Phonetic detail and lateralization of reading-related inner speech and of auditory and somatosensory feedback processing during overt reading

Abstract

Phonetic detail and lateralization of inner speech during covert sentence reading as well as overt reading in 32 right-handed healthy participants undergoing 3T fMRI were investigated. The number of voiceless and voiced consonants in the processed sentences was systematically varied. Participants listened to sentences, read them covertly, silently mouthed them while reading, and read them overtly. Condition comparisons allowed for the study of effects of externally versus self-generated auditory input and of somatosensory feedback related to or independent of voicing. In every condition, increased voicing modulated bilateral voice-selective regions in the superior temporal sulcus without any lateralization. The enhanced temporal modulation and/or higher spectral frequencies of sentences rich in voiceless consonants induced left-lateralized activation of phonological regions in the posterior temporal lobe, regardless of condition. These results provide evidence that inner speech during reading codes detail as fine as consonant voicing. Our findings suggest that the fronto-temporal internal loops underlying inner speech target different temporal regions. These regions differ in their sensitivity to inner or overt acoustic speech features. More slowly varying acoustic parameters are represented more anteriorly and bilaterally in the temporal lobe while quickly changing acoustic features are processed in more posterior left temporal cortices. Furthermore, processing of external auditory feedback during overt sentence reading was sensitive to consonant voicing only in the left superior temporal cortex. Voicing did not modulate left-lateralized processing of somatosensory feedback during articulation or bilateral motor processing. This suggests voicing is primarily monitored in the auditory rather than in the somatosensory feedback channel. Hum Brain Mapp 38:493-508, 2017. © 2016 Wiley Periodicals, Inc.

Keywords: consonant voicing; fMRI; hemispheric asymmetry; mouthing; sentence reading; silent articulation.

Figure 1

Representative example of the acoustic measures obtained from a sentence rich in voiced (left panels) and a sentence rich in voiceless consonants (right panels) of the male speaker. The presence or absence of phonation (voicing) as measured as fundamental frequency in PRAAT is displayed as 1 or 0 (red horizontal lines, left red y‐axis). The overall amount of phonation is given as a percentage on top of the two upper panels. Changes in Intensity (upper panels) and spectral Center of Gravity (COG, lower panels) during the sentence are displayed in black (right y‐axes). The x‐axes correspond to Time in seconds. [Color figure can be viewed at http://wileyonlinelibrary.com.]

Figure 2

Condition‐independent parametric modulation by occurrence of phonation. Activity increase with voicing (gold) and activity increase with increasing numbers of voiceless consonants (purple) at P < 0.05, FWE corrected.

Figure 3

Condition effects. All activations (conditions contrasted against silent baseline) are depicted at P < 0.05, FWE corrected. Green: Overt reading, Blue: Mouthing while reading. Red: Covert reading. Yellow: Listening while viewing wingding characters from left to right.

Figure 4

Condition differences related with sensory feedback processing. Upper panel: The contrast overt reading > mouthing while reading reveals bilateral activations associated with auditory feedback processing (green). Activity in the bilateral anterior auditory cortices and in the ventromesial prefrontal cortex is suppressed during overt reading compared with listening (yellow). Lower panel: Activations related with sensorimotor processing as revealed by the contrast mouthing while reading > covert reading (blue). Note the left‐lateralization in secondary somatosensory cortices. All activations at P < 0.05, FWE corrected.