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The predictability problem
(2007)
We try to determine whether it is possible to approximate the subjective Cloze predictability measure with two types of objective measures, semantic and word n-gram measures, based on the statistical properties of text corpora. The semantic measures are constructed either by querying Internet search engines or by applying Latent Semantic Analysis, while the word n-gram measures solely depend on the results of Internet search engines. We also analyse the role of Cloze predictability in the SWIFT eye movement model, and evaluate whether other parameters might be able to take the place of predictability. Our results suggest that a computational model that generates predictability values not only needs to use measures that can determine the relatedness of a word to its context; the presence of measures that assert unrelatedness is just as important. In spite of the fact, however, that we only have similarity measures, we predict that SWIFT should perform just as well when we replace Cloze predictability with our measures.
To investigate eye-movement control in reading, the present thesis examined three phenomena related to the eyes’ landing position within words, (1) the optimal viewing position (OVP), (2) the preferred viewing location (PVL), and (3) the Fixation-Duration Inverted-Optimal Viewing Position (IOVP) Effect. Based on a corpus-analytical approach (Exp. 1), the influence of variables word length, launch site distance, and word frequency was systematically explored. In addition, five experimental manipulations were conducted. First, word center was identified as the OVP, that is the position within a word where refixation probability is minimal. With increasing launch site distance, however, the OVP was found to move towards the word beginning. Several possible causes of refixations were discussed. The issue of refixation saccade programming was extensively investigated, suggesting that pre-planned and directly controlled refixation saccades coexist. Second, PVL curves, that is landing position distributions, show that the eyes are systematically deviated from the OVP, due to visuomotor constraints. By far the largest influence on mean and standard deviation of the Gaussian PVL curve was exhibited by launch site distance. Third, it was investigated how fixation durations vary as a function of landing position. The IOVP effect was replicated: Fixations located at word center are longer than those falling near the edges of a word. The effect of word frequency and/or launch site distance on the IOVP function mainly consisted in a vertical displacement of the curve. The Fixation-Duration IOVP effect is intriguing because word center (the OVP) would appear to be the best place to fixate and process a word. A critical part of the current work was devoted to investigate the origin of the effect. It was suggested that the IOVP effect arises as a consequence of mislocated fixations, i.e. fixations on unintended words, which are caused by saccadic errors. An algorithm for estimating the proportion of mislocated fixations from empirical data was developed, based on extrapolations of landing position distributions beyond word boundaries. As a new central theoretical claim it was suggested that a new saccade program is started immediately if the intended target word is missed. On average, this will lead to decreased durations for mislocated fixations. Because mislocated fixations were shown to be most prevalent at the beginning and end of words, the proposed mechanism generated the inverted U-shape for fixation durations when computed as a function of landing position. The proposed mechanism for generating the effect is generally compatible with both oculomotor and cognitive models of eye-movement control in reading.