Ventral visual cortex exhibits highly organized and selective patterns of functional activity associated with visual processing. However, this specialization decreases in normal aging, with functional responses to different visual stimuli becoming more similar with age, a phenomenon termed “dedifferentiation.” The current study tested the hypothesis that age-related degradation of the inferior longitudinal fasciculus (ILF), a white matter pathway involved in visual perception, could account for dedifferentiation of both localized and distributed brain activity in ventral visual cortex. Participants included 281 adults, ages 20–89 years, from the Dallas Lifespan Brain Study who underwent diffusion-weighted imaging to measure white matter diffusivity, as well as fMRI to measure functional selectivity to viewing photographs from different categories (e.g., faces, houses). In general, decreased ILF anisotropy significantly predicted both focal and broad functional dedifferentiation. Specifically, there was a localized effect of structure on function, such that decreased anisotropy in a smaller mid-fusiform region of ILF predicted less selective (i.e., more dedifferentiated) response to viewing faces in a proximal face-responsive region of fusiform. On the other hand, the whole ILF predicted less selective response across broader ventral visual cortex for viewing animate (e.g., human faces, animals) versus inanimate (e.g., houses, chairs) images. This structure–function relationship became weaker with age and was no longer significant after the age of 70 years. These findings indicate that decreased white matter anisotropy is associated with maladaptive differences in proximal brain function and is an important variable to consider when interpreting age differences in functional selectivity.

The visual recognition of letters dissociates from the recognition of numbers at both the behavioral and neural level. In this article, using fMRI, we investigate whether the visual recognition of numbers dissociates from letters, thereby establishing a double dissociation. In Experiment 1, participants viewed strings of consonants and Arabic numerals. We found that letters activated the left midfusiform and inferior temporal gyri more than numbers, replicating previous studies, whereas numbers activated a right lateral occipital area more than letters at the group level. Because the distinction between letters and numbers is culturally defined and relatively arbitrary, this double dissociation provides some of the strongest evidence to date that a neural dissociation can emerge as a result of experience. We then investigated a potential source of the observed neural dissociation. Specifically, we tested the hypothesis that lateralization of visual number recognition depends on lateralization of higher-order numerical processing. In Experiment 2, the same participants performed addition, subtraction, and counting on arrays of nonsymbolic stimuli varying in numerosity, which produced neural activity in and around the intraparietal sulcus, a region associated with higher-order numerical processing. We found that individual differences in the lateralization of number activity in visual cortex could be explained by individual differences in the lateralization of numerical processing in parietal cortex, suggesting a functional relationship between the two regions. Together, these results demonstrate a neural double dissociation between letter and number recognition and suggest that higher-level numerical processing in parietal cortex may influence the neural organization of number processing in visual cortex.

We investigated the hypothesis that increased prefrontal activations in older adults are compensatory for decreases in medial-temporal activations that occur with age. Because scene encoding engages both hippocampal and prefrontal sites, we examined incidental encoding of scenes by 14 young and 13 older adults in a subsequent memory paradigm using functional magnetic resonance imaging (fMRI). Behavioral results indicated that there were equivalent numbers of remembered and forgotten items, which did not vary as a function of age. In an fMRI analysis subtracting forgotten items from remembered items, younger and older adults both activated inferior frontal and lateral occipital regions bilaterally; however, older adults showed less activation than young adults in the left and right parahippocampus and more activation than young adults in the middle frontal cortex. Moreover, correlations between inferior frontal and parahippocampal activity were significantly negative for old but not young, suggesting that those older adults who showed the least engagement of the parahippocampus activated inferior frontal areas the most. Because the analyses included only the unique activations associated with remembered items, these data suggest that prefrontal regions could serve a compensatory role for declines in medial-temporal activations with age.

Age differences in frontal and hippocampal activations in working memory were investigated during a maintenance and subsequent probe interval in an event-related fMRI design. Younger and older adults either viewed or maintained photographs of real-world scenes (extended visual or maintenance conditions) over a 4-sec interval before responding to a probe fragment from the studied picture. Behavioral accuracy was largely equivalent across age and conditions on the probe task, but underlying neural activations differed. Younger but not older adults showed increased left anterior hippocampal activations in the extended visual compared with the maintenance condition. Onthesubsequent probeinterval, however, older adultsshowed more left and right inferior frontal activations than younger adults. The increased frontal activations at probe in older adults may have been compensatory for the decreased hippocampal activations during maintenance, but alternatively could have reflected the increased difficulty of the probe task for the older subjects. Thus, we demonstrate qualitatively different engagement of both frontal and hippocampal structures in older adults in a working memory task, despite behavioral equivalence.