Auditory Cortex "What" and "Where" Processing Streams

Researcher: Iiro Jääskeläinen, Jyrki Ahveninen

Recent studies have provided evidence for the existence of segregated pathways within the auditory cortex for processing of auditory object/content and location features. These processing pathways have been termed as the “what” and “where” processing streams, analogously to the visual system. The “what” processing stream has been suggested to progress anteriorly/ laterally from the koniocortex as a function of increasing complexity of auditory stimulation from simple sinusoids to more complex auditory patterns, such as phonetic sounds. Conversely, increased specificity for “where” information has been observed in areas posterior to the koniocortex. Supporting evidence has been detected by cellular-level measurements in non-human primates, human non-invasive imaging studies, as well as by abnormalities of spatial hearing in patients with lesions in posterior auditory cortex. This segregation of the processing streams is also reflected in areas outside of the temporal lobe; the posterior “where” processing stream was observed to involve parietal cortex and superior aspects of prefrontal cortex, whereas the “what” processing stream was noted to involve activation of the inferior frontal gyrus. Also, in monkey tracer studies the “what” and “where” processing streams were connected anatomically to different areas of the prefrontal and parietal cortices.

In our ongoing collaboration with Massachusetts General Hospital / Harvard Medical School NMR Center, we aim at elucidating and modelling the neural mechanisms underlying processing of object and spatial location information. In our recent study, electromagnetic activity was localized using our fMRI/MEG/EEG analysis techniques to areas corresponding with those implicated in previous work to underlie the “what” and “where” processing streams. We observed differential frequency-tuning in the posterior and anterior auditorycortex areas that could reflect the “what” and “where” processing streams, the neurons in the posterior “where” pathway being more broadly tuned on sound frequency than neurons in the anterior “what” pathway involved in fine discrimination of object features. Further, we observed a close relationship between the suppression of the posterior auditory-cortex N1 activity and reduced behavioral distractibility, implicating that the “where” pathway conducts a relatively fast and coarse stimulus novelty analysis, which could be intimately linked to behavioral “flight-or-fight” responding. Conversely, the anterior N1 activity is presumably generated in areas processing the “what” information. This is also suggested by our preliminary results showing that the anterior N1 activity corresponds to the mismatch response, which in turn predicts attentional discrimination of minute differences in sound frequency, a hallmark of the “what” processing.

In our further studies, we have tentatively confirmed that neural ensembles occupying regions posterior to the human primary auditory cortex are specifically tuned to 3-D sound location cues. Further, we observed that selective attention to sound location features selectively "sharpened" the 3-D location tuning of the underlying neurons (see Fig.48). These studies were conducted at our collaborative laboratory, the Massachusetts General Hospital - Harvard Medical School - Massachusetts Institute of Technology Athinoula A. Martinos Center for Biomedical Imaging in collaboration with Drs. Jyrki Ahveninen, Tommi Raij, Sari Levänen, Matti Hämäläinen and John W. Belliveau.

Figure 48

Figure 48: Grand-average ECD amplitude waveforms for the posterior (Top) and anterior (Bottom) N1m sources in the right hemisphere of 5 subjects. The subjects were here attending to the sound location. Note that the amplitude of the posterior N1m to the second sound (Top) was clearly larger when the sound location changed in relation to the first sound.