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CHAPTER 13 Looking At Vision andHearing 241
THE OTHER GANGLION CELLS: MOVEMENT
AND LOW CONTRAST
The On and O magnocellular ganglion (M) cells have large cell bodies and are less
numerous than parvocellular cells. They’re selective for low contrast, motion, and
depth of objects, but they don’t transmit color. Magnocellular cells are anatomi-
cally named parasol cells because of their size and extensive dendrites. Magnocel-
lular cell axons project to the magnocellular layer in LGN. Amacrine cell input
contributes more to the responses of magnocellular cells than parvocellular cells.
The koniocellular ganglion cells send their axons to the koniocellular layers in the
LGN and many other retinal targets. They mediate some aspects of color vision
particularly for blue cones. They may have a role in integration of somatosensory
system-proprioceptive information with visual perception.
Intrinsically photosensitive ganglion cells express the photopigment melanopsin.
They’re depolarized by light and are involved in detecting and transmitting infor-
mation about overall light levels. Their main targets are the suprachiasmatic
nucleus in the hypothalamus for circadian rhythm and pretectum in the midbrain
involved in the pupillary light reex.
Covering distance with action potentials
Within the retina, the processes of photoreceptors, horizontal, bipolar, and most
amacrine cells extend less than 1 millimeter. These distances don’t need action
potentials. But ganglion cell axons are many centimeters long.
Increasing and decreasing action potential output
When an On ganglion cell responds to light in the center, its depolarization
increases action potential ring and output. But when an O ganglion cell is
hyperpolarized by light in the center it increases in ring also. Therefore, these
two classes of ganglion cells can provide information about light above and below
the mean light level through increases in action potential output.
Coloring it in: Photons and color vision
Color vision requires having receptors with dierent spectral sensitivities (the prob-
ability of absorbing light as a function of the wavelength). That’s because the
same molecular cascade that modulates glutamate release happens no matter
what the wavelength of the photon that is absorbed. So, if you have only one pho-
toreceptor type, you can’t tell from its output if it responded to a small number of
photons with a wavelength that is well absorbed or a larger number of photons
with a wavelength less well absorbed.
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242 PART 3 Understanding Perception and Cognition
Comparing responses from dierent cones
Figure13-4 shows the rod and cone spectral sensitivity curves (the probability of
absorbing a photon as a function of wavelength). Every wavelength results in a
unique ratio of activity of the three cone types.
Many ganglion cells in the retina are what’s called color-opponent. The activity of
one cone type— say, red— is opposed to the activity of another type — say,
green— so that the ganglion cell is excited only by red light and inhibited by
green, which is called color contrast.
Mismatching your socks: Color blindness
and anomalies
Humans and some other primates with three cone types are called trichromats
(three colors). So, what’s special about having three cones— why not two or four?
Most mammals and many partially colorblind humans have only two cones and
are called dichromats (two colors). A dichromat only has good color discrimination
in the area where the spectral sensitivities of the two cones overlap, so that the
ratio of the outputs of the two cones changes a lot with any change in
wavelength.
FIGURE13-4:
Cone spectral
sensitivity curves.
© John Wiley & Sons, Inc.
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CHAPTER 13 Looking At Vision andHearing 243
With three cones good wavelength discrimination occurs in most of the spectrum.
Humans and other primates appear to have developed trichromacy by splitting the
ancestral long-wavelength cone into green and red cones, giving good color dis-
crimination for ripe red fruit against a green leafy background, among other things.
Some vertebrates have four cone types, and some invertebrates have even more.
Humans who are missing a cone type are colorblind, although the term is a mis-
nomer because these people are only color decient in one part of the spectrum.
The most common colorblindness is missing a red cone (protanope) or green cone
(deuteranope). This occurs in about one 1in 20 men, but only one 1in 400 women.
The gene for the red and green cones is on the X chromosome. Because males have
only one X chromosome, one mutation gives the defect to a male. If you’re miss-
ing the blue cone, you’re a tritanope that is much less common than red-green
colorblindness.
Sending Messages to the Brain
An estimated 30 ganglion cell subclasses establish many parallel pathways leaving
the retina. The human retina projects primarily to the LGN of the thalamus, which
projects to the occipital lobe of the cortex for image analysis and conscious visual
perception. Subcortical brain areas process unconscious aspects of visual behav-
ior. We describe some of these brain regions in the next sections.
