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V Menon

Publications and source records attributed to V Menon.

At least 73 records · Page 4Linked to original sources
PubMed2000

Analysis of a distributed neural system involved in spatial information, novelty, and memory processing.

Perceiving a complex visual scene and encoding it into memory involves a hierarchical distributed network of brain regions, most notably the hippocampus (HIPP), parahippocampal gyrus (PHG), lingual gyrus (LNG), and inferior frontal gyrus (IFG). Lesion and imaging studies in humans have suggested that these regions are involved in spatial information processing as well as novelty and memory encoding; however, the relative contributions of these regions of interest (ROIs) are poorly understood. This study investigated regional dissociations in spatial information and novelty processing in the context of memory encoding using a 2 x 2 factorial design with factors Novelty (novel vs. repeated) and Stimulus (viewing scenes with rich vs. poor spatial information). Greater activation was observed in the right than left hemisphere; however, hemispheric effects did not differ across regions, novelty, or stimulus type. Significant novelty effects were observed in all four regions. A significant ROI x Stimulus interaction was observed - spatial information processing effects were largest effects in the LNG, significant in the PHG and HIPP and nonsignificant in the IFG. Novelty processing was stimulus dependent in the LNG and stimulus independent in the PHG, HIPP, and IFG. Analysis of the profile of Novelty x Stimulus interaction across ROIs provided evidence for a hierarchical independence in novelty processing characterized by increased dissociation from spatial information processing. Despite these differences in spatial information processing, memory performance for novel scenes with rich and poor spatial information was not significantly different. Memory performance was inversely correlated with right IFG activation, suggesting the involvement of this region in strategically flawed encoding effort. Stepwise regression analysis revealed that memory encoding accounted for only a small fraction of the variance (< 16%) in medial temporal lobe activation. The implications of these results for spatial information, novelty, and memory processing in each stage of the distributed network are discussed.

Adolescent↗
PubMed1999

Failures of automatic and strategic processing in schizophrenia: comparisons of event-related brain potential and startle blink modification.

Noises elicit startle blinks that are inhibited when immediately (approximately 100 ms) preceded by non-startling prepulses, perhaps reflecting automatic sensory gating. Startle blinks are facilitated when preceded by prepulses at longer lead intervals, perhaps reflecting strategic processes. Event-related brain potentials (ERPs) and startle blinks were used to investigate the well-documented prepulse inhibition failure in schizophrenia. Blinks and ERPs were recorded from 15 schizophrenic men and 20 age-matched controls to noises alone and to noises preceded by prepulses at 120 (PP120), 500 (PP500) and 4000 ms (PP4000) lead intervals. Neither blinks nor any of the ERP components elicited by the noise alone differentiated schizophrenics from controls, although responses to noises were modified by prepulses differently in the two groups. With the N1 component of the ERP, patients showed normal inhibition but lacked facilitation, and with P2, patients lacked inhibition, but showed normal facilitation. With reflex blinks and P300, inhibition was seen in both groups, but no facilitation. These results suggest that different neural circuits are involved in blink and cortical reflections of startle modification in schizophrenics and controls, with both automatic and strategic processes being impaired in schizophrenia.

Acoustic Stimulation↗
PubMed1998

Differential activation of dorsal basal ganglia during externally and self paced sequences of arm movements.

The basal ganglia are thought to be critically involved in motor control. However, the relative contributions of the various sub-components are not known. Although, in principle, functional magnetic resonance imaging (fMRI) provides adequate resolution to image the basal ganglia at the spatial scale of the individual nuclei, activating these nuclei with fMRI has proven to be difficult. Here we report two tasks, involving externally and self paced sequences of arm movements, which resulted in significant activation of contralateral posterior (post-commissural) putamen and globus pallidus. This activation did not significantly differ between the tasks. In contrast, significant activation of the contralateral and ipsilateral anterior caudate and anterior putamen was observed only during externally paced arm movements. These results suggest a dissociation in the roles of the anterior and posterior dorsal basal ganglia: the anterior caudate and putamen may be involved in sensory to motor mapping and the posterior putamen and globus pallidus may be involved in the motor response itself. The findings support the hypothesis that the basal ganglia may be involved in gating sensory influences onto motor areas.

