be humorous but it
is the idea that certain cells react to the visual
image of certain people.





The lights people see are mixtures of different
wavelengths. Perceived color is primarily a
function of the dominant wavelength in these
mixtures.
Lights with the longest wavelengths appear red,
whereas those with shortest appear violet.
Most of the diverse variations are the result of
mixing a few basic colors, there are two kinds of
color mixtures: subtractive and additive.
Subtractive- color mixing works by removing
some wavelengths of light, leaving less light than
was originally there.
Additive- color mixing works by superimposing
lights, putting more light in the mixture than
exists in any one light by itself





The lights people see are mixtures of different
wavelengths. Perceived color is primarily a
function of the dominant wavelength in these
mixtures.
Lights with the longest wavelengths appear red,
whereas those with shortest appear violet.
Most of the diverse variations are the result of
mixing a few basic colors, there are two kinds of
color mixtures: subtractive and additive.
Subtractive- color mixing works by removing
some wavelengths of light, leaving less light than
was originally there.
Additive- color mixing works by superimposing
lights, putting more light in the mixture than
exists in any one light by itself





the trichromatic theory of color vision was first stated
by Thomas Young and later modified by Hermann von
Helmholtz.
Trichromatic Theory- holds that the human eye has
three types of receptors with differing sensitivities to
different light wavelengths.
Helmholtz believed that the eye that had specialized
receptors that were sensitive to the wavelengths
associated with red, green, or blue.
Color Blindness- encompasses a variety of deficiencies
in the ability to distinguish among colors.
Blindness is misleading because it is very rare to find
someone to be completely blind of all colors they
usually are dichromats meaning they see two of the
three.






Complementary colors- are pairs of colors that
produce gray tones when mixed together.
The various pairs of complementary colors can be
arranged in a color circle.
Afterimage- a visual image that persists after a
stimulus is removed.
the color of the afterimage will be the complement of
the color you originally started at.
Opponent process theory- holds that color perception
depends on receptors that make antagonistic
responses to three pairs of colors.
The antagonistic processes in this theory provide
plausible explanations for complementary afterimages
and the need for four names to describe colors.(red,
blue, green, yellow)
Activity one- blindfold
Exploring perceptual processes in vision
How does the brain make sense of forms,
objects, depth, etc.?
 The brain makes sense of it all through visual
perceptions
For example.
A Reversible figure is made intentionally made
to be ambiguous. It is usually a drawing that is
compatible with two different interpretations
that can shift back and forth.
What is Feature Analysis?
 It is the process of detecting specific
elements in visual input and assembling them
into a more complex form
For example.
 A person starts with the components od a
form, such as lines, and then builds them into
perceptions of objects, such as squares or
triangles. (Form Perception)
What is Form Perception?
 Form Perception refers to our ability to
visually perceive objects in the world in
response to the patterns of light that they
caste on our retinas
Feature analysis assumes that form perception
involves:
 (1)Bottom-Up processing
 (2) Top- Down processing
What is Bottom-Up processing?
 Bottom-Up processing is a progression of
individual elements to the whole.
For example.
 We attend to or perceive elements by starting
with the smaller, more fine details of that
element and then building upward until we have
a solid representation of it in our minds.
What is Top- Down processing?
 Top-Down processing is a progression from
the whole to the elements.
For example.
 The is evidence that people can perceive a
word before its individual letters, a
phenomenon that has to reflect top-down
processing



Is a tendency to experience a stable
perception in the face of continually changing
sensory input
Your brain is able to keep an object the same
far away as up close
We are able to distinguish; size, color, shape,
brightness, and location



visual illusion- involves an apparently
inexplicable discrepancy between the
appearance of a visual stimulus.
Involves a misinterpretation of perception
Common visual Illusions: Mueller-Lyer
Illusion, the Ponzo Illusion, and Monster
Illusion



Two Lines appear different lengths
Reality they are same
Misperception of depth and size constancies
lead to illusion.





Horizontal lines same length
Seem different because of the vertical lines
Vertical lines slant towards each other
Monster Illusion
Same concept and both same size



Are objects that can be represented on a two
dimensional drawing, but cannot be on a
three dimensional drawing
First glance they look fine, but after close
examination they don’t add up or actually
work
One perception about hypothesis in vision is
inconsistent while another would be consiten



Reveals about formulating hypothesis
Reveal about when hypothesis is wrong
Is completely subjective to what the person
sees and only certain things can be objective.
•
•
The ability to hear what is
happening around you.
Must be processed by the
brain.
 Triggered
by any sound producing
a sensory input in the form of
sound waves.
 Sound waves are the stimuli
starting the hearing experience.
Vibrating molecules produce sound.
(Sound waves)
 Sound is Characterized by the
following:
 Amplitude
 Wave Length
 Sound Purity

Amplitude: the amount of energy in a
sound wave.
 Wave Length: the distance from one
sound wave to the next. Measured
from Crest to Crest
 Sound Purity: measured in “Pitch”. It is
also know as the tone of the sound.



