Chandan J. Vaidya

Immature Frontal Lobe Contributions to Cognitive Control in Children: Evidence from fMRI

Event-related fMRI was employed to characterize differences in brain activation between children ages 8-12 and adults related to two forms of cognitive control: interference suppression and response inhibition. Children were more susceptible to interference and less able to inhibit inappropriate responses than were adults. Effective interference suppression in children was associated with prefrontal activation in the opposite hemisphere relative to adults. In contrast, effective response inhibition in children was associated with activation of posterior, but not prefrontal, regions activated by adults. Children failed to activate a region in right ventrolateral prefrontal cortex that was recruited for both types of cognitive control by adults. Thus, children exhibited immature prefrontal activation that varied according to the type of cognitive control required.

Myelination and organization of the frontal white matter in children: a diffusion tensor MRI study.

Myelination is critical for the functional development of the brain, but the time course of myelination during childhood is not well known. Diffusion tensor MR imaging (DTI) provides a new method for estimating myelination in vivo. Myelin restricts diffusion of water transverse to the axons, causing diffusion to be anisotropic. By quantifying the anisotropy, the progressive myelination of axons can be studied. Central white matter of the frontal lobe was studied in seven children (mean age 10 years) and five adults (mean age 27 years). Anisotropy in the frontal white matter was significantly lower in children than in adults, suggesting less myelination in children. Measurement of the coherence of white matter revealed that the right frontal lobe had a more regular organization of axons than the left frontal lobe, in both children and adults. The results demonstrate that maturation of the frontal white matter continues into the second decade of life. The time course of prefrontal maturation makes it possible that myelination is a basis for the gradual development of prefrontal functions, such as increased working memory capacity.

Functional Magnetic Resonance Imaging of Semantic Memory Processes in the Frontal Lobes

Frontal-lobe activation during semantic memory performance was examined using functional magnetic resonance imaging (fMRI), a noninvasive technique for localizing neural activity associated with cognitive function Left inferior prefrontal cortex was more activated for semantic than for perceptual encoding of words, and for initial than for repeated semantic encoding of words Decreased activation for semantic encoding of repeated words reflects repetition priming, that is, implicit retrieval of memory gained in the initial semantic encoding of a word The left inferior prefrontal region may subserve semantic working memory processes that participate in semantic encoding and that have decreased demands when such encoding can be facilitated by recent semantic experience These results demonstrate that fMRI can visualize changes in an individuals brain function associated with the encoding and retrieval of new memories

Age-Related Differences in Multiple Measures of White Matter Integrity: A Diffusion Tensor Imaging Study of Healthy Aging

Diffusion tensor imaging (DTI) measures diffusion of molecular water, which can be used to calculate indices of white matter integrity. Early DTI studies of aging primarily focused on two global measures of integrity; the average rate (mean diffusivity, MD) and orientation coherence (fractional anisotropy, FA) of diffusion. More recent studies have added measures of water movement parallel (axial diffusivity, AD) and perpendicular (radial diffusivity, RD) to the primary diffusion direction, which are thought to reflect the neural bases of age differences in diffusion (i.e., axonal shrinkage and demyelination, respectively). In this study, patterns of age differences in white matter integrity were assessed by comparing younger and healthy older adults on multiple measures of integrity (FA, AD, and RD). Results revealed two commonly reported patterns (Radial Increase Only and Radial/Axial Increase), and one relatively novel pattern (Radial Increase/Axial Decrease) that varied by brain region and may reflect differential aging of microstructural (e.g., degree of myelination) and macrostructural (e.g., coherence of fiber orientation) properties of white matter. In addition, larger age differences in FA in frontal white matter were consistent with the anterior–posterior gradient of age differences in white matter integrity. Together, these findings complement other recent studies in providing information about patterns of diffusivity that are characteristic of healthy aging. Hum Brain Mapp, 2010.

Brain Hyperconnectivity in Children with Autism and its Links to Social Deficits

Autism spectrum disorder (ASD), a neurodevelopmental disorder affecting nearly 1 in 88 children, is thought to result from aberrant brain connectivity. Remarkably, there have been no systematic attempts to characterize whole-brain connectivity in children with ASD. Here, we use neuroimaging to show that there are more instances of greater functional connectivity in the brains of children with ASD in comparison to those of typically developing children. Hyperconnectivity in ASD was observed at the whole-brain and subsystems levels, across long- and short-range connections, and was associated with higher levels of fluctuations in regional brain signals. Brain hyperconnectivity predicted symptom severity in ASD, such that children with greater functional connectivity exhibited more severe social deficits. We replicated these findings in two additional independent cohorts, demonstrating again that at earlier ages, the brain of children with ASD is largely functionally hyperconnected in ways that contribute to social dysfunction. Our findings provide unique insights into brain mechanisms underlying childhood autism.

Evidence for cortical encoding specificity in episodic memory: memory-induced re-activation of picture processing areas

Functional magnetic resonance imaging (fMRI) was used to examine whether neural pathways used to encode pictures into memory were re-activated during retrieval of those memories. At encoding, subjects semantically classified common objects presented as pictures or words. At retrieval, subjects performed yes/no recognition memory judgments on words that had been encoded as pictures or as words. The retrieval test probed memory for the encoded item, but not memory for the modality of the encoded item (picture/word). Results revealed that a subset of the brain regions involved specifically in encoding of pictures were also engaged during recognition memory for the encoded pictures. Specifically, encoding of pictures relative to words engaged bilateral extrastriate visual cortex, namely fusiform, lingual, middle occipital, and inferior temporal gyri (Broadman area (BA) 18/19/37). Recognition memory judgments about words that were encoded as pictures relative to those that were encoded as words activated fusiform and inferior temporal gyri primarily in the left hemisphere. Thus, cortical areas originally involved in perception of a visual experience become part of the long-term memory trace for that experience. These findings suggest a neural basis for encoding specificity and transfer appropriate processing in human memory.

Evidence for multiple mechanisms of conceptual priming on implicit memory tests

The authors examined effects of encoding manipulations on 4 conceptual-implicit memory tasks: word-cued association, category-cued association, category verification, and abstract/concrete classification. Study-phase conceptual elaboration enhanced priming for word-cued association with weakly associated words (Experiment 3), and for category-cued association with high- and low-dominance exemplars (Experiments 4 and 5), but did not enhance priming for word-cued association with strongly associated words (Experiments 1 and 2), for category verification with high- and low-dominance exemplars (Experiment 5), or for abstract/concrete classification (Experiment 7). Forms of priming that were unaffected by conceptual elaboration were not mediated by perceptual processes because they were unaffected by study-test modality changes (Experiments 6 and 8). The dissociative effects of conceptual elaboration on conceptual-implicit tasks suggest that at least 2 dissociable mechanisms mediate conceptual priming.

Convergent behavioral and neuropsychological evidence for a distinction between identification and production forms of repetition priming

Four experiments examined a distinction between kinds of repetition priming which involve either the identification of the form or meaning of a stimulus or the production of a response on the basis of a cue. Patients with Alzheimers disease had intact priming on picture-naming and category-exemplar identification tasks and impaired priming on word-stem completion and category-exemplar production tasks. Division of study-phase attention in healthy participants reduced priming on word-stem completion and category-exemplar production tasks but not on picture-naming and category-exemplar identification tasks. The parallel dissociations in normal and abnormal memory cannot be explained by implicit-explicit or perceptual-conceptual distinctions but are explained by an identification-production distinction. There may be separable cognitive and neural bases for implicit modulation of identification and production forms of knowledge.

Intact Implicit Learning of Spatial Context and Temporal Sequences in Childhood Autism Spectrum Disorder

Autism spectrum disorder (ASD) is defined by atypicalities in domains that are posited to rely on implicit learning processes such as social communication, language, and motor behavior. The authors examined 2 forms of implicit learning in 14 children with high-functioning ASD (10 of whom were diagnosed with Aspergers syndrome) and 14 control children, learning of spatial context known to be mediated by the medial temporal lobes (using the contextual cueing task) and of sequences known to be mediated by frontal-striatal and frontal-cerebellar circuits (using the alternating serial reaction time task). Both forms of learning were unimpaired in ASD. Spatial contextual implicit learning was spared in ASD despite slower visual search of spatial displays. The present findings provide evidence for the integrity of learning processes dependent on integration of spatial and sequential contextual information in high-functioning children with ASD.

Long-Term Memory for the Terrorist Attack of September 11: Flashbulb Memories, Event Memories, and the Factors that Influence Their Retention.

More than 3,000 individuals from 7 U.S. cities reported on their memories of learning of the terrorist attacks of September 11, as well as details about the attack, 1 week, 11 months, and/or 35 months after the assault. Some studies of flashbulb memories examining long-term retention show slowing in the rate of forgetting after a year, whereas others demonstrate accelerated forgetting. This article indicates that (a) the rate of forgetting for flashbulb memories and event memory (memory for details about the event itself) slows after a year, (b) the strong emotional reactions elicited by flashbulb events are remembered poorly, worse than nonemotional features such as where and from whom one learned of the attack, and (c) the content of flashbulb and event memories stabilizes after a year. The results are discussed in terms of community memory practices.