Effective cerebral connectivity during silent speech reading revealed by functional magnetic resonance imaging

Abstract

Seeing the articulatory gestures of the speaker ("speech reading") enhances speech perception especially in noisy conditions. Recent neuroimaging studies tentatively suggest that speech reading activates speech motor system, which then influences superior-posterior temporal lobe auditory areas via an efference copy. Here, nineteen healthy volunteers were presented with silent videoclips of a person articulating Finnish vowels /a/, /i/ (non-targets), and /o/ (targets) during event-related functional magnetic resonance imaging (fMRI). Speech reading significantly activated visual cortex, posterior fusiform gyrus (pFG), posterior superior temporal gyrus and sulcus (pSTG/S), and the speech motor areas, including premotor cortex, parts of the inferior (IFG) and middle (MFG) frontal gyri extending into frontal polar (FP) structures, somatosensory areas, and supramarginal gyrus (SMG). Structural equation modelling (SEM) of these data suggested that information flows first from extrastriate visual cortex to pFS, and from there, in parallel, to pSTG/S and MFG/FP. From pSTG/S information flow continues to IFG or SMG and eventually somatosensory areas. Feedback connectivity was estimated to run from MFG/FP to IFG, and pSTG/S. The direct functional connection from pFG to MFG/FP and feedback connection from MFG/FP to pSTG/S and IFG support the hypothesis of prefrontal speech motor areas influencing auditory speech processing in pSTG/S via an efference copy.

Figure 1. The stimuli that were used in the present study.

Shown are individual frames, from the fifth frame with steps of five frames to the 30^th frame, from the videoclips of /i/, /a/, and /o/.

Figure 2. Brain areas significantly activated by viewing videoclips of face articulating non-target (/a/ and /i/) and target (/o/) vowels.

The third row shows brain areas that were significantly more active in the target condition than in the non-target condition, and the fourth row the areas that were significantly more active in the non-target condition than the target condition, respectively. Color corresponds to the t statistics.

Figure 3. Top panel: the directionally connected model for the SEM analysis on the effective connectivity during speech perception.

Note that a path from BA10→BA 22 was added because of this path loaded with the highest MI value (for details, see Materials and Methods). Middle panels: the estimated path coefficients, the standard errors of the mean (SEM), and the associated p-values (in parenthesis) in the /a/+/i/ non-target condition and the /o/ target condition. Statistically significant (p≤0.05) and insignificant (p>0.05) paths were rendered in solid and dashed lines, respectively. Bottom panel: pair-wise comparison of the path coefficients between target /o/ and non-target /a/+/i/ conditions. Statistically significantly (p≤0.05) and insignificantly (p>0.05) different paths were rendered in solid and dashed lines, respectively.