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 Human research

 Functional organization of the auditory cortex (parcellation, hemispheric specialization)

To date, there is no unifying concept that comprises different hypotheses on the functional organization of the auditory cortex with respect to a dichotomic specialization of the hemispheres (speech vs. music, linguistic vs. affective, temporal vs. spectral) or the dichotomic processing pathways within one hemisphere (dorsal/ventral, anterior/posterior). Furthermore, the functional parcellation of the auditory cortex is still essentially unclear. Our main aim is to systematically tackle these problems by studying the processing of fundamental stimulus properties in combination with specific tasks. Using this approach several studies revealed that fMRI activation is strongly dependent on the task and less on the stimulus properties. These findings also led to a detailed picture of hemispheric specialization. In this context, the specialization of the right auditory cortex on the processing of the direction of pitch-changes and of the left auditory cortex on the sequential processing of discrete acoustic events can be considered as important cases.

The integration of electrophysiological (EEG, MEG) with BOLD-dependent data (fMRI) is of crucial importance in order to obtain detailed information about both the temporal course of centres of activity as well as their spatial location. Results obtained from anatomical and functional MRI studies form the basis for source modelling in MEG- and EEG studies. Subsequently, dynamic interaction of areas within a hemisphere as well as across hemispheres can be described in the framework of a coherence analysis of those electrophysiological data.

Parcellation of the auditory cortex

At present the only potential correlates of different auditory cortex fields in humans are multiple foci of activation which can be observed with fMRI in individual subjects. But in view of the large anatomical variability (see Fig.), it is a major problem to establish identity of activation clusters across individuals. Using a combination of anatomical landmarks and clusters of functional activation, we have distinguished four territories. Two territories cover the characteristic stripes of activation on Heschl's gyrus (HG). T1 on the antero-medial aspect contains primary auditory cortex fields and shows the strongest level-dependent activation (Brechmann et al., 2002). T2 is centered on Heschl‘s sulcus and contains secondary fields (presumably lateral belt areas). TA is located anterior to HG on the planum polare and is specifically involved in auditory foreground-background decomposition tasks (Scheich et al., 1998; Brechmann et al., 2002). T3 with several clusters of activation mainly covers the planum temporale. On the right hemisphere, we revealed a movement-sensitive area (Baumgart et al. 1999) and a specific involvement in the processing of the direction of frequency-modulated tones (Brechmann & Scheich, 2005). T3 on the left hemisphere is part of Wernicke's area and thus responsible for the processing of semantic aspects of speech. However, left T3 also responds to pure tones (see Fig.) and is differently activated by consonant vs. dissonant musical tunes (Passynkova et al., 2007).



S tr 1: Sulcus transversus primus,
SI: Sulcus intermedius
HS: Heschl's Sulcus
CS: Circular Sulcus
HG: Heschl's Gyrus
PT: Planum temporale

Hemispheric specialization

Hemispheric specialization of auditory cortex for sounds including speech and music has been viewed as analytical dispositions for spectral or temporal sound properties. However, any sound contains spectral and temporal information. The significance of one or the other for a receiver may change with attention or with conceptual listening involving e.g. discrimination or categorization. Using an identical stimulus-set of frequency-modulated tones (FM) we showed that different cognitive tasks can modify the representation of spectral and temporal sound properties in the two hemispheres compared to simple exposure (Brechmann & Scheich, 2005, Fig. 1). A complementary MEG study provided further insight into the temporal course of the hemispheric differences in response to FM tones (König et al., 2008). Two further studies revealed that certain tasks involving the timbre of sounds lead to specific activation of left auditory cortex areas (Deike et al. 2004, Passynkova et al., 2007) although it is widely believed that the processing of timbre is mainly a function of the right hemisphere.
To systematically characterize such task-dependent hemispheric specialization we developed a method taking advantage of the contralateral organization of the auditory pathway (Behne et al., 2005). In principle we use monaural stimulation of the left or right ear and compare the activation in each hemisphere under the same conditions in which in addition white noise is presented to the contralateral ear (Fig. 2). As to the processing of FM-direction we showed that the influence of noise on the activation of the contralateral auditory cortex is restricted to the right hemisphere which is specialized for the processing of FM-direction. This method is currently used to evaluate other kinds of hemispheric specializations of the auditory cortex.

Fig. 1: The use of an identical stimulus-set of frequency-modulated tones (FM) involving different cognitive tasks leads to differential involvement of the two hemispheres . (A) categorization of FM-duration (short vs. long), (B) categorization of FM-direction (rising vs. falling).



Fig. 2: Information about FM tones presented to the right ear in (A) does not directly reach the “proper” hemisphere via the excitatory pathway but presumably via the contralateral hemisphere. Thus, FM direction has to be processed in competition to white noise which directly arrives at the right auditory cortex. This competition which leads to an increase in activation does not occur when the FM tones are presented to the left and noise to the right ear (B).

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