Poster Slam Session B, Tuesday, August 20, 2019, 3:00 – 3:15 pm, Finlandia Hall, Angela Grant
Inner speech in silent reading evokes theta phase-locked responses in the auditory cortex
Bo Yao1, Jason Taylor1, Briony Banks2, Sonja Kotz1,3;1Division of Neuroscience and Experimental Psychology, School of Biological Sciences, Faculty of Biology, Medicine and Health, University of Manchester, 2Department of Psychology, Lancaster University, 3Departments of Neuropsychology & Psychopharmacology, Maastricht University
A growing body of research shows that theta-band (4-7Hz) neural oscillations in the auditory cortex track (or are phase-locked to) the rhythm of natural speech (Giraud & Poeppel, 2012; Luo & Poeppel, 2007). It seems to reflect a neural mechanism for segmenting and coding continuous speech signals into hierarchical linguistic units for comprehension (Ding et al., 2015; Peelle et al., 2013). However, the role of neural oscillations remains unknown for inner speech processing - a pervasive mental phenomenon observed in a range of cognitive tasks including thinking, problem solving, working memory, reading and writing. Inner speech is an inwardly audible speech experience without outward articulation. Like overt speech, inner speech can be characterised by acoustic features such as tempo (Alexander & Nygaard, 2008; Yao & Scheepers, 2011) and activates the same cortical areas as overt speech (Brück et al., 2014; Yao et al., 2011). The shared acoustic features and neural substrates between inner and overt speech suggest that they may be governed by a common oscillation-based neural mechanism. The present EEG study explored for the first time whether inner speech may be associated with theta oscillations in the auditory cortex. Thirty-two native speakers of English silently read 120 short stories (+60 fillers) that contained either a direct speech quotation (e.g., Mary said: “This dress is lovely”) or a linguistically-matched indirect speech quotation (e.g., Mary said that the dress was lovely). The stories were presented sentence-by-sentence for self-paced reading, with the critical speech quotations (e.g., “This dress is lovely”) presented last. EEG data were recorded throughout the experiment using a 64-channel Biosemi Active-Two system. The EEG data were pre-processed and epoched to the presentation onsets of the critical speech quotations. We decomposed EEG data in time-frequency space using a 7-cycle Morlet wavelet in SPM12 (www.fil.ion.ucl.ac.uk/spm/software/spm12/). We calculated evoked power and phase-locking values (PLVs; i.e. inter-trial phase coherence) in theta frequencies (4-7Hz), which were converted into 3D spatial parametric maps (2D scalp map × time) by conditions for 2nd level group analysis. Group analyses revealed significantly higher phase-locked activity (power and phase-locking) at ~250-500ms when reading direct (relative to indirect) speech quotations, implying a stronger theta phase reset at the beginning of inner speech processing. This phase-locked activity was left lateralised (fronto-temporo-parietal) and was source localised to the left occipito-temporal and fusiform area (roughly BA37), bilateral temporal areas (mostly BA20/21) and the left inferior and middle frontal area (BA45/46), part of the phonological processing network in reading. Our results demonstrate the importance of theta oscillations in silent reading, and suggest for the first time that inner speech may result from more synchronous oscillatory sampling of phonological representations that are decoded during silent reading.
Themes: Phonology and Phonological Working Memory, Reading
Method: Electrophysiology (MEG/EEG/ECOG)
Poster B63