Wavelengths of sound are described in
terms of their frequency.
Frequency
◦ is measured in cycles per second (Hertz)
◦ Higher Frequency results in higher pitch
◦ Lower Frequency results in lower pitch



Humans can hear sounds as low as 20 Hz and
as high as 20,000 Hz.
The Human ear is most sensitive around
2000 Hz.
Humans are more sensitive to a single
frequency rather than a complex mixture of
many separate frequencies.

Amplitude
(Decibels)

The ear is divided into three different
sections that function by using different
moving parts.

The External Ear-Vibration of air molecules

The Middle Ear-Vibration of movable bones

The Inner Ear- Waves in fluid


The external ear consists of the Pinna. The
Pinna is basically a cone that collects the
sounds.
Sound waves are picked up by the Pinna and
funneled through the auditory canal into the
eardrum which is a vibrating membrane.
http://www.google.com/search?q=external+ear+pinna+and+ear+drum&hl=en&sa=X&tbm=isch&tbo=u&source=univ&ei=KDJPUbDrGsyLqQGt14H4C w&ved=
0CDsQsAQ&biw=1350&bih=679#imgrc=E3Ic_XBo-M-GKM%3A%3BIgia4gzAQG3WM%3Bhttp%253A%252F%252Fwww.cochlea.org%252Fspe%252Fen%252Foreillecochlee%252Fimages%252Fe_oreille_ext.gif%3Bhttp%253A%252F%252Fwww.cochlea.org%252Fen%252Fspe%252Fexternal -ear.html%3B380%3B362


Within the middle ear the vibrations of the
eardrum are transmitted to vibrate 3 tiny
bones in the ear (hammer, anvil and stirrup)
together these bones are referred to as the
Ossicles.
The Ossicles pick up small changes in air
pressure.

The inner ear consists mainly of the Cochlea.
The Cochlea is a fluid filled, coiled tunnel that
contains receptors for hearing. The Cochlea is
vibrated by the Ossicles and the signals are
routed through the thalamus and auditory
cortex.

There are TWO main theories of hearing and
these are the Place Theory and Frequency
Theory
•Herman Von Helmholtz
theorized that a specific sound
frequency would vibrate in it’s
own specific manor. This in turn
would cause the basilar
membrane to only vibrate in
specific patterns.
•This would be similar to
plucking a specific guitar string
makes a specific pitch.


In place theory it is said that perception of
pitch corresponds to the vibration of different
portions or places along the basilar
membrane.
It is assumed in this theory that hair cells at
various locations vibrate independently by
different frequencies. Then that the brain
detects the different frequency according to
which area is most active.



Frequency Theory holds that perception of
pitch corresponds to the rate or frequency
that the basilar membrane vibrates.
In this theory rather than a guitar the Basilar
Membrane is compared to a drumhead.
The whole membrane is said to vibrate in
response to sound

It is said that a specific frequency causes the
basilar membrane to vibrate a corresponding
number of times in a second. 3,000Hz would
vibrate 3,000 times a second.


It has been discovered that both place and
frequency theory were correct in part but
both were flawed.
Place theory was correct other than the
thinking that the hairs are independent. They
are not and they actually move together in
the way suggested by frequency theory.




Pitch perception depends on both place and
frequency coding of vibrations along the
basilar membrane.
Sounds under 1,000Hz appear to be
translated into frequency pitch.
Sounds between 1,000 and 5,000Hz appear
to depend on a combo of frequency and place
coding.
Sounds over 5,000 seem to depend on place
coding only.

Sound Expieriment


The Physical Stimuli for Taste: any chemical
substance that dissolves in water.
Gustatory Receptors: Clusters of taste cells
found in the taste buds that line the trenches
around tiny bumps on the tongue.


Primary Tastes: Sweet, Sour, Bitter, and Salty
Sensory Adaptation: a gradual decline in the
sensitivity to prolonged stimulation.


The Physical Stimuli for Smell: chemical
substances that can evaporate into the air.
Olfactory Cilia: the receptors for the nose,
hair like structures located in the upper
portion of the nasal passages.


Smell is the only sensory system that is not
routed through the thalamas before it
projects onto the cortex.
Oders can affect peoples moods

The Physical Stimuli for Touch: consists of
mechanical, thermal, and chemical energy
that comes into contact with the skin
◦ Produce perceptions of tactile stimulation, warmth,
cold, and pain.