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Transcript
Prepare videos:
• Current injection into a FFA of a patient:
http://www.jneurosci.org/content/32/43/149
15.full
• https://youtu.be/7s1VAVcM8s8 (start at
4.4min to 6 min)
Sensory Neuroscience
What realities of the world can we feel?
• five traditionally recognized (Aristotle, 350BC):
• sight (ophthalmoception, rods=B&W, cones=color vision),
• hearing (audioception),
• taste (gustaoception),
• smell (olfacoception or olfacception),
• touch (tactioception)
Do our receptors cover the complete spectrum of informative
radiation?
Can other animals feel thing that
humans cannot?
• Ultraviolet radiation
(birds and insects).
• Ultrasound (bats) UV
• Electrical sense
(sharks)
• Radioactivity
(rodents).
• Magnetoception
(the ability to
detect the direction
one is facing based
on the Earth's
magnetic field birds)
IR
• Sensors: convert external signal (radiation,
molecules, pressure) to electrical signal.
• How do they encode the intensity of the signal?
• All sensory receptors first transmit to thalamus,
then to cortex (except olfaction)
Thalamus
• The active relay
station of the brain.
• Every sensory system
(with the exception of
the olfactory system)
includes a thalamic
nucleus that receives
sensory signals and
sends them to the
associated primary
cortical area.
• Located in the walls of
the 3d ventricle
Taste
What is more important: sensory
receptors or the cortical neurons?
• Taste is responsible for evaluating food items.
• Sweet and bitter are two of the most important
sensory percepts for humans and other animals
– Sweet taste allows the identification of energyrich nutrients
– Bitter warns against the intake of potentially
noxious chemicals.
• information from taste receptor cells in the
tongue is transmitted through multiple neural
stations to the primary gustatory cortex.
• In the primary gustatory cortex sweet and bitter
are represented by neurons organized in a
spatial map with each taste quality encoded by
distinct cortical fields.
• EXPERIMENT: activate the brain field
representing bitter taste and give mouse sweet
water - will mouse drink?
• activate the brain field representing sweet taste
and give mouse bitter water - will mouse drink?
Perception is in the cortex!
• The activity in the
primary gustatory cortex
directly controls an
animal’s internal
perception of sweet and
bitter taste and drives
behavioral actions.
• Essentially, it does not
matter what sensory
receptors are reporting.
You perceive only the
cortical neurons activity.
Vision
Hearing
Touch
Thalamus - active relay station
• Every sensory system (with the
exception of the olfactory system)
includes a thalamic nucleus that
receives sensory signals and
sends them to the associated
primary cortical area.
• Connections run both ways:
from thalamus to cortex and
from cortex to thalamus.
• This allows the thalamus to actively
amplify relevant information.
• Major role in regulating the level of
awareness (e.g. meditation
experiment).
• Damage to the thalamus
can lead to permanent coma.
Olfaction
• Smell receptors
connect directly to
the brain
• That explains the
ability of certain
smells to trigger
vivid memories
(e.g. of a
grandmother)
• 5% of DNA in
mammals is
encoding smell
receptors (only
40% of these
genes are
expressed in
humans)
• Olfactory bulb  phylogenetically old structures in temporal lobe:
• primary olfactory area in piriform cortex,
• the entorhinal cortex
• the amygdala
•  Orbitofrontal olfactory area in frontal lobe
For all non-primate mammals olfaction is the
main sense
• The neocortex (Latin for
"new bark"), also called the
neopallium ("new mantle")
can be viewed as a large
outgrowth of neurons
around the hippocampus.
• It is the invention of
mammals
• A sheet of simple cortexlike structure in between
hippocampus and olfactory
lobe (piriform area) in the
brain of the reptile is
replaced in mammals by the
6-layered neocortex in
mammals.
Hearing
• The vestibular system provides
the sense of balance and spatial
orientation.
