Karin H James | PubMed Health

PubMed2006

Letter processing automatically recruits a sensory-motor brain network.

Behavioral, neuropsychological and neuroimaging research suggest a distributed network that is recruited when we interact with letters. For the first time, we combine several letter processing tasks in a single experiment to study why letters seem to engage such disparate processing areas. Using fMRI, we investigate how the brain responds to letters using tasks that should recruit systems for letter perception, letter writing, letter copying and letter imagery. We describe a network of five cortical regions including the left fusiform gyrus, two left pre-central areas, left cuneus and the left inferior frontal gyrus that are all selectively engaged during a 1-back matching paradigm with letters. Our results suggest involvement of these regions to different extents in different tasks. However, the regions also form an integrated network such that letter perception also engages motor regions while writing recruits letter-specific visual regions as well. We suggest that this distributed network is a direct result of our sensory-motor interactions with letters.

Adult↗

PubMed2005

Letter processing in the visual system: different activation patterns for single letters and strings.

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 nonpronounceable letter strings cannot be assumed to be equivalent to single-letter perception.

Adult↗