PSB CH5 - VISION

CHAPTER 5
VISION
I. VISUAL CODING
General Principles of Perception

Law of specific nerve energies statement that whatever excites a particular nerve always sends the same kind of information to the brain
1. Animals need to perceive the things around them. Objects emit energy that
stimulates the receptors that transmit information to an animal’s brain.
2. One’s brain codes the information that but that information does not resemble what one actually sees until it is interpreted.
3. Each receptor is specialized to absorb one kind of energy and transduce it into an
electrochemical pattern in the brain. Impulses in certain neurons indicate light and
impulses in other neurons indicate sound.
4. Another aspect of coding is frequency of a response, or how fast a neuron is firing.
This controls for the intensity of a feeling, like pain.

The Eye and Its Connections to the Brain

Pupil: An opening in the center of the iris (a band of tissue that gives our eyes their
color) in which light enters the eye.

—The pupil is focused by the lens (adjustable) and cornea (not adjustable) and projected to the retina.

Retina: Rear surface of the eye, which is lined with visual receptors.

—Lights from the left side of the world strikes the right half of the retina and vice versa; light from below strikes the top half of the retina and vice versa.

The Route within the Retina

Receptors →bipolar cells → ganglion cells → back of brain
Within the vertebrate retina, receptors send their messages to bipolar cells
(neurons located close to the center of the eye).

Bipolar cells send their message to ganglion cells (neurons located even closer
to the center of the eye). Ganglion cell axons join together, and then loop around and travel back to the brain.

Amacrine cells get information from bipolar cells and send it to other bipolar cells, other amacrine cells, or ganglion cells. They are important for complex processing of visual information.

The ganglion cells join together to form the optic nerve (or optic tract). The
point at which the optic nerve leaves the eye is known as the blind spot, because it has no visual receptors.

Fovea and Periphery of the Retina

Fovea: Central portion of the macula specialized for acute, detailed vision.

The fovea has the least impeded vision, as blood vessels and ganglion cells are
almost absent. Further aiding the detailed vision of the fovea, each receptor connects to a single bipolar cell, which in turn connects to a single ganglion cell.

Midget ganglion cells: The ganglion cells in the fovea of humans and other primates.
These cells are small and each receives an input from a single cone. Important for perceiving detail.
—Each cone in the fovea has a direct line to the brain and can register the exact
location of any point of light on the fovea.

Toward the periphery of the retina, more and more receptors converge onto bipolar and ganglion cells. As a result, the brain cannot detect the exact location or shape of a peripheral light source.

In the periphery, your ability to detect detail is limited by interference from other nearby objects.

Because the midget ganglion cells provide 70 percent of the input to the brain, our vision is dominated by what we see in the fovea.
In many bird species, the eyes occupy most of the head, compared to 5% of the
head in humans. These birds have two foveas per eye to enhance perception of detail in th periphery.
The density of receptors in the retina may depend on the needs of the organism. For instance, hawks have a greater density of receptors in the top half of the retina to see below while flying, while rats have a greater density in the bottom half of the retina to locate predators above them.

Visual Receptors: Rods and Cones
Two types of receptors exist in the vertebrate retina: rods and cones.

Rods are abundant in the periphery of the retina; they are involved in both peripheral and night vision.

Cones are found primarily in the fovea; they are involved in both visual acuity and color vision.

In humans, the ratio of rods to cones is 20-to-1.

Though, cones provide about 90 percent of the brain’s input.
2. Rods and cones contain photopigments (chemicals that release energy when struck
by light). Photopigments consist of 11-cis-retinal bound to proteins called opsins which modify the photopigments’ sensitivity to different wavelengths of light.

Color Vision
1. Color vision requires comparing the responses of different kinds of cones. Animals
like rats, which have one type of cone, cannot discriminate one color from another.
2. In the human visual system, the shortest visible wavelengths (about 350 nm) are
perceived as violet; progressively longer wavelengths are perceived as blue, green,
yellow, and red near 700 nm.
3. Color vision requires a special coding system in the nervous system. Two major
interpretations of color vision were proposed in the 1800s: the trichromatic theory
and the opponent-process theory.

