Our language faculties allow us to produce and understand a potentially infinite set of word combinations (Chomsky, 1965). However, this does not necessarily imply that the normal usage of language always occurs via a full parsing of whatever linguistic information we are presented with. There are indeed word sequences that, because of some peculiar characteristics, are likely to be stored as a whole. Some examples are collocations, i.e., expressions of multiple words which commonly co-occur in a given order (e.g., “at the end of,” “in relation to”), idioms, i.e., word phrases that have a meaning different from their literal meaning (e.g., “to kick the bucket,” “to spill the beans”), compounds, i.e., words made up by other existing words (e.g., “black hole,” “pickpocket”), and irreversible binomials (IBs), i.e., word pairs linked by a conjunction (e.g., “hit and run,” “dead or alive”).

There are two main reasons why our mental lexicon could store word sequences: frequency of usage (Bybee, 2007), and the opaque relationship between the meaning of the single words and the meaning of whole structures, when the meaning of the whole sequence cannot be derived analytically by the meaning of the constitutive elements (Libben, 1998).

The present study focuses on IBs, which are complex linguistic constructions found in both Romance and Germanic languages. In Italian and in English, as well as in other languages, IBs usually consist of two words conjoined by a copulative or disjunctive conjunction (e.g., “bianco e nero,” black and white; “ora o mai più,” now or never), in some cases accompanied by one or more prepositions (e.g., “in fretta e furia,” in a rush, lit. in rush and fury; Malkiel, 1959; Masini, 2006). The two constituents must belong to the same lexical class.

As the adjective irreversible suggests, the order of the elements in an IB cannot be reversed. The Italian sentences in Figure 1 can be used as examples.

In Figure 1A the IB “acqua e sapone” (lit. water and soap) shows an idiomatic, non-compositional meaning. The reversed order is unacceptable.

In Figure 1B the same sequence “acqua e sapone” becomes completely different because of its referential, non-idiomatic, meaning (water and soap), and the order can be freely reversed.

For IBs (as for all other idiomatic word sequences) it is logically necessary that the information about the whole sequence meaning is stored somewhere in the long-term memory. In light of this consideration, a crucial issue concerns how this information is stored and accessed. Evidence related to multi-word sequence representation and processing mainly comes from studies of idioms (e.g., “to cry over spilled milk”) and compounds (e.g., “black hole”).

The “lexical” models, which were initially proposed for idiom processing, postulated that idioms are stored in the mental lexicon as multi-words sequences and can be retrieved as such (Bobrow and Bell, 1973). On the other hand, according to “non-lexical” theories, idioms are represented as configurations of lexical items but whole-sequence representations are not stored in the mental lexicon. For example, according to the Configuration Hypothesis (Cacciari and Tabossi, 1988), an idiom initially undergoes a word-by-word processing (just as a non-idiomatic sentence) and the idiomatic meaning is retrieved only when enough information is accumulated to identify the word sequence as an idiom. According to the hybrid model of idiom representation (Sprenger et al., 2006) idioms are not treated as unitary lexical items at the word form level, but at a higher level of representation (i.e., superlemma, Sprenger et al., 2006). Recent findings on idiom processing seem to support non-lexical models, since there is evidence that syntactic analysis occurs also after the idiomatic expression is recognized (Peterson et al., 2001).

Theories on the representation and processing of compound words mainly derive from the theories on morphologically complex words. Traditionally two competing classes of theories about the representation of complex words have been proposed: the “full-listing” theories (Butterworth, 1983; Bybee, 1995) and the “decomposition” or “full parsing” theories (Taft and Forster, 1976; Libben et al., 1999; McKinnon et al., 2003; Taft, 2004). After an initial debate about these opposing theories, a compromise known as “dual-route” theories, has gained ground. There are several variants of “dual-route” theories, but all of them share the claim that words can be either stored as a whole or be decomposed into their morphological constituents (Caramazza et al., 1988; Sandra, 1990; Zwitserlood, 1994; Baayen et al., 1997; Isel et al., 2003). Different types of complex words can be processed preferentially via one route rather than another. For example, very frequently used items and, in the particular case of compounds, opaque ones, can be stored and processed more efficiently in their full form. In contrast, less frequent items and transparent compounds can be preferably subject to decomposition. Compounds have largely been studied using different experimental paradigms, such as lexical decision (e.g., Juhasz et al., 2003; Duñabeitia et al., 2007) or priming (e.g., Jarema et al., 1999), with different experimental techniques such as recording eye movements (e.g., Juhasz et al., 2003; Inhoff et al., 2008; Kuperman et al., 2008), or ERPs (e.g., Koester et al., 2004; Koester et al., 2007; El Yagoubi et al., 2008), and with patients who have brain lesions (for recent reviews see Chiarelli et al., 2007; Semenza and Mondini, 2010; Semenza et al., 2011b). The large majority of these studies converge in suggesting that compounds are decomposed at an early stage of their processing, regardless of their transparency. However, the evidence of early parsing often does not exclude the role of a whole-word form. Indeed, recent results suggest that together with compound parsing, whole-word forms may affect early processing, and that multiple sources of information are taken into account in compound processing (Kuperman et al., 2008).

