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...movements reading (Liversedge & Findlay, 2000; Rayner, 1978, 1998), a number of issues remain disputed (Starr & Rayner, 2001). One of these controversies deals with the question of what precisely influences the eye guidance system when it decides not to make a saccade to the next word but to fixate the following word (i.e. word skipping). In this article, we focus on the relative contributions of visual vs. linguistic factors in this decision, an issue we will show to be at the core of the ongoing debate concerning the skipping of words in reading.
Although eye movement patterns during reading seem almost incomprehensibly complex, much can be gained from considering every eye movement as an individual decision of where and when to move the eyes. Interestingly, there is a big difference between the variables that influence these two decisions. The decision of when to move the eyes away from a word (reflected in fixation times on that word) is primarily determined by the processing ease of the word. A very robust finding is that readers will spend more time looking at a low-frequency word than at a high-frequency word (e.g. Inhoff & Rayner, 1986; Rayner & Duffy, 1986; Schilling, Rayner, & Chumbley, 1998). Likewise, a word that is predictable from the preceding context will be looked at for a shorter time than a neutral word (e.g. Balota, Pollatsek, & Rayner, 1985; Binder, Pollatsek, & Rayner, 1999; Ehrlich & Rayner, 1981; Rayner & Well, 1996; Vitu, 1991). Even though visual factors can influence the gaze duration on a word (e.g. a longer word will receive a longer gaze duration; Rayner, Sereno, & Raney, 1996), the linguistic properties of the word account for quite a large part of the variance in fixation times. On the other hand, the decision of where to move the eyes seems to be mostly determined by visual factors, such as the length of the currently fixated word and the lengths of the next words. For instance, Rayner (1979) showed that readers have a tendency to make the first fixation on a word slightly left of the centre of that word (but see White & Liversedge, 2004 for an example of a linguistic influence on landing site).
A first indication of the disputed nature of the word skipping phenomenon is that it is difficult to place within this commonly used when/where dichotomy. Robust influences of both a low-level visual nature and a high-level linguistic nature have been shown to affect skipping behaviour. A typical low-level visual effect on word skipping is the effect of launch site: the closer the eyes are to the parafoveal word, the higher the probability that this word will be skipped on the next saccade (Kerr, 1992; Rayner et al., 1996; Vitu, O'Regan, Inhoff, & Topolski, 1995). The most robust empirical finding in word skipping related to the characteristics of the word itself is the effect of word length: readers tend to skip short words more often than long words (e.g. Rayner, 1979; Rayner & McConkie, 1976; Vitu et al., 1995). Interestingly, this observation has also been made when readers are asked to fake eye movements while scanning through z-strings. Based on the similarities between the skipping patterns of string scanning and normal reading Vitu et al. concluded that predetermined oculomotor strategies are an important determinant of eye movement control in normal reading. However, these conclusions have been questioned by subsequent research (Rayner & Fischer, 1996), and experimental manipulations have been shown to affect skipping of z-strings and normal words in different ways (Drieghe, Brysbaert, & Desmet, 2005). In this latter study, Drieghe et al. observed that whereas adding an extra blank space after a z-string increased the fixation probability of that string by 10%, the same manipulation had no effect whatsoever on the fixation probability of an actual word. Observations such as these cast serious doubts on the generalizability of the findings on skipping in z-string scanning to normal reading.
The fact that short words are skipped more often than long words could be due to either the length of the word or the processing ease of the word (many short words are high-frequency words or syntactic function words). Processing ease has also been shown to influence skipping behaviour when word length is controlled for. A word that is predictable from the preceding context is skipped more often than a word that is not predictable (e.g. Balota et al., 1985; Drieghe, Brysbaert, Desmet, & Debaecke, 2004; Ehrlich & Rayner, 1981; Rayner, Binder, Ashby, & Pollatsek, 2001; Rayner & Well, 1996) and a high-frequency word is skipped more often than a low-frequency word (e.g. Henderson & Ferreira, 1993; Radach & Kempe, 1993; Rayner & Fischer, 1996; Rayner et al., 1996). The predictability and frequency effects suggest that a skipped word has already been processed to a certain degree while the eyes were fixating on the previous word. Considerable debate in the eye movement literature focuses precisely on the extent to which a word can be processed in parafoveal vision and what effects this has on eye movement control (e.g. Radach & Kennedy, 2004). Whereas some models posit that the eye movement system can skip a word only when the word has been completely recognized on the prior fixation or when full recognition is imminent (e.g. the E-Z Reader model, Reichle, Rayner, & Pollatsek, 2003), other models claim that word skipping is based on coarser information and that it entails an educated guess, taking into account factors such as word length and only very partial word identification. In the EOVP model (Brysbaert & Vitu, 1998), for instance, the main determiners of the decision to skip a word are the length of the word and the experience the system has built up with how often a word of a certain length at a certain distance can be skipped without hindering overall text comprehension. This decision can be made with very limited information about the identity of the word.
Other models, such as the SWIFT model (Engbert, Longtin, & Kliegl, 2002; Engbert, Nuthmann, Richter, & Kliegl, 2005) or the Glenmore model (Reilly & Radach, 2003, 2006) can be placed somewhere between the two views mentioned above in terms of how much parafoveal processing they assume to take place prior to skipping. In the SWIFT model, the lexical processing associated with a word is assumed to build up gradually until it has reached a maximum value, after which the lexical activity associated with that word declines. Saccades will be oriented towards words that have the highest level of excitation, which occurs at the peak of lexical processing of that word. As a consequence, the more processing of the parafoveal word (word n + 1) has occurred, the higher the chances that the level of excitation for this word will already have passed its peak and be surpassed by the level of excitation of the subsequent word (word n + 2). Word n + 2 will then win the competition of becoming the target for the next saccade and word n + 1 will be skipped. As a consequence, SWIFT allows a word to be skipped even when the level of lexical processing of word n + 1 has not yet reached the amount assumed by the E-Z Reader model (e.g. a word n + 2 with a very high level of excitation would increase the chances of the word n + 1 being skipped, regardless of the level of excitation associated with word n + 1). On the other hand, SWIFT usually assumes much...
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