[Pd] OT- FFT and human auditory cortex

[Chuckk Hubbard]

On 5/25/06, padawan12 <padawan12 at obiwannabe.co.uk> wrote:
> Very interesting. You two Chuck(k)s certainly got me thinking
> over some old stuff, but this left right thing is really
> fascinating. For one thing it makes me think maybe there's
> more to localisation than we know about right now.

Glad to have awakened something interesting.

>
> > There has been a long standing debate over functional specialization
> of the different hemispheres.
>
> Can only comment from a cog sci perspective, but I remember a general
> theme in Fodor and some other texts that we often see complementary
> behaviour with one hemispherical faculty taking up the loose ends or
> blindspots left by the other. In that regard all our senses are
> "stereoscopic", or at least not as discrete per organ as we assume,
> we like to get two points of view from which perception emerges.
> (eg taste is linked to smell)

It makes sense this way, since the system grew around itself so to
speak, improvements cropping up and lingering without ever overhauling
the whole thing.  I was happy that my perception teacher acknowledged
that there are more than 5 senses.  He was reluctant to give a number.
 I think it's inane to teach children they have 5 senses and ignore
equilibrium and temperature.  Like children wouldn't be able to
comprehend these things.

Doesn't it also make sense, however, that even if two sides, or just
two areas, performed some function fairly similarly at one time,
gradually whichever side or area was more acute or earlier in the line
in more individuals would be emphasized, and the importance of the
other area performing that function would decrease?  Some birds have
photosensitive cells in their pituitary glands!  We've no use for
that, the thing being so insulated in there.

>
> Since time and frequency acuity are mutually exclusive at the limit
> wouldn't it make sense that the brain would evolve to have one side
> process things using a complementary principle than the other?

I don't know much about the brain, but I don't see why it would have
to be the sides in that case, it could just as well be other
structures in the brain.


>
> > The cochlea is actually a dynamic organ, in and of itself
>
> And can exhibit very localised and very fast adaptive behaviour almost
> within itself as a protection mechanism. This is how it achieves such
> an awesome dynamic range,  after lying in a soundproof isolation
> tank for many hours subjects report hearing a hissing noise which we
> assume is brownian motion of air molecules. On the other hand an impulse
> at 130dBU can effectively shut down the auditory system and dilate the
> tympanic membrane with the same kind of reflex to an eyelid closing.
> I've always found this amazing. That our senses are capable of picking
> up the smallest practically measurable "quanta" of information (eg
> the eye retina can detect a single photon of light)

And the ossicles can actually be twisted to angles of less amplification.
Our perception teacher pointed out that we put our clothes on at the
beginning of the day and almost immediately stop feeling them.
Leonard B. Meyer said that emotional response to music comes from
"inhibited tendencies".  His example was of a man reaching into his
shirt pocket for a cigarette.  He pulls the pack out, removes a
cigarette, lights it and smokes it, and isn't aware of any of it.
Might not even be able to remember whether he smoked a cigarette
later.  But if he reaches in and there's no cigarettes, he experiences
immediate awareness and emotional affect.  Same with drones vs.
modulations in music.  This is why I love Frank Zappa.

>
> > tonotopically
>
> What do you mean by that term please? In simple language.

In this context:
----------------
"The IEPs showed that the right hemisphere tonotopically encodes pitch
information->there were position different "signatures" like
event-related potentials that varied with respect to frequency.  In
the left hemisphere, there was no tonotopic organization, the areas of
the brain under study responded equally to a wide band of
frequencies."
----------------

I take "tonotopically" to refer to different frequencies exciting
different physical places.  Like on the basilar membrane.  It isn't
only the frequency of excitation, it's also the area of most
excitation.  See below...

>
> > I'm searching my brain for a better term than timing
> > encoding, but not sure
>
> PPM (pulse position modulation/encoding)? Well, that's what I'd call
> it with my electronic engineers hat on.

