Human cortical functions in auditory change detection evaluated with multiple brain research methods

The present thesis examined in human subjects the brain mechanisms involved in the detection of unattended auditory changes. According to previous results, the auditory change-detection mechanism consists of several anatomical and functional units which are activated within the first 200 ms after the onset of sound change. In order to investigate this mechanism with high temporal and spatial resolution, electroencephalography, magnetoencephalography, functional magnetic resonance imaging, and the new method of recording the event-related optical signal were used. Näätänen’s model of auditory change detection assumes that change detection is based on a memory representation of the past auditory events, which contain information about the physical characteristics (e.g., frequency) and abstract relations of sounds (e.g., ascending vs. descending tone pair). Furthermore, the model assumes that change detection occurs independently of attentional resources and may lead to a switch of attention to the change occurring in unattended sounds. The results of the present studies were in concordance with these assumptions: First, it was shown that the memory system underlying auditory change detection operates also on categorical speech information. Second, the relation of the change-detection mechanism and volitional control functions was further clarified by showing that the subject’s foreknowledge of sound changes does not affect the functioning of the change-detection mechanism. Third, anatomical information about the temporal-frontal lobe network of brain areas involved in auditory change detection was provided. (The exact location of the brain areas in the frontal lobe involved in change detection was not known previously.) Furthermore, it was shown that the temporal-frontal lobe network was activated in an order that is congruent with the assumption that a temporal-lobe change-detection process triggers subsequent processes in the frontal lobe associated with the initiation of an attention switch. Finally, based on the present results, an updated version of Näätänen’s model was proposed.