Spatiotemporal Signatures of Lexical-Semantic Prediction

Abstract

Although there is broad agreement that top-down expectations can facilitate lexical-semantic processing, the mechanisms driving these effects are still unclear. In particular, while previous electroencephalography (EEG) research has demonstrated a reduction in the N400 response to words in a supportive context, it is often challenging to dissociate facilitation due to bottom-up spreading activation from facilitation due to top-down expectations. The goal of the current study was to specifically determine the cortical areas associated with facilitation due to top-down prediction, using magnetoencephalography (MEG) recordings supplemented by EEG and functional magnetic resonance imaging (fMRI) in a semantic priming paradigm. In order to modulate expectation processes while holding context constant, we manipulated the proportion of related pairs across 2 blocks (10 and 50% related). Event-related potential results demonstrated a larger N400 reduction when a related word was predicted, and MEG source localization of activity in this time-window (350-450 ms) localized the differential responses to left anterior temporal cortex. fMRI data from the same participants support the MEG localization, showing contextual facilitation in left anterior superior temporal gyrus for the high expectation block only. Together, these results provide strong evidence that facilitatory effects of lexical-semantic prediction on the electrophysiological response 350-450 ms postonset reflect modulation of activity in left anterior temporal cortex.

Keywords: N400; anterior temporal cortex; expectation; magnetoencephalography; semantic.

Figure 1.

Group-averaged EEG and MEG sensor-level signals, time-locked to the target word, for low and high expectation unrelated minus related contrast. (A) Mean EEG-evoked responses across the 4 quadrant regions in which EEG sensor analyses were conducted. (B) Root mean square MEG-evoked responses, calculated across planar gradiometers only, in frontal, parietal, temporal, and occipital regions.

Figure 2.

MEG statistical map illustrating t-values for the unrelated-associated contrast of dSPM source estimates between 350 and 450 ms, thresholded at P < 0.05. Yellow outline indicates the left anterior temporal cluster that demonstrated a significant reduction for the related condition relative to the unrelated condition in the high expectation block. Lighter shading indicates the search region used in the cluster-level permutation test. Inset: cortical fMRI statistical map illustrating left anterior superior temporal region demonstrating a significant reduction for the related condition relative to the unrelated condition in the high expectation block, FDR corrected at P < 0.05. Waveform plots show the mean MEG dSPM activity estimates for the left anterior temporal cluster plotted across the −100:600 ms time-window in each condition, for illustration only.

Figure 3.

Mean EEG sensor and MEG source amplitudes, respectively, across unrelated and related conditions at each level of expectation for the right posterior EEG sensors at which the N400 effect was at its peak and for the left anterior temporal region demonstrating a significant effect of relatedness in the high expectation block in MEG.