Functional Magnetic Resonance Imaging Blood Oxygenation Level-Dependent Signal and Magnetoencephalography Evoked Responses Yield Different Neural Functionality in Reading

Abstract

It is often implicitly assumed that the neural activation patterns revealed by hemodynamic methods, such as functional magnetic resonance imaging (fMRI), and electrophysiological methods, such as magnetoencephalography (MEG) and electroencephalography (EEG), are comparable. In early sensory processing that seems to be the case, but the assumption may not be correct in high-level cognitive tasks. For example, MEG and fMRI literature of single-word reading suggests differences in cortical activation, but direct comparisons are lacking. Here, while the same human participants performed the same reading task, analysis of MEG evoked responses and fMRI blood oxygenation level-dependent (BOLD) signals revealed marked functional and spatial differences in several cortical areas outside the visual cortex. Divergent patterns of activation were observed in the frontal and temporal cortex, in accordance with previous separate MEG and fMRI studies of reading. Furthermore, opposite stimulus effects in the MEG and fMRI measures were detected in the left occipitotemporal cortex: MEG evoked responses were stronger to letter than symbol strings, whereas the fMRI BOLD signal was stronger to symbol than letter strings. The EEG recorded simultaneously during MEG and fMRI did not indicate neurophysiological differences that could explain the observed functional discrepancies between the MEG and fMRI results. Acknowledgment of the complementary nature of hemodynamic and electrophysiological measures, as reported here in a cognitive task using evoked response analysis in MEG and BOLD signal analysis in fMRI, represents an essential step toward an informed use of multimodal imaging that reaches beyond mere combination of location and timing of neural activation.

How Silent Is Silent Reading? Intracerebral Evidence for Top-Down Activation of Temporal Voice Areas during Reading

“…The neural response involves high-frequency activity, already shown to be exquisitely sensitive to auditory stimuli, including speech (Crone et al, 2001;Towle et al, 2008;Chang et al, 2010). This result is reminiscent of an earlier study by our group, reporting that a region in the left anterior superior temporal gyrus (STG) was active during a silent reading task , in line with an earlier suggestion that the left STG might comprise an auditory analog of the word form area (Cohen et al, 2004).…”

Neurocomputational account of how the human brain decides when to have a break

“…This complex neural signal with precise spatiotemporal characteristics was identified with both fMRI and MEG using standard analytical tools, which validates the usual assumptions about the relationships between electromagnetic and hemodynamic activity (18)(19)(20)(21)(22). Due to its high temporal resolution, MEG brought evidence that the electromagnetic activity emitted by our regions of interest indeed followed the neural dynamics that was modeled and convolved with hemodynamic response to fit fMRI data.…”

Consistency and similarity of MEG- and fMRI-signal time courses during movie viewing

“…Moradi et al, 2003;Brookes et al, 2005;Nangini et al, 2009;Stevenson et al, 2011) whereas the MEG and fMRI spatial patterns often differ during cognitive tasks (e.g. more than 15 mm in various regions as shown by Liljestr€ om et al, 2009; more than 10 mm outside occipital cortex; Vartiainen et al, 2011). Moreover, the association between BOLD and MEG signals can be region-and frequency-dependent (Kujala et al, 2014) as well as task-related (Furey et al, 2006).…”