25-08-2017, 09:32 PM
Amplitude Modulation Detection in the auditory brainstem
Amplitude Modulation Detection in the auditory brainstem.docx (Size: 28.08 KB / Downloads: 27)
The diagram shown in figure 1 shows the elements of the model. The cochlea filters
the incoming sound signal. Since the width of the pass-band of a cochlear band-pass filter is proportional to its cut-off frequency, the filters will not be able to resolve the individual harmonics of a high frequency carrier (>3kHz) amplitude modulated at a low rate (<500Hz). The outputs of the cochlear filters that have their cut-off frequency slightly above the carrier frequency of the signal will therefore still be modulated in amplitude at the original modulation frequency. This modulation component will therefore synchronize a certain group of Chopper cells. The synchronization of this group of Chopper cells can be detected using a coincidence detecting neuron, and signals the presence of a particular amplitude modulation frequency. This model is biologically plausible, because it is known that the choppers synchronize to a particular amplitude modulation frequency and that they project their output towards the Inferior Colliculus (amongst others). Furthermore, neurons that can function as coincidence detectors are shown to be present in the Inferior Colliculus and the rate of firing of these neurons is a band-pass function of the amplitude modulation rate. It is not known to date however if the choppers actually project to these coincidence detector neurons.
To understand why the Choppers will synchronize for a certain amplitude modulation frequency, one has to look at the signal envelope, which contains temporal information on a time scale that can influence the spiking neurons. The 5kHz carrier itself will not contain any temporal information that influences the spiking neuron in an important way. Consider the case when the modulation frequency is similar to the chopping frequency (figure 2). If a Chopper then spikes during the rising flank of the envelope, it will come out of its refractory period just before the next rising flank of the envelope. If the driven chopping frequency is a bit too low, the Chopper will come out of its refractory period a bit later, therefore it receives a higher average stimulation and it spikes a little higher on the rising flank of the envelope. This in turn increases the chopping frequency and thus provides a form of negative feedback on the chopping frequency. This therefore makes spiking on a certain point on the rising flank of the envelope a stable situation. With the same reasoning one can show that spiking on the falling flank is therefore an unstable situation. Furthermore, it is not possible to stabilize a cell driven above its maximum chopping rate, nor is it possible to stabilize a cell that fires more than once per modulation period. Since a group of similar choppers will stabilize at about the same point on the rising flank, their spikes will thus coincide when the modulation frequency allows them to.