Neural Coding + Sound Localization

Neural code

The way that stimulus information is represented by neurons

Receptive field (types?)

The specific set of input patterns a neuron responds to

Low and high level, broad and narrow

What will bind and stimulate a neuron

Dynamic range

The range of a stimulus (intensity, frequency etc) reflected by neuronal activity.

(ex: auditory neurons responsive to stimulus intensity typically inc firing in 20-50 db range, then becomes saturated)

Matching firing rate to frequency

What do neuron tuning curves show?

Plot of frequency response so that we can see the resonant frequency of neurons- different neurons will have different frequency responses

resonant frequency

The frequency that is most strongly enhanced by resonance. The resonance frequency of a closed tube is determined by the length of the tube.

CF/characteristic frequency

The frequency at which the least energy is needed to stimulate it. Different nerves have different CFs, thresholds, and bandwidths

The CF is or is not roughly the same as the resonant frequency of the part of the BM it is attached to?

is! aka, preserving the tonotopic nature

The threshold is the...

minimum stimulus that causes a change in signal transduction

How does ototoxicity affect the neuron tuning curve?

Can cause elevated thresholds; widened bandwidth; and shift of CF to lower frequencies

Tuning curve

a graph of the responses of a single auditory nerve fiber or neuron to sounds that vary in frequency and intensity

What do tuning curves of auditory nerve neurons tell us?

The response of auditory nerve neurons is directly located to their location along the basilar membrane. Neurons closer to the apex= greatest activity at a low frequency

What are the three neural coding strategies?

Labeled line code, rate code, and temporal code

Labeled line code

Certain cells specifically respond to certain stimuli (frequency, smell, etc)

Basically saying there's a 1:1 relationship for neuron to stimulus. Limitation: Not enough neurons

Rate code

A neuronal coding principle in which the firing rate of a neuron carries information about the stimulus. It is often associated with neural coding of magnitude (intensity of pressure, stretch, light intensity, or loudness).

Rate depends on stimulus intensity (rate increases for 20-50 dB range aka neuronal dynamic range) - limited past 50 dB due to firing rate/dynamic range

Temporal code

Neuronal firing is locked to the periodicity of the stimulus (the frequency)-- locked to a part of the sine wave limited due to firing rate

Place theory

in hearing, the theory that links the pitch we hear with the place where the cochlea's membrane is stimulated

Temporal theory

asserts that frequency is coded by the activity level of a sensory neuron

can be tuned to the frequency of certain stimuli-- aka phase locking

Limitation to place and temporal theory

Phase locking in temporal theory only works up to 1,000 Hz

Place theory--does it tell us about intensity?

We need both for time and for frequency info

Phase locking

Firing of auditory neurons in synchrony with the phase of an auditory stimulus

up to 1,000 Hz

Part of temporal theory

Volley Principle

groups of neurons or hair cells working collectively to use phase locking by sharing the responsibility of representing each part of the stimuli.

Which theory explains the benefits and limitations of a CI?

Place theory

Widening of cochlear tuning with increasing intensity causes-

improved reproduction of the temporal; pattern while degrading the spatial pattern

The sharp tuning observed at lower intensities facilitates

spatial resolution at the expense of temporal

Humans are sensitive to changes in sound source of as little as ___ degrees

1-2 in forward direction

Two types of info to derive info about localization

ITD and ILD

ITD

interaural time difference= difference in arrival times of stimuli at left and right ears

ILD

(Interaural Level Difference) The difference in level (intensity) between a sound arriving at one ear versus the other

head shadow plays a role

A change in 1-2 degrees in location of a stimulus corresponds to an ITD of about

13 microseconds (versus neurons working at 1 ms! way more time so it seems impossible)

Level differences are most effective for what frequencies?

high

MSO contributes to...

best for which frequencies

ITD

Inputs come in from both sides, best for differentiating low freuencies

LSO/MNTB contributes to...

ILD

LSO receives bilateral inputs, but the contra inputs pass through MNTB and become inhibitory signals to amplify level differences

Codes and limitations

Place theory/labeled line code- we simply don't have enough neurons to be able to have groups that respond to the infinite number of stimuli in the world

Rate code/temporal code- because of refractory periods there are limits to how fast neurons can fire, so this only works up to about 1,000 Hz and then works less

How do we overcome the limitations of phase locking?

The volley principle- small groups of neurons work together to convey info about frequency

CI users- which theories help?

Electrode chains- we can be place specific about what we were stimulating

But we need temporal theory to get more specificity - we can choose how to stimulate the system.

Temporal coding strategy in apical regions of cochlear and place theory carries more weight in higher frequency regions. Mid-regions both working together with volley principle helping,

Which theory works better for low and high frequencies?

Place theory= best for high frequencies

Temporal theory= low frequencies

Ribbon synapse

Specialized transport system that assist in signal precision/fidelity

Endbulbs of Held

assist in signal precision/fidelity

Calyces of held

assist in signal precision/fidelity

MSO uses ITD to best localize what frequencies?

low frequencies

LSO uses ILD to best localize what frequencies?

high frequencies > 3k

Where in frequency is human localization poor, since neither ITD or ILD accounts for this range?

around 2k

What do synaptic specializations (ribbons and bulbs of held) do?

promote fidelity and accuracy of stimulus reproduction

Resonance

a state of a system in which an abnormally large vibration is produced in response to an external stimulus, occurring when the frequency of the stimulus is the same as the natural vibration frequency of the system.