• The cochlea
converts sound
pressure patterns
from the outer
ear into electrical
impulses which
are passed on to
the brain via the
auditory nerve.
• In the spiralled cochlea, waves propagate from the base (near the middle ear and
the oval window) to the apex (the top of the spiral). The stiffness of the basilar
membrane determines the mechanical wave propagation properties (“The place–
frequency map” = tonotopic organization):
• High frequencies lead to maximum vibrations at the basal end of the cochlear coil,
where the membrane is narrow and stiff,
• Low frequencies lead to maximum vibrations at the apical end of the cochlear coil,
where the membrane is wider and more compliant.
•A core component of the
cochlea is the Organ of
Corti, the sensory organ of
hearing
• The hair cells in the organ
of Corti are tuned to certain
sound frequencies by way
of their location in the
cochlea, due to the degree
of stiffness in the basilar
membrane
Basilar
membrane
Mechanically-gated channels (cochlea)
• cochlear
prosthesis are
easy because of
cochlear
tonotopic
organization
Vision
• About half of neocortex in humans is
devoted to vision (Barton, 1998).
• Photoreceptor, the protein sensitive to photons, evolved
once. Different types of eyes evolved over 100 times in
animals.
• E.g. in mammals, receptors for light are in the back of the eye;
In octopus receptors face light
• A lot of computational processing happens in the
neurons located right in an eye:
• if every retinal receptor had its own axon going into the
brain, the optic nerve would be over one inch thick
red, green
and blue
• Rods and
Cones
• Fovea=only 2
degrees of
visual field
(sun=0.5
degrees)
• Rods are extremely sensitive, and can be triggered by a
single photon. At very low light levels, visual
experience is based solely on the rod signal.
• Cones require significantly brighter light (i.e., a larger
numbers of photons) in order to produce a signal.
IR
actual
Perceived
• Most mammals, including
dogs and cats, have only two
different kinds of
UV
color receptors (cones).
• Why?
• A remote vertebrate ancestor of
all mammals possessed 4 color
receptors, but nocturnal
mammalian ancestors lost two of
four cones in the retina at
the time of dinosaurs.
• Most fish, reptiles and birds still
have 4 color receptors while all
mammals, with the exception of
some primates still have only 2
color receptors.
• Some primates (including apes)
have acquired the third color
receptors.
• Why?
• Some people (2.4% of males) have
two color receptors – cannot
distinguish between red, orange,
yellow and green.
• Visual signal is fed into two neuronal pathways:
• 1. Into neocortex via lateral geniculate nucleolus in the
thalamus (Conscious)
• 2. Into superior colliculus in the midbrain (Unconscious)
• RECALL: Neurons in the
cortex are organized
territorially based on
their function:
• motor neurons are
located in the motor
cortex,
• neurons sensitive to
touch in the
somatosensory cortex,
• neurons responsible for
language
comprehension are
concentrated in
Wernicke’s area,
• neurons responsible for
language production are
located in Broca’s area.
• The visual system also consists of
multiple departments:
perception of motion, color and
depth is handled by different
departments.
• The primary visual cortex, V1, is
located in the occipital lobe.
• It receives information from the
retina via the lateral geniculate
nucleus in the thalamus.
• The primary visual cortex
is the first cortical area that
receives visual information.
• Bilateral damage to the primary
visual cortex results in blindness;
this disorder is often referred to
as cortical blindness, to
distinguish it from retinal
blindness.
Visual System
From the primary visual cortex, the
visual information is passed in two
directions, or streams:
• In the first stream, the information flows from
the primary visual cortex to the inferior temporal cortex.
• This stream includes the departments that deal with object recognition
• Due to the stream’s direction from the back of the brain towards the front
of the brain (along “the brain’s belly”), it is referred to as the ventral
pathway from venter, the Latin word for the abdomen.
• The second where stream deals with an object’s position in space.
