Okay, just had this material in a class this semester...and I've done some research... There has been a long standing debate over functional specialization of the different hemispheres. One hypothesis is that the left hemisphere deals with high frequency information, and the right deals with low frequency information. This does not mean that the auditory information is actually divided disjointly to the hemispheres, but as a general theme in lateralization, the left hemisphere resolves higher frequency information... This hypothesis explains that the left hemisphere can represent fine timing information by having a more full set of frequencies, and that the right hemisphere resolves pitch contour by pertaining more to slow changes in frequency.
but....the neurological side doesn't really compare with this hypothesis. It's not something that can be boiled down to a single hypothesis. What's really going on is that subcortical structures in the auditory pathway differentially project to the right and left hemispheres.
in an article by Ligeois-Chavel (2001, New York Academy of Sciences), intracerebrally recorded evoked potentials (IEPs) were taking during a simple pitch experiment. 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.
While the coding in the auditory cortex is "like" an FFT, it is NOT an FFT. there are many fine differences....the whole auditory pathway is not real clear cut where functions are localized, exactly. At different stages in the auditory pathway, frequencies can be tonotopically or rate encoded. The cochlea is actually a dynamic organ, in and of itself->it's not a passive organ with a system of resonators and transducers, it's like a bank of critically tuned resonators at unstable equilibrium (like a hair trigger). The cochlea encodes a train of phase locked pulses, which are transmitted by the auditory nerve, which contains both tonotopically and timing encoded (fine timing) information. The cochlea can represent frequencies with timing encoding (I'm searching my brain for a better term than timing encoding, but not sure) up to around 5 kHz (it seemed a little high to me, when I first read the number...I thought it would be around 1 kHz). As we proceed through the auditory pathway, the ability to encode timing decreases, so that at the cortical level, timing encoding only pertains to the low <100-200 Hz frequencies. Hence, the primary auditory cortex accomplishes things by rate encoding (i.e. the strength of the tone is represented by the frequency of neuron action potentials, and not by timing between them)
(I can't remember these too well at the moment, I just graduated this spring...and my brains a little fried...there may be more. I just feel like I've left some out) okay, it goes cochlea->auditory meatus (midbrain ~ pons/mesencephalon)->lateral lemniscus (of pons)->superior olivary complex (of pons)->inferior colliculus->medial geniculate nucleus (of thalamus)->primary auditory cortex. (the named areas occur on both sides of the brain. There is also, of course, and auditory chiasm taking place somewhere around the superior olivary complex)
It's not real clear what each of these things does, and how they contribute to specific functions of the auditory system. There are some differences in encoding at different areas, such as the inferior colliculus. The IC is organized into isofrequency layers, each layer encodes a different band of frequencies (this is more "like" a wavelet transform), and fine timing information here works to accomplish sound localization (also has to do with eye position information from superior colliculus) There may be other information that is encoded at the level of the IC (I'm leaning strongly towards pitch in the IC). Different kinds of information are projected from the subcortical structures to the cortical levels...it just depends upon what kind of processing is taking place, and how it's organized.
but the point (there IS a point) is that there are many different representations of frequency information along the auditory pathway. Each structure named does some kind of processing and passes the processing along to the next structure. Ultimately, there are several different ways that sound is encoding up to and including the primary auditory cortex. It is "like" and FFT, but it is also "like" a wavelet transform, and it is also "like" a bank of hair-trigger resonators.
Chuck
On 5/24/06, Chuckk Hubbard badmuthahubbard@gmail.com wrote:
http://cercor.oxfordjournals.org/cgi/content/full/11/10/946
I came across this fact researching my final paper for Perception class: the left auditory cortex is known to resolve temporal changes in sound better, while the right auditory cortex resolves tonal and harmonic information more finely. As soon as I read this I thought of FFT. Could it be the difference between the brain hemispheres is related to the auditory cortices having different block sizes? -Chuckk
-- "Far and away the best prize that life has to offer is the chance to work hard at work worth doing." -Theodore Roosevelt
PD-list@iem.at mailing list UNSUBSCRIBE and account-management -> http://lists.puredata.info/listinfo/pd-list