Chapter 6
Vision
General Principles of Perception
• Each of our senses has specialized receptors
that are sensitive to a particular kind of
energy
• Receptors for vision are sensitive to light
• Receptors “transduce” (convert) energy into
electrochemical patterns so that the brain can
perceive sights, sounds, smells, etc.
General Principles of Perception
(cont’d.)
• Law of specific nerve energies states that
activity by a particular nerve always conveys
the same type of information to the brain
– Example: impulses in one neuron indicate
light; impulses in another neuron indicate
sound
• Which neurons respond, the amount of
response, and the timing of response
influence what we perceive
The Eye and Its Connections to the
Brain
• Light enters the eye through an opening in
the center of the iris called the pupil
• Light is focused by the lens and the cornea
onto the rear surface of the eye known as the
retina
– The retina is lined with visual receptors
• Light from the left side of the world strikes the
right side of the retina and vice versa
The Eye and Its Connections to the
Brain (cont’d.)
• Visual receptors send messages to neurons
called bipolar cells, located closer to the
center of the eye
• Bipolar cells send messages to ganglion cells
that are even closer to the center of the eye
– The axons of ganglion cells join one
another to form the optic nerve that travels
to the brain
The Eye and Its Connections to the
Brain (cont’d.)
• Amacrine cells are additional cells that
receive information from bipolar cells and
send it to other bipolar, ganglion, or amacrine
cells
• Amacrine cells control the ability of the
ganglion cells to respond to shapes,
movements, or other specific aspects of
visual stimuli
The Eye and Its Connections to the
Brain (cont’d.)
• The optic nerve consists of the axons of
ganglion cells that band together and exit
through the back of the eye and travel to the
brain
• The point at which the optic nerve leaves the
back of the eye is called the blind spot
because it contains no receptors
The Eye and Its Connections to the
Brain (cont’d.)
• The central portion of the retina is the fovea
and allows for acute and detailed vision
– Packed tight with receptors
– Nearly free of ganglion axons and blood
vessels
The Eye and Its Connections to the
Brain (cont’d.)
• Each receptor in the fovea attaches to a
single bipolar cell and a single ganglion cell
known as a midget ganglion cell
• Each cone in the fovea has a direct line to the
brain which allows the registering of the exact
location of input
• Our vision is dominated by what we see in the
fovea
The Eye and Its Connections to the
Brain (cont’d.)
• In the periphery of the retina, a greater
number of receptors converge into ganglion
and bipolar cells
– Detailed vision is less in peripheral vision
– Allows for the greater perception of much
fainter light in peripheral vision
The Eye and Its Connections to the
Brain
• The arrangement of visual receptors in the
eye is highly adaptive
– Example: predatory birds have a greater
density of receptors on the top of the eye;
rats have a greater density on the bottom
of the eye
Visual Receptors: Rods and Cones
• The vertebrate retina consists of two kinds of
receptors:
– Rods: most abundant in the periphery of
the eye and respond to faint light (120
million per retina)
– Cones: most abundant in and around the
fovea (6 million per retina)
• Essential for color vision & more useful
in bright light
Visual Receptors: Rods and Cones
(cont’d.)
• Though cones are outnumbered, they provide
about 90% of the brain’s input
• The average number of axons in the optic
nerve is one million, but some people may
have two or three times as many
• Heightened visual responses are important in
many activities
– Example: top tennis, squash, and fencing
athletes show faster brain responses to
visual stimuli
Visual Receptors: Rods and Cones
(cont’d.)
• Photopigments: chemicals contained by both
rods and cones that release energy when
struck by light
– Consist of 11-cis-retinal bound to proteins
called opsins
• Light energy converts 11-cis-retinal quickly
into all-trans-retinal
• Light is thus absorbed and energy is released
that activates second messengers within the
cell
Color Vision
• The perception of color is dependent upon
the wavelength of the light
• “Visible” wavelengths are dependent upon
the species’ receptors
• The shortest wavelength humans can
perceive is 400 nanometers (violet)
• The longest wavelength that humans can
perceive is 700 nanometers (red)
Color Vision (cont’d.)
