neuroimaging that also make computational interpretations difficult—for instance, the changes measured with neuroimaging do not distinguish between distant and local synaptic connections, or between excitatory and inhibitory connections.
The potential ambiguity in relating the models to neuroimaging results is well illustrated by considering the activation pattern observed in the left frontal operculum. This pattern of activation can be summarized as nonsignificant activation for low-frequency consistent words, and robust activation for low-frequency exception words and nonwords (see Table 2). On the one hand, this pattern of activation agrees nicely with inferences about the “activation” of the phonological layer in connectionist models (or possibly the hidden layer between orthography and phonology). Representations sufficient to produce a correct response should be formed rapidly (producing less “activation” per stimulus) for low-frequency consistent words, because the representations benefit not only from prior exposure to the exact word, but also from prior exposure to other words with similar spelling-to-sound patterns. Representations for low-frequency exception words and nonwords should be formed more slowly (producing more “activation”). For exception words, this is because the representations must depend largely on prior exposure to the exact word, because only limited exposure to similar exception words will have occurred. For nonwords, the converse is true—there is no benefit from prior exposure to the stimulus, and instead a representation must be based on similarity to previously presented words.
Dual-route models (44–46) can also account for the left frontal opercular activation pattern, but for very different reasons. In this framework, the left frontal operculum could be involved in the assembled route, in which a rule-based process is used to “sound out” a pronunciation on the basis of correspondences between individual graphemes and phonemes, or the operculum could be a recipient of information from both the assembled and the direct routes (such as the phonological buffer proposed in ref. 46). “Activation” of the assembled route in the nonword condition makes sense, because the direct route (which is based on associations between existing orthographic and phonological representations of whole words) cannot support generalizations to nonwords. Low “activation” of this route for consistent words is also interpret able, because the pronunciation of low-frequency consistent words can be driven beneficially by output from both the direct and the assembled route. For low-frequency exception words, this is not the case: the output from the assembled route will actually produce incorrect information (e.g., that “pint” should be pronounced as if it rhymes with “hint”). Until the competition with the output from the direct route can be resolved and a single pronunciation established, continued “activation” of the assembled route, or a recipient buffer, might be necessary. It is this competition that is thought to cause subjects to read low-frequency exception words more slowly than low-frequency consistent words (44–46).
It is thus possible to form interpretations of the neuroimaging data that can account for the results by using either a dual-route or a connectionist framework, though as noted this involves a set of inferences that have yet to be evaluated empirically. At some level, it is hardly surprising that multiple interpretations can be formed, because the activation pattern in the left frontal operculum parallels the behavioral effects of consistency on reaction time, and it is these behavioral effects that the models seek to explain. But the existence of multiple interpretations does not negate the value of having additional constraints for consideration in the further development of theoretical models. For instance, one intriguing aspect of the proposed interpretations is that they lead to different predictions about the effects of left frontal brain damage. Specifically, in the connectionist framework, the greater activation of this region when subjects read aloud exception as opposed to consistent words can be thought of in some sense as a reflection of the fact that the transformation is slower. Hence, the prediction would be that patients with left frontal damage should have the greatest difficulty reading low-frequency exception and nonwords, and that they should still read low-frequency exception words more slowly than low-frequency consistent words (although in severe cases, phonology may be accessed almost entirely via semantics, and hence the magnitude of the consistency effect would be reduced; ref. 19). But in the dual-route framework, the activation of the left frontal operculum for exception words is not only unnecessary, it is actually counterproductive. Hence, the prediction would be that the patients should read low-frequency exception words just as rapidly as low-frequency consistent words, because in dual-route models the elevated reaction time seen in normal subjects emerges from competition between the direct and assembled routes.
Another important point to keep in mind is that although this review focused on the pattern of activation observed in the left frontal operculum, other regions showed different patterns of stimulus-related differences that also place constraints on theoretical accounts of reading. For instance, both the left frontal operculum and the left posterior superior temporal cortex (BA 22) have been implicated in phonological processes, but of these two regions, only the left frontal operculum showed any effects of consistency. This suggests that the nature of the phonological processes supported by these two regions differs. On the other hand, effects of consistency observed in primary motor cortex (greater activation for low-frequency consistent words and nonwords than low-frequency exception words) are difficult to reconcile with existing models of reading (Fiez et al., unpublished data). This result suggests that the transformation from orthography to phonology involves processing that is not accounted for within existing models, or that motor cortex is involved in a process outside the scope of the implemented models, such as the transformation from phonological to articulatory representations (Fiez, et al., unpublished data).
In the first section of this review, the results from nine neuroimaging studies were reviewed to give a “big picture” of the brain regions that are active during reading. Though such localization is important, it is only the beginning. Neuroimaging can also be used to reveal differences in the activation of brain regions, and this in turn opens up the possibility that neuroimaging can provide new insights into both the cognitive processes involved in reading and the location of these cognitive processes. Although there is much work to be done, results from neuroimaging can already be used to provide new perspectives on the results obtained in using other methodologies, and there is every reason to believe that neuroimaging will continue to be a value component of an interdisciplinary effort to understanding reading.
This work was supported by in part by NIH grants NS06833, EY08775, HL13851, NS32979.
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