What could go wrong? Linguistic illusions and incremental interpretation

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The systems underlying incremental sentence comprehension are, in general, highly successful - comprehenders typically understand sentences of their native language quickly and accurately. The occasional failure of the system to deliver an appropriate representation of a sentence is therefore potentially illuminating. There are many ways the comprehender's general success could in principle be accomplished; the systematic pattern of failures places some constraints on the possible algorithms. This dissertation explores two cases of systematic failure, negative polarity illusions and substitution illusions (sometimes called "Moses illusions") with the goal of identifying the specific circumstances under which the illusion arises, and, as a consequence, the specific constraints placed on possible implementations of linguistic knowledge.

In the first part of this dissertation, I explore the profile of the negative polarity illusion, a case in which a sentence containing an unlicensed negative polarity item and a preceding, but not structurally-relevant licensor is perceived as if it is acceptable, at least in early stages of processing. I consider various proposals for the grammatical knowledge that determines the restricted distribution of negative polarity items, and possible algorithms for using that grammatical knowledge in real time to process a sentence containing a negative polarity item. I also discuss possible parallels between negative polarity illusions and superficially-similar illusory phenomena in other domains, such as subject-verb agreement. Across sixteen experiments, I show that the profile of the illusion is more restricted than previously thought. Illusions do not always arise when an unlicensed negative polarity item is preceded by a structurally-irrelevant licensor, and the circumstances under which they do arise are quite specific. These findings suggest that the negative polarity illusion may be meaningfully distinct from other illusory phenomena, though this conclusion does not necessarily require stipulating a separate mechanism for every illusion. I discuss the implications of these findings for possible real-time implementations of grammatical knowledge.

In the second part of this dissertation, I turn to the substitution illusion, a case in which a word in a trivia fact is swapped out for another word, making the sentence a world knowledge violation, but comprehenders do not consciously detect the anomalous nature of the sentence. Here I attempt to develop specific and testable hypotheses about the source of the illusion, paying particular attention to how the same mechanism that "fails" in illusion sentences (in that it does not allow the comprehender to detect the anomaly) serves the comprehender well in other circumstances. I demonstrate that the substitution illusion, like the negative polarity illusion, is more restricted than previously thought - some stimuli yield very high illusion rates while others yield very low illusion rates, and this variability appears to be non-random. In seven experiments, I pursue both a correlational approach and an experimental manipulation of illusion rates, in order to narrow the space of possible explanations for the illusion.

These investigations collectively demonstrate that occasional errors in comprehension do not necessarily reflect the use of "shortcuts" in sentence processing, and can be explained by the interaction of the linguistic system with non-linguistic components of the cognitive architecture, such as memory and attention. While neither illusion phenomenon is ultimately fully explained, the research presented here constitutes an important step forward in our understanding of both domains and their broader implications.