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Differential processing of sounds with varying spectral and temporal complexity in bilateral temporal cortex
J Mehnert1, S Telkemeyer1, C Schmitz1,2, J Steinbrink1, H Obrig1,3, I Wartenburger1,4
1Berlin NeuroImaging Center, Charité, Berlin, Germany/2NIRx Medizintechnik GmbH, Berlin, Germany/3Day Clinic for Cognitive Neurology, University Hospital, Leipzig, Germany/4Department of Linguistics, University of Potsdam, Potsdam, Germany

Introduction: The extraction of linguistic content from acoustic signals requires the analysis and decoding of the spectral and temporal information. To characterize the cerebral basis of processing acoustic signals we presented 10 noise-like stimuli containing different grades of temporal or spectral complexity (described in Schönwiesner et al., 2005) while measuring the cerebral oxygenation changes using Near-Infrared Spectroscopy (NIRS). Psychoacoustic models assume a differential sensitivity of the left and right auditory cortices to spectro-temporal features of speech (Zatorre and Belin, 2001; Hickok and Poeppel, 2007). We therefore hypothesized hemispheric asymmetries and parametric graduations in the cortical oxygenation levels to vary parametrically in either the number of spectral components (spectral complexity, 4-16 spectral components) or the temporal modulation rate (temporal complexity, 2-40 Hz temporal components).

Methods: 23 healthy, right-handed adults (15 female) listened to pseudo-randomized stimuli (duration: 3 sec, variable interstimulus interval) and had to judge the identity of the successive stimuli in a one-back manner. Cortical oxygenation changes were measured by optical tomography using a NIRS imaging system (DYNOT 232 by NIRx Medizintechnik GmbH, Berlin, Germany). In total, 16 emitting and 30 detecting fiber optic probes (optodes) were positioned in two rectilinear grids (2 cm probe separation) over the left and right hemispheric temporal cortices. The setup affords simultaneous recording of NIRS signals for different inter-optode distances. We used close-spaced (2 cm) channels to correct the signal from farther spaced (2.8 cm) fiber pairs to reduce the influence of extra-cortical hemodynamics (e.g., oxygenation changes in the skin). An (semi-) automated movement artifact correction was applied. Attenuation changes measured at 760 nm and 830 nm were converted into changes of oxygenated [oxy-Hb] and deoxygenated hemoglobin [deoxy-Hb] based on a modified Beer–Lambert law (Cope and Delpy, 1988). A General Linear Model based analysis provided beta-values, t-values, and time courses for each stimulus and channel.

Results: Behavioral: judging the identity of two successive stimuli was more difficult when stimuli differed in spectral complexity (5 out of 10 pairs around chance level) than in temporal variation (2 out of 10 pairs around chance level). NIRS results: the analysis of variance showed a significant main effect of tempo in 4 channels with a parametric variation of signal changes. For spectral variation we found a clear main effect of hemisphere with greater signal changes over the right hemisphere in 2 channels and a main effect of spectral variation.

Conclusions: We show that the modulation of temporal and spectral complexity in noise-like stimuli, which are parametrically varied with respect to their spectral and temporal complexity, elicits a lateralized and focal response. The underlying vascular response is reflected by oxygenation changes in temporal cortices as detected by NIRS. The data are in line with recent models postulating a distributed network sub serving the analysis of specific acoustic properties. These models also link the lateralization and specialization of the auditory cortices to basic properties of the brain structures relevant for language processing.

References:
Cope, M (1988), 'System for long-term measurement of cerebral blood and tissue oxygenation on newborn infants by near infra-red transillumination', Medical & Biological Engineering & Computing, vol. 26, no. 3, pp. 289-294.
Hickok, G (2007), 'The cortical organization of speech processing', Nature Reviews Neuroscience, vol. 8, no. 5, pp. 393-402.
Schönwiesner, M (2005), 'Spectral and Temporal Processing in the Human Auditory Cortex – Revisited', Annals of the New York Academy of Sciences, vol. 1060, no. , pp. 89-92.
Zatorre, R (2001), 'Spectral and temporal processing in human auditory cortex', Cerebral Cortex, vol. 11, no. 10, pp. 946-953.