Academic journal article Cognitive, Affective and Behavioral Neuroscience

Letter Processing in the Visual System: Different Activation Patterns for Single Letters and Strings

Academic journal article Cognitive, Affective and Behavioral Neuroscience

Letter Processing in the Visual System: Different Activation Patterns for Single Letters and Strings

Article excerpt

One would expect that a lifetime of experience recognizing letters would have an important influence on the visual system. Surprisingly, there is limited evidence of a specific neural response to letters over visual control stimuli. We measured brain activation during a sequential matching task using isolated characters (Roman letters, digits, and Chinese characters) and strings of characters. We localized the visual word form area (VWFA) by contrasting the response to pseudowords against that for letter strings, but this region did not show any other sign of visual specialization for letters. In addition, a left fusiform area posterior to the VWFA was selective for letter strings, whereas a more anterior left fusiform region showed selectivity for single letters. The results of different analyses using both large regions of interest and inspections of individual patterns of response reveal a dissociation between selectivity for letter strings and selectivity for single letters. The results suggest that reading experience fine-tunes visual representations at different levels of processing. An important conclusion is that the processing of non-pronounceable letter strings cannot be assumed to be equivalent to single-letter perception.

Extensive experience with an object class can result in exquisite perceptual skills. Car experts can recognize at a glance the makes and models of hundreds of cars, and bird-watchers can identify the species of a bird briefly seen through foliage. The acquisition of such perceptual expertise with objects can lead to functional specialization within the brain systems dedicated to visual processing. Faces are commonly used to study the phenomenon of category-specific neural specialization (Kanwisher, 2000; Kanwisher, Chun, & McDermott, 1996), but neural specialization has also been demonstrated for other expert object classes, including cars, birds, and even novel objects (e.g., "Greebles"; Gauthier, Tarr, Anderson, Skudlarski, & Gore, 1999; Rossion, Gauthier, Goffaux, Tarr, & Crommelinck, 2002). Most people are also perceptual experts with words. For adults, much of modern life is spent reading, and adult readers can recognize words with amazing efficiency (LaBerge & Samuels, 1974). As children, most of us once dedicated a considerable amount of time to learning letters, and each day a literate person must process thousands of letters just to interact normally with the modern environment. Here, we hypothesize that this extensive experience results in letter-specific neural specialization of the visual system.

Most of the neuroimaging studies conducted with printed text were designed for the purpose of understanding the processes involved in reading. Therefore, it may not be surprising that there is only limited evidence speaking directly to the neural substrates involved in recognizing single letters. Several large circuits appear to underlie various aspects of reading. For example, a dorsal posterior system (angular gyrus, supramarginal gyrus, and superior temporal sulcus) is thought to subserve orthography-to-phonology correspondences (Black & Behrmann, 1994). Semantic analyses of words are believed to occur predominantly in the left inferior frontal lobe and posterior superior temporal sulcus (Bookheimer, 2002; Gabrieli, Poldrack, & Desmond, 1998), and context comprehension is considered a right hemisphere task (Kircher, Brammer, Tous, Williams, & McGuire, 2001). A posterior ventral network including the occipitotemporal region is thought to underlie visual processing of printed text and to be responsible for the late-developing skills of rapid word recognition that result from increased reading experience (Frackowiak, Friston, Frith, Dolan, & Mazziotta, 1997; Pugh et al., 2001). Within this system, the region that has received the most attention as a candidate area for early visual processing of printed text is the left fusiform gyrus.

Part of the left midfusiform gyrus shows higher activation for words than for consonant strings of the same length (Cohen et al. …

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