Phonology, semantics, and the role of the left inferior prefrontal cortex

Herbster et al. (1997) contribute to the growing literature on the functional neuroanatomy of word reading by evaluating stimulus-specific activation differences. The stimuli—regular words, irregular words, and nonwords—were specifically chosen to help tease apart contributions of orthography, phonology, and semantics to word pronunciation. The results, along with those from related studies [Fiez et al., 1993; Rumsey et al., 1997] have direct relevance for competing models of the transformation between orthography (how words look) and phonology (how words sound). Herbster et al. [1997] conclude that their results are consistent with connectionist models of word reading, and more specifically hypothesize that the left inferior frontal gyrus may be part of a phonological pathwaywhich supportsword reading, whereas the fusiform gyrus may be part of a semantic pathway. Zatorre et al. [1996] and Demonet et al. [1996] recently reviewed a series of neuroimaging studies that suggest the left inferior frontal gyrus is involved in phonological processing. For instance, greater activation in the left inferior frontal gyrus was found when subjects performed a phonological versus a pitch decision about pairs of auditory presented syllables [Zatorre et al., 1992], and when subjects performed a phonological versus an orthographic discrimination task on visually and auditorily presented words [Fiez et al., 1995]. Verbal working memory studies have also implicated this area in articulatory/phonological processes [e.g., see Awh et al., 1996; Paulesu et al., 1993]. However, evidence reviewed below also indicates that the left inferior frontal gyrus contributes to semantic processing. With respect to the conclusions reached by Herbster et al. [1997], this fact raises two questions: 1) How do the contributions of frontal and posterior regions (e.g., the fusiform gyrus) to semantic processing differ, and 2) are phonological versus semantic interpretations of left inferior prefrontal activation competing or complementary interpretations of the neuroimaging data?