• Discrimination among colors depend upon
the combination of responses by different
neurons
• Two major interpretations of color vision
include the following:
– Trichromatic theory/Young-Helmholtz
theory
– Opponent-process theory
Color Vision (cont’d.)
• Trichromatic theory: color perception occurs
through the relative rates of response by
three kinds of cones
– Short wavelength, medium-wavelength,
long-wavelength
Color Vision (cont’d.)
• Trichromatic theory explained:
– Each cone responds to a broad range of
wavelengths, but some more than others
– The ratio of activity across the three types
of cones determines the color
– More intense light increases the brightness
of the color but does not change the ratio
• Incomplete theory of color vision
– Example: negative color afterimage
Color Vision (cont’d.)
• The opponent-process theory suggests that
we perceive color in terms of paired
opposites
– The brain has a mechanism that perceives
color on a continuum from red to green and
another from yellow to blue
– A possible mechanism for the theory is that
bipolar cells are excited by one set of
wavelengths and inhibited by another
Color Vision (cont’d.)
• Both the opponent-process and trichromatic
theory have limitations
• Color constancy, the ability to recognize color
despite changes in lighting, is not easily
explained by these theories
• Retinex theory suggests the cortex compares
information from various parts of the retina to
determine the brightness and color for each
area
– Better explains color and brightness
constancy
Color Vision (cont’d.)
• Color vision deficiency is an impairment in
perceiving color differences
• Gene responsible is contained on the X
chromosome (~8% of men & <1% of women)
• Caused by either the lack of a type of cone or
a cone has abnormal properties
• Most common form is difficulty distinguishing
between red and green
– Results from the long- and mediumwavelength cones having the same
photopigment
An Overview of the Mammalian Visual
System
• Rods and cones of the retina make synaptic
contact with horizontal cells and bipolar cells
• Horizontal cells are cells in the eye that make
inhibitory contact onto bipolar cells
• Bipolar cells make synapses onto amacrine
cells and ganglion cells
• The different cells are specialized for different
visual functions
An Overview of the Mammalian Visual
System (cont’d.)
• Ganglion cell axons form the optic nerve
• The optic chiasm is the place where the two
optic nerves leaving the eye meet
• In humans, half of the axons from each eye
cross to the other side of the brain
• Most ganglion cell axons go to the lateral
geniculate nucleus, a smaller amount to the
superior colliculus, and fewer to other areas
The Neural Basis of Visual Perception
• The lateral geniculate nucleus is part of the
thalamus specialized for visual perception
– Destination for most ganglion cell axons
– Sends axons to other parts of the thalamus
and to the visual areas of the occipital
cortex
– Cortex and thalamus feed information back
and forth to each other
Processing in the Retina
• Lateral inhibition is the reduction of activity in
one neuron by activity in neighboring neurons
• The response of cells in the visual system
depends upon the net result of excitatory and
inhibitory messages it receives
• Lateral inhibition is the retina’s way
responsible of sharpening contrasts to
emphasize the borders of objects
Further Processing
• The receptive field refers to the part of the
visual field that either excites or inhibits a cell
in the visual system of the brain
• For a receptor, the receptive field is the point
in space from which light strikes it
• For other visual cells, receptive fields are
derived from the visual field of cells that either
excite or inhibit
– Example: ganglion cells converge to form
the receptive field of the next level of cells
Further Processing (cont’d.)
• Ganglion cells of primates generally fall into
three categories:
– Parvocellular neurons
– Magnocellular neurons
– Koniocellular neurons
Further Processing (cont’d.)
• Parvocellular neurons:
– Mostly located in or near the fovea
– Have smaller cell bodies and small
receptive fields
– Are highly sensitive to detect color and
visual detail
Further Processing (cont’d.)
