Inner versus Overt Speech Production: Does This Make a Difference in the Developing Brain?

Abstract

Studies in adults showed differential neural processing between overt and inner speech. So far, it is unclear whether inner and overt speech are processed differentially in children. The present study examines the pre-activation of the speech network in order to disentangle domain-general executive control from linguistic control of inner and overt speech production in 6- to 7-year-olds by simultaneously applying electroencephalography (EEG) and functional near-infrared spectroscopy (fNIRS). Children underwent a picture-naming task in which the pure preparation of a subsequent speech production and the actual execution of speech can be differentiated. The preparation phase does not represent speech per se but it resembles the setting up of the language production network. Only the fNIRS revealed a larger activation for overt, compared to inner, speech over bilateral prefrontal to parietal regions during the preparation phase. Findings suggest that the children's brain can prepare the subsequent speech production. The preparation for overt and inner speech requires different domain-general executive control. In contrast to adults, the children´s brain did not show differences between inner and overt speech when a concrete linguistic content occurs and a concrete execution is required. This might indicate that domain-specific executive control processes are still under development.

Keywords: event-related brain potentials (ERPs); functional near-infrared spectroscopy (fNIRS); inner speech production; overt speech production; preparation of speech production.

Figure A1

FNIRS time courses for [oxy-Hb] contrasting overt and inner speech for the Preparation phase (A) and the Execution phase (B).

Figure A2

FNIRS time courses including standard errors of the mean (SEMs) for [oxy-Hb] contrasting overt and inner speech for the Preparation phase (A) and the Execution phase (B).

Figure A3

FNIRS time courses for [deoxy-Hb] including standard errors of the mean (SEMs) contrasting overt and inner speech for the Preparation phase (A) and the Execution phase (B).

Figure A4

Event-related brain potentials (ERPs). (A) Grand averages for the Preparation phase. (B) Grand averages for the Execution phase. Negative polarity is plotted upward. The x-axis ranges from −200 to 1500 ms. No low-pass filter was applied for presentation purposes.

Figure A5

Event-related brain potentials (ERPs) for the Preparation phase including standard deviations. (A) Grand averages for the Overt Speech Preparation (B) Grand averages for the Inner Speech Preparation. Negative polarity is plotted upward. The x-axis ranges from −200 to 1500 ms. No low-pass filter was applied for presentation purposes.

Figure A6

Event-related brain potentials (ERP) for the Execution phase including standard deviations. (A) Grand averages for the Overt Speech Execution (B) Grand averages for the Inner Speech Execution. Negative polarity is plotted upward. The x-axis ranges from −200 to 1500 ms. No low-pass filter was applied for presentation purposes.

Figure 1

Design of the study. In 16 mini-blocks, 40 different colored pictures were presented twice (in inner and overt speech condition). Every mini-block contained 5 trials of the same condition. During the Preparation Phase, the pictures were cued by a red speech (overt speech condition) or a blue thinking (inner speech condition) bubble. During the Execution Phase, participants had to name the pictures (e.g., the rhinoceros) aloud or silently. The pictures were taken from Rossion and Pourtois [49] with images courtesy of the authors. + indicates a fixation cross; ø indicates the mean duration of each trial (15 s) and inter-stimulus-interval (ISI 8 s).

Figure 2

Simultaneous electroencephalography- (EEG)-electrodes and functional near-infrared spectroscopy- (fNIRS)-channel placement. (A) 32 EEG-electrodes and fNIRS probe arrangement; stars indicate 8 fNIRS light emitters; dots indicate 8 fNIRS detectors; purple ellipses indicate regions of interest (ROIs) of the EEG which entered statistical analyses. (B) fNIRS-channel placement: L1-8 show 8 left-hemispheric fNIRS-channels; R1-8 show 8 right-hemispheric fNIRS channels. fNIRS optodes which were positioned in a standard EEG electrode position are labeled, e.g., AF3. Grey squares refer to the regions of interest (ROIs) of the fNIRS channels which were used for statistical analyses.

Figure 3

fNIRS results for (deoxy-Hb). (A) Time courses for the Preparation phase. (B) Beta-values comparing overt and inner speech for the Preparation phase (left) and Execution phase (right) merged over all ROIs. The asterisk indicates statistically significant differences between overt versus inner speech. Please note that a more negative value for (deoxy-Hb) indicates increased activations. (C) Time courses for the Execution phase. See Appendix A Figure A3 for time courses for the Preparation and Execution phase in (deoxy-Hb) including standard errors of the mean (SEMs).