Wow, now there's a term that would get a blank stare from me!  I'd say
temporal, depending on the context.  Or "frequency of impulses".  I'd
also use "localized" as opposed to "tonotopic", but that's just my own
vocab.

> See below vis Shannon, timing is increasingly less relevant higher up
> the tree.

"increasingly less" lol
The things we type when we find something stimulating.


>
> > so that at the cortical level, timing
> > encoding only pertains to the low <100-200 Hz frequencies.
>
> The magic number at ten to twenty milliseconds has fascinated me
> for ages. It comes up time and again, in Garbors work, in Warren,
> Jones and Lee (all cognitive scientists - check McAdams and Bigand
> compilation "Thinking in Sound"). Something very important is
> happening here. It is the essence of granular synthesis and marks
> the important point where discontinuity becomes continuity
> (eg grains "fuse" at this point and we have to move to a wavelet
> like model where position and frequency become combined) Can you
> throw any hypothesis at this speaking as neurobiologist?
> Do you think maybe one side of the brain is taking over from
> the other?

Here is what I've gathered from both my own reading and my perception
class.  I'm sure Charles can elucidate, and I suspect this is part of
what he said somewhere in the parts I didn't quite follow.
The nerves triggered by the cilia in the basilar membrane can only
fire 300-500 times a second (in my notes I have written 333x, but I'm
not sure why).  But they are innervated in bundles of 10, and so it is
possible to hear sounds up to 3k-5k just from frequency of nerve
impulses (is that what he means by timing encoding? awful coincidence
of numbers there).  Fish and amphibians only have this method of pitch
recognition, and can't hear any higher frequencies.  At low
frequencies-- up to 100 Hz is the magic number I read somewhere-- the
entire basilar membrane undulates.  But we can hear famously up to 20k
(24k according to my teacher).  At higher frequencies, less and less
of the basilar membrane undulates, and specific points of excitation
register as specific pitches.
Later, when we discussed sensitivity to frequencies, I asked my
teacher if the overlap of methods between about 100 Hz and 3kHz has
anything to do with our hearing being most sensitive in that range,
and that being where speech tends to occur.  He didn't know.  Ha!
If I understand correctly, Charles has had some experience with how
the brain actually processes these two frequency detection devices
differently.

I also wonder if this entire-membrane-excitation-at-low-frequencies
thing has to do with why people are so moved by bass.  But I'm sure
there are many reasons for that.

>The crazy thing about this adaptive
> system is that it's the brain itself which is deciding (in a dynamic
> fashion) what these salient features are according to context.
> I remember from somewhere that in 60ms a human or monkey can tell the
> difference between a bolt of lightning, a twig snapping and a raindrop
> on a leaf. All this information must be in the very small attack of
> a signal. It happens long before the frontal brain can classify
> and tag the sound with a word. While it's obvious from a survival
> POV in evolutionary biology it also indicates that adaptive feedback
> must be occurring at a very low level.

I've read of how these shortcuts appear in response to emotional
situations.  You jump when the snake starts moving, only understanding
what it was afterward.  I was thinking of this in assuming that there
would be a difference as to which ear the sound comes in, even though
the information is shared between the hemispheres.  It gets to the
area that does the processing before the area that receives messages
from that area.

Just to throw another anecdote out there,
http://www.mit.edu/~perfors/oldhotornot.htm
She says women found men more attractive if their names had
higher-formant vowels.  In that case I'm doubly screwed with my first
and last name...
I thought her hypothesis was ridiculous, that women today are more
attracted to guys who seem smaller and less threatening.  I suggested
to her it had to do with the complexity of the tone, like birdsong.
Rock singers have always been loved for high-pitched screaming,
guitarists for searing solos, and it isn't because it makes them seem
small.  It could also just be increased muscle tension in the mouth
saying some words.

To get somewhat on-topic, Charles, what brings you to Pd?  Have you
used it in research at all?  For auditory stuff or DSP?  I think this
list could benefit from people whose primary interest is other than
programming or composing.

-Chuckk