Resting-state functional connectivity changes following audio-tactile speech training

Abstract

Understanding speech in background noise is a challenging task, especially when the signal is also distorted. In a series of previous studies, we have shown that comprehension can improve if, simultaneously with auditory speech, the person receives speech-extracted low-frequency signals on their fingertips. The effect increases after short audio-tactile speech training. In this study, we used resting-state functional magnetic resonance imaging (rsfMRI) to measure spontaneous low-frequency oscillations in the brain while at rest to assess training-induced changes in functional connectivity. We observed enhanced functional connectivity (FC) within a right-hemisphere cluster corresponding to the middle temporal motion area (MT), the extrastriate body area (EBA), and the lateral occipital cortex (LOC), which, before the training, was found to be more connected to the bilateral dorsal anterior insula. Furthermore, early visual areas demonstrated a switch from increased connectivity with the auditory cortex before training to increased connectivity with a sensory/multisensory association parietal hub, contralateral to the palm receiving vibrotactile inputs, after training. In addition, the right sensorimotor cortex, including finger representations, was more connected internally after the training. The results altogether can be interpreted within two main complementary frameworks. The first, speech-specific, factor relates to the pre-existing brain connectivity for audio-visual speech processing, including early visual, motion, and body regions involved in lip-reading and gesture analysis under difficult acoustic conditions, upon which the new audio-tactile speech network might be built. The other framework refers to spatial/body awareness and audio-tactile integration, both of which are necessary for performing the task, including in the revealed parietal and insular regions. It is possible that an extended training period is necessary to directly strengthen functional connections between the auditory and the sensorimotor brain regions for the utterly novel multisensory task. The results contribute to a better understanding of the largely unknown neuronal mechanisms underlying tactile speech benefits for speech comprehension and may be relevant for rehabilitation in the hearing-impaired population.

Keywords: cochlear implants; fMRI; multisensory training; resting-state functional MRI; speech comprehension; tactile aid.

Figure 1

(A) Left-hemisphere and (B) right-hemisphere aspects of the 40 independent components (ICs) revealed in the IC analysis. The color bar represents voxel-to-voxel correlation z-values (threshold at z = 2); yellow to red in the color bar indicates a positive correlation, and green to blue indicates a negative correlation.

Figure 2

Subsequent steps of the statistical analysis: (A) extraction of the signal from an independent component in the session before (PRE) and after (POST) training, (B) functional connectivity (FC) analysis in the PRE and POST sessions separately with the signal in all brain voxels, (C) unthresholded FC comparison between PRE and POST, (D) thresholded FC comparison between PRE and POST. IC3 (early visual cortex) is used as an example.

Figure 3

Results of the statistical analysis. (A) for IC3, FC was found increased with a cluster in the left SPL/aSMG/PostCG after training; (B) for IC6, FC was found decreased with the right sensorimotor cortex after training; (C) for IC4, FC was found decreased with the right primary visual cortex; (D) for IC2, FC was found both increased with a cluster in the right LOC/pMTG (red-yellow) and decreased with the insula (purple-pink) after training. The color bars represent t-values of the statistical comparison of functional connectivity POST vs. PRE training; t-values above zero (red-yellow) indicate increased FC after training, while t-values below zero (purple-pink) indicate decreased FC after training. IC, Independent Component; LOC, lateral occipital cortex; MTG, middle temporal gyrus; pSTS, posterior superior temporal sulcus.