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Hi Ben,
One example of envelope based auditory processing is
the cochlear implant. Envelopes are usually extracted from bandpass filter
outputs by full-wave rectification and low pass filtering. If it's any use
to you, the filter cutoff frequency commonly used for this is 200 Hz. (And
some experimental CI processing strategies have used
half-wave rectification without any smoothing at all).
I'm not sure if there is a neural modeling or
physiological reason for the cutoff frequency. 200 Hz retains the
temporal modulations of the speech envelope, which some CI listeners are able to
hear as a cue to fundamental frequency.
As you say, in normal hearing, neurons can follow much
higher frequency stimuli. Although it's not exactly my area, I would assume
you would only use a low (e.g. 50 Hz) cutoff frequency if you wanted to model
auditory processing without temporal cues. For a
physiological model, I don't know of any reason for 50 Hz. In addition to
neural refractory effects, there are lowpass cutoff frequencies related to
the synaptic delay and re-sequestering of neuro-transmitter by the inner
hair cells. But that too is much faster, I think.
Sorry I don't have any references. Maybe someone else can
comment.
Regards,
Daniel Taft
From: AUDITORY - Research in Auditory Perception [mailto:AUDITORY@xxxxxxxxxxxxxxx] On Behalf Of Hornsby, Benjamin Wade Young Sent: Tuesday, 24 February 2009 3:55 AM To: AUDITORY@xxxxxxxxxxxxxxx Subject: [AUDITORY] envelope extraction Hi all, I hope the answer to this question
is not too obvious but,… We have recently been talking about auditory processing
models, in specific, temporal processing models, many of which incorporate an
envelope extraction stage. To do this I’ve seen in many cases the use of half
wave rectification followed by a low pass filter. The cutoff for this filter is
generally a pretty low frequency, say around 50 Hz or so. A colleague suggested
that the actual cutoff frequency should be based on the assumed time constant of
the system or subsystem being evaluated. I’ve been trying to determine the
physiologic rational for such a low frequency filter (slow time constant) in
models of auditory processing. Neural processing limitations like the refractory
period of the neuron are much faster than this. Anyone have a reference or two
that might help explain the physiologic rational for this low frequency filter
cutoff? Thanks
much, Ben |