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Biomedical subjects

C J Vaidya

Publications and source records attributed to C J Vaidya.

10 recordsLinked to original sources
PubMed1999

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.

Adult↗
PubMed1999

Dissociation between two forms of conceptual priming in Alzheimer's disease.

Patients with Alzheimer's disease (AD) and healthy control participants performed 2 conceptual repetition priming tasks, word-associate production and category-exemplar production. Both tasks had identical study-phases of reading target words aloud, had the most common responses as target items, and required production of a single response. Patients with AD showed normal priming on word-associate production but impaired priming on category-exemplar production. This dissociation in AD suggests that conceptual priming is not a unitary form of memory but rather is mediated by separable memory systems.

Aged↗
PubMed1998

Selective effects of methylphenidate in attention deficit hyperactivity disorder: a functional magnetic resonance study.

Functional MRI revealed differences between children with Attention Deficit Hyperactivity Disorder (ADHD) and healthy controls in their frontal-striatal function and its modulation by methylphenidate during response inhibition. Children performed two go/no-go tasks with and without drug. ADHD children had impaired inhibitory control on both tasks. Off-drug frontal-striatal activation during response inhibition differed between ADHD and healthy children: ADHD children had greater frontal activation on one task and reduced striatal activation on the other task. Drug effects differed between ADHD and healthy children: The drug improved response inhibition in both groups on one task and only in ADHD children on the other task. The drug modulated brain activation during response inhibition on only one task: It increased frontal activation to an equal extent in both groups. In contrast, it increased striatal activation in ADHD children but reduced it in healthy children. These results suggest that ADHD is characterized by atypical frontal-striatal function and that methylphenidate affects striatal activation differently in ADHD than in healthy children.

Adolescent↗
PubMed1998

Word-identification priming for ignored and attended words.

Three experiments examined contributions of study phase awareness of word identity to subsequent word-identification priming by manipulating visual attention to words at study. In Experiment 1, word-identification priming was reduced for ignored relative to attended words, even though ignored words were identified sufficiently to produce negative priming in the study phase. Word-identification priming was also reduced after color naming relative to emotional valence rating (Experiment 2) or word reading (Experiment 3), even though an effect of emotional valence upon color naming (Experiment 2) indicated that words were identified at study. Thus, word-identification priming was reduced even when word identification occurred at study. Word-identification priming may depend on awareness of word identity at the time of study.

Adolescent↗
PubMed1998

Object decision priming in Alzheimer's disease.

Priming for line drawings of real and nonreal objects was examined in an object decision task for 16 patients with Alzheimer's disease (AD) and 16 normal elderly control (NC) participants. In two study phases, participants decided if objects were real or nonreal. In an implicit test phase, real/nonreal decisions were made for studied and unstudied objects, and priming was measured as the difference in decision speed or accuracy between studied and unstudied objects. In an explicit test phase, yes/no recognition was measured for real and nonreal objects. AD patients had impaired explicit memory for real and nonreal objects and intact repetition priming for real objects. By the latency measure, both AD and NC groups showed priming for nonreal objects but in opposite ways. Classification decisions about studied relative to nonstudied nonreal objects were slower for the AD patients, whereas such decisions were faster for the NC participants. Classification decisions of both groups were less accurate for repeated nonreal objects. These results support the claim that AD patients with mild cognitive impairment show normal perceptual priming. The AD inhibition for studied nonreal objects is discussed in terms of the decision conflict that occurs when recollection of source is not available to counter the influence of familiarity.

Aged↗
PubMed1998

Font-specific priming following global amnesia and occipital lobe damage.

Font-specificity in visual word-stem completion priming was examined in patients with global amnesia and Patient M.S., who had a right-occipital lobectomy. Word-stems appeared in the same or different font as study words. Amnesic patients showed normal font-specific priming (greater priming for words studied in the same than different font as test), despite impaired word-stem cued recall. Patient M.S. failed to exhibit font-specific priming, despite preserved declarative memory. Therefore, perceptual specificity in visual priming depends on visual processes mediated by the right-occipital lobe rather than medial temporal and diencephalic regions involved in declarative memory.

Adult↗
PubMed1997

A dissociation between perceptual explicit and implicit memory processes.

Patient M.S., who underwent right-occipital lobe resection to treat intractable epilepsy, has intact recall and recognition memory for words, but impaired repetition priming in word identification and visual stem-completion tasks. This mirror dissociation to amnesia suggests that explicit recognition and visuoperceptual repetition priming are mediated by distinct neural systems. In prior studies, however, M.S.' recognition memory was tested only with tasks that drew upon his intact verbal knowledge. The present study examined M.S.' recognition memory for nonverbal perceptual information, namely, the modality and font of word presentation and line patterns. M.S.' recognition memory was intact, providing further evidence that perceptual explicit and implicit memory processes are subserved by functionally and neurally independent memory systems.

Adult↗
PubMed1997

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.

Humans↗
PubMed1995

Semantic encoding and retrieval in the left inferior prefrontal cortex: a functional MRI study of task difficulty and process specificity.

Prefrontal cortical function was examined during semantic encoding and repetition priming using functional magnetic resonance imaging (fMRI), a noninvasive technique for localizing regional changes in blood oxygenation, a correlate of neural activity. Words studied in a semantic (deep) encoding condition were better remembered than words studied in both easier and more difficult nonsemantic (shallow) encoding conditions, with difficulty indexed by response time. The left inferior prefrontal cortex (LIPC) (Brodmann's areas 45, 46, 47) showed increased activation during semantic encoding relative to nonsemantic encoding regardless of the relative difficulty of the nonsemantic encoding task. Therefore, LIPC activation appears to be related to semantic encoding and not task difficulty. Semantic encoding decisions are performed faster the second time words are presented. This represents semantic repetition priming, a facilitation in semantic processing for previously encoded words that is not dependent on intentional recollection. The same LIPC area activated during semantic encoding showed decreased activation during repeated semantic encoding relative to initial semantic encoding of the same words. This decrease in activation during repeated encoding was process specific; it occurred when words were semantically reprocessed but not when words were nonsemantically reprocessed. The results were apparent in both individual and averaged functional maps. These findings suggest that the LIPC is part of a semantic executive system that contributes to the on-line retrieval of semantic information.

Adolescent↗