The effect of lexical predictability on distributions of eye fixation durations

A word's predictability in context has a well-established effect on fixation durations in reading. To investigate how this effect is manifested in distributional terms, an experiment was carried out in which subjects read each of 50 target words twice, once in a high-predictability context and once in a low-predictability context. The ex-Gaussian distribution was fit to each subject's first-fixation durations and single-fixation durations. For both measures, the mu parameter increased when a word was unpredictable, while the tau parameter was not significantly affected, indicating that a predictability manipulation shifts the distribution of fixation durations but does not affect the degree of skew. Vincentile plots showed that the mean ex-Gaussian parameters described the typical distribution shapes extremely well. These results suggest that the predictability and frequency effects are functionally distinct, since a frequency manipulation has been shown to influence both mu and tau. The results may also be seen as consistent with the finding from single-word recognition paradigms that semantic priming affects only mu.