Development of Ventral Stream Representations for Single Letters

Abstract

Visual form recognition is mediated by the ventral extrastriate processing stream. Some regions of ventral stream cortex show preferential activity for specific stimulus categories, but little is known about how this regional specialization develops. Acquisition of letter-naming skill is of particular interest because letter recognition serves as the gateway to visual processing of words, and fluent letter naming predicts children's reading success. For this reason, we examined the school-age development of visual letter processing using fMRI. In a 2 x 2 design, we compared ventral stream BOLD activity in two groups, children (n= 22, age 6-11) and adults (n= 15, age 20-22), during two tasks: naming of single letters and naming of simple line drawings of objects. We hypothesized that, based on adults' greater experience with letters, the posterior left fusiform gyrus would be activated more in adults for letter naming than it would be in children. We found that bilateral areas of ventral stream cortex during letter naming were activated in both children and adults and that the midposterior areas of the fusiform gyrus in both hemispheres were activated to a greater degree in adults than in children. There were no areas within the ventral stream in either hemisphere that were activated preferentially for letters over line drawings, nor were there any significant differences in the developmental changes observed for letter naming compared to object naming. These findings indicate that visual processing of single letters continues to develop in both hemispheres during grade school. However, we found no evidence for development of areas specialized for single letter processing. Rather, our findings suggest that letter recognition is performed using the same general form recognition systems as are used to process other visually similar stimuli.