Relaying at the thalamus
The main output of the retina responsible for conscious perception is through the
thalamus. Figure13-5 shows how the retinal ganglion cell axons leave each eye as
the optic nerve, but then combine a few centimeters later at a structure called the
optic chiasm, which means “optic crossing.” About half of the ganglion cell axons
from each eye cross at the chiasm and go to the opposite side of the brain. About
half don’t cross and stay on the same side.
SEEING IN DIFFERENT COLORS
Abnormal color vision occurs when a person has cones with a dierent spectral sensitiv-
ity than a typical person. This produces poorer color vision in most cases, but in some
cases, it produces better color vision. Specically, because females have two X chromo-
somes, they can be eectively a tetrachromat (four colors) if they have two dierent
genes for the same cone but with dierent spectral sensitivities.
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244 PART 3 Understanding Perception and Cognition
Figure13-5 illustrates that the half of the right retina closest to the nose (the nasal
retina) receives images from the right visual eld, while the half of the right retina
farthest from the nose (the temporal retina) receives input from the left visual eld.
At the optic chiasm, the axons rearrange so that information received from the
nasal retina projects to the left thalamus, while the temporal retinal axons project
to the right thalamus. A similar but opposite arrangement occurs for the left eye.
The right thalamus receives ganglion cell axons from both eyes that see the left
visual eld, and the left thalamus receives axons that see the right visual eld.
Axons leaving the optic chiasm are called the optic tracts. Unlike the optic nerve,
each optic tract has ganglion cell axons from both eyes. The destination of about
FIGURE13-5:
Retinal
projections to
the thalamus
and visual cortex.
© John Wiley & Sons, Inc.
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CHAPTER 13 Looking At Vision andHearing 245
80 percent of these axons is the LGN in the thalamus, while about 20 percent of
axons travel to the midbrain or hypothalamus.
Parallel processing in visual
centers in mammals
The most important visual destinations of ganglion cell axons are as follows:
»
LGN in thalamus: This is the most important destination for image
processing.
»
Superior colliculus in midbrain: It controls eye movements for orienting the
eyes toward stimuli of interest.
»
Pretectum in midbrain: It includes the nucleus of the optic tract (NOT),
pretectal nucleus (PO), and posterior pretectal nucleus (PP). It controls the
pupillary light reex. The Edinger-Westphal nucleus receives inputs from
intrinsically photosensitive ganglion cells. The pretectum coordinates eye
movements during smooth pursuit and mediates the accommodation reex
by which the eye maintains focus.
»
Suprachiasmatic nucleus in the hypothalamus: This target receives inputs
from intrinsically photosensitive ganglion cells and controls circadian rhythms.
»
Accessory optic system: It consists of several areas involved in attention, eye
movements, and the coordination of vestibular with visual input for balance. It
includes the medial, lateral, and terminal nucleus.
Fanning Out: From Thalamus
to Occipital Lobe
In the LGN, each thalamic relay cell receives inputs from one or a few similar gan-
glion cells. The LGN is layered, and some layers receive inputs only from parvocel-
lular ganglion cells, whereas others receive inputs only from magnocellular
ganglion cells.
Although the relay cells in the thalamus respond very much like their ganglion cell
inputs, other inputs to the LGN from other parts of the brain (including visual
cortex) allow gating functions that are associated with attention. These inputs
modulate the strength of a neuron’s responses to any particular stimulus based on
the context— the importance of that stimulus.
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246 PART 3 Understanding Perception and Cognition
The thalamic relay neurons send their axons to the visual area of the neocortex at
the back of the brain in the occipital lobe. This ber tract of axons leaving the LGN
is called the optic radiation because the axons look like the axons are fanning out
from a bundle. This pathway mediates almost all the vision of which you’re con-
scious (versus vision functions, such as control of the pupil diameter and some
eye movements, which you’re neither conscious of nor able to voluntarily con-
trol). Here we describe the ow of visual information from the thalamus to and
within the primary visual cortex.
Layering and concurrent processing in V1
The area of the occipital lobe that receives the input from the thalamic relay neu-
rons is called visual area 1 (V1), the primary visual cortex. It’s also referred to as
Brodmann area 17 and striate cortex because of a dense stripe in this area that
appears in histology.
