A Comprehensive Three-dimensional Cortical Map of Vowel Space

Abstract

Mammalian cortex is known to contain various kinds of spatial encoding schemes for sensory information including retinotopic, somatosensory, and tonotopic maps. Tonotopic maps are especially interesting for human speech sound processing because they encode linguistically salient acoustic properties. In this study, we mapped the entire vowel space of a language (Turkish) onto cortical locations by using the magnetic N1 (M100), an auditory-evoked component that peaks approximately 100 msec after auditory stimulus onset. We found that dipole locations could be structured into two distinct maps, one for vowels produced with the tongue positioned toward the front of the mouth (front vowels) and one for vowels produced in the back of the mouth (back vowels). Furthermore, we found spatial gradients in lateral-medial, anterior-posterior, and inferior-superior dimensions that encoded the phonetic, categorical distinctions between all the vowels of Turkish. Statistical model comparisons of the dipole locations suggest that the spatial encoding scheme is not entirely based on acoustic bottom-up information but crucially involves featural-phonetic top-down modulation. Thus, multiple areas of excitation along the unidimensional basilar membrane are mapped into higher dimensional representations in auditory cortex.

Linguistic category structure influences early auditory processing: Converging evidence from mismatch responses and cortical oscillations

“…This implies different source configurations depending on the vowel stimuli, which is consistent with previous observations of dipole differences as a function of vowel type (e.g. Obleser et al, 2004; Scharinger et al, 2011). These differences in configuration may also underlie the MMN response.…”

Categorical effects in fricative perception are reflected in cortical source information

“…The number of channels was increased in order to improve the fit of the ECD solutions. Fitting was done as described in previous MEG studies (Obleser et al, 2004; Scharinger et al, 2011) using the program MEGLAB (Kit, Japan). For each standard type in each hemisphere, this resulted in mean coordinates of the corresponding dipole, with x-values describing the medial-lateral axis, y-values the anterior-posterior axis, and z-values the superior-inferior axis.…”

Electromagnetic recording of the auditory system

“…In the domain of speech and language, typical studies have presented single vowels (e.g., Obleser et al, 2004;Scharinger et al, 2011) and syllables (e.g., Poeppel et al, 1996;Bonte et al, 2006;Tavabi et al, 2007), spoken words (e.g., Marinkovic et al, 2003;Uusvuori et al, 2008), and sentences (e.g., Ahissar et al, 2001;Luo and Poeppel, 2007). The geometric alignment of these parts of cortex make it very accessible to MEG-based recording.…”

“…For example, the M100, associated with early cortical analyses of stimuli, has been argued to be generated on the planum temporale immediately adjacent to Heschl's gyrus (the medial portion of which is putative core (or primary) auditory cortex (see Lütkenh€ oner and Poeppel, 2011, for discussion). For example, the precise source localization of the M100 elicited by different vowels has been employed to examine whether vowels are represented in a topographically systematic manner in auditory cortex (e.g., Obleser et al, 2004;Scharinger et al, 2011). Two responses that are used especially often in auditory research and language research, respectively, are the M100 (or N100m) and the M400 (or N400m).…”