Abstract
A wide variety of evidence, from neurophysiology, neuroanatomy, and imaging studies in humans and animals, suggests that human auditory cortex is in part tonotopically organized. Here we present a new means of resolving this spatial organization using a combination of non-invasive observables (EEG, MEG, and MRI), model-based estimates of spectrotemporal patterns of neural activation, and multivariate pattern analysis. The method exploits both the fine-grained temporal patterning of auditory cortical responses and the millisecond scale temporal resolution of EEG and MEG. Participants listened to 400 English words while MEG and scalp EEG were measured simultaneously. We estimated the location of cortical sources using the MRI anatomically constrained minimum norm estimate (MNE) procedure. We then combined a form of multivariate pattern analysis (representational similarity analysis) with a spatiotemporal searchlight approach to successfully decode information about patterns of neuronal frequency preference and selectivity in bilateral superior temporal cortex. Observed frequency preferences in and around Heschl's gyrus matched current proposals for the organization of tonotopic gradients in primary acoustic cortex, while the distribution of narrow frequency selectivity similarly matched results from the fMRI literature. The spatial maps generated by this novel combination of techniques seem comparable to those that have emerged from fMRI or ECOG studies, and a considerable advance over earlier MEG results.
Highlights
For most sensory systems, the spatial organization of cortical neuronal responses resembles that of the sensory surfaces, for example, retinotopy in vision, cochleotopy in audition, and somatotopy in the cutaneous senses
FREQUENCY RESPONSE DISTRIBUTIONS Figures 6A,B show the distributions of preferred frequencies for all vertices in bilateral superior temporal cortex, summarizing the results compiled over the 200 ms from stimulus onset
The peak of the distribution on the left is at around 250–350 Hz, which is in the range of the fundamental frequency of the female voice
Summary
The spatial organization of cortical neuronal responses resembles that of the sensory surfaces, for example, retinotopy in vision, cochleotopy in audition, and somatotopy in the cutaneous senses. Non-invasive methods, primarily MEG and EEG, have played an increasingly important role in neuroimaging investigations of auditory processes ranging from elementary auditory perception to complex speech processing (e.g., Luo and Poeppel, 2007). A pioneering neuromagnetic study by Romani et al (1982), for example, used a single gradiometer device to provide evidence suggesting tonotopic organization in human auditory cortex. Most subsequent MEG studies used equivalent current dipole (ECD) methods to model the MEG sources for the N100 component (Sarvas, 1987), typically finding a gradient from high to low frequency in auditory cortex. When multiple sources are likely to exist in auditory cortex, it is unrealistic to assume that there is a single dipole in this region and results in unreliable conclusions about the organization of the auditory system (Lutkenhoner et al, 2003). The exclusive focus on the N100 component in many earlier MEG studies militated against exploring frequency responses in other time windows— other auditory components have been examined in some studies (e.g., Pantev et al, 1995)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
