Functional characterisation of letter-specific responses in time, space and current polarity using magnetoencephalography

Abstract

Recent neurophysiological evidence suggests that a hierarchical neural network of low-to-high level processing subserves written language comprehension. While a considerable amount of research has identified distinct regions and stages of processing, the relations between them and to this hierarchical model remain unclear. Magnetoencephalography (MEG) is a technique frequently employed in such investigations; however, no studies have sought to test whether the conventional method of reconstructing currents at the source of the magnetic field is best suited for such across-subject designs. The present study details the results of three MEG experiments addressing these issues. Neuronal populations supporting responses to low-level orthographic properties were housed posteriorly near the primary visual cortex. More anterior regions along the fusiform gyrus encoded higher-level processes and became active ~80ms later. A functional localiser of these early letter-specific responses was developed for the production of functional regions of interest in future studies. Previously established response components were successfully grouped based on proximity to the localiser, which characterised location, latency and functional sensitivity. Unconventional anatomically constrained signed minimum norm estimates of MEG data were most sensitive to the primary experimental manipulation, suggesting that the conventional unsigned unconstrained method is sub-optimal for studying written word processing.

In spoken word recognition the future predicts the past

“…This sample size was selected based on previous studies using the same MEG machine (e.g. (Gwilliams and Marantz, 2015;Gwilliams et al, 2016)). They were recruited from the New York University Abu Dhabi community and were compensated for their time.…”

Information properties of morphologically complex words modulate brain activity during word reading

“…Previous studies, on English words, have found support for automatic form-based decomposition (Fruchter, Stockall, & Marantz, 2013), morphological decomposition indexed by TPL (Solomyak & Marantz, 2010), as well as effect of morphological complexity in a corresponding right hemispheric response (Zweig & Pylkkänen, 2009 the first would be associated with orthographic processing and the second with more abstract lexical processing (Gwilliams et al, 2016). In the present study, to ensure robust across-participants matching, the letter-string response was modeled as a single source in the more traditional fashion (Tarkiainen et al, 1999), and it may thus not capture a possible second component in a more anterior region that might be linked to lexicality or morphology.…”

“…Thus, the functional role of the occipito‐temporal activity in morphological processing seems to remain somewhat unclear. Interestingly, a recent study, using English words as stimuli and applying a special variant of distributed source modeling, suggests that the occipito‐temporal response may in fact consist of two functionally and temporally distinct components: the first would be associated with orthographic processing and the second with more abstract lexical processing (Gwilliams et al, 2016). In the present study, to ensure robust across‐participants matching, the letter‐string response was modeled as a single source in the more traditional fashion (Tarkiainen et al, 1999), and it may thus not capture a possible second component in a more anterior region that might be linked to lexicality or morphology.…”

“…A midline occipital response peaking at around 100 ms after stimulus onset captures processing related to basic visual features (Tarkiainen, Helenius, Hansen, Cornelissen & Salmelin, 1999; Cornelissen, Tarkiainen, Helenius, & Salmelin, 2003; Wydell, Vuorinen, Helenius, & Salmelin, 2003). It is followed by a letter‐string sensitive activation peaking at around 150 ms in the occipito‐temporal cortex with left‐hemispheric dominance (Tarkiainen et al, 1999; Gwilliams, Lewis, & Marantz, 2016). Activity in the inferior temporal cortex and fusiform area between 150 and 200 ms has also been reported to be sensitive to orthographic and morphological properties of letter strings, supporting the so‐called morpho‐orthographic segmentation hypothesis in which decomposition is based on visual word forms and takes place prior to lexical access (Solomyak & Marantz, 2010; Zweig & Pylkkänen, 2009).…”

“…A midline occipital response peaking at around 100 ms after stimulus onset captures processing related to basic visual features (Tarkiainen, Helenius, Hansen, Cornelissen & Salmelin, 1999;Cornelissen, Tarkiainen, Helenius, & Salmelin, 2003;Wydell, Vuorinen, Helenius, & Salmelin, 2003). It is followed by a letter-string sensitive activation peaking at around 150 ms in the occipito-temporal cortex with left-hemispheric dominance (Tarkiainen et al, 1999;Gwilliams, Lewis, & Marantz, 2016).…”