Asymmetrical neural processing of amplitude modulated sounds: a psychophysical, fMRI and TMS investigation

Human speech is the most behaviourally important and complex signal that the human brain is required to process yet it does so with remarkable ease. Speech is composed of highly complex amplitude modulations over time and these modulations are known to be crucial for intelligibility. There is evidence for hemispheric asymmetries in processing auditory modulations over different timescales and multiple models have been proposed to account for these. The procedure by which the auditory system extracts and processes these modulations is not fully understood. Psychophysical, neuroimaging and non-invasive neurostimulation techniques can be combined in complementary ways to potentially provide unique insights into this problem. Functional magnetic resonance imaging (fMRI) and transcranial magnetic stimulation (TMS) are relatively novel methods that have not been previously applied in combination to investigate amplitude modulation processing. Three psychophysical and fMRI-guided TMS studies were conducted in order to address the following research questions. Firstly, is fMRI-guided TMS an effective method for modulating AM processing? Secondly, are different TMS protocols more or less effective at modulating AM processing? Finally, is fMRI-guided TMS an effective method for further understanding the functional asymmetry of speech processing? Online dual pulse TMS to right auditory cortex was shown to be effective at modulating 4 Hz AM detection accuracy. State-dependent TMS to left auditory cortex was shown to be effective at modulating 40 Hz AM detection accuracy, but the effects were complex. Continuous theta burst stimulation was not shown to be effective at modulating AM depth discrimination ability. It was thus found that fMRI-guided TMS can be an effective tool for modulating AM processing, however, efficacy differs depending on the specific TMS protocol used. Further, fMRI-guided TMS can be used to investigate functional asymmetry of speech processing, however some important caveats apply.