To our knowledge no previous study has ever investigated the mental representation of IBs. One study (Siyanova-Chanturia et al., 2011) focused on non-idiomatic “reversible binomials,” word phrases composed of two words and a conjunction. Whereas the word order in IBs is fixed, in reversible binomials it is not, and reversing the order of constituents does not change the overall meaning of the word pair (e.g., “groom and bride,” “bride and groom”). However, one word order is usually preferred to the other, and occurs more frequently (Masini, 2006). In their study, Siyanova-Chanturia et al. (2011) investigated the effects of several psycholinguistic variables on different eye movement measures. Their main findings were about the effects of binomial frequency, indicating that a whole-sequence representation of reversible binomials affects reading.

The data presented in the relevant literature point to two opposite hypotheses. According to theories on idioms, a whole-sequence representation would play a significant role only in the later stages of processing (e.g., Cacciari and Tabossi, 1988; Sprenger et al., 2006). In contrast, recent theories on compounds (e.g., Kuperman et al., 2008) suggest an early influence of representation of both constituents and whole words. The very recent study of Siyanova-Chanturia et al. (2011) on reversible binomials supports this latter hypothesis in that only a frequency effect of the whole binomial (and not of single words) was found.

The present study is the first neuropsychological investigation concerning the problem of the mental representation and processing of IBs (preliminary data on a lesser number of participants have been presented at the Academy of Aphasia conference held in Athens in 2010, see Arcara et al., 2010). In order to test the lexical status of IBs, the phenomenon of neglect dyslexia is exploited. Reading IBs in terms of neglect dyslexia may indeed contribute to understanding if and when a whole representation can be accessed during lexical processing.

Neglect dyslexia is a deficit that commonly follows right hemisphere damage: it is characterized by an impairment in reading of letters, words, or strings of words located in the contra-lesional (i.e., left) visual space. Errors produced in word reading are often omissions and substitutions of the leftmost portion of the word or of the string of words. The literature on neglect dyslexia suggests that errors may be influenced by the lexical status of the target. The nature of stored lexical knowledge seems to partially compensate for attentional problems (Arduino et al., 2002). More conspicuous components of a word, such as, for instance, the head of a compound, seem more resistant to the deficit (Semenza et al., 2011a). A similar investigation related to IBs is thus conducted here.

Thirty participants with left neglect were recruited for the present research. All patients were Italian native speakers who showed signs of neglect dyslexia for the left side of the visual space as a consequence of a right hemisphere lesion. They were recruited, for the most part, through the I.R.C.C.S¹ Ospedale S. Camillo, Lido di Venezia (Italy) and a few participants were tested in their homes. Neglect was assessed through the Behavioral Inattention Test (BIT). Table 1 reports descriptive data about participants. The location of the bulk of the lesions is reported as the best approximation allowed by available neuroimaging data (CT scans or MRI).

People with left-sided neglect dyslexia demonstrate a clear lexical effect. They can read IBs better when components are presented in their correct order. This may be taken as an indication that they treat IBs as lexical, not decomposable, elements. This finding is therefore a strong evidence that IBs tend to be stored in the mental lexicon as a whole and that this whole form is preferably addressed in the retrieval process. Even though it should be possible to activate each constituent of an IB singularly, this experiment shows the primacy of the lexicality of whole binomials in terms of their processing (examples of recent results supporting whole-sequence representation and processing are given in Tremblay and Baayen, 2010; Siyanova-Chanturia et al., 2011; Tremblay et al., 2011). It is unlikely that the observable advantages of IBs are related to a guessing strategy which may be adopted by patients with neglect dyslexia. Only once did a patient produce an IB in place of the corresponding NB (e.g., the NB “caduta e fuga,” fall and run, read as the IB “toccata e fuga,” hit and run). Secondly, no significant effect of the probability of guessing the IBs giving the second constituent was found (no significant effect of PROB_SECOND). Thus, it is unlikely that patients used the available information on the rightmost part of the stimulus to guess what the neglected part was.

The difference between IBs and RBs supports the conclusion of the effect of a whole-sequence representation. Both IBs and RBs include the same words but arranged in a different order (e.g., “toccata e fuga,” tr. hit and run and “fuga e toccata,” tr. run and hit). The only difference between these two conditions is the order of constituents. The advantages of IBs cannot be explained, but assuming that a whole-binomial representation affects the early stages of word processing, which may be impaired in patients with neglect dyslexia (Vallar et al., 2010). The advantages of IB reading may also be explained in terms of frequency: IBs in the correct order are obviously more frequent then RBs and NBs. However, when considering only IBs, we did not find any effect of frequency (the probability of reading correctly IBs is not related to their frequency) suggesting that it is not the frequency that mainly determines if an IB is stored as a whole, but its lexical status.

