Congratulations to Dr. Kevin Robert Smith, who just this morning successfully defended his dissertation here at Talking Brains West (aka Hickok Lab at UC Irvine). Kevin's thesis started out asking the question, What is the nature of the human auditory "where" stream? But ended up concluding that there might not be a "where"...I had originally gotten interested in spatial hearing, motion in particular, because folks like Josef Rauchecker and Tim Griffiths were finding "motion" sensitive activations in the human planum temporale, darn near our beloved Area Spt. This meant there were two presumed dorsal stream functions (spatial "where" and sensory-motor "how") co-mingling in the same neural neighborhood. I wondered whether the dorsal stream might be composed of two anatomically separate, and functionally independent systems, or whether the very same neural real estate was occupied by spatial and sensory-motor systems.
Enter Kevin Smith (my grad student) and Kourosh Saberi (my colleague and collaborator). Together we decided first to make sure we could replicate the auditory motion effects in the planum and then see how it relates to area Spt. Kourosh, our local auditory guru, suggested that we build a control into our first experiment. While other folks had contrasted moving with non-moving sounds and found PT activation, no one had tried to assess the effects of non-moving but spatially varying sound sources. So we had the usual moving condition and had another condition in which stationary sounds randomly appeared at different locations during the activation block (other studies used blocks of stationary sounds that only appeared at one location). To our surprise, the non-moving but spatially varying stimuli activated the PT "motion area" just as robustly as the moving stimuli. Kevin's second experiment searched for a motion-selective area using an event-related/adaptation design. Same result: PT regions that respond to motion also respond just as well to non-moving but spatially varying stimuli.
So there's no motion area. But clearly there is still a "where" pathway right? After all, in both of Kevin's expeirments, manipulating the location of a sound source causes activation in the PT. Well, Robert Zatorre for one, might argue otherwise. In a 2002 paper, Zatorre found that putative "spatial" activation effects were only evident when spatial information provided cues to auditory object identity. He suggested that there is no pure "where" pathway. Instead, where interacts extensively with "what."
We were not so sure, so in Kevin's third experiment (almost submitted, right Kevin?), he compared activation during listening to a single talker that was either presented at a single location, or bounced around between three locations. He found more PT activation for the three location condition than the one location condition. A clear spatial effect, right? Yes, but... He also had a three-talker condition: three voices presented simultaneously. These voices were presented either at a single location (and stayed put) or at three different locations (and also stayed put at their respective locations). We found more activation for the three location condition than the one location condition, which might be viewed as a spatial effect, except that this 3-talker/3-location condition produced significantly more activation than the 1-talker/3-location condition. This is odd according to a pure spatial account because the 3-talker/3-location condition doesn't involve any spatial change -- all sound sources stay put -- whereas the 1-talker/3-location condition involved a lot of spatial change (new location every second). It seems that the increase in activation for the 3-talker/3-location condition results from the interaction for spatial and object information.
In other words, I think Zatorre is right. There is no pure auditory "where" system, but rather a system that uses spatial information (perhaps computed subcortically?) to segregate auditory objects.
So what is the auditory dorsal stream doing? I would say sensory-motor integration is the best characterization, except that I have suggested that such a system may not be part of the auditory system proper (see "The Auditory Stream May Not Be Auditory"). Maybe the "stream" concept is nearing the end of its usefulness. Rather than thinking about processing streams within a sensory modality, maybe we need to start thinking about interfaces between sensory systems and other systems, such as a sensory-motor interface and a sensory-conceptual interface. So where does that leave "where"? Who knows.
References
Smith KR, Saberi K, Hickok G.
An event-related fMRI study of auditory motion perception: no evidence for a
specialized cortical system.
Brain Res. 2007 May 30;1150:94-9. Epub 2007 Mar 7.
Smith KR, Okada K, Saberi K, Hickok G.
Human cortical auditory motion areas are not motion selective.
Neuroreport. 2004 Jun 28;15(9):1523-6.
Zatorre RJ, Bouffard M, Ahad P, Belin P.
Where is 'where' in the human auditory cortex?
Nat Neurosci. 2002 Sep;5(9):905-9.
No comments:
Post a Comment