Approaches to the cortical analysis of auditory objects.

We describe work that addresses the cortical basis for the analysis of auditory objects using 'generic' sounds that do not correspond to any particular events or sources (like vowels or voices) that have semantic association. The experiments involve the manipulation of synthetic sounds to produce systematic changes of stimulus features, such as spectral envelope. Conventional analyses of normal functional imaging data demonstrate that the analysis of spectral envelope and perceived timbral change involves a network consisting of planum temporale (PT) bilaterally and the right superior temporal sulcus (STS). Further analysis of imaging data using dynamic causal modelling (DCM) and Bayesian model selection was carried out in the right hemisphere areas to determine the effective connectivity between these auditory areas. Specifically, the objective was to determine if the analysis of spectral envelope in the network is done in a serial fashion (that is from HG to PT to STS) or parallel fashion (that is PT and STS receives input from HG simultaneously). Two families of models, serial and parallel (16 in total) that represent different hypotheses about the connectivity between HG, PT and STS were selected. The models within a family differ with respect to the pathway that is modulated by the analysis of spectral envelope. After the models are identified, Bayesian model selection procedure is then used to select the 'optimal' model from the specified models. The data strongly support a particular serial model containing modulation of the HG to PT effective connectivity during spectral envelope variation. Parallel work in neurological subjects addresses the effect of lesions to different parts of this network. We have recently studied in detail subjects with 'dystimbria': an alteration in the perceived quality of auditory objects distinct from pitch or loudness change. The subjects have lesions of the normal network described above with normal perception of pitch strength but abnormal perception of the analysis of spectral envelope change.