Negation Processing

Using negation in natural communication requires a specific type of context. If someone states that "a robin is not a chair," for instance, you may find yourself wondering about whether anyone thought that robins were chairs in the first place. In fact, in most naturally-occurring cases, we use negation to talk about things that are already in evidence, such as when correcting someone's mistaken beliefs about something ("the lab meeting is not at 11, it's at 10:30!"). But one of the things that remains unclear about how we comprehend negation is when and how this kind of contextual information is used by our brains - are there any differences in how negation is processed in natural vs. unnatural conditions? At EPL, we're pursuing this issue by applying uni- and multivariate pattern analyses to EEG data.

People generally find it quite easy to construct and understand a negated sentence such as This car is not a Ford. We utter sentences like this many times a day. Yet exactly how negation is processed is a matter of debate among linguists. When we hear a sentence like This car is not a Ford, do we compute the meaning of the structure one word at a time in the exact order the words appear? Or do we compute the meaning of the “inner proposition” first – This car is a Ford – and then, as a second step, negate this proposition?

Using ERPs, we test whether negative sentences are processed in the same way as the positive sentences, or whether processing negation is a two-step process where negation is evaluated after the inner proposition is understood first. 

Previous studies have tested these two competing hypotheses by using negation in a pragmatically licensed environment (Nieuwland and Kuperberg, 2008), or by presenting negated sentences out of the blue (Fischler et al.,1983). In this study, we use a slightly modified version of Fischler et al.’s stimuli embedded in a context. We hope to find an answer to a few questions by evaluating the effect of context on the processing of negation:

• Is negation processed incrementally or non-incrementally (in a two-step fashion)? Does contextually-constrained negation elicit N400?

• Do contextually constrained pseudo-words trigger N400 on a semantically incongruous category word?

We look for an N400 – an ERP response elicited in response to semantic incongruity (Kutas and Hillyard, 1980, 1984; Polich et al. 1981 among several others). By manipulating the truth or falsity of the proposition, we can determine whether the brain computes the meaning of the inner proposition first, or whether it computers the meaning of the words in the order they appear. 

The final word of the sentence will either be congruous or incongruous, depending on whether it is negated. A robin is a bird is perfectly congruous, but A robin is not a bird is false and unexpected! By contrast, A hammer is a bird is incongruous, but A hammer is not a bird is true and perfectly congruous. The brain’s response to this final word will tell us whether negation is computed before or after this word is integrated into the meaning of the sentence.

If negation is computed before reaching the final word of the sentence, only the false sentences (regardless of negation) will elicit an N400. If negation is computed after the meaning of the inner proposition has been computed, then the N400 is expected in false-affirmative (A robin is a tree) and true-negative sentences (A robin is not a tree).

We test two groups of participants. One group will read sentences using real words (robin, hammer). The other group will read complementary versions of these sentences with pseudo-words (dwopse, twizen). We are interested in whether context will affect the order in which negation is computed, and whether this process will be the same for words that have to be learned in real time.

Our preliminary results support non-incremental processing of negation in the pseudo-word group (N=13). This also indicates that the semantic mapping of a newly learned word in a context is very fast – the newly learned words can elicit an N400 immediately after initial exposure.