Lexical and Semantic Processing

Lexical decisions do not imply the processing of the word meaning; phonological patterns can be correctly recognized as words even if their meaning is not known.

Yet, evidence for semantic priming at a short SOA following task-induced letter-level processing of the prime suggests that the access to the semantic network and the processing of the word’s meaning is the default action of the word perception mechanism (see Smith, Bentin, & Spalek, submitted, for a comprehensive discussion).

Consequently, using only performance measures, it is very difficult to disentangle lexical/phonological and semantic processes in single word recognition when only performance measures are used. It is not surprising, therefore, that influential models tend to devalue the role of phonological processing in word recognition, suggesting that following the orthographic analysis (on the basis of which, for example, a logogen is activated, Morton, 1969), the activation of the word’s meaning in the cognitive/semantic system is a “direct” next step. One of the aims of the present study was to explore the possibility of distinguishing between lexical/phonological and semantic processing by taking advantage of the time continuous measure provided by ERPs. A comparison between the ERPs elicited in the rhyme, lexical decision, and semantic decision experiments suggests that phonological and semantic processes are indeed distinct in time course and possibly also in their functional neuroanatomy.

Recall that lexical decision processes were examined in the present study in three separate oddball experiments that differed in the characteristics of the distinction between targets and nontarget stimuli. We assumed that the cognitive processes required for each distinction modulated the ERPs elicited by the nontargets in each experiment. As in the rhyme task, the ERPs in the lexical decision tasks distinguished mostly between the nonwords (which required only a shallow, orthographic process to be categorized) and the phonologically legal stimuli. Although the ERPs elicited by words and pseudowords were apparently distinguished better in the lexical decision task than in the rhyme task, this difference failed to reach statistical significance (p□= 0.085). In contrast, a significant distinction was found between words and pseudowords in the semantic decision task. The difference between the effect of stimulus type in the lexical and semantic decision tasks might be explained by assuming that different cognitive processes were necessary for making each kind of decision. For example, whereas lexical decisions may be based primarily on activating phonological units in the lexicon, semantic decisions probably require a more extensive and deeper elaboration of the word’s meaning. Consequently, although the activation of word meaning may start in parallel with phonological matching and may even help the lexical decision process, semantic decisions elicit cortical activation that should usually last longer. Indeed, in the present study, the onset of the difference between the ERPs elicited by each stimulus type in the lexical and semantic decision tasks were not very far apart, whereas the epoch during which different ERPs were elicited by each stimulus type was longer in the semantic than in the lexical decision task. Differences between the functional neuroanatomy of the semantic and lexical activity is suggested by the significantly different distribution of the N350 and N450, the two most prominent negative potentials that were elicited in the lexical decision and semantic tasks, respectively. Whereas the N350 was largest at T3 and was distributed over the midtemporal and supratemporal regions, the semantic decision seemed to involve, in addition, more anterior and superior areas of the temporal lobes and adjacent regions in the left frontal lobe. This distribution (particularly its left-hemisphere-dominant asymmetry) is different from that usually found for N400 potentials in sentences (Kutas & Hillyard, 1982) or lexical decision tasks (e.g., Holcomb, 1993). It is, however consistent with PET findings in tasks that require semantic activity (e.g., Demonet et al., 1992) and fMRI studies of word generation (McCarthy et al., 1994). At the very least, this distribution supports a dissociation between pure phonological and semantic activity, consistent with neurological studies that have described a double dissociation between dyslexic patients who can read words without understanding their meaning (e.g., Schwartz, Saffran, & MArin, 1980), and patients who understand the meaning of spoken words but are unable to read them (for a recent review see Ellis & Young, 1996).

Comparing phonologically legal and illegal orthographic patterns across all tasks suggests that the linguistic-related ERP activity (in single-word processing) was reflected in negativities whose peak latency preceded the P3b. This finding is congruent with ample evidence that has been published since the discovery of the N400 (Kutas & Hillyard, 1980). In the present lexical decision task, the most prominent negativity peaked at 350 msec. As mentioned above, words and pseudowords elicited similar ERPs at this latency.

This finding seems to contradict the well-established RT difference between words and pseudowords in lexical decision tasks. Our data, however, were derived from a lexical decision paradigm different from the ordinary word/nonword decision tasks. First, it required no speeded RTs, and therefore some of the factors influencing the RTs in lexical decision tasks were inconsequential in the present paradigm. Second, and more important, the ERPs measured for the present comparisons were not elicited by the target stimuli.

Both the words and the pseudowords were equally ir-relevant for the subject’s task and were therefore members of the same task-related response category. Indeed, the amplitude of the P300 elicited by the words was significantly higher than that elicited by the pseudowords. In conclusion, we suggest that the N350 may be associated with the phonological analysis of the orthographic pattern applied to both words and pseudowords.

In the semantic task, the difference between words and pseudowords was apparently divided into two distinct epochs. The first ended at about 350 msec from stimulus onset (the peak latency of the negative potential in the lexical decision tasks). During this epoch the ERPs elicited by words and pseudowords did not significantly differ one from another. Therefore we suggest that the ERP activity elicited during this period is associated with phonological processes that are similar in the lexical decision and the semantic tasks. During the second epoch, words and pseudowords were clearly different. This difference started at about 350 msec and culminated at the peak of the N450, which is not seen in the lexical decision task (Figures 2E and 5).

Surprisingly, the N450 elicited by pseudowords was significantly larger than that elicited by words. In general, we (as well as others) assume that larger negativities reflect more extensive processing that, in this experiment, was semantic (cf. the modulation of the N400 by semantic priming, Bentin et al., 1985, or by repetition, Rugg, 1985, in lexical decision). Because the task was to distinguish between abstract and concrete words, one approach could have been to perform a lexical decision first and then continue the semantic processing only for words. Such an approach should have resulted in a larger N450 for words than for pseudowords. A second approach was also possible, however. In this approach the reader would attempt to decide directly whether a phonological legal orthographic pattern is an abstract word or not (i.e., without making a word/pseudoword distinction first). If this approach is taken, deciding that a (known) concrete word is not abstract may be easier (and faster) than deciding that a pseudoword is not an (infrequent) abstract word. Apparently our subjects chose the second decision strategy. Admittedly, this interpretation is post hoc. It is, however, consistent with the larger P300 observed for words (which might have been the source of the word/pseudoword difference) and not (a priori) implausible.