Neuromagnetic evidence for the timing of lexical activation: an MEG component sensitive to phonotactic probability but not to neighborhood density

Evidence from electrophysiological measures such as ERPs (event-related potentials) and MEG (magnetoencephalography) suggest that the first evoked brain response component sensitive to stimulus properties affecting reaction times in word recognition tasks occurs at 300-400 ms. The present study used the stimulus manipulation of Vitevich and Luce (1999) to investigate whether the M350, an MEG response component peaking at 300-400 ms, reflects lexical or postlexical processing. Stimuli were simultaneously varied in phonotactic probability, which facilitates lexical activation, and in phonological neighborhood density, which inhibits the lexical decision process. The present results indicate that the M350 shows facilitation by phonotactic probability rather than inhibition by neighborhood density. Thus the M350 cannot be a postlexical component.     

Before the N400: effects of lexical-semantic violations in visual cortex

There exists an increasing body of research demonstrating that language processing is aided by context-based predictions. Recent findings suggest that the brain generates estimates about the likely physical appearance of upcoming words based on syntactic predictions: words that do not physically look like the expected syntactic category show increased amplitudes in the visual M100 component, the first salient MEG response to visual stimulation. This research asks whether violations of predictions based on lexical-semantic information might similarly generate early visual effects. In a picture-noun matching task, we found early visual effects for words that did not accurately describe the preceding pictures. These results demonstrate that, just like syntactic predictions, lexical-semantic predictions can affect early visual processing around ～100ms, suggesting that the M100 response is not exclusively tuned to recognizing visual features relevant to syntactic category analysis. Rather, the brain might generate predictions about upcoming visual input whenever it can. However, visual effects of lexical-semantic violations only occurred when a single lexical item could be predicted. We argue that this may be due to the fact that in natural language processing, there is typically no straightforward mapping between lexical-semantic fields (e.g., flowers) and visual or auditory forms (e.g., tulip, rose, magnolia). For syntactic categories, in contrast, certain form features do reliably correlate with category membership. This difference may, in part, explain why certain syntactic effects typically occur much earlier than lexical-semantic effects.     

Neural correlates of the effects of morphological family frequency and family size: an MEG study

Schreuder and Baayen (Schreuder. R., & Baayen, R. H. (1997). How complex simplex words can be. Journal of Memory and Language 37, 118-139) report that lexical decision times to nouns are not sensitive to the cumulative frequency of the noun's morphological derivatives in its "morphological family", even though such a cumulative frequency effect is obtained in the domain of inflection. Under a decomposition view of derivational morphology, this constitutes a puzzling exception to the robust finding that lexical frequency is one of the major determinants of behavioral response latencies. If morphologically complex words are decomposed, each occurrence of a member of a noun's morphological family should add to its root-frequency. We investigated the effects of morphological family frequency on the magnetoencephalographic response component M350, which shows sensitivity to factors affecting early stages of lexical processing, including lexical frequency. We hypothesized that high morphological family frequency should have a facilitory effect on the M350, even though no such effect can be seen in response time, presumably due to competition among possible root-affix combinations. Contrary to this hypothesis, we found that high family frequency elicits an M350 inhibition, suggesting that competition among morphological family members occurs at the M350. The result is significant, since there is evidence that competition among phonologically similar words occurs after, not at, the M350. Thus, our results suggest that competition within a morphological family precedes competition within a phonological similarity neighborhood.     

Tracking the time course of word recognition with MEG

Twenty years ago it was discovered that recognition of semantically unexpected words is associated with a special ERP signature - the N400. Pinpointing the precise functional significance of the N400 has, however, been difficult. Recent MEG studies of word processing reveal that, in fact, the N400 decomposes into several functionally distinct subcomponents, allowing for more fine-grained investigation of its significance.     

The Anterior Midline Field: coercion or decision making?

To study the neural bases of semantic composition in language processing without confounds from syntactic composition, recent magnetoencephalography (MEG) studies have investigated the processing of constructions that exhibit some type of syntax-semantics mismatch. The most studied case of such a mismatch is complement coercion; expressions such as the author began the book, where an entity-denoting noun phrase is coerced into an eventive meaning in order to match the semantic properties of the event-selecting verb (e.g., ‘the author began reading/writing the book’). These expressions have been found to elicit increased activity in the Anterior Midline Field (AMF), an MEG component elicited at frontomedial sensors at 400 ms after the onset of the coercing noun [Pylkkänen, L., & McElree, B. (2007). An MEG study of silent meaning. Journal of Cognitive Neuroscience, 19, 11]. Thus, the AMF constitutes a potential neural correlate of coercion. However, the AMF was generated in ventromedial prefrontal regions, which are heavily associated with decision-making. This raises the possibility that, instead of semantic processing, the AMF effect may have been related to the experimental task, which was a sensicality judgment. We tested this hypothesis by assessing the effect of coercion when subjects were simply reading for comprehension, without a decision-task. Additionally, we investigated coercion in an adjectival rather than a verbal environment to further generalize the findings. Our results show that an AMF effect of coercion is elicited without a decision-task and that the effect also extends to this novel syntactic environment. We conclude that in addition to its role in non-linguistic higher cognition, ventromedial prefrontal regions contribute to the resolution of syntax-semantics mismatches in language processing.