Download Eagleman Ch 6. Other Senses

6: Other Senses
Cognitive Neuroscience
David Eagleman
Jonathan Downar
Chapter Outline
Detecting Data from the World
 Hearing
 The Somatosensory System
 Chemical Senses
 The Brain is Multisensory
 Time Perception

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Detecting Data from the World
We use five senses to interact with the
outside world, as traditionally defined.
 There are many subdivisions of those five,
plus sensations from inside the body.
 For all senses, there are specialized
receptors to transduce the stimulus.
 All senses project to primary sensory
cortex, and have some form of mapping.

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Hearing
The Outer and Middle Ear
 Converting Mechanical Information into
Electrical Signals: The Inner Ear
 The Auditory Nerve and Primary Auditory
Cortex
 The Hierarchy of Sound Processing
 Sound Localization
 Balance

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The Outer and Middle Ear
Sounds are vibrations carried through air
or water as waves
 We interpret the frequency (measured in
Hz) of the vibration as the pitch of the
sound.
 The amplitude of the vibration is
interpreted as the loudness of the sound.

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The Outer and Middle Ear
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The Outer and Middle Ear
The pinna collects and amplifies certain
frequencies of sound and directs that
sound down the ear canal.
 The sound energy strikes the tympanic
membrane and causes it to vibrate at the
same frequency as the sound wave.

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The Outer and Middle Ear
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The Outer and Middle Ear

In the middle ear, vibrations of the
tympanic membrane cause movement of
the three bones of the middle ear.
 Malleus
(Hammer)
 Incus (Anvil)
 Stapes (Stirrup)

Movement of these bones causes
movement of the oval window.
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Converting Mechanical
Information into Electrical Signals
The oval window is part of the cochlea,
which is the inner ear.
 The cochlea contains three fluid-filled
tubes, wound around a central axis, like a
snail shell.
 Inside the cochlea is the basilar
membrane, which vibrates in time with the
sound wave.

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Converting Mechanical
Information into Electrical Signals
The basilar membrane is tighter at one
end (the base) and looser at the other end
(the apex).
 This difference means there is a tonotopic
map of frequencies along the basilar
membrane

 High
frequencies near the base.
 Low frequencies near the apex.
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Converting Mechanical
Information into Electrical Signals
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Converting Mechanical
Information into Electrical Signals
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Converting Mechanical
Information into Electrical Signals
Inner hair cells along the basilar
membrane transduce sound into electrical
signals.
 The vibration of the membrane causes the
stereocilia to flex closer together or further
apart.

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Converting Mechanical
Information into Electrical Signals
Tip links on the stereocilia cause ion
channels to be pulled open, depolarizing
the cell, when the cilia move one direction.
 When cilia move the other direction, the
channels close and the cell is
hyperpolarized.

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Converting Mechanical
Information into Electrical Signals
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Converting Mechanical
Information into Electrical Signals
The outer hair cells help
to amplify and sharpen
the incoming sound.
 The basilar membrane
can break apart a
complex sound into the
component frequencies.

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The Auditory Nerve and Primary
Auditory Cortex
The auditory (cochlear) nerve carries
information from the inner hair cells to the
cochlear nucleus of the brainstem.
 Each fiber is a labeled line, carrying
information about only one frequency.
 Information travels from the cochlear
nucleus through a number of nuclei to the
primary auditory cortex in the temporal
lobe.

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The Auditory Nerve and Primary
Auditory Cortex
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The Hierarchy of Sound
Processing
Deafness can result from damage to the
outer, middle, or inner ear.
 Damage that occurs to the auditory
pathway after the inner ear typically results
in damage to the ability to process sounds.
 Higher auditory areas are involved in the
interpretation of sounds.

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The Hierarchy of Sound
Processing
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Sound Localization
We can localize sounds that occur around
us based on characteristics of the sound
and interaural differences.
 Interaural timing differences are used to
identify the location of a sharp, brief
sound.
 Interaural phase differences are used to
localize a continuous sound.

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Sound Localization
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Balance

The vestibular system, with three
semicircular canals and two otolith organs,
provides information about orientation.
 The
semicircular canals detect head rotation
and angular acceleration.
 The otolith organs detect linear acceleration.
 Both systems detect movement by the
displacement of hair cells, similar to the
auditory system.
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Balance
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Balance
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The Somatosensory System
Touch
 Temperature
 Pain
 Proprioception
 Interoception
 The Somatosensory Pathway

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Touch
The somatosensory system tells us about
the external world, where our limbs are in
space, and about our internal world.
 Receptors are found all over our skin and
within our internal organs.

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Touch
Touch receptors are mechanoreceptors,
which respond to stretching or bending.
 Meissner’s corpuscles and Merkel’s disks
are located close to the surface of the skin
and have small receptive fields.
 Pacinian corpuscles and Ruffini’s endings
are located deeper in the skin and have
large receptive fields.

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Touch
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Temperature
Thermoreceptors are mechanoreceptors
that convey temperature information.
 These receptors carry information about
how the stimulus differs from the
temperature of the skin.
 One population carries information about
stimuli warmer than the skin and a
separate population carries information
about stimuli cooler than the skin.

