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questions: (i) Does the neural architecture of vision include a brain area for letter recognition (as opposed to digit recognition) and (ii) if so, how might such an area arise? We will first describe a functional neuroimaging experiment that did indeed find evidence that letter and digit recognition depend on different neural substrates. We will then describe one potential hypothesis that could account for the localization of noninnate functions. Finally, we will present a behavioral study that confirms one critical prediction of that hypothesis with respect to letter and digit recognition.

FUNCTIONAL NEUROIMAGING

In a previous experiment (T.A.P., M.Stallcup, G.K.Aguirre, D.Alsop, M.D’Esposito, J.Detre & M.A.F., unpublished work) we used functional MRI to test whether letter recognition is localized in the brain and, in particular, whether its neural substrate is segregated from that of digit recognition.

Subjects passively viewed blocks of letter strings, blocks of digit strings, blocks of geometric shape strings, and blocks of fixation points (baseline). One subject participated twice (6 weeks apart) for a total of six sessions. The strings of letters, digits, and shapes were matched in length and size, and the letters and digits were presented in the same font. A surface coil was placed over the left occipitotemporal cortex.

Statistically significant segregation was observed in individual subjects. In four of the six sessions, an area in the left inferior occipitotemporal cortex responded significantly more to letters than digits (Fig. 1). Two sessions were run on the same subject (H.B.), 6 weeks apart, and showed activation in the same area both times. The two subjects who did not show significant activation in the letter vs. digit comparison both showed subthreshold activation in the same left inferior occipitotemporal area [K.H. had 17 contiguous voxels above Z= 2.5 around the Talairach coordinate (–33, –34, –4); M.S. had 19 contiguous voxels above Z=2.5 around the Talairach coordinate (–35, –38, –4)]. The digit vs. letter comparison did not show any significant activations at the P<0.016 level (0.05 after correcting for three planned comparisons) in any subject although this comparison did show one activation at the P<0.03 level (0.1 corrected) in one subject (J.N.). The shape vs. letter/digit comparison showed significant areas of activation in four of the six sessions. Some posterior areas were activated significantly by all three stimulus types relative to fixation. It is also important to point out that using a surface coil over the left hemisphere disrupts the signal in the right hemisphere. One should therefore not conclude that there were no right hemisphere activations or that there were no differences in the comparisons that failed to show significant activations.

FIG. 1. Significant differences in the blood-oxygenation level-dependent (BOLD) MRI signal during passive viewing of letters vs. digits (L-D), digits vs. letters (D-L), and shapes vs. letters and digits (S-LD) (T.A.P., M.Stallcup, G.K.Aguirre, D.Alsop, M.D’Esposito, J.Detre & M.A.F., unpublished work). For each of the three comparisons in each of the six sessions, the single horizontal brain slice that showed the most significantly activated voxels for that comparison is shown. Voxels in yellow were significant at the P<0.05 level after correcting for all the voxels as well as for the three planned comparisons. Voxels in red were significant at the P<0.1 level, corrected. The left hemisphere appears on the left and the right hemispheres on the right.



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