The 1.2 million ganglion cells synapse on about the same number of LGN relay
neurons. But the LGN neurons synapse on about 200 million cortical neurons in
V1. V1 neuronal responses become more specialized, such as only responding to
edges that have a certain orientation (horizontal, vertical, or other), or very spe-
cic colors. After the signal reaches V1, visual processing pathways diverge.
Selecting for orientation and movement
V1 neurons extract local visual features in a small region of the retina. V1 cells are
almost all sensitive to the orientation of the stimulus, such as a line or an edge.
Several ganglion cells arranged in a line and of a particular direction are activated
to represent the image.
All stimulus orientations (vertical, horizontal, and everything in between) are
represented in V1. And so, a small group of ganglion cells in one area of the retina
must give rise to a much larger number of V1 cortical cells, each of which responds
only to a particular orientation of an edge in that area.
Other V1 neurons only respond to certain directions of motion of an edge, as
though a particular sequence of ganglion cells had to be stimulated in a certain
order. As with orientation, all directions are encoded, each by a particular set of
cells. Other V1 cells are sensitive to the dierences between the image that the
right eye sees compared to what the left eye sees. This binocular disparity is because
the eyes are oset in the head and so receive oset images of the world. This cod-
ing is used for depth perception.
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CHAPTER 13 Looking At Vision andHearing 247
Area V1 is the rst place in the visual hierarchy where individual neurons receive
inputs from both eyes. Area V1 represents dierent extracted features in columns.
A column has a range of neurons sensitive to particular orientations and eye dom-
inance. Moving across the cortex, the orientation preference changes from verti-
cal to horizontal, then back to vertical. Neighboring columns spanning all
orientations, and going from left to right eye dominance, are sometimes called
hypercolumns.
Streaming the Message to the Temporal
and Parietal Lobes
Area V1, at the posterior pole of the occipital lobe, projects to higher areas of the
visual cortex. Just anterior to V1 is area V2, and anterior to V2 is V3. Neurons in
these areas tend to respond in similar ways.
Virtually all the occipital lobe, much of the parietal lobe, and the inferior temporal
lobe contain neurons that respond to specic patterns of visual inputs. After V2
and V3 the processing stream splits into two major pathways: the ventral stream
and the dorsal stream that have functional dierences. The following sections
discuss the ow of visual information along the ventral and dorsal streams in
more detail.
Seeing complex shapes, colors, and
objects in the ventral stream
The ventral stream (also called the ventral pathway) is the projection of V1-3 to the
inferior aspect of the temporal lobe (called the infero-temporal cortex or IT). The
ventral pathway is typically called the “what” pathway. Parvocellular ganglion
cells via LGN relay neurons dominate this pathway.
Ventral stream neurons prefer particular patterns or colors (almost all neurons
in ventral stream area V4, for example, are color selective). As you move in an
anterior direction along IT, neurons respond only to increasingly complex
patterns— such as the shape of a hand. Closer to the anterior end (pole) of the
temporal lobe is a cortical area called the fusiform face area that contains neurons
that respond to faces.
Damage to the fusiform face area can cause a disorder called prosopagnosia, in
which people can’t recognize faces, even their own, to varying degrees.
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Seeing where and how-to in
the dorsal stream
The dorsal stream (also called the dorsal pathway) is the projection of V1-3 to the
parietal lobe. Cortical areas in the dorsal stream include areas called middle tem-
poral (MT) and medial superior temporal (MST). Magnocellular ganglion cells are
the primary controllers of this pathway. Neurons in these areas respond only to
particular patterns of movement.
MST neurons respond to image movement patterns created by self-movement,
such as tilting the head, resulting in the rotation of the entire visual eld, which
is the expanding motion pattern generated by forward movement, such as typi-
cally shown in movies for spacecraft traveling through star elds. Some dorsal
stream respond to motion parallax, the larger shift of closer than more than distant
objects during movement.
The dorsal stream mediates vision for actions that rely on vision, such as catching
a ball or running through the woods without bumping into trees. Damage to this
area results in apraxis— the inability to skillfully do visually guided tasks.
Communicating between dorsal
and ventral streams
Despite the dierence in the functions and location of the dorsal and ventral
streams, they do converge in the frontal lobe and also communicate — or
crosstalk— with each other. Here are two examples:
»
Deciphering structure from movement: In structure from motion experi-
ments, researchers illuminate only a few reector spots on various body parts of
actors otherwise in the dark. By watching the reector motion patterns, observ-
ers can tell that the dots are on the bodies of people, what the people are doing,
and even the people’s gender. Motion-detecting neurons from the dorsal stream
must send information to object-analyzing neurons in the ventral pathway.
