Developmental differences for word processing in the ventral stream

Abstract

The visual word form system (VWFS), located in the occipito-temporal cortex, is involved in orthographic processing of visually presented words (Cohen et al., 2002). Recent fMRI studies in children and adults have demonstrated a gradient of increasing word-selectivity along the posterior-to-anterior axis of this system (Vinckier et al., 2007), yet whether this pattern is modified by the increased reading experience afforded by age is still in question. In this study, we employed fMRI and an implicit word-processing task, and then used a region of interest analysis approach along the occipito-temporal cortex to test the prediction that the selectivity for words along the extent of the VWFS differs between older experienced and younger novice readers. Our results showed differences between children and adults during word processing in the anterior left occipito-temporal cortex, providing evidence of developmental refinement for word recognition along the VWFS.

Figure 1

fMRI Implicit Reading feature detection task. Each run consisted of alternating epoch of real-word (RW) and false-font (FF) feature detection. Task epochs were interspersed with fixation periods (Fix) during which a cross-hair was displayed.

Figure 2

Group Whole Brain Activation maps. Left/Middle Within group activation maps for each of the task conditions relative to fixation (top and middle rows – p < 0.001; cluster-size corrected) and differential activation between real words and false-fonts (bottom – p < 0.001; uncorrected) for children and adults, surface rendered on the standardized MNI SPM template. Right: Between-group activations within the bilateral occipito-temporal cortices for each of the computed contrasts (p < 0.001 uncorrected). Z-coordinates were chosen based on the location of the peak activation within this region. L: denotes the left hemisphere, R: Right hemisphere. A full list of activation peaks is presented in Table 3.

Figure 3

Gradient images illustrating the lay-out of spatial sensitivity to real words (red) and false-fonts (blue) in children and adults. An image of differential activity between words and false-fonts was obtained and masked within the occipito-temporal cortex including the lingual gyrus, fusiform gyrus and the inferior occipital gyrus.

Figure 4

Region of interest analysis in the occipito-temporal cortex. Top: Spherical regions of interest selected in bilateral occipito-temporal regions. Each 4mm³ sphere contained approximately 33 voxels. ROI 4 (third from top - green) represents the co-ordinates located closest to the reported peak of the Visual Word Form Area. Middle/Bottom: Mean percent signal change values within the eight 4mm³ spherical regions of interest within left and right occipito-temporal regions for real words minus false-fonts in children (solid) and adults (clear - A) and for real-words (red) and false-fonts (blue) in children (B) and adults (C) separately. Error bars represent 1 standard error measurement.

Figure 5

Relationship between measures of reading ability and real word activation. Correlation of word-selective fMRI activity in ROI 8 with naming fluency across the span of subjects (left; children: solid; adults: clear) and with real word reading ability in adults (right).