Sensory receptors and neural coding of sensation

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Sensory receptors and neural coding of
sensation How is it done? Specific receptor
mechanisms exist for each system and will be
discussed in subsequent lectures Lets look
at common organizational principles common to
all systems for stimulus detection,
transduction, and information coding of signals
to the brain.
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Psychophysics demonstrated that perception and
behavior could be described and studied
empirically 1) Studies relationship between
physical stimulus and the attributes of sensory
experience 2) Early psychophysicists such as
Ernst Weber and Gustav Fechner believed that
they could explain the minds perception by
describing the sensory stimulus experience.
They did not succeed, but were conceptually
correct
Gustav Fechner
Ernst Weber
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Sensory function basics 1) In the 19th
century, Weber and Fechner used psychophysics to
discover that all sensory systems encode four
attributes of a stimulus. a) modality vision,
touch, audition, taste, smell vestibular
(motion) b) intensity level of
stimulation c) duration temporal
sequence d) location spatial topography
2) When combined, these four attributes give rise
to sensation.
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Psychophysics established a relationship between
intensity of sensation and stimulus magnitude
1) Intensity of sensation depends upon strength
of stimulus Webers law S K x S where
S minimal difference in strength between a
reference stimulus S and a second stimulus
that can be discriminated, K scaling
constant This is known as the just noticeable
difference (jnd) 2) Later, it was found that
the intensity of sensation is best described by a
power function. Stevens law I K(S So)n
where I intensity of sensation, S
stimulus strength, So threshold amplitude
of the stimulus, n system exponent
n 2.8
n 1
n 0.67
This is useful, since sensory systems must
respond to stimuli that vary over several orders
of magnitude in strength (non-linear)
n 0.4
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What is sensory threshold (So) and how is it
determined? Psychometric functions
Threshold lowers (attention, priors)
Threshold increases (tolerance dysfunction)
0.75
Detection threshold
0.25
Stimulus magnitude
Several methods used for determining
threshold Method of limits (yes/no
response) Two alternative forced choice task
(choose A or B)
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Detection threshold used for audiometrics
Method of limits staircase procedure
Normal hearing
High frequency loss (left ear) Moderate broadband
loss (right ear)
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Sensory physiology 1) Studies relationship
between physical stimulus and neural
response 2) Quantifies stimulation parameters,
transduction, neural coding,
and information processing in the CNS
Today Merging of psychophysics with sensory
physiology has produced new understandings into
how neural systems give rise to perception,
decision making, and behavior.
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Linear and rotational acceleration
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Common properties of receptors 1)
Specialized cells (neural and non-neural) that
perform a transduction from modal stimulus to
change in membrane voltage. 2) Connects
to primary afferent neurons, bipolar or
pseudo- polar cells, whose somas lie in ganglia,
then project to CNS 3) Receptors are
ordered across the sensory epithelium in a
topographic map. Neurons innervating
receptors project to CNS and maintain
topography
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Action potentials are the elementary units of the
neural code
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Receptors are modality specific 1) Johannes
Muller (1826) formulated the Doctrine
of Specific Nerve Energies. Receptors when
stimulated with any type of stimulus (e.g.,
pressure, electric shock) will always signal
specific modality. Useful for prosthetics like
cochlear implants .
2) However, multiple modality receptors can
also converge to give rise to new
sensations, such as wet or sticky even
though specific receptors for these
qualities dont exist.
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Sensory systems encode 4 basic attributes of
stimuli produce sensation
Small probe to center of receptive field
slow adaptation
rapid adaptation
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Photoreceptors .
Rods respond to black/white dim illumination
(night vision) Cones respond to color wavelengths
(3 types)
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Chemoreceptors Olfaction and Taste .
Transduction via direct channel or 2nd messenger
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Hair cells are mechanoreceptors that are
spatially tuned
Stereocilia movement opens ion channels. Toward
large stereocilia produces depolarization. Away
from large stereocilia produces
hyperpolarization. Orthogonal displacement does
not change membrane potential.
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Neural adaptation important principle of neural
coding
All sensory receptors and nerve fibers adapt to
constant stimulation. Adaptation varies
from slow to rapid. Slow sustained
stimuli Rapid on/off, time derivative
of stimulation (velocity,
acceleration of stimulus)
Examples of adaptation in touch nerve fibers
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Receptors and neurons are tuned with different
bandwidths
Cochlear hair cells
VIIIth nerve fibers (auditory branch)
Chorda tympani fibers (VII)
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Receptive fields vary across the surface
Two-point discrimination
Receptive fields
Mean threshold (mm)
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Spatial resolution determined by density of
receptors and receptive field size
In the human retina, there are 120 million
rods and 7 million cones
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Spatial distribution of receptors topographic
representation
Salamander olfactory bulb
Topographic activity codes for hedonics
limonene
Optical imaging with voltage sensitive
dyes. Hotter colors indicate more neural
activity.
Receptive field determines location Each receptor
only responds to stimuli in its receptive
field
ethyl-n-butyrate
21
Neurons have noise
Neural noise gives rise to uncertainty. Must use
population of cells to better discriminate.
50 trials of identical visual stimulus while
recording from fly visual neuron H1
PSTH represents probability r(t) per unit
time that neuron will fire
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Neural sensitivity and noise level affects signal
code
Number of observations
Low sensitivity
Stim A
Stim B
Regular firing neurons temporal code?
Spikes/sec
Number of observations
High sensitivity
Stim A
Stim B
Irregular firing neurons rate code?
Spikes/sec
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Neural Coding Simple interactions Note
linear summation of receptor potential
Pacinian corpuscle responds to pressure
Spatial summation A and B applied at same
location C applied at different location
Temporal summation A and C applied at different
relative times
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Ascending neural coding complex interactions
Convergence produces large receptive fields
and loss of specificity, but can also
increase sensitivity
Inhibitory interneurons increase contrast
and sharpen signal/noise
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Neural mechanisms underlying psychophysics
Neural firing rate, duration, and number of cells
in synchrony together encode intensity and
temporal resolution of sensation
Response of somato-mechanoreceptor fiber
Subjects verbal report of stimulus magnitude
Neural firing frequency (spikes/sec)
Threshold 80mm
The correlations between sensation and neural
firing rate suggest that the psychometric and
neurometric functions are in register
I K(S So)n
n 1