»
Perceiving depth: The visual system estimates distance to objects in the
environment using a variety of cues. Some of these cues are based on ventral
stream analysis of patterns, such as overlap, whereas other cues involve
movement, such as motion parallax. Ventral and dorsal pathway cues
together create depth perception.
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CHAPTER 13 Looking At Vision andHearing 249
Losing vision: Blindness
Blindness is visual impairment in which a person has limited or no ability to per-
ceive light, shapes, or form. Various factors can cause it, including congenital
conditions, eye diseases, injuries, or neurological factors. Blindness can be cate-
gorized into various types based on its cause and severity, and include total, par-
tial, functional, cortical, and color blindness.
Many conditions can lead to impaired vision:
»
Cataracts: They’re caused by clouding of the lens and can be treated by
surgical replacement of the lens.
»
Glaucoma: This disease of the optic nerve is caused by increased intraocular
pressure that can be controlled by drugs that reduce that pressure.
»
Diabetic retinopathy: Diabetes mellitus and increased blood glucose cause
this disorder. The leading cause of blindness in developed countries, diabetic
retinopathy can lead to blood vessel growth, swelling of the macula, and
scarring. It can be treated bylaser surgery and with diet and drugs that
control blood glucose levels.
»
Retinitis pigmentosa (RP): RP isa group of rare genetic eye disorders that
aect the retina’s photoreceptors. It typically begins with the loss of night
vision in childhood, but later can aect peripheral and/or central vision from
eects on both photoreceptors and retinal pigment epithelial cells. More
than 60 genes have been implicated in RP.Mutations in the rhodopsin gene
is the most common dominant form.
»
Aging related macular degeneration (AMD): AMD is caused by of photore-
ceptor cells in the macula. Both genetic and environmental factors can
contribute. This loss of vision in the center of the visual eld aects the ability
to recognize people, read, and drive.
Two types have been identied:
•
Dry AMD: A buildup of the protein drusen in the macula leads to damage
to photoreceptors. The FDA recently approvedthe rst drugs for treating
dry AMD.
•
Wet AMD: New blood vessels intrude under the macula, causing blood
and uid to leak into the retina and damage photoreceptors. An eective
therapy for wet AMD is anti-VEGF drug injections, which stop bleeding and
leaking from blood vessels and slows vascular growth (angiogenesis).
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Seeing without meaning: Agnosias
Agnosias are losses of specic visual abilities, usually because of damage in higher-
order areas of the visual cortex, and examples include the following:
»
Blindsight: Patients with injury in V1 can make saccades to the source of a light
or other visual stimulus without being able to consciously perceive the light.
»
Pattern vision: Damage to ventral stream areas causes loss of pattern vision,
such as facial agnosia where a person can’t recognize faces.
»
Sense of color: Another well-studied ventral stream agnosia is a loss of color
sense, called achromatopsia, due to damage to area V4in the ventral stream.
Unlike more common colorblindness, which comes from the retina, people
with achromatopsia report that they can see dierences between areas of
dierent colors. They don’t perceive the areas as having color, but rather
being dierent shades of gray.
»
Motion detection: Damage to area MT in the dorsal stream causes a motion
detection defect called akinetopsia.
»
Left-sided vision: Damage to the parietal lobe, particularly on the right side
of the brain, results in hemispatial neglect, in which people don’t see or pay
attention to anything in their left visual eld.
Sensory substitution devices are being developed that transform the representa-
tion visual sensory input into a new representation with a dierent sensory form
for blind individuals. For example, a visual feed from a camera can be transformed
into a touch stimulus, with tactile sensation that can be used for visual
perception.
Listening In: Capturing Sound Waves
Hearing, like vision, captures energy from the environment. Your ears detect
vibrations in the air that are created by objects vibrating in the world, which are a
series of high- and low-pressure air pulses. Sound is the perception from receiv-
ing this energy.
The auditory system is unbelievably sensitive. The threshold for the auditory sys-
tem to detect pressure waves in the air is very close to the threshold of constant,
random air motion itself. You can’t do much better than that. Your sense of hear-
ing allows you not only to detect that sound exists, but also what made it, what it
means, and from where it came. In the next sections we describe how sound waves
are converted into uid waves.
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