Direct causal evidence for auditory cortical "what" and "where" processing streams provided by transcranial magnetic stimulation
Since initial observations in animal models, there has been accumulating evidence suggesting that sound identity and location information is processed in parallel anterior and posterior auditory-cortex streams in humans. Human neuroimaging evidence has, however, not been indisputable since posterior auditory cortical areas have been observed to be sensitive to also other than auditory spatial features. Furthermore, while neuroimaging findings are beyond any doubt highly informative, they cannot not per se provide causal evidence for the involvement of anterior and posterior auditory cortical areas in processing of “what” and “where” auditory information. Transcranial magnetic stimulation guided by magnetic resonance imaging is a method that, by making it possible to transiently deactivate specific cortical areas, allows causal testing of the involvement of cortical regions in task performance.
In their recent study, Dr. Jyrki Ahveninen et al. (2013) transiently inhibited bilateral anterior and posterior auditory cortical areas in healthy volunteers when they were performing sound localization and sound identity discrimination tasks. The transient inhibition was accomplished with paired-pulse transcranial magnetic stimulation guided by magnetic resonance imaging, with the pulses delivered 55-145 ms following the to-be-discriminated auditory stimuli. The anatomical areas targeted by the transcranial magnetic stimulation were further confirmed with individual-level cortical electric field estimates. It was observed that transient inhibition of posterior auditory cortical regions delayed reaction times significantly more during sound location than sound identity discrimination. In contrast, transient inhibition of anterior auditory cortical regions delayed reaction times significantly more during sound identity than sound location discrimination.
These highly exciting findings provide direct causal evidence in support of the parallel auditory cortex “what” vs. “where” processing pathways in humans. These results not only nicely help clarify the still-debated issue of whether the posterior human auditory cortex participates in auditory space processing, but methodologically the findings further demonstrate the feasibility of using paired-pulse transcranial magnetic stimulation in targeting cortical areas that are located very close to one another. The introduction of methods that allow precise estimation of the cortical targets of transcranial magnetic stimulation also provides an important methodological advance.
Reference: Ahveninen J, Huang S, Nummenmaa A, Belliveau JW, Hung A-Y, Jaaskelainen IP, Rauschecker JP, Rossi S, Tiitinen H, Raij T. Evidence for distinct auditory cortex regions for sound location versus identity processing. Nature Communications (2013) 4: 2585. http://dx.doi.org/10.1038/ncomms3585