• Magnocellular neurons:
– Are distributed evenly throughout the retina
– Have larger cell bodies and visual fields
– Are highly sensitive to large overall pattern
and moving stimuli
Further Processing (cont’d.)
• Koniocellular neurons:
– Have small cell bodies
– Are found throughout the retina
– Have several functions, and their axons
terminate in many different places
Further Processing (cont’d.)
• Cells of the lateral geniculate have a
receptive field similar to those of ganglion
cells:
– An excitatory or inhibitory central portion
and a surrounding ring of the opposite
effect
The Primary Visual Cortex
• Pattern recognition in the cerebral cortex
occurs in a few places
• The primary visual cortex (area V1) receives
information from the lateral geniculate
nucleus and is the area responsible for the
first stage of visual processing
• Some people with damage to V1 show
blindsight: an ability to respond to visual
stimuli that they report not seeing
The Primary Visual Cortex (cont’d.)
• Hubel and Weisel (1959, 1998) distinguished
various types of cells in the visual cortex:
– Simple cells
– Complex cells
– End-stopped/hypercomplex cells
The Primary Visual Cortex (cont’d.)
• Simple cells:
– Fixed excitatory and inhibitory zones
– The more light that shines in the excitatory
zone, the more the cell responds
– The more in the inhibitory zone, the less
the cell responds
– Bar-shaped or edge-shaped receptive
fields with vertical and horizontal
orientations outnumbering diagonal ones
The Primary Visual Cortex (cont’d.)
• Complex cells:
– Located in either V1or V2
– Have large receptive field that can not be
mapped into fixed excitatory or inhibitory
zones
– Responds to a pattern of light in a
particular orientation and most strongly to a
moving stimulus
The Primary Visual Cortex (cont’d.)
• End-stopped or hypercomplex cells:
– Are similar to complex cells but with a
strong inhibitory area at one end of its bar
shaped receptive field
– Respond to a bar-shaped pattern of light
anywhere in its large receptive field,
provided the bar does not extend beyond a
certain point
The Primary Visual Cortex (cont’d.)
• In the visual cortex, cells are grouped
together in columns perpendicular to the
surface
• Cells within a given column process similar
information
– Respond either mostly to the right or left
eye, or respond to both eyes equally
– Do not consistently fire at the same time
The Primary Visual Cortex (cont’d.)
• Cells in the visual cortex may be feature
detectors, neurons whose response indicate
the presence of a particular feature/ stimuli
• Prolonged exposure to a given visual feature
decreases sensitivity to that feature
Development of the Visual Cortex
• Animal studies have greatly contributed to the
understanding of the development of vision
• Early lack of stimulation of one eye: leads to
synapses in the visual cortex becoming
gradually unresponsive to input from that eye
• Early lack of stimulation of both eyes: cortical
responses become sluggish but do not cause
blindness
Development of the Visual Cortex
(cont’d.)
• Sensitive/critical periods are periods of time
during the lifespan when experiences have a
particularly strong and enduring effect
• Critical period ends with the onset of
chemicals that inhibit axonal sprouting
• Changes that occur during critical period
require both excitation and inhibition of some
neurons
• Cortical plasticity is greatest in early life, but
never ends
Development of the Visual Cortex
(cont’d.)
• Stereoscopic depth perception is a method of
perceiving distance in which the brain
compares slightly different inputs from the two
eyes
• Relies on retinal disparity or the discrepancy
between what the left and the right eye sees
• The ability of cortical neurons to adjust their
connections to detect retinal disparity is
shaped through experience
Development of the Visual Cortex
(cont’d.)
• Strabismus is a condition in which the eyes
do not point in the same direction
– Usually develops in childhood
– Also known as “lazy eye”
• If two eyes carry unrelated messages, cortical
cell strengthens connections with only one
eye
• Development of stereoscopic depth
perception is impaired
Development of the Visual Cortex
(cont’d.)
