Phonological decoding involves left posterior fusiform gyrus

Abstract

Aloud reading of novel words is achieved by phonological decoding, a process in which grapheme-to-phoneme conversion rules are applied to "sound out" a word's spoken representation. Numerous brain imaging studies have examined the neural bases of phonological decoding by contrasting pseudoword (pronounceable nonwords) to real word reading. However, only a few investigations have examined pseudoword reading under both aloud and silent conditions, task parameters that are likely to significantly alter the functional anatomy of phonological decoding. Subjects participated in an fMRI study of aloud pseudoword, aloud real word, silent pseudoword, and silent real word reading. Using this two-by-two design, we examined effects of word-type (real words vs. pseudowords) and response-modality (silent vs. aloud) and their interactions. We found 1) four regions to be invariantly active across the four reading conditions: the anterior aspect of the left precentral gyrus (Brodmann's Area (BA) 6), and three areas within the left ventral occipitotemporal cortex; 2) a main effect of word-type (pseudowords > words) in left inferior frontal gyrus and left intraparietal sulcus; 3) a main effect of response-modality (aloud > silent) that included bilateral motor, auditory, and extrastriate cortex; and 4) a single left hemisphere extrastriate region showing a word-type by response-modality interaction effect. This region, within the posterior fusiform cortex at BA 19, was uniquely modulated by varying phonological processing demands. This result suggests that when reading, word forms are subject to phonological analysis at the point they are first recognized as alphabetic stimuli and BA 19 is involved in processing the phonological properties of words.

Figure 1

Experimental paradigm. Subjects underwent two experimental runs: reading single words (nonwords and real words) aloud and silently. No stimuli were repeated across the two runs. Each run consisted of blocks during which subjects read words (48 s) or blocks during which they only fixated (24 s). During the reading blocks three words (or nonwords) were presented during an 8‐s trial, followed by a 4‐s acquisition period. Each block contained four epochs (trial plus acquisitions) each epoch lasting 12 s, the duration of the TR. Real word or pseudoword trials consisted of a 200 ms word presentations, followed by a pause of 2450 ms in which a crosshair was presented and the subjects responded aloud or silently.

Figure 2

Transverse sections illustrating areas of significant activity for the four task conditions relative to the fixation baseline. From left to right: aloud pseudoword reading; aloud real word reading; silent pseudoword reading; silent real word reading; and the conjunction of all four tasks showing regions of activity common to all tasks. For all figures the z‐coordinate given is in the coordinate space of the atlas of Talairach and Tournoux [1988]. Images are portrayed in the radiological convention, with the left side of the brain (L) represented on the right side of the figure and anterior is towards to top of the figure. For visualization purposes, activity is displayed at a critical threshold of Z > 2.33 (P < 0.01, uncorrected).

Figure 3

Transverse slices of the three cortical regions showing a main effect of lexicality (pseudowords > real words, collapsed across aloud and silent modalities). The z‐coordinate of each slice matches those in Table II.

Figure 4

Transverse slices showing a main effect of response‐modality (aloud > silent, collapsed across pseudoword and real word‐types). The maxima of these areas are presented in Table III.

Figure 5

Transverse (a), sagittal (b), and coronal (c) views of the single region, located in the left posterior fusiform gyrus (BA 19), revealed by the interaction between word‐type (lexicality) and response‐modality. The bar graph illustrates the relative contribution (mean voxel intensity given in arbitrary units) of each of the four task conditions to the 142‐voxel region revealed by the interaction analysis. Post‐hoc paired t‐tests showed significantly greater activation for the aloud pseudoword reading condition relative to the aloud real word, silent pseudoword, and silent real word reading conditions (P < 0.05). There were no significant differences between the aloud and silent real word reading conditions, or between the silent pseudoword and silent real word conditions.