An Overview

The interpretation of the present results and their implications for the psycholinguistic and neural mechanisms involved in processing individual words are valid to the extent that (1) our tasks implicated, indeed, the presumed perceptual and linguistic processes and (2) the scalp-recorded ERPs were modulated by these processes.

Although none of the above caveats can be easily overridden, we accepted both assumptions as working hypotheses. With these caveats in mind, we can continue our discussion and suggest some interpretations.

The ERPs elicited by the different stimuli across tasks displayed several important patterns. First, regardless of task and phonological values, orthographic patterns elicit fairly similar activity at the occipital and occipitotemporal scalp regions, predominantly in the left hemisphere (Figure 6). This pattern suggests that letters automatically activate visual modules that are tuned to detect orthographic material prior to any deeper linguistic process. Orthographic stimuli that allow phonological and/or phonetic processing activate language-processing-specific areas in the midtemporal and supratemporal regions, predominantly in the left hemisphere (Figure 8). These areas are probably involved in phonological and phonetic processing. In addition, semantic activity elicits ERPs that are distributed over the anterior-temporal fronto-central scalp areas. In the present study we used only orthographic patterns. Other studies, however, showed similar ERP distribution in response to visually presented objects (Barrett & Rugg, 1989) and even nonlinguistic stimuli such as unfamiliar human faces (Barrett & Rugg, 1989; Bentin & McCarthy, 1994). Hence, the fronto-central areas activated in the semantic decision task in the present study may be part of a conceptual semantic memory system that may include, but does not necessarily totally overlap with, the words’ meaning network. Interestingly, there seems to be a correlation between the site of activity on the anterior-posterior dimension, on the one hand, and the depth of processing in general and linguistic processing in particular, on the other (Figure 2B and 2E). Apparently deeper processing of the orthographic patterns is associated with activity in more anterior regions of the temporal lobe. A similar conclusion has been reached by McCarthy and his colleagues using intracranial recordings (McCarthy et al., 1995; Nobre & McCarthy, 1995), and it is congruent with the functional organization of the “ventral pathway” of the visual system described by several authors (e.g., Felleman & Van Essen, 1991; MAunsell & Newsome, 1987; Van Essen & DeYoe, 1995).

The scalp distribution of the ERP activity in the different tasks and their onset and time course is incongruent with either a unified brain mechanism for word perception or a serial model of processing. The scalp distribution of the negative peaks, although overlapping to some extent, was sufficiently distinct (across peaks) to suggest that different neural networks may be involved in each type of process. Overall, such a pattern may support a word-recognition mechanism based on a network of interrelated neural modules working in synchrony, each of which is responsible for a particular aspect of the word-recognition process. The peaks of the negative components associated with each level of processing were different, later in deeper processing tasks than in more shallow ones. Yet, the epochs during which the ERPs were modulated by each task overlapped in time to a great extent. Although the duration of an ERP does not necessarily equal the processing time, the two are probably connected. Therefore, the overlap between the ERPs elicited in different tasks suggests the onset of deeper levels of processing does not wait for the shallower process to conclude. Such a pattern should be more congruent with a cascade (McClelland, 1979) than with a serial-processing model of word recognition.

Although suggestive, this research is obviously not conclusive. It opens the door, however, for the investigation of the existence of separate functional “modules” involved in word recognition by providing converging evidence for their functional neuroanatomical dissociation and describing their relative time course of activation.