Speech motor brain regions are differentially recruited during perception of native and foreign-accented phonemes for first and second language listeners

Abstract

Brain imaging studies indicate that speech motor areas are recruited for auditory speech perception, especially when intelligibility is low due to environmental noise or when speech is accented. The purpose of the present study was to determine the relative contribution of brain regions to the processing of speech containing phonetic categories from one's own language, speech with accented samples of one's native phonetic categories, and speech with unfamiliar phonetic categories. To that end, native English and Japanese speakers identified the speech sounds /r/ and /l/ that were produced by native English speakers (unaccented) and Japanese speakers (foreign-accented) while functional magnetic resonance imaging measured their brain activity. For native English speakers, the Japanese accented speech was more difficult to categorize than the unaccented English speech. In contrast, Japanese speakers have difficulty distinguishing between /r/ and /l/, so both the Japanese accented and English unaccented speech were difficult to categorize. Brain regions involved with listening to foreign-accented productions of a first language included primarily the right cerebellum, left ventral inferior premotor cortex PMvi, and Broca's area. Brain regions most involved with listening to a second-language phonetic contrast (foreign-accented and unaccented productions) also included the left PMvi and the right cerebellum. Additionally, increased activity was observed in the right PMvi, the left and right ventral superior premotor cortex PMvs, and the left cerebellum. These results support a role for speech motor regions during the perception of foreign-accented native speech and for perception of difficult second-language phonetic contrasts.

Keywords: Broca's area; accent; cerebellum; fMRI; internal model; non-native speech; premotor; speech perception.

Figure 1

Mean percent correct behavioral phonetic (/r/ vs. /l/) identification performance for the English (blue) and Japanese (red) groups for unaccented and foreign-accented speech. Standard error of the mean is given above each bar. All conditions were significantly above chance performance of 50%. See text for additional contrasts that were statistically significant.

Figure 2

Significant brain activity (thresholded at pFDR < 0.05 corrected) for the contrast of (A) Eng (unaccented), (B) Jpn (unaccented), (C) Eng (accented), and (D) Jpn (accented). All contrasts showed activity bilaterally in premotor cortex and Broca's area, the superior temporal gyrus/sulcus, the inferior parietal lobule, the pre- supplementary motor area pre-SMA, and the cerebellum. The conjunction analysis, shown in (G), confirmed these regions were active for all conditions (E). The contrast of accented Relative to unaccented thresholded at p < 0.001 uncorrected for Jpn showed activity in the left inferior frontal gyrus in Broca's area 44, the right dorsal premotor cortex, the pre-SMA, and the cerebellum bilaterally (F). The contrast of accented relative to unaccented for the Jpn group did not show any significant activity thresholded at p < 0.001 uncorrected. The main effect of language group (Japanese vs. English) is shown in (H), red corresponds to activity thresholded at p < 0.001 for Japanese > English and blue corresponds to activity for English > Japanese.

Figure 3

Significant brain activity (thresholded at p < 0.001 uncorrected) for the interaction of language group and accent. This contrast focused on the activity involved with perception of foreign-accented productions of a first-language phonetic category. (A) Significant brain activity rendered on the surface of the brain for the contrast of Eng(accented-unaccented) – Jpn(accented-unaccented) showing activity in pre- supplementary motor area pre-SMA, left and right Broca's area BA45, right dorsolateral prefrontal cortex DLPFC, and left and right cerebellum. (B–E) shows contrast estimates and standard error of the SPM analysis relative to rest for the four conditions in selected regions: (B) Pre-SMA, (C) PMvi, (D) Broca's, (E) Cerebellum.

Figure 4

Region of interest (ROI) analysis for the contrast of Eng(accented-unaccented) – Jpn(accented-unaccented) using small volume correction (p < 0.05) for multiple comparisons. (A) left PMvi/Broca's area. (B) Right Cerebellum Lobule VI. MNI X, Y, Z coordinates are given at the top of each brain slice. Negative × MNI coordinates denote left hemisphere and positive × values denote right hemisphere activity. The SPM contrast estimates and standard error relative to rest for all four conditions are given on the left side of each ROI rendered image.

Figure 5

(A) Contrast investigating specific brain regions involved with the perception of foreign-accented productions of a second-language phonetic category, Jpn(accented) – Eng(accented). Activity is present in the right ventral inferior premotor cortex including Broca's area PMvi/Broca's right ventral superior premotor cortex PMvs. (B) Activity for perception of foreign-accented productions of a second language phonetic category that may not be specific Jpn(unaccented) – Eng(unaccented) was found in the left and right PMvi/Broca's, the right PMvs/PMd, the right Broca's area BA45, the left inferior frontal gyrus BA47, the left postcentral gyrus, the left inferior parietal lobule, and the left and right cerebellum.

Figure 6

Region of interest analysis for the contrast of Jpn(accented) – Eng(accented) using small volume correction (p < 0.05) for multiple comparisons. (A) Right PMvi/Broca's area. (B) Right PMvs/PMd. (C) Left Cerebellum Lobule VI. MNI X, Y, Z coordinates are given at the top of each brain slice. Negative × MNI coordinates denote left hemisphere and positive × values denote right hemisphere activity. The SPM contrast estimates and standard error relative to rest for all four conditions are given on the left side of each ROI rendered image.

Figure 7

Region of interest analysis for the contrast of Jpn(unaccented) – Eng(unaccented) using small volume correction (p < 0.05) for multiple comparisons. (A) Left PMvi/Broca's area. (B) Right PMvi/Broca's area. (C) Right PMvs/PMd. (D) Left Cerebellum Lobule VI. (E) Right Cerebellum Lobule VI. MNI X, Y, Z coordinates are given at the top of each brain slice. Negative × MNI coordinates denote left hemisphere and positive × values denote right hemisphere activity. The SPM contrast estimates and standard error relative to rest for all four conditions are given on the left side of each ROI rendered image.