• The departments along the where stream are concerned with an object’s
motion, location, as well as controlling the eyes and arms in the specialized
tasks of grasping nearby objects.
• Due to the stream’s direction from the back of the brain towards the
parietal lobe (along the “the brain’s back”), it is referred to as the dorsal
pathway from dorsum, the Latin word for the back. The dorsal stream has
been described as providing “vision for action.”
• E.g. area V5/MT (middle
temporal) is in the dorsal where
stream:
• Lesion: patient unable to see
motion, seeing the world in a
series of static "frames“
• Consider a 40-something woman
is Switzerland who suffered a
parietal lobe stroke in 1978: she
become unable to see motion,
seeing the world in a series of
static "frames“; e.g. when pouring
tea into a cup the moving liquid
appeared frozen. She would then
find it difficult to see when to stop
pouring. She also found it difficult
to cross the road because the cars
appeared suddenly.
• MT/V5 integrates local visual
motion signals (from V1) into the
global motion of complex objects
Departments along the
ventral “WHAT” stream: V1
• The departments become more
specialized the farther the information moves
along the visual pathway.
• Neurons in the lateral geniculate nuclei can
be activated by visual stimulation from either
one eye or the other but not both eyes. They
respond to any change in activity of the
retinal neuron that they are connected to.
• Neurons in V1 can be activated by either eye,
are sensitive to specific attributes, such as
the orientation of line segments and color.
• Some neurons in V1 respond to differences in
the position of stimuli in the images from the
left and from the right eyes. This deference is
called the binocular disparity.
• Majority of neurons are sensitive to simple
motion: some tuned to line moving up/down’
others are tuned to line moving to left/right
V1 relies on topographical
representation of spatial information
Neurons in V1:
•organized topographically
•can be activated by either eye
•sensitive to orientation of line segments
•color
•binocular disparity
Adapted from Tootell, 1982
Device for blind by Paul Bach y Rita:
Electrodes on the tong.
• https://youtu.be/7s1VAVcM8s8 (start at
4.4min to 6 min)
• Scan brain activity in visual area 
Neurologically this input indistinguishable
from sight.
• Psychologically too patients experience the
tong data as vision.
• Blind patient perceived objects as out there
in space in front of them. They escaped from
ball flanged at them and could sense when
objects moved closer and farther. They even
experienced waterfall illusion.
• Similarly, Daniel Kish (see TED) who uses
clicks to navigate the space shows strong
activity in visual cortex while listening to
reflected auditory clicks
Departments along the ventral what stream:
• The departments become more specialized
the farther the information moves
along the visual pathway.
• Neurons in the lateral geniculate nuclei can
be activated by visual stimulation from
either one eye or the other but not both
eyes. They respond to any change in activity
of the retinal neuron that they are
connected to.
• Neurons in V1, which can usually be
activated by either eye, are sensitive to
specific attributes, such as the orientation of
line segments, color, and binocular disparity.
•
Neurons in V2 are more specific. They
•
respond to:
• short lines and corners,
• contours,
•
• small spots of color within larger
receptive fields
• to synchronized movement of contrast
borders and rows of spots against a
background.
Neurons in V4 are even more specific. They
respond selectively to aspects of visual
stimuli critical to shape identification.
Neurons in the inferior temporal lobe are
most specific. They respond only when an
entire object (such as a face) is present
within the visual field.
Fusiform gyrus
• Have you ever wondered why do we have
photos of a face used for passport identifications?
Not finger prints, not iris of the eye, not smell.
• Because we have a lot real estate in the brain
dedicated to processing of faces.
• Fusiform face area is important for recognition of birds, cars, dogs, Chinese
characters, i.e. whenever we need to parse a narrow class of nearly
identical things.
• Current injection into a FFA of a patient:
http://www.jneurosci.org/content/32/43/14915.full
• The surgeon can be heard in a video, talking to the patient, “Look at my face
and tell me what happens when I do this.”
