Neuroimaging studies of word reading

Abstract

This review discusses how neuroimaging can contribute to our understanding of a fundamental aspect of skilled reading: the ability to pronounce a visually presented word. One contribution of neuroimaging is that it provides a tool for localizing brain regions that are active during word reading. To assess the extent to which similar results are obtained across studies, a quantitative review of nine neuroimaging investigations of word reading was conducted. Across these studies, the results converge to reveal a set of areas active during word reading, including left-lateralized regions in occipital and occipitotemporal cortex, the left frontal operculum, bilateral regions within the cerebellum, primary motor cortex, and the superior and middle temporal cortex, and medial regions in the supplementary motor area and anterior cingulate. Beyond localization, the challenge is to use neuroimaging as a tool for understanding how reading is accomplished. Central to this challenge will be the integration of neuroimaging results with information from other methodologies. To illustrate this point, this review will highlight the importance of spelling-to-sound consistency in the transformation from orthographic (word form) to phonological (word sound) representations, and then explore results from three neuroimaging studies in which the spelling-to-sound consistency of the stimuli was deliberately varied. Emphasis is placed on the pattern of activation observed within the left frontal cortex, because the results provide an example of the issues and benefits involved in relating neuroimaging results to behavioral results in normal and brain damaged subjects, and to theoretical models of reading.

Figure 1

Schematic diagram showing mean location of positive foci across studies of word reading (see Table 1). Foci in the right hemisphere are shown with filled squares, foci in the left hemisphere are shown with filled circles, three foci near the midline are shown in the left hemisphere with an “M,” and two subcortical foci near the thalamus, basal ganglia, and insular cortex are shown with “Lsc” and “Rsc.” It should be noted that these are only mean locations, that the distribution of locations in some cases covered a fairly wide range (as indicated by the standard deviations listed in Table 1), and that each mean location does not necessarily signify the center of a single functional area.