Visual/Orthographic Processing

The most important outcome of the analysis of the ERPs elicited in the size-decision task was that orthographic and nonorthographic stimuli elicited significantly different responses without further distinction within each category. This pattern is similar to the results obtained intracranially by Nobre and colleagues (1994), suggesting that early in the course of visual processing, before phonological analysis occurs, the brain may distinguish between orthographic and nonorthographic visual information. Unlike the intracranial ERPs, however, in which the distinction between the two categories was limited to the N200, on the scalp the distinction between categories at the peak of N170 was followed by a longer lasting epoch during which the ERPs elicited by orthographic and nonorthographic stimuli were distinct.

Furthermore, whereas intracranially orthographic and nonorthographic stimuli elicit N200 potentials in adjacent but not overlapping regions of the middle fusiform gyrus, this pattern may have been reflected at T5 and T6 as an interaction between the stimulus category and the hemispheric asymmetry: The N170 elicited by orthographic stimuli was larger than that elicited by nonorthographic stimuli in the left posterior-temporal/occipital regions of the scalp (T5) and smaller in the right posterior-temporal/occipital regions of the scalp (T6). Moreover, the difference between the two categories began considerably earlier at T5 (140-msec) rather than T6 (210-msec). This difference suggests that although both hemispheres probably respond to both orthographic and nonorthographic visual information, the well-documented superiority of the left hemisphere for processing language-related stimuli may affect early visual processing. In fact, the response of the right hemisphere may have been initiated by activity starting first on the left. Such a system could account, for example, for pure alexia resulting from lesions in the left occipital cortex that also include the splenium of the corpus callosum (e.g., Benton, 1975; Campbell & Regard, 1986; Damasio & Damasio, 1983; Henderson, 1986).

Assuming that, at least for orthography, processing specificity cannot be innate, the early distinction in the visual system between orthographic and nonorthographic information (as well as the demonstrated specificity of adjacent regions for human faces, Bentin et al., 1996; George et al., 1996) suggests that different parts of the visual system can learn to tune themselves to respond selectively to specific (probably ecologically important) visual information.

Although far-field recorded, in conjunction with intracranial recordings and neuroimaging data, the orthographic specificity observed in the present ERP results may also provide a better understanding of the functional neuroanatomy of the orthographic lexicon. They suggest the existence of a functionally specialized stream within the ventral visual pathway, specifically involved in the perceptual processing of orthographic stimuli. Moreover, consistent with PET findings, the present results suggest that this process is particularly conspicuous in the left hemisphere. PET studies led some researchers to suggest that written word forms are processed (or at least initiated) in the occipital lobes (Petersen et al., 1989; Petersen et al., 1990; Posner & Petersen, 1990; Posner, Petersen, Fox, & Raichle, 1988; Posner & Raichle, 1994). Other researchers suggest that the extrastriate cortex responds to any complex visual stimulus whereas the specificity for visual word forms starts only in the midtemporal regions (Beauregard et al., 1997; Bookheimer, Zeffiro, Blaxton, Gaillard, & Theodore, 1995; Chertkow, Bub, Beauregard, Hosein, & Evans, in press; Howard et al., 1992; Price et al., 1994). The lateral-occipital scalp distribution of the ERPs and the SCD calculated on the basis of the ERPs elicited in the orthographic task supports a suggestion, based on intracranial recordings, that regions in the extrastriate cortex respond preferentially to orthographic information, and this process may be the first step toward the formation of a word visual pattern (e.g., Allison et al., 1994; Nobre et al., 1994). However, these regions do not distinguish between legal and illegal word forms and therefore cannot be the sole mechanism that subserves the orthographic lexicon. We will return to this issue when discussing the pattern of the ERPs elicited in the lexical decision stages of the present study.