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Pain
The perception of pain by nociceptors is
necessary for our survival.
 There are three types of nociceptors.

 Mechanical
nociceptors are activated by
physical damage.
 Thermal nociceptors are activated by very
high or very low temperatures.
 Chemical nociceptors are activated by
particular chemicals.
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Pain
Some nociceptors are responsive to more
than one type of stimuli and are called
polymodal.
 Silent nociceptors respond to the body’s
own chemical signals are can play a role
in the increased sensitivity to stimulation
following injury.

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Pain
Nociceptors appear to be free nerve
endings.
 Different nociceptors transmit their signals
at different rates.

C
fibers are small and unmyelinated, carrying
the signal slowly.
 A delta fibers are myelinated and carry
mechanical and thermal pain signals quickly.
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Proprioception
Proprioception is the sense of position and
movement of our own body.
 Muscle spindles detect the length of the
muscle and the speed of stretching.
 Golgi tendon organs provide information
about muscle tension.

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Proprioception
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Interoception
Interoception is our ability to perceive the
internal state of our body, such as hunger,
thirst, and mood.
 Receptors include stretch receptors and
nociceptors.

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Interoception
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Interoception
Gate control theory describes why we
sometimes notice pain and sometimes do
not, depending on the situation.
 If information from the interoceptors
arrives at the central nervous system at
the same time as nociceptive information,
this can overwhelm the CNS, and the pain
signals are blocked.

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The Somatosensory Pathway
Information from the head and face is
carried by the trigeminal cranial nerve.
 Information from each dermatome of the
body is carried into the dorsal horn of the
spinal cord.
 Different types of somatosensory
information follow different pathways to the
brain.

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The Somatosensory Pathway
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The Somatosensory Pathway
Somatosensory information decussates
and is relayed by the thalamus to the
primary somatosensory cortex (S1)
 S1 is in the parietal lobe, immediately
posterior to the central sulcus.
 There is a somatotopic map of the body,
known as the homunculus.

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The Somatosensory Pathway
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Chemical Senses
Taste
 Smell
 The Sense of Flavor
 Pheromones

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Taste
For both taste and smell, molecules bind
to receptors and trigger the release of
neurotransmitters.
 Taste receptors are found on the tongue,
palate, pharynx, epiglottis, and
esophagus.
 Taste cells are grouped into taste buds,
which are grouped into papillae.

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Taste
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Taste
There are currently five basic tastes:
Sweet, Sour, Bitter, Salty, Umami
 Salty and sour trigger ionotropic receptors.
 Sweet, bitter, and umami mostly trigger
metabotropic receptors, but activate some
ionotropic receptors as well.

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Taste
Most taste receptors appear to have a
preferred type of taste they respond to, but
can respond to other tastes in high
concentration.
 Gustatory afferent neurons relay taste
information to the brainstem and on to the
frontal operculum.

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Taste
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Smell
The olfactory epithelium is found in the
back of the nasal cavity.
 Odorants dissolve in the mucus covering
the epithelium and bind to receptors on the
cilia of the olfactory receptor cells.
 Each olfactory receptor cell expresses
only one type of receptor.
 Receptors are all GPCRs.

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Smell
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Smell

Olfactory receptor cells project to the
glomeruli in the olfactory bulb.
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Smell

Olfactory pathway travels from the
olfactory bulb to the primary olfactory
cortex in the rhinencephalon.
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The Sense of Flavor
Flavor is a combination of taste, smell,
temperature, and texture.
 Higher-level processing within the brain
evokes memories of past experiences.

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Pheromones
Pheromones are chemicals released to
transmit information to and influence
another member of the same species.
 Pheromones are detected by the
vomeronasal organ.
 At this time, it is not clear to what extent
humans use pheromones.

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The Brain is Multisensory
Synesthesia
 Combining Sensory Information
 The Binding Problem
 The Internal Model of the World

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Synesthesia
The brain needs to bring together
information from different senses.
 Synesthesia is a condition where different
senses are integrated inappropriately,
such as when a letter is associated with a
particular color.
 It appears to be due to heightened
connectivity between brain regions.

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Synesthesia
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Combining Sensory Information
Many neurons in the brain are more
responsive when presented with more
than one sensation at a time.
 In the McGurk effect, what a subject hears
can be influenced by what they see at the
same time.

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Combining Sensory Information
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The Binding Problem
We do not perceive a stimulus as having
separate visual and auditory components,
but as a unified stimulus.
 How the sensory signals are integrated is
known as the binding problem.
 Recurrent connections between the
sensory systems likely are important for
solving the binding problem.

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The Binding Problem
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The Internal Model of the World
Our final perception does not rely only on
stimuli from the outside world, but also on
expectations from past experiences.
 In anosognosia, a patient is apparently
unaware of their own physical limitations.

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Time Perception
Time involves input from all of the sensory
systems.
 Time perception is a construct of the brain.
 Time appears to slow down in crucial
situations because the amygdala encodes
memories more thoroughly.
 These more dense memories make it
seem like the event took longer.

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