• Early exposure to a limited array of patterns
leads to nearly all of the visual cortex cells
becoming responsive to only that pattern
• Astigmatism refers to a blurring of vision for
lines in one direction caused by an
asymmetric curvature of the eyes
– 70% of infants have astigmatism
Development of the Visual Cortex
(cont’d.)
• Study of people born with cataracts but had
them removed at age 7 or 12 indicate that
vision can be restored gradually, but
problems persist:
– Difficulty in recognizing objects
– Unable to tell that components are part of a
whole
– Best prognosis is for children whose vision
problems are corrected early in life
The “What” and “Where” Paths
• The secondary visual cortex (area V2)
receives information from area V1, processes
information further, and sends it to other
areas
• Information is transferred between area V1
and V2 in a reciprocal nature
The “What” and “Where” Paths
(cont’d.)
• The ventral stream refers to the path that
goes through temporal cortex; the “what” path
– Specialized for identifying and recognizing
objects
• The dorsal stream refers to the visual path in
the parietal cortex; the “where” path
– Helps the motor system to find objects and
move towards them
The “What” and “Where” Paths
(cont’d.)
• The two streams communicate
– Each participates in identifying what and
where an object is
• Damaging either stream will produce different
deficits
– Ventral stream damage: can see where
objects are but cannot identify them
– Dorsal stream damage: can identify objects
but not know where they are
Detailed Analysis of Shape
• Receptive fields become larger and more
specialized as visual information goes from
simple cells to later areas of visual
processing
• The inferior temporal cortex contains cells
that respond selectively to complex shapes
but are insensitive to distinctions that are
critical to other cells
• Cells in this cortex respond to identifiable
objects
Detailed Analysis of Shape (cont’d.)
• Shape constancy is the ability to recognize an
object’s shape despite changes in direction or
size
• The inferior temporal neuron’s ability to
ignore changes in size and direction
contributes to our capacity for shape
constancy
• Damage to the pattern pathways of the cortex
can lead to deficits in object recognition
Detailed Analysis of Shape (cont’d.)
• Visual agnosia is the inability to recognize
objects despite satisfactory vision
– Caused by damage to the pattern pathway
usually in the temporal cortex
Detailed Analysis of Shape (cont’d.)
• Face recognition occurs relatively soon after
birth
• People with cataracts removed at 2-6 months
develop nearly normal vision but have slight
difficulties in distinguishing faces
• Newborns show strong preference for a rightside-up face and support idea of a built-in
face recognition system
• Facial recognition continues to develop
gradually into adolescence
Detailed Analysis of Shape (cont’d.)
• Prosopagnosia is the inability to recognize
faces
– Occurs after damage to the fusiform gyrus
of the inferior temporal cortex
– The fusiform gyrus responds much more
strongly to faces than anything else
Color Perception
• Color perception depends on both the light
reflected on an object and how it compares
with objects around it
– Area V4 may be responsible for color
constancy and visual attention
– Color constancy: the ability to recognize
something as being the same color despite
changes in lighting
Motion Perception
• Motion perception involves a variety of brain
areas in all four lobes of the cerebral cortex
• The middle-temporal cortex (MT/V5)
responds to a stimulus moving in a particular
direction
• Cells in the dorsal part of the medial superior
temporal cortex (MST) respond to expansion,
contraction or rotation of a visual stimulus
• Both receive input from the magnocellular
path; color-insensitive
Motion Perception (cont’d.)
• Motion blindness refers to the inability to
determine the direction, speed and whether
objects are moving
– Likely caused by damage in area MT
• Some people are blind except for the ability to
detect which direction something is moving
– Area MT probably gets some visual input
despite significant damage to area V1
Motion Perception (cont’d.)
• Several mechanisms prevent confusion or
blurring of images during eye movements
– Saccades are a decrease in the activity of
the visual cortex during quick eye
movements
– Neural activity and blood flow decrease 75
ms before and during eye movements