To predict or not to predict: The role of context constraint and truth-value in negation processing.

Studies on negation processing often report a Polarity by Truth interaction: False affirmative sentences show longer response times and larger N400 ERPs relative to true affirmative sentences, while for negative sentences the effect of truth-value is typically reversed. This interaction has repeatedly been linked to variations in lexical associations, predictability, or to the need of constructing two subsequent mental representations during the comprehension of negative sentences. In five ERP experiments, employing a picture-sentence verification paradigm, we investigate how negation, truth and predictability interact in sentence processing. Predictability was manipulated by varying the number of alternative sentence continuations provided by the context to make it equivalent for both sentence polarities. For both sentence polarities, true sentences yielded a smaller N400 response in the strongly constraining context, where the processor could form a unique prediction, relative to the weakly constraining context, where no clear prediction could be made. For false sentences, the effect of context was reversed for both sentence polarities. Furthermore, the effect of Truth was dependent on the predictability rather than sentence polarity: Both affirmative and negative sentences showed the same direction of the effect of Truth, i.e., a larger N400 for false rather than true sentences in the strongly constraining context, and a reversed effect in the in the weakly constraining context, although differences in the size of these effects between the two polarities were apparent. In addition, we observe a long lasting positivity effect for negation, in both context conditions, for both truth-values and across all five experiments, which is discussed in terms of inhibition mechanisms caused by negation.