UNCERTAINTY AND EXPECTATION IN SENTENCE PROCESSING 1 Running head : Uncertainty and expectation in sentence processing Uncertainty and expectation in sentence processing

There is now considerable evidence that human sentence processing is expectation based: As people read a sentence, they use their statistical experience with their language to generate predictions about upcoming syntactic structure. This study examines how sentence processing is affected by readers’ uncertainty in their expectations about upcoming structure. In a self-paced reading study, we manipulate both expectations and the uncertainty about them factorially, based on lexical subcategorization information. We compare two types of uncertainty: uncertainty about the verb’s complement, reflecting the next prediction step; and uncertainty about the full sentence, reflecting an unbounded number of prediction steps. We find that uncertainty about the full structure, but not about the next step, was a significant predictor of processing difficulty: Greater reduction in uncertainty was correlated with increase reading times. We additionally replicated previously observed effects of expectation violation (surprisal), orthogonal to the effect of uncertainty. This suggests that both surprisal and uncertainty have a role to play in explaining human reading times. We discuss the consequences for theories of sentence comprehension.