Neuroimaging paradigms for tonotopic mapping (I): the influence of sound stimulus type

Although a consensus is emerging in the literature regarding the tonotopic organisation of auditory cortex in humans, previous studies employed a vast array of different neuroimaging protocols. In the present functional magnetic resonance imaging (fMRI) study, we made a systematic comparison between stimulus protocols involving jittered tone sequences with either a narrowband, broadband, or sweep character in order to evaluate their suitability for the purpose of tonotopic mapping. Data-driven analysis techniques were used to identify cortical maps related to sound-evoked activation and tonotopic frequency tuning. Principal component analysis (PCA) was used to extract the dominant response patterns in each of the three protocols separately, and generalised canonical correlation analysis (CCA) to assess the commonalities between protocols. Generally speaking, all three types of stimuli evoked similarly distributed response patterns and resulted in qualitatively similar tonotopic maps. However, quantitatively, we found that broadband stimuli are most efficient at evoking responses in auditory cortex, whereas narrowband and sweep stimuli offer the best sensitivity to differences in frequency tuning. Based on these results, we make several recommendations regarding optimal stimulus protocols, and conclude that an experimental design based on narrowband stimuli provides the best sensitivity to frequency-dependent responses to determine tonotopic maps. We forward that the resulting protocol is suitable to act as a localiser of tonotopic cortical fields in individuals, or to make quantitative comparisons between maps in dedicated tonotopic mapping studies.