Adult↗
PubMed1997

Deficits in multiple systems of working memory in schizophrenia.

Working memory, the ability to hold and manipulate information 'on-line' in a temporary memory store, is impaired in schizophrenia. This impairment may be characterized within the framework of two opposing theoretical models: (1) central executive as coordinator of component processes of working memory or (2) multiple independent systems of spatial and object memory. In order to test which of these models better explains the working memory deficit of schizophrenia, 14 schizophrenic patients and 12 age- and gender-matched control subjects performed tests of spatial memory (dot location), object memory (shapes, color dots) and a dual paradigm (dot location + shapes). If schizophrenia impairs the central executive, a group-by-task interaction would demonstrate excessively worse performance on the dual than single tasks in schizophrenics relative to controls; however, the absence of an interaction would be consistent with deficits in the multiple working memory systems. The schizophrenic group was significantly impaired on all measures, and both the schizophrenic and control performance was worse on the dual than the single tasks. Despite the schizophrenic group performance deficits on the single tasks, the extent of such deficit did not appear additive and contributive to the dual tasks. The lack of a group-by-task interaction provided no support for the central executive model of dysfunction. Rather, the results uphold the model of working memory deficits arising from compromise of multiple (here spatial and object), relatively independent systems, both of which are affected in schizophrenia.

Adult↗
PubMed1997

Combined event-related fMRI and EEG evidence for temporal-parietal cortex activation during target detection.

Target detection is the process of bringing a salient stimulus into conscious awareness. Target detection evokes a prominent event-related potential (ERP) component (P3) in the electroencephalogram (EEG). We combined the high spatial resolution of functional magnetic resonance imaging (fMRI) with the high temporal resolution of EEG to investigate the neural generators of the P3. Event-related brain activation (ERBA) and ERPs were computed by time-locked averaging of fMRI and EEG, respectively, recorded using the same paradigm in the same subjects. Target detection elicited significantly greater ERBAs bilaterally in the temporal-parietal cortex, thalamus and anterior cingulate. Spatio-temporal modelling of ERPs based on dipole locations derived from the ERBAs indicated that bilateral sources in the temporal-parietal cortex are the main generators of the P3. The findings provide convergent fMRI and EEG evidence for significant activation of the temporal-parietal cortex 285-610 ms after stimulus onset during target detection. The methods developed here provide a novel multimodal neuroimaging technique to investigate the spatio-temporal aspects of processes underlying brain function.

Adult↗
PubMed1997

Chemoprevention of oral leukoplakia with vitamin A and beta carotene: an assessment.

We conducted a double-blind placebo-controlled trial to evaluate the chemopreventive potential of either vitamin A alone or beta carotene alone in subjects with oral leukoplakia in Kerala, India. We randomised 160 fishermen and women with oral precancerous lesions to receive oral vitamin A (retinyl acetate 300,000 IU/week x 12 months, n = 50), or beta carotene (360 mg/week x 12 months, n = 55), or placebo (n = 55). Blood, saliva and urine samples were collected at baseline and at exit to study serum micronutrients and mutagenicity assays. Biopsies of the mucosal lesions at entry were performed for histopathological exclusion of malignancy. The subjects were examined once every 2 months to establish clinical response of lesions and toxicity, if any. The results are based on 43 complaint subjects on placebo, 42 on vitamin A and 46 on beta carotene. The complete regression rates were: 10% in the placebo arm, 52% with vitamin A and 33% with beta carotene (P < 0.0001). Homogeneous leukoplakias and smaller lesions responded better than non-homogeneous and larger lesions. No major toxicities were observed. Half of the responders with beta carotene and two thirds with vitamin A relapsed after stopping supplementation. Serum beta carotene concentration increased substantially with beta carotene administration while with vitamin A supplementation there was no change in serum retinol levels. In the vitamin A treated group there was a significant decrease in serum alpha tocopherol. Vitamin A administration resulted in a significant remission of oral leukoplakia without any side effects of prolonged vitamin A supplementation. The results of this study, as well as those from previous studies, appear to provide strong supporting evidence to justify long term trials with vitamin A in subjects with high-risk leukoplakias with oral cancer as an endpoint.

Adult↗
PubMed1997

Physicochemical characterization of nedocromil bivalent metal salt hydrates. 3. Nedocromil calcium.

A crystalline pentahydrate and a crystalline 8/3 hydrate of nedocromil calcium (NC) were prepared. The relationships between these solid phases and the nature of the water interactions in their structures were studied through characterization by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Karl Fischer titrimetry (KFT), hot-stage microscopy (HSM), ambient- or variable-temperature powder X-ray diffraction (PXRD), Fourier-transform infrared (FTIR) spectroscopy, solid-state nuclear magnetic resonance (SSNMR) spectroscopy, water uptake at various relative humidities (RH), intrinsic dissolution rate (IDR) and solubility measurements. The solubility and intrinsic dissolution rate of the pentahydrate in water at 25 degrees C are approximately 17% greater than the corresponding values for the 8/3 hydrate, corresponding to a greater Gibbs free energy of only 380 J.mol-1 (91 cal.mol-1) for the pentahydrate. The results of DSC, TGA, and FTIR and SSNMR spectroscopy indicate that the water of hydration is more loosely bound in the pentahydrate than in the 8/3 hydrate. On increasing the temperature in open-pan DSC and TGA, the water in the pentahydrate is released in four steps (three steps in crimped pans), whereas the water in the 8/3 hydrate is released in three steps (three steps also in crimped pans). These three stepwise dehydrations are fundamentally explained by their different water environments in the crystal structure of the 8/3 hydrate, which was determined by single-crystal XRD [crystal data: triclinic, space group P1, a = 13.2381(3) A, b = 13.3650(2) A, c = 17.8224(2) A, alpha = 68.202(1) degrees, beta = 86.894(1) degrees, gamma = 82.969(1) degrees, Z = 6]. The asymmetric unit contains three nedocromil anions and three calcium cations associated with eight water molecules. The nedocromil anions act as polyfunctional ligands to the Ca2+ ions, coordinating through both the carbonyl oxygen and the carboxylate oxygen atoms. The molecular conformations of the three nedocromil anions in the asymmetric unit are almost identical. However, the crystal structure contains two different calcium environments, one of which has the Ca2+ ion hydrated by four water molecules in the equatorial plane and by two carbonyl oxygens in its axial coordination sites. In the second environment, the Ca2+ ion has four carboxylate oxygen atoms in its equatorial plane and two water molecules in its axial coordination sites. Two of the carboxylate ligands are twisted out of the tricyclic ring, and the other two carboxylate ligands are nearly coplanar with the tricyclic ring. All of the eight water molecules in the 8/3 hydrate are linked to calcium and carboxylate ions and none are linked to other water molecules.

Calcium↗
PubMed1997

Changes in myocardial density during postinfarction healing: effect on estimation of in vivo left ventricular mass by echocardiographic imaging.

To determine whether changes in density (rho) of infarct and noninfarct zones during healing and remodeling after myocardial infarction influence estimates of left ventricular mass and detection of temporal changes by imaging, we measured weights (g) and volumes (mL) of infarct, noninfarct, and mixed tissue in hearts removed 1 to 42 days after anterior infarction in three groups of dogs: nonreperfused infarction treated with placebo or captopril, or infarcts reperfused after 2 h. In vivo mass was calculated from in vivo diastolic myocardial volumes (echocardiograms) and an assumed density of 1.05 g/mL or actual values derived from tissue weights and volumes. Over the 42 days, actual density deviated more from the assumed value of 1.05 in infarct than noninfarct zones, and the overall density was higher for reperfused than nonreperfused ventricles (1.09 vs. 1.06 g/mL, p < 0.01). Correction for density improved the correlation between absolute in vivo and postmortem mass slightly but not the detection of relative changes in mass in control, captopril, or reperfusion groups. These findings suggest that (i) densities of infarct and noninfarct zones differ and change during healing, especially after reperfusion, and (ii) correction for density provides more accurate estimates of volume-derived mass in reperfused hearts.

Angiotensin-Converting Enzyme Inhibitors↗
PubMed1997

Randot stereoacuity at various binocular combinations of Snellen acuity.

Binocular combinations of Snellen acuity were varied from 6/60 to 6/6 in standard steps in twenty normal adults and Randot stereoacuity was measured at each level. Using the 560 unique stereoacuity scores thus obtained, a nomogram was constructed which provides expected stereoacuity scores at all unique binocular combinations of Snellen acuity. It was seen that there is a linear correlation between binocular isoacuity (at level 6/36 or better) and Randot stereoacuity.

Adolescent↗
PubMed1996

Spatio-temporal correlations in human gamma band electrocorticograms.

Animal electrocorticogram (ECoG) studies have shown that spatial patterns in the gamma band (>20 Hz) reflect perceptual categorization. Spatio-temporal correlations were investigated in the 20-50 Hz range in search for similar phenomena in human ECoG. ECoGs were recorded in a somatosensory discrimination task from 64-electrode subdural grid arrays, with inter-electrode spacing of 1 cm, overlying somatosensory, motor and superior temporal cortices in 2 patients with intractable epilepsy. Bootstrap techniques were devised to analyze the spatial and temporal characteristics of the correlations. Despite an extensive search, no evidence was found for globally correlated activity related to behavior either in narrow (1.e., 35-45 Hz) or broad (i.e., 20-50 Hz) bands. Spatial patterns, extracted using principal component analysis, could not be classified with respect to stimulus type in any time interval. Instead, spatially and temporally intermittent synchronization was observed between pairs of electrodes in 1 cm X 1 cm regions with high variability within and across trials. The distribution of correlation coefficients differed substantially from background levels at inter-electrode distances of 1 cm and 1.4 cm but not 2 cm or more. The minimum duration of correlation, the decorrelation time, of the ECoG was about 50 msec; the average correlation duration at 1 cm inter-electrode distance was about 150 msec; and the recurrence rate of significant correlation peaks was about 1.3/sec. The findings suggest that the surface diameters of domains of spatially correlated activity underlying perceptual categorization in human gamma band ECoG are limited to less than 2 cm and that the intermittent synchronization observed across separations of 1 cm and 1.4 cm is not solely due to volume conduction. Thus, if such gamma band spatial patterns exist in the human brain, no existing technology would be capable of measuring them at the scalp, and subdural electrode arrays for cortical surface recording would have to have spacings under 5 mm.

Adult↗
PubMed1996

Decreased Ca2+ sensitivity of isometric tension in skinned cardiac myocytes from tail-suspended rats.

Tail suspension in rats causes a cephalic shift in blood, resulting in a volume load on the heart similar to that observed during microgravity spaceflight or mild heart failure. The present study determined the influence of increased cardiac hemodynamic load on myofilament isometric tension as a function of Ca2+ concentration in skinned cardiac myocytes of control and 7-day head-down tilt Sprague-Dawley rats. Isometric force of single skinned myocytes was measured by attaching cells with adhesive to a force transducer and piezoelectric translator. A significant decrease in the Ca2+ sensitivity of tension was observed in cardiac myocytes from suspended rats [pCa of half-maximal tension (pCa50) of 5.83 +/- 0.03] compared with control rats (pCa50 of 5.94 +/- 0.03). Maximum tension generation and slope of the tension-pCa relationship were unaffected by head-down tilt. Electrophoretic analysis of myofilament proteins indicates differences in expression of proteins in the 50-60 and 100-120 kDa ranges; immunoblot analysis of tubulin (50 kDa) expression indicates no change in the ratio of beta-tubulin to light chain 1 or tropomyosin. Decreased force-producing ability at a given submaximum Ca2+ concentration in cardiac myocytes from suspended rats suggests a decrease in contractility possibly due to changes in cardiac myofilament protein expression following chronic elevated volume load on the heart.

Animals↗
PubMed1996

Fractal analysis of cytoskeleton rearrangement in cardiac muscle during head-down tilt.

Head-down tilt by tail suspension of the rat produces a volume, but not pressure, load on the heart. One response of the heart is cytoskeleton rearrangement, a phenomenon commonly referred to as disruption. In these experiments, we used fractal analysis as a means to measure complexity of the microtubule structures at 8 and 18 h after imposition of head-down tilt. Microtubules in whole tissue cardiac myocytes were stained with fluorescein colchicine and were visualized by confocal microscopy. The fractal dimensions (D) of the structures were calculated by the dilation method, which involves successively dilating the outline perimeter of the microtubule structures and measuring the area enclosed. The head-down tilt resulted in a progressive decrease in D (decreased complexity) when measured at small dilations of the perimeter, but the maximum D (maximum complexity) of the microtubule structures did not change with treatment. Analysis of the fold change in complexity as a function of the dilation indicates an almost twofold decrease in microtubule complexity at small kernel dilations. This decrease in complexity is associated with a more Gaussian distribution of microtubule diameters, indicating a less structured microtubule cytoskeleton. We interpret these data as a microtubule rearrangement, rather than erosion, because total tubulin flourescence was not different between groups. This conclusion is supported by F-actin fluorescence data indicating a dispersed structure without loss of actin.

Actins↗
PubMed1995

Interaction of neuronal populations with delay: effect of frequency mismatch and feedback gain.

The effect of frequency mismatch, signal transduction delay and inter-population feedback gain on the interaction of neuronal populations, mediated by long-range excitation, is investigated using physiologically realistic system parameters. Self-consistent solutions for the frequency, amplitude, and relative phase of the component signals are derived for limit cycle oscillations. These solutions predict important qualitative features including discontinuous changes in frequency of oscillation and phase reversal between symmetric and antisymmetric limit cycles. A singularity in the solutions is used to predict parameter regions in which limit cycles do not exist. If limit cycles exist at zero delay, it is shown that limit cycles and quasi-periodic attractors alternate as a function of delay. The implications of these results for estimating physiologically meaningful delays from observed phase shifts in EEG time series are discussed. Spectral peaks for the quasi-periodic attractor occur at m nu 1 +/- n nu 2, where the difference nu 1 - nu 2 is approximately equal to the intrinsic population frequency mismatch delta nu. The cross-correlation function is amplitude modulated with a frequency equal to delta nu/2, indicating that the two populations slip in and out of phase with a mean correlation duration equal to 1/delta nu. These findings underpin the dynamical basis of delay induced "desynchronization" of oscillations reported in computer simulations. Bifurcation diagrams indicate that quasi-periodic attractors exist for a wide range of parameters in the presence of delay in long-range excitation and non-zero frequency mismatch. If the frequency mismatch is sufficiently large and the feedback gain is sufficiently small, quasi-periodic attractors exist for all delays. Delays of a few milliseconds, much smaller than the system time scale, can destabilize limit cycle oscillations. The role of synaptic change in inducing bifurcations of limit cycles to quasi-periodic attractors and vice versa is discussed. The implication of these findings for the generation of chaos in distributed neural systems is discussed.

Electroencephalography↗