Side effects of constituent length were also found. The effects of word length are commonly found in many studies on neglect dyslexia patients (see the review by Vallar et al., 2010). In the present study, the analysis of all stimuli shows that, as the length of both the first and the second constituent increases, reading accuracy decreases. The lengths of the first and second constituents could affect the stimulus processing in different ways. In the case of the first constituent, the longer it is, the more it falls on the neglected side. It is more difficult to interpret the effect of the length of the second constituent. We can speculate that, the longer it is, the more it may capture the attention of neglect patients on the right (Vallar et al., 2010), further compromising their reading performance of the left portion.

In the analysis restricted to IBs, only an effect of the second constituent length was found. As its length increases, reading performance decreases. Again, the effect of attentional capture could explain this result. The absence of an effect of first constituent length only on IBs supports the importance of a whole-sequence representation in the analysis. In fact, if the attention of neglect patients is captured by the second constituent, then the first one is more often dropped (see the difference in the proportion of omissions and substitutions of the first constituent in IBs, Table 3), but if the patient directs his/her attention also toward the leftmost part of the stimulus, then the whole sequence can be retrieved (hence, the first constituent length plays no role).

These findings extend our knowledge of the phenomena which occur as part of the syndrome of neglect dyslexia, showing the extent to which stored lexical knowledge interferes with defective visuo-spatial processing and partially compensates for attentional problems. The lexical effects in neglect dyslexia demonstrated so far are typically exemplified by the word superiority effect (a proportion of such patients read better words than non-words, Sieroff et al., 1988; Behrmann et al., 1990; Brunn and Farah, 1991). Another finding in neglect dyslexia is that non-words consisting of a real root and a real affix are easier to read than when they contain neither of these constituents (Arduino et al., 2002). Moreover, words with more orthographic neighbors are more difficult to read than words with few orthographic neighbors (Riddoch et al., 1990; Arguin and Bub, 1997). A similar effect has not been so far described for strings of words. The present findings thus underline the pervasiveness of lexical effects in neglect dyslexia and highlight the nature of this deficit. Orthographic information is easier to process when related to salient lexical items. Thus, words presented in the context of an IB would require a relatively lighter processing load. They would benefit from top-down facilitation (see Lavie, 1995; Brand-D’Abrescia and Lavie, 2007 for theories about the role of attention in word recognition) with respect to when they are presented in the reverse order or when they have no established relation to each other in the lexicon. Strings of words with no order relation to each other appear instead to be more prone to attention lapses. As suggested by effects obtained from substitution errors, probably the strongest competition is from distracters. The lexical effects of neglect dyslexia can be interpreted (Arduino et al., 2002) as supporting “late selection” views of attentional processing (Deutsch and Deutsch, 1963; Behrmann et al., 1991; Umiltà, 2001). They suggest that spatial attentional components, the impairment of which leads to neglect dyslexia, also operate at a later stage of processing, after the information presented in the unattended visual area has undergone higher-level analyses, including lexical and semantic processing.

In reference to theories of lexical representations, the performance of neglect patients in reading IBs suggests that a whole-binomial representation is stored at the level of the input orthographic lexicon (Vallar et al., 2010). In reference to the theory by Levelt and collaborators (Levelt et al., 1999; Sprenger et al., 2006), IB reading can imply the activation of a whole-sequence representation being already at a peripheral input level, i.e., the word form level. We cannot exclude the possibility that these effects are also related to an activation of semantic representations. However, the modulation of the attentional deficit, shown here in analogy with other lexical effects in neglect dyslexia, seems to suggest that the representation of the whole sequence is stored at the level of the input orthographic lexicon.

These results are in line with theories that assume the possibility of an early influence of the whole-sequence representation in lexical processing. A recent mathematical model that embraces such view is the probabilistic model of information sources (PROMISE) (Kuperman et al., 2008). According to the PROMISE model, several probabilistic sources of information are taken into account in morphological processing. Among these sources, one is the whole-word probability. Even if this model has been introduced for morphologically complex words (as compounds) it could also be valid for explaining IB processing. Indeed, increasing evidence supports models that take into account word sequence probabilities in reading (e.g., McDonald and Shillcock, 2003). It is more difficult to integrate the current results with the major established views of idiom processing (Cacciari and Tabossi, 1988; Sprenger et al., 2006), which advocate that a whole-idiom representation should play a role only in later stages of processing or be represented at a superlemma level (hence, not in the word form, but at a supralexical level). However, it is possible to conciliate the present results with these theories by assuming as follows: since IBs are small constructions with poor syntactic information, they are more likely to be stored in the input orthographic lexicon as a whole (as compounds) in comparison to idiomatic sentences.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.