Electrophysiological indices of brain activity to content and function words in discourse

An increase in positivity of event-related potentials (ERPs) at the lateral anterior sites has been hypothesized to be an index of semantic and discourse processing, with the right lateral anterior positivity (LAP) showing particular sensitivity to discourse factors. However, the research investigating the LAP is limited; it is unclear whether the effect is driven by word class (function word versus content word) or by a more general process of structure building triggered by elements of a determiner phrase (DP).

To examine the neurophysiological indices of semantic/discourse integration using two different word categories (function versus content word) in the discourse contexts and to contrast processing of these word categories in meaningful versus nonsense contexts.

Planned comparisons of ERPs time locked to a function word stimulus 'the' and a content word stimulus 'cats' in sentence-initial position were conducted in both discourse and nonsense contexts to examine the time course of processing following these word forms.

A repeated-measures analysis of variance (ANOVA) for the Discourse context revealed a significant interaction of condition and site due to greater positivity for 'the' relative to 'cats' at anterior and superior sites. In the Nonsense context, there was a significant interaction of condition, time and site due to greater positivity for 'the' relative to 'cats' at anterior sites from 150 to 350 ms post-stimulus offset and at superior sites from 150 to 200 ms post-stimulus offset. Overall, greater positivity for both 'the' and 'cats' was observed in the discourse relative to the nonsense context beginning approximately 150 ms post-stimulus offset. Additionally, topographical analyses were highly correlated for the two word categories when processing meaningful discourse. This topographical pattern could be characterized as a prominent right LAP. The LAP was attenuated when the target stimulus word initiated a nonsense context.

The results of this study support the view that the right LAP is an index of general discourse processing rather than an index of word class. These findings demonstrate that the LAP can be used to study discourse processing in populations with compromised metalinguistic skills, such as adults with aphasia or traumatic brain injury.