The Trichromatic (Young-Helmholtz) Theory
Thomas Young first recognized that color was not understood by examining light, but
instead through biology. He believed that there are a few types of receptors,
and each was sensitive to a different range of wavelengths.
According to this theory of color vision, humans have three different types of
cones, each sensitive to a different set of wavelengths. We discriminate among
wavelengths by the ratio of activity across the three types of cones.
Individuals differ in regards to where the short, medium, and long-wavelength
cones are distributed in the retina.
Visual Field: The part of your world that you see.

The Opponent-Process Theory
Negative color afterimages: Visual phenomena that occur when you stare at a
colored object under a bright light without moving your head and then look at
a plain white surface. You would see a replacement of the red you had been
staring at with green, green with red, yellow and blue with each other, and
black and white with each other.
To explain negative color afterimages and other visual phenomena, the
opponent-process theory was proposed. According to this theory, we perceive
color in terms of paired opposites: white-black, red-green, and yellow-blue.

Opponent-process theory states that negative afterimages result from fatiguing
a response by opponent-process cells (e.g., a cell that responds to green light
becomes fatigued after prolonged stimulation, which results in a red afterimage
when the green light is removed).

Retinex Theory
Color Constancy: The ability to recognize the color of objects despite changes
in lighting. This ability is not explained by the trichromatic theory or the
opponent-process theory.
Retinex Theory: Theory proposed to account for color constancy. When
information from various parts of the retina reaches the cortex, the cortex
compares each of the inputs to determine the brightness and color perception
for each area.

Color Vision Deficiency
Color vision deficiency is also sometimes known as color blindness, and is
characterized by the inability to perceive color differences as most people do.
Note that complete colorblindness (perception of only black and white) is rare.
Some people lack one or two of the three types of cones. Some have three
types of cones but one is abnormal.
Red-green color blindness is the most common form of this disorder (primarily
seen in males).

In red-green color deficiency people have trouble distinguishing red from green because their long- and medium-wavelength cones have the same photopigment instead of different ones.

II. How the Brain Processes Visual Information
A. An Overview of the Mammalian Visual System
1. Rods and cones make synaptic connections with horizontal cells and bipolar cells.
Horizontal cells make inhibitory contact onto bipolar cells, which in turn synapse
with amacrine cells and ganglion cells. All these cells are in the eye.
2. Axons of the ganglion cells from each eye form the optic nerves. The optic nerves
from the left and right eyes meet at the optic chiasm, where in humans half of the
axons from each eye cross to the opposite side of the brain. Most of the ganglion
cell axons go to the lateral geniculate nucleus (LGN) of the thalamus. Most axons
from the LGN synapse are sent to the visual areas of the cerebral cortex.

Processing in the Retina
Because the retina contains such a large of number of receptors (120 million rods
and 6 million cones), we have cells that respond to a particular pattern of visual
information to extract meaningful visual data. An example of this is lateral
inhibition.
Lateral inhibition: The reduction of activity in one neuron by activity in
neighboring neurons); a technique of the retina to sharpen the boundaries of visual
objects.

Horizontal cells play a crucial role in lateral inhibition by connecting the photoreceptors to the bipolar cells, enhancing contrast and improving the perception of edges in the visual field.

Further Processing
Receptive Field: The portion of the visual field that excites or inhibits a specific
cell in the visual system of the brain.
Most primate ganglion cells are either parvocellular neurons (small cell bodies
located in or near the fovea), magnocellular neurons (larger cell bodies distributed
evenly throughout the retina), or koniocellular neurons (similar in size to
parvocellular neurons, but distributed throughout the retina).

a. Parvocellular neurons have small receptive fields and respond best to visual
details and color. These cells synapse only onto cells of the LGN.
b. Magnocellular neurons have larger receptive fields and respond best to moving
stimuli. Most of these cells synapse onto cells of the LGN, but a few connect to
other areas of the thalamus.
c. Koniocellular neurons have several different functions and their axons connect
to the LGN, other areas of the thalamus, and the superior colliculus.

The Primary Visual Cortex
An area known as area V1 or the striate cortex located in the occipital cortex,
responsible for the first stage of visual processing.