• On the first trial, the physician pretends to inject current, and the patient
just shakes his head and mutters, “Nothing.”
• But when a four-milliampere current is sent through the electrodes, he
says, “You just turned into someone else. Your face metamorphosed. Your
nose got saggy and went to the left. You almost looked like somebody I'd
seen before but somebody different. That was a trip.”
How do we
recognize an object?
-- we guess!
Self-organization of a
neuronal ensemble
Bottom-up activation
Perception of the object
• Recognition occurs based on Bayesian statistics:
the brain evaluates the probability of a hypothesis, using
some prior probability,
• i.e. the brain is not calculating but finding the closest
matching neuronal ensemble: neurons from multiple visual
areas synchronize and that results in perception (conscious
closure).
• Neuronal ensemble self-organization is made possible by
feedback axons. The number of feedback axons is an order of
magnitude greater than feed-forward axons.
Visual recognition is
matching to the closest
neuronal ensemble!
• M. Gazzaniga explains Hawkins’s hypothesis: “Computer scientists have been
modeling intelligence as if it were the result of computations—a one- way process.
They think of the brain as if it, too, were a computer doing tons of computations.
They attribute human intelligence to our massively parallel connections, all running
at the same time and spitting out an answer. They reason that once computers can
match the amount of parallel connections in the brain, they will have the
equivalent of human intelligence. But Hawkins points out a fallacy in this reasoning,
which he calls the hundred-step rule. He gives this example: When a human is
shown a picture and asked to press a button if a cat is in the picture, it takes about
a half second or less. This task is either very difficult or impossible for a computer
to do. We already know that neurons are much slower than a computer, and in that
half second, information entering the brain can traverse only a chain of one
hundred neurons. You can come up with the answer with only one hundred steps.
A digital computer would take billions of steps to come up with the answer. So how
do we do it?
• “The brain doesn’t ‘compute’ the answers to problems; it retrieves the answers
from memory. In essence, the answers were stored in memory a long time ago. It
only takes a few steps to retrieve something from memory. Slow neurons are not
only fast enough [to] do this, but they constitute the memory themselves. The
entire cortex is a memory system. It isn’t a computer at all.”
It is matching!
• Hawkins: “For many years most scientists ignored these
feedback connections. If your understanding of the brain
focused on how the cortex took input, processed it, and
then acted on it, you didn’t need feedback. All you needed
were feed forward connections leading from sensory to
motor sections of the cortex. But when you begin to realize
that the cortex’s core function is to make predictions, then
you have to put feedback into the model: the brain has to
send information flowing back toward the region that first
receives the inputs. Prediction requires a comparison
between what is happening and what you expect to
happen. What is actually happening flows up, and what you
expect to happen flows down “
It is matching!
• M. Gazzaniga: “So back to the visual processing of the face that
we started with: Inferior Temporal Lobe is firing away
about identifying a face pattern, sending this info forward to
the frontal lobes, but also back down the hierarchy. “I'm
getting a face code, still there, still there, ahh . . . , OK, it's gone,
I'm out.” But V4 had already put most of the info together, and
while it sent it up to IT, it also yelled back down to V2, "I betcha
that's a face. I got it almost I got it almost pieced together, and
the last ninety-five out of one hundred times the pieces were
like this, it was a face, so I betcha that's what we got now, too!”
And V2 is yelling, “I knew it! It seemed so familiar. I was so
guessing the same damn thing. I told V1 as soon as it started
sending me stuff. Like I am so hot!”
• Stop
Brain comparisons
Historically thinkers have always compared the brain
to the technological marvels of the age:
• Roman doctors likened it to aqueducts.
• Descartes saw a cathedral organ.
• Scientists of the Industrial revolution spoke of mills,
looms, clocks.
• Thinkers of early 1900s saw the telephone
switchboard.
• Nowadays the brain is most compared to a
computer.
• The brain is not a computer!