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Biomedical subjects

K D Singh

Publications and source records attributed to K D Singh.

At least 37 records · Page 2Linked to original sources
PubMed1997

Magnetoencephalographic investigation of human cortical area V1 using color stimuli.

The aim of this study was to determine the response properties of the human visual cortex to chromatic stimuli using magnetoencephalography (MEG). Evoked responses were recorded to isoluminant red/green sinusoidal gratings for a wide range of spatial and temporal frequencies. For each condition the response was dominated by a single major component which was well modeled by an equivalent current dipole. Coregistration of MEG and MRI data provided evidence that the principal evoked cortical activity originated from visual area V1. To investigate the chromatic response properties of this area, the maximum global field power of the evoked response was plotted as a function of stimulus spatial and temporal frequency. The spatial-frequency tuning was lowpass and