—People with damage to this area report no conscious vision or visual imagery, even in their dreams.
Blindsight: The ability to respond in some way to visual information after
extensive damage to area V1. People with blindsight will respond to the stimuli but
will report that they cannot see it.
Research suggests two reasons for the possible existence of blindsight: 1) Small
islands of healthy tissue remain in an otherwise damaged visual cortext, but not
large enough to provide conscious perception. 2) The thalamus sends visual input
to several other brain areas besides area V1, which are strengthened after V1 is
damaged.

Simple and Complex Receptive Fields
David Hubel and Torsten Wiesel distinguished three categories of neurons in
the visual cortex: simple, complex, and end-stopped or hyper-complex cells.
Simple cells: Neurons with fixed excitatory and inhibitory zones in their
receptive fields; these cells are found only in the primary visual cortex (V1).
Most simple cells have bar-shaped or edge-shaped receptive fields.
Complex cells: Located in either V1 or V2, these neurons have receptive fields
that respond to particular orientations of light but cannot be mapped into fixed
excitatory and inhibitory zones. Complex cells receive their input from a
combination of simple cells.
End-stopped (hypercomplex) cells: Resemble complex cells but also have a
strong inhibitory area at one end of their bar-shaped receptive field.

The Columnar Organization of the Visual Cortex
Cells in the visual cortex are grouped together in columns perpendicular to the
surface according to their responsiveness to specific stimuli. For example, cells
in a particular column may respond only to visual input from the left or right
eye, or from both eyes about equally. Also, cells in some columns respond best
to stimuli of a single orientation.

Are Visual Cortex Cells Feature Detectors?

Feature Detectors: Neurons whose responses indicate the presence of a
particular feature. The fact that prolonged exposure to a given visual feature
decreases sensitivity to that feature supports this concept.
For example, the waterfall illusion is when you stare at a waterfall for a minute
or more then look away, the rocks and trees nearby look like they’re flowing
upwards.

Development of the Visual Cortex

During early development, the visual system forms synaptic connections similar to
the rest of the brain and the newborn mammal has a lateral geniculate and visual
cortex that already resembles an adult’s. However, the visual system needs normal
visual experience to maintain and fine-tune its connections.

Deprived Experience in One Eye
Most neurons in the visual cortex of cats and primates receive stimulation from both eyes.
If a kitten is deprived of visual stimulation in one eye early in life, the kitten will become almost blind in that eye because synapses in the visual cortex gradually become unresponsive to input from the deprived eye.

Deprived Experience in Both Eyes
If both eyes are deprived of stimulation, cortical cells will remain responsive
(albeit sluggishly) to both eyes.
There is a sensitive period of visual experience when experiences have a
particularly strong and enduring influence. However, even after the sensitive
period, prolonged lack of visual experience can weaken the visual cortex.

Uncorrelated Stimulation in the Two Eyes
Most neurons in the human visual cortex responds to approximately
corresponding areas of both eyes. By comparing the inputs from the two eyes,
you achieve stereoscopic depth perception.
Stereoscopic depth perception requires the brain to detect retinal disparity,
the discrepancy between what the left and right eyes see.
c. Abnormal experiences can disrupt binocular vision.
d. Strabismus (or strabismic amblyopia), or lazy eye: Condition in which the
eyes do not point in the same direction. Individuals born with this disorder
cannot perceive depth better with two eyes than with one.
e. This disorder is treated by putting a patch over the active eye, forcing the child
to use the ignored eye. The patch is most effective if used early, as many
children refuse to wear it later in life.
f. A promising therapy for the disorder is asking the child to play action video
games, which require the attention of both eyes.

5. Early Exposure to a Limited Array of Patterns
a. A kitten that spends its entire early sensitive period wearing goggles with
horizontal lines painted on them will have nearly all its visual cortex cells
become responsive only to horizontal lines. After months of normal
experience, these kittens will not respond to vertical lines.
b. Astigmatism: A blurring of vision for lines in one direction; this disorder is
caused by an asymmetric curvature of the eyes. Corrective lenses during early
childhood (before age 3-4 years) improve visual capacity in adulthood.
6. Impaired Infant Vision and Long-Term Consequences
a. People who are born with cataracts (cloudy lenses) but have them surgically
repaired at age 2-6 months eventually develop nearly normal vision but have
subtle problems processing some visual information.
b. An early cataract in the left eye will cause problems, because it limits visual
information to the right hemisphere needed for normal visual processing (such
as face recognition).

III. Parallel Processing in the Visual Cortex
The Ventral and Dorsal Paths
1. The primary visual cortex (area V1) sends information to the secondary visual
cortex (area V2), which is responsible for the second stage of processing. The
connections between V1 and V2 are reciprocal.
2. Ventral Stream: These pathways are also called the “what” pathways because they
are specialized for identifying and recognizing objects.
3. Dorsal Stream: This pathway is the “where” or “how” pathway: It helps the motor
system find objects, move toward them, grasp them, and so forth

4. Damage in one stream of the other will result in different deficits.
a. Damage to the dorsal stream (parietal cortex) seems to have the most normal
vision. They can read, recognize faces, and describe objects in detail. They
know what things are but not where they are. They cannot accurately reach out
to grab an object.
Damage to the ventral stream sees where but not what. They can see where
objects are and grab them, but cannot make sense of a television program
because they have trouble identifying what things are.

B. Detailed Analysis of Shape
1. As information travels from V1 to V2, it becomes more complex. The receptive
field becomes more specialized. Many cells respond only to circles, lines that meet
at a right angle, or even more complex properties.

Inferior Temporal Cortex
a. Cells in the inferior temporal cortex respond to identifiable objects.
b. Cells also respond to what the viewer perceives and not to what the stimulus is
physically (figure versus background). Cells continue responding the same
way despite changes in the shape’s position, size, and angle.
The area is important for shape constancy (the ability to recognize an object’s
shape even as it approaches, retreats, or rotates).
d. Damage to the shape pathway of the cortex leads to specialized deficits. An
inability to recognize objects despite otherwise satisfactory vision is called
visual agnosia (meaning “visual lack of knowledge”). It usually results from
damage in the temporal cortex.
e. fMRI studies have shown that the fusiform gyrus of the inferior temporal
cortex is largely specialized for face recognition. The fusiform gyrus is also
activated when identifying car models, bird species, and so forth.

3. Recognizing Faces
a. Human newborns are predisposed to pay attention to faces more so than other
stationary objects. However, infants show a strong preference for a realistic,
right-side-up face over an upside-down face or a distorted face.
b. Precision of face recognition is best when those faces are familiar. It has been
shown that people are better at recognizing faces of their own ethnicity.
c. Face recognition depends on several brain areas, including the occipital face
area, the amygdala, and parts of the temporal cortex, including the fusiform
gyrus, especially in the right hemisphere.

d. Prosopagnosia: The impaired ability to recognize faces without an overall loss
of vision or memory. People with prosopagnosia can identify if a person is
young or old, male or female, but they do not recognize who they are.

C. Color Perception
1. Area V4 and other nearby brain regions are believed to be important for color
constancy. Area V4 also contributes to visual attention.
2. Damage to area V4 would result in, for example, someone not being able to find a
yellow highlighter if the overhead lighting changed.

Motion Perception
The Middle Temporal Cortex
Area MT (middle-temporal cortex, also known as area V5) and adjacent area
MST (medial superior temporal cortex) are important for motion detection.
b. Cells in the MT respond selectively to a stimulus moving in a particular
direction, acceleration, deceleration, and somewhat to still photographs that
imply movement. Cells in the MST respond best to complex stimuli such as
the expansion, contraction, or rotation of a large visual scene. For example, it
responds to objects that move relative to their backgrounds.
2. Motion Blindness
a. Areas MT and MST respond strongly to moving objects, and only to moving
objects.
b. Damage to, or around, these areas results in motion blindness (ability to see
objects but impairment at seeing whether they are moving or, if so, which
direction and how fast).
c. You do not see your own eyes move because area MT and parts of the parietal
cortex decrease activity during voluntary eye movements, known as saccades.
d. The opposite of motion blindness also occurs: Some people are blind except
for the ability to detect which direction something is moving.