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Burst Application: Auditory Evoked Responses
Inside the brain are "billions and billions" of neurons which are involved in all types of mental, motor, sensory, and regulatory activity. A neuron is like a simple logic gate in a computer: it produces an output pulse when the proper input conditions are met. The firing rate of a neuron typically increases when it receives excitatory input signals, or a reduction of inhibitory inputs, from other neurons.
Electrodes placed on the scalp can detect the gross effect of the moment-to-moment firing of large populations of neurons. At one instant the sum of all the neural pulses in the brain region under an electrode may make it slightly more positive than an electrode placed over some other region. An instant later the sum may be less than at the other electrode, so the relative polarity reverses.
The tiny scalp potentials (voltages) thus observed tend to look very much like random noise, with perhaps some slow overall rhythm. The randomness is only apparent, however. It is similar to what you would hear if you could listen to all the radio stations in the world at the same time: You couldn't hear any individual program because it would be drowned out by all the other unrelated programs.
An evoked potential is a way of recording just one "program" from the scalp electrodes, namely the reponse of the brain to a specific stimulus. The desired reponse may last tens or hundreds of milliseconds after the stimulus, but it is swamped by the other background activity. However, the assumption is that if the exact same stimulus is given repeatedly, the target response will be the same each time. On the other hand, the vast majority of brain activity is unrelated to the target response, so it just goes about its normal activity and is not the same each time... it is just "random" noise.
So, if we record the scalp potentials for some fixed interval after each stimulus, and add all these little recordings using synchronous waveform averaging so they are time-aligned with the stimulus, the uncorrelated background activity will average toward zero while the target responses add together constructively to give the desired waveform.
To measure an auditory evoked response, the stimulus is typically a series of brief tone bursts. The recovered response waveform can be used to indicate how well the subject heard the bursts. This method can be used even when the subject is unconscious or otherwise unable to speak, such as a pre-lingual infant or non-human animal.
For auditory evoked responses, rise and fall times of about 1 msec are commonly used. If you use shorter times, the subject may appear to be more sensitive because the wider spectrum stimulates the firing of more neurons from adjacent frequency regions. This gives less information about the true sensitivity at the frequency of the tone.
As you lengthen the rise and fall times, however, the response gets progressively smaller. This is because an evoked response is the summation of many neurons firing at once. Since the individual firing rates and threshold sensitivities of these neurons are not all alike, to get them to fire at the same time you must give a stimulus that quickly goes from below most neuron thresholds to above many neuron thresholds.
Each neuron will thus start its output pulse train at the same time, so all the initial pulses will be superimposed to form the evoked response peak. But subsequent neural pulses will not be in synchrony, so the response rapidly falls. With a slow rise time, the different neural thresholds are reached at differing times, so the number of pulses superimposing to form the evoked response is reduced.
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