Biomedical subjects

Massihullah Hamidi

Publications and source records attributed to Massihullah Hamidi.

3 recordsLinked to original sources

PubMed2006

Nonvisual codes and nonvisual brain areas support visual working memory.

Systems models hold working memory to depend on specialized, domain-specific storage buffers. Here, however, we demonstrate that short-term retention of the identity or location of visually presented stimuli is disrupted by nonvisual secondary tasks performed in the dark-passive listening to nouns or endogenous generation of saccades, respectively. This indicates that the short-term retention of visual information relies on multiple mental codes, some of them nonvisual. Event-related functional magnetic resonance imaging (fMRI) reveals the neural correlates of these interference effects to be more complex and more regionally specific than previously described. Although nonspecific dual-task effects produce a generalized decrease of task-evoked fMRI response across many brain regions, the interference-specific effect is a relative increase of activity localized to regions associated with the secondary task in question: left hemisphere perisylvian cortex in the case of passive listening distraction and frontal oculomotor regions in the case of saccadic distraction. Within these regions, the neural interference effects are specific to voxels that show delay-period activity on unfilled memory trials. They also predict individual differences in the magnitude of the behavioral interference effect. These results indicate that nonvisual processes supported by nonvisual brain areas contribute importantly to "visual" working memory performance.

Adult↗

PubMed2006

Repetitive transcranial magnetic stimulation dissociates working memory manipulation from retention functions in the prefrontal, but not posterior parietal, cortex.

Understanding the contributions of the prefrontal cortex (PFC) to working memory is central to understanding the neural bases of high-level cognition. One question that remains controversial is whether the same areas of the dorsolateral PFC (dlPFC) that participate in the manipulation of information in working memory also contribute to its short-term retention (STR). We evaluated this question by first identifying, with functional magnetic resonance imaging (fMRI), brain areas involved in manipulation. Next, these areas were targeted with repetitive transcranial magnetic stimulation (rTMS) while subjects performed tasks requiring only the STR or the STR plus manipulation of information in working memory. fMRI indicated that manipulation-related activity was independent of retention-related activity in both the PFC and superior parietal lobule (SPL). rTMS, however, yielded a different pattern of results. Although rTMS of the dlPFC selectively disrupted manipulation, rTMS of the SPL disrupted manipulation and STR to the same extent. rTMS of the postcentral gyrus (a control region) had no effect on performance. The implications of these results are twofold. In the PFC, they are consistent with the view that this region contributes more importantly to the control of information in working memory than to its STR. In the SPL, they illustrate the importance of supplementing the fundamentally correlational data from neuroimaging with a disruptive method, which affords stronger inference about structure-function relations.

Adult↗

PubMed2004

Glial reduction in amygdala in major depressive disorder is due to oligodendrocytes.

BACKGROUND: A previous study reported reductions in glial density and glia/neuron ratio in the amygdala of individuals with major depressive disorder (MDD), without a change in neuronal density. It is not known, however, whether this glial loss is due to astrocytes, oligodendrocytes, or microglia. METHODS: Tissue samples, equally from the right and left hemispheres, were obtained from subjects diagnosed with MDD (n = 8), bipolar disorder (BD) (n = 9), or no psychiatric disorders (n = 10). Sections were stained immunohistochemically for S-100beta (for astrocytes) and human leukocyte antigen (for microglia), and with the Nissl method. In Nissl-stained sections, oligodendrocytes have more compact, darker-stained nuclei, whereas astrocytes and microglia have larger, lighter-stained nuclei, with more granular chromatin. Neurons are larger, with a nucleolus and stained cytoplasm. The density of glia was determined with stereologic methods. RESULTS: The density of total glia and oligodendrocytes in the amygdala was significantly lower in MDD than in control subjects, but not significantly lower in BD compared with control subjects. The decreases were largely accounted for by differences in the left hemisphere. There was no significant decrease in astrocyte or microglia density in MDD or BD subjects. CONCLUSIONS: The glial cell reduction previously found in the amygdala in MDD is primarily due to oligodendrocytes.

Adolescent↗