Biomedical subjects

J B Demb

Publications and source records attributed to J B Demb.

6 recordsLinked to original sources

PubMed1998

Functional magnetic resonance imaging of early visual pathways in dyslexia.

We measured brain activity, perceptual thresholds, and reading performance in a group of dyslexic and normal readers to test the hypothesis that dyslexia is associated with an abnormality in the magnocellular (M) pathway of the early visual system. Functional magnetic resonance imaging (fMRI) was used to measure brain activity in conditions designed to preferentially stimulate the M pathway. Speed discrimination thresholds, which measure the minimal increase in stimulus speed that is just noticeable, were acquired in a paradigm modeled after a previous study of M pathway-lesioned monkeys. Dyslexics showed reduced brain activity compared with controls both in primary visual cortex (V1) and in several extrastriate areas, including area MT and adjacent motion-sensitive areas (MT+) that are believed to receive a predominant M pathway input. There was a strong three-way correlation between brain activity, speed discrimination thresholds, and reading speed. Subjects with higher V1 and MT+ responses had lower perceptual thresholds (better performance) and were faster readers. These results support the hypothesis for an M pathway abnormality in dyslexia and imply strong relationships between the integrity of the M pathway, visual motion perception, and reading ability.

Adult↗

PubMed1998

Psychophysical evidence for a magnocellular pathway deficit in dyslexia.

The relationship between reading ability and psychophysical performance was examined to test the hypothesis that dyslexia is associated with a deficit in the magnocellular (M) pathway. Speed discrimination thresholds and contrast detection thresholds were measured under conditions (low mean luminance, low spatial frequency, high temporal frequency) for which psychophysical performance presumably depends on M pathway integrity. Dyslexic subjects had higher psychophysical thresholds than controls in both the speed discrimination and contrast detection tasks, but only the differences in speed thresholds were statistically significant. In addition, there was a strong correlation between individual differences in speed thresholds and reading rates. These results support the hypothesis for an M pathway abnormality in dyslexia, and suggest that motion discrimination may be a more sensitive psychophysical predictor of dyslexia than contrast sensitivity.

Adult↗

PubMed1997

Brain activity in visual cortex predicts individual differences in reading performance.

The relationship between brain activity and reading performance was examined to test the hypothesis that dyslexia involves a deficit in a specific visual pathway known as the magnocellular (M) pathway. Functional magnetic resonance imaging was used to measure brain activity in dyslexic and control subjects in conditions designed to preferentially stimulate the M pathway. Dyslexics showed reduced activity compared with controls both in the primary visual cortex and in a secondary cortical visual area (MT+) that is believed to receive a strong M pathway input. Most importantly, significant correlations were found between individual differences in reading rate and brain activity. These results support the hypothesis for an M pathway abnormality in dyslexia and imply a strong relationship between the integrity of the M pathway and reading ability.

Dyslexia↗

PubMed1995

Functional MRI measurement of language lateralization in Wada-tested patients.

In this study the use of functional MRI (fMRI) for measuring language lateralization non-invasively was examined. The subjects were seven patients with histories of temporal lobe epilepsy who had undergone Wada testing for pre-surgical evaluation. Four patients were left-hemisphere-dominant and three were right-hemisphere-dominant for language. They received fMRI scans while they made semantic or perceptual judgments about visually presented words. Regions of the inferior frontal gyrus (pars triangularis and pars orbitalis) and neighbouring orbital cortex, corresponding to portions of Brodmann areas 45, 46 and 47, exhibited significant increases in activation during semantic relative to perceptual judgments. Lateralization of the increases in activation were consistent with the Wada test assessments of hemispheric language dominance in each of the seven patients. These results suggest that, in addition to providing a tool for investigating human cognitive processes, fMRI has significant clinical potential as a non-invasive measure of language lateralization.

Adult↗

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↗

PubMed1994

Implicit and explicit memory for compound words.

Subjects studied visually presented compound words (e.g., TOOTHPASTE, HEARTACHE) and then received a recognition, perceptual identification, or word-fragment completion test that contained old, recombined (e.g., TOOTHACHE), and partially and completely new words. False recognitions increased with the increasing number of previously studied components; however, priming in perceptual identification occurred only for old words. Priming in word-fragment completion occurred for old and recombined words. Reducing the time available to solve word fragments, from 20 sec to 5 sec, did not affect the pattern of results; it is therefore unlikely that priming for recombined words resulted from the use of a recollection-based strategy. Memory tasks that involve a conceptual component access memories that are constructed from parts; memory tasks that are primarily perceptual do not access such memories.

Humans↗