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in a left frontal region. Rather than providing a broad review of the suspected functions of each area active during word reading, the goal will be to use the left frontal region as a “case study” to illustrate the challenges and benefits that may arise from attempts to relate neuroimaging results to those obtained by using other methodologies.

Do Results Converge Across Studies?

As noted above, one of the most obvious uses of neuroimaging is that it provides tools for localizing brain regions that are active during the performance of a cognitive task. But how good are these tools, and what has been learned so far about the location of brain regions that are active during reading? To address this question, results were reviewed from nine studies in which subjects read aloud single words.§ In six of the studies, word reading was compared with a passive control condition in which subjects either rested with their eyes closed (2, 3), maintained fixation on a crosshair (refs. 4 and 5; J.A.F., D.A. Balota, M.E.Raichle, and S.E.P., unpublished data), or viewed meaningless line drawings (6). In the remaining three studies, word reading was compared with a sensorimotor control condition in which subjects produced an utterance (e.g., “hiya”) in response to letter-like “false font” strings (7, 8) or consonant strings (9).

In evaluating these nine studies, the first issue to be confronted is that the results vary. For instance, the number of significant changes found in any given study ranges from 2 (8) to 32 (6), and within an anatomical region the locations of activation vary (e.g., peak changes that localize to primary motor cortex are separated by as much as 14 mm). This variability arises from many sources, including: (i) limitations of neuroimaging technology, especially the fact that in both positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) the ratio of the signal to the noise is low. (ii) differences in the procedures used to analyze neuroimaging data, and (iii) anatomical and cognitive differences between subjects. An additional and potentially avoidable source of variability across studies is task-related. For instance, in all nine of the studies described above subjects read words, but many features of this task varied (presentation rate, stimuli, etc.), as did the control task to which reading was compared (as described above). Because the task comparisons thus are only approximately the same, there is no reason to expect the results to be more than approximately the same. In fact, as will be discussed below, such variations can be exploited to understand the specific contributions of each region involved in reading.

Although a full account of the functional neuroanatomy of reading will require an understanding of how the results are affected by each source of variability, it is also useful to understand what, if anything, is common across studies. To get at this question, a quantitative review of the nine studies of word reading was conducted. On average, in each study 16 significant activations were reported as coordinate locations (foci) in the Talairach and Tournoux brain atlas (10). yielding a total of 147 foci. Of these 147 foci, 104 were determined to represent a commonly found activation. These foci fell into clusters,** as summarized in Table 1 and Fig. 1. Left-lateralized groups of foci were found in the ventral occipitotemporal cortex in the fusiform and lingual gyri (near BA 18 and 37), and in left frontal operculum (near BAs 44, 45, and the anterior insula). Given that language functions generally are viewed as strongly left-lateralized, it is interesting to note that bilateral activation was observed in many areas, including anterior and posterior portions of the superior temporal gyrus (near BA 22), dorsal and more ventral portions of the postcentral gyrus (near BA 4). the region near the basal ganglia and insular cortex, and the cerebellar hemispheres. Medially, activation was found in the medial frontal gyrus [at or near the supplementary motor area (SMA)], the anterior cingulate gyrus (near BA 32), and the cerebellum. These results are broadly consistent with neuroanatomical expectations derived from the neurological literature (e.g., see ref. 1).

Overall, the outcome of this quantitative review is encouraging, because it demonstrates that neuroimaging results can and do converge across studies to identify brain areas that are active during reading. However, several cautionary points should be noted. First, the foci within each cluster were in some cases separated by as much as 20 mm. This raises serious questions about whether each identified region should be treated as a single functional area. Second, many foci did not meet the criteria for common activation, but this does not mean they should be dismissed as noise. By emphasizing regions of common activation, other important areas that are detected less consistently, or that are very susceptible to task-related differences, may have been overlooked.

Fractionating Regions and Processes: Making Broad Cuts

As discussed in the Introduction, the goal of neuroimaging extends beyond localization. A more interesting challenge is to use neuroimaging to understand how reading is accomplished by using a set of localized areas distributed throughout the brain. In this section, the focus will be on task and control comparisons that can help illuminate which of the regions active during reading are involved in more basic sensory and motor functions, and which may be specialized for more central components of reading, such as the orthographic, phonological, and semantic processing.

Two different general classes of control conditions were used in the nine studies reviewed above: (i) baseline conditions in which reading was compared with a simple visual task (e.g., fixate on a crosshair) or to a rest task (e.g., “close your eyes and empty your mind”), and (ii) sensorimotor control conditions in which subjects viewed nonlinguistic but comparably complex visual stimulus (e.g., strings of “false fonts”) and also produced an articulatory output for each trial. Evaluation of Table 1 reveals several regions of activation that were almost always found when a baseline control task was used, but not when a

§  

All neuroimaging studies of reading that met the following criteria were included in this analysis: (i) subjects read aloud a series of individually displayed words, (ii) data were compared with a relatively passive or sensorimotor control condition (see text for further description), (iii) the majority of the brain was imaged, and (iv) regions of activation were reported as coordinate locations (foci) in terms of the Talairach and Tournoux atlas space (10). In all nine studies that met these criteria, the data were acquired by using PET.

  

In several of the studies word reading was compared with different control conditions, or different task variations were compared with the same control condition. In such studies, activation of the same region often was found in multiple comparisons. As an alternative to dealing with each focus separately, or arbitrarily selecting only one comparison to evaluate, similarly located foci within an individual study were averaged together.

  

A focus from any given study was judged to represent commonly found activation if it was near (within 20 mm) foci from a majority of studies. In other words, each focus had to be near a focus from at least four of the other eight studies, or near a focus from at least three of the six studies that used a passive control condition (to avoid excluding areas involved in visual processing and speech production). The 20-mm distance criterion was chosen because it is near the resolution of most analyzed PET images, and it is beyond the range of the typical response variability found between subjects and studies.

**  

Foci were assigned to clusters based on the Brodmann area (BA)/gyral location to which each focus plotted. In some cases, more than one focus from a given study fell within the same Brodmann region. When this tendency appeared across studies, the foci were divided into two clusters (e.g., in many of the studies both a dorsal and a more ventral focus were identified within BA 4). Finally, the mean location of the foci falling within each cluster was computed.



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