Unlike decisions regarding stimulus size, which can be made just as well on orthographic and nonorthographic stimuli, rhyme decisions based on written stimuli usually require the transformation of orthographic patterns into phonological patterns from which phonetic codes can be discerned. 6Consequently, in the rhyme task we used only orthographic stimuli that, as expected, elicited an N170 evident particularly at the posterior temporal and occipital sites (see Figure 5, TP7 and TP8). As anticipated on the basis of the results in the size task, the ERPs elicited by the three orthographic stimulus types were not distinguishable at the level of the N170. One hundred milliseconds later, however, two categories of stimuli were evidently processed differently. One included the words and the pseudowords for which the formation of a phonological pattern was possible and on the basis of which the phonetic decision could have been made.
The second category comprised the nonwords that could not be transformed into a coherent phonological structure and consequently allowed a negative decision based on shallow orthographic analysis. The difference between the ERPs elicited by pronounceable and nonpronounceable stimuli was probably associated with the difference in processing the two stimulus categories, as well as to a difference in decision-making strategies.
Whereas, following the N170, the ERPs elicited by nonwords were dominated by a positive-going potential (possibly a P300 associated with the fast and easy reaching of a negative decision), the ERPs elicited by pronounceable stimuli were comprised first of a negative potential (N320), at the resolution of which the P300- like potential was observed.
Because the cognitive and linguistic processes required for making rhyme decisions are not evident in performance, it is impossible to unequivocally link the N320 to a particular cognitive event. For example, although the full activation of the lexicon is not necessary for generating phonetic codes, 7as revealed by the correct decisions made for pseudowords, phonology probably mediates between orthography and phonetics.
Furthermore, it may be possible to decide whether two orthographic patterns rhyme on the basis of matching their abstract phonological realizations (i.e., without converting the phonemes to phones). Therefore, we cannot discard the possibility that the activity reflected by the ERPs in the rhyme-decision task was only phonological. Indeed, although the spatial distributions of the N320 and N350 did not completely overlap, both potentials were maximal at T3. Moreover, the onset □of the difference between phonologically legal and illegal stimuli began slightly sooner in the lexical decision task (270 msec at T3) than in the phonetic decision task (295 msec at T3). This difference, however, was not significant, a fact that is hardly surprising if we assume that phonological processes were involved in both processes. Yet, compared with the N320, the distribution of the N350 is slightly more anterior in the left temporal lobe and clearly broader in circumference, including parietal and fronto-parietal areas that were not activated in the phonetic task. The difference in latency and scalp distribution between the N320 and the N350 that was observed in the lexical decision tasks suggests that the cognitive processes involved in these two tasks did not entirely overlap. It is possible that the N320 is associated with the phonetic transformation performed on pronounceable orthographic patterns, a process that began not earlier than 270 msec from stimulus onset, following the initiation of the orthographic analysis. Assuming that ERPs are, at least partially, associated with cognitive events and reflect their time course and, to some extent, their underlying neural basis, the precedence of the N320 over the N350 and the partial overlap in scalp distribution suggests that while phonological units were activated in both tasks, the formation of phonetic codes is faster (concluded sooner) than the additional lexical (or postlexical) processes required to reach a lexical decision. The scalp distribution of the N320 was very different from that of the N170, being particularly conspicuous at midtemporal-parietal sites, predominantly over the left hemisphere. This pattern is inconsistent with the findings of Rugg (1984) (see also Praamstra, Meyer, & Levelt, 1994; Rugg & Barrett, 1987), who reported a right-hemisphere dominant N450 in a rhyme-matching task. Yet, both the scalp distribution and
the considerably shorter latency of the N320 relative to the N450 suggest that two different cognitive phenomena were tapped in the two studies. The N450 may be associated with a relatively late, postlexical phonological process, whereas the N320 could represent an early lexical or prelexical process of grapheme-to-phoneme-to-phone translation. This suggestion is supported by the distribution of the N320, which roughly corresponds to Wernicke’s area. This distribution is consistent with the data reported in several PET studies in which temporo-parietal activation was found when subjects performed rhyme-detection tasks on visually or auditory presented words (Petersen & Fiez, 1993; Petersen et al., 1989).
Interestingly, these temporal/temporo-parietal regions were not activated by simple auditory stimuli, including tones, clicks, or rapidly presented synthetic syllables (Lauter, Herscovitch, Formby, & Raichle, 1985; MAzziotta, Phelps, Carson, & Kuhl, 1982). Moreover, clinical neuro-psychological literature reported that lesions surrounding the left sylvian fissure (Wernicke’s area, insular cortex, supramarginal gyrus) may cause a deficit in sound categorization and an inability to arrange sounds into coherent speech (MArshall, 1986). Hence, the ERP data in the present study concur with previous neuroimaging and neuropsychological evidence regarding the neuroanatomical distribution of areas associated with phonetic processing, suggesting that the phonetic analysis of written words starts at about 270 msec from stimulus onset, about 150 msec after the onset of orthographic analysis.
We chose a target that can rhyme with words ending in different spellings. Hence, subjects could not have performed this task by simply matching the orthographic patterns (see Methods).
Some models of word recognition, however, suggest that the phonological structures of pseudowords are derived by activating lexical entries of analogous real words (e.g., Glushko, 1979).