Functional Characterization of the Human Speech Articulation Network

Abstract

A number of brain regions have been implicated in articulation, but their precise computations remain debated. Using functional magnetic resonance imaging, we examine the degree of functional specificity of articulation-responsive brain regions to constrain hypotheses about their contributions to speech production. We find that articulation-responsive regions (1) are sensitive to articulatory complexity, but (2) are largely nonoverlapping with nearby domain-general regions that support diverse goal-directed behaviors. Furthermore, premotor articulation regions show selectivity for speech production over some related tasks (respiration control), but not others (nonspeech oral-motor [NSO] movements). This overlap between speech and nonspeech movements concords with electrocorticographic evidence that these regions encode articulators and their states, and with patient evidence whereby articulatory deficits are often accompanied by oral-motor deficits. In contrast, the superior temporal regions show strong selectivity for articulation relative to nonspeech movements, suggesting that these regions play a specific role in speech planning/production. Finally, articulation-responsive portions of posterior inferior frontal gyrus show some selectivity for articulation, in line with the hypothesis that this region prepares an articulatory code that is passed to the premotor cortex. Taken together, these results inform the architecture of the human articulation system.

Figure 1.

Sample trials from Experiments 2 and 3. (a) Example vowel. (b) Example nonspeech oral-motor movement. (c) A hard trial from the spatial working memory task.

Figure 2.

Articulation-responsive brain regions. (a) A whole-brain probabilistic activation overlap map for 20 participants, with the parcels discovered by the GSS analysis (Fedorenko et al. 2010) overlaid on top (note that 1 of the 11 parcels—L pSTG2—is not visible on the surface; see Fig. 3). (b) fROIs in the left and right hemisphere of 4 sample individual participants. These fROIs were defined by intersecting the individual activation maps for the “hard articulation > fixation” contrast with the parcels, and selecting the top 10% of most responsive voxels within each parcel.

Figure 3.

Responses to the articulation and spatial WM conditions of each articulation-responsive fROI. Left: The parcels used to define articulation-responsive fROIs. Right: Responses of the individually defined articulation-responsive fROIs to each condition across Experiments 1–3.

Figure 4.

Responses to the articulation and spatial WM conditions as a function of fROI type within the macroanatomical regions that show sensitivity to both articulation and general cognitive demand. Left column: The anatomical regions (a subset of the MD parcels), which include L and R IFGop, PrCG, Insula, and SMA. Middle columns: Responses to the articulation and spatial WM conditions in the articulation-responsive fROIs (defined by the hard articulation > fixation contrast; the “Artic fROIs” column) and in the MD fROIs (defined by the hard > easy spatial WM contrast; the “MD fROIs” column). Right column: Voxel counts for the hard articulation > fixation contrast, the hard > easy spatial WM contrast at the P < 0.001, uncorrected at the whole-brain level threshold, and voxels that show significant effects for both contrasts.