Coordination of Word Recognition and Oculomotor Control During Reading: The Role of Implicit Lexical Decisions

Recognizing words so that their meanings can be retrieved is critical to successful reading comprehension. These lexical processes must be coordinated with other perceptual, motor, and attentive processes in order for eye movements during reading to be controlled efficiently. During reading the eyes alternate between fixations (periods when the eyes are nearly stationary) and saccades (rapid ballistic movements); for skilled readers saccades typically occur four to five times per second with most words receiving only a single first-pass fixation (Rayner, 1998). Oculomotor processes, which are constrained by differential acuity across the retina and by the nature of ocular motor control, are major determinants of eye movements during reading. The contribution of these processes can be seen in how physical characteristics of text, such as the length of words, influence the pattern of fixations and saccades. Language-based processes that relate to comprehension of the text also exert a strong influence on when – and occasionally where – the eyes move during reading. The contribution of these processes can be seen in how eye movements are influenced by non-physical aspects of the text such as word frequency or the predictability of a word in context. Very different explanations have been advanced for how oculomotor and language-based processes are coordinated. At one extreme it is argued that language-based effects on eye-movements lead to only slight modulation of a more basic oculomotor process that generates forward saccades of a standard length at a regular rhythm (McConkie, Kerr, Reddix, & Zola, 1988; O’Regan, 1990; Yang & McConkie, 2001). At the other extreme it is argued that, while oculomotor processes determine most characteristics of the targeting of forward saccades, word recognition determines when saccades are initiated and in some instances their targeting (Morrison, 1984; Pollatsek, Reichle & Rayner, 2006; Reichle, Pollatsek, Fisher, & Rayner, 1998). Other models take more intermediate positions (Engbert, Nuthmann, Richter, & Kliegl, 2005; Reichle, Rayner, & Pollatsek, 2003). The phenomenon of skipping, where a word is not directly fixated during the first pass of the eyes over the text, has been central to the evaluation of these alternative models. Word length has a very strong effect on skipping, with short words skipped more frequently than long words (Kerr, 1992; Vitu, O’Regan, Inhoff, & Topolski, 1995; for a review see Brysbaert, Drieghe, & Vitu, 2005). This effect can be readily understood as resulting from visual and motor processes. A short word to the right of the fixated word is seen more clearly than a long word because it is more likely to fit within the fovea. Further, having the eyes land on short words tends to require shorter saccades than having them land on long words, which increases the likelihood that short words will be skipped because motor error in saccade targeting takes the form of overshoot for short saccades and undershoot for long saccades (McConkie, Kerr, & Dyre, 1994; McConkie et al., 1988). Language-based factors also affect skipping, such that frequent and predictable words are more likely to be skipped than infrequent or unpredictable words (Ehrlich & Rayner, 1981; Rayner & Fisher, 1996; Rayner & Raney, 1996; Rayner & Well, 1996; Schilling, Rayner, & Chumbley, 1998; White, 2008). Skipping effects are interesting in part because they are the only case where language factors related to word recognition affect the targeting of forward saccades rather than just their timing (Drieghe, Rayner, & Pollatsek, 2005). As such, skipping has been an important arena for addressing the manner in which lexical processing is coordinated with processes of attention, vision and motor control during reading. Models of eye movement control that attribute only a small role to language-based processing relative to oculomotor processing account for language effects on skipping in a number of ways. For example, according to the Extended Optimal Viewing Position (EOVP) model, language-based skips are in essence an educated guess based on factors such as word length and very partial word identification (Brysbaert & Vitu, 1998). The main determinants of the decision to skip a word are the length of the word in the parafovea 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. Consequently that decision is made with very limited information about the identity of the skipped word. Models of eye-movement control that attribute a more substantial role to language-based (or cognitive) factors have gone through a number of iterations and refinements, but have retained a core set of mechanisms to account for language-based skipping as described by the EZ Reader model (Morrison, 1984; Pollatsek et al., 2006; Reichle et al., 1998). These models posit a serial-attention-shifting mechanism where words in the text are processed one at a time (though recognition of a word is achieved by matching the stimulus to multiple lexical and sublexical representations in parallel). Once the fixated word is recognized, attention shifts to processing parafoveal information about the next word. Attention shifts prior to movement of the eyes because of limits on the rate at which saccades can be generated. In the majority of cases the oculomotor system then executes a planned saccade which has been targeted to the next word on the basis of a parallel process of visual analysis that locates word boundaries. The processing that occurs prior to fixation on that word provides a parafoveal-preview benefit that facilitates recognition, a spillover effect that can be observed in first-pass reading times (Henderson & Ferreira, 1990; Kennison & Clifton, 1995; White, Rayner, & Liversedge, 2005). In a minority of cases, recognition of the word in the parafovea occurs quickly enough that the planned saccade to that word can be cancelled and a new saccade can be programmed to a more distant word resulting in a skip of the word in the parafovea. Thus, according to the EZ Reader model the ease of recognizing the current word affects both the amount of parafoveal-preview benefit on the next word and, together with the ease of recognizing the next word, determines whether that next word is skipped. A strong association between ease of word recognition, parafoveal preview benefit and skipping has been found for word frequency and predictability (e.g., Drieghe et al. 2005; Rayner & Fisher, 1996) though manipulations of the visual characteristics of text (Drieghe, 2008) and the phonological length of words (Fitzsimmons & Drieghe, 2011) have shown dissociations between these factors. An essential feature of the serial-attention-shifting models is that the same mechanisms are used for recognizing words in the fovea and parafovea. In both cases the initiation of saccadic processes is linked to processing words to a level where the words are highly likely to be fully recognized soon (Pollatsek et al., 2006). Gordon, Plummer and Choi (in press) tested this model by using repetition to manipulate the ease of recognizing a target word while using the boundary technique (Rayner, 1975) to manipulate the information about that word available during parafoveal preview. The boundary technique is a gaze-contingent method where the stimulus word changes when the eyes cross an invisible boundary on the screen, thereby allowing manipulation of the relationship between parafoveal and foveal information about a word. In Gordon et al. the string in the parafovea could either be the target word itself (e.g., the name Herman) or a nonword created by transposing two internal letters of the target word (e.g., Hreman). When used as primes in masked-priming studies or as preview strings in eye-movement studies, such transposed letter (TL) nonwords produce substantial facilitation in the subsequent processing of the target word as compared to substituted letter (SL) nonwords that have the same number of letter-in-position differences with the target word (Johnson, Perea, & Rayner, 2007; Perea & Lupker, 2003a, 2003b;). However, when TL nonwords are directly fixated they disrupt reading (causing longer first-pass reading times and greater likelihood of refixation) because they are not words (Rayner, White, Johnson, & Liversedge, 2006; White, Johnson, Liversedge, & Rayner, 2008). Gordon et al. found that facilitating lexical access through repetition increased skipping when the parafoveal preview string was the target word itself, but that repetition had no effect on skipping when the parafoveal preview string was a highly-similar TL nonword. This pattern supports serial-attention-shifting models, such as EZ Reader, by showing that the letter string in the parafovea must be recognized as a word in order for lexical priming to increase skipping above the baseline rate that occurs due to oculomotor processes. Thus, it is not the case that contextual facilitation due to repetition priming constitutes a bias that combines with partial visual information from the periphery to increase skipping (see Engbert et al., 2005; Rayner, Slattery, Drieghe & Liversedge, 2011 for advocacy of such a mechanism in the case of word predictability). The Gordon et al. findings are consistent with models of eye-movement control during reading where the key link between language-based and oculomotor processes is an implicit lexical decision about whether the letter string in the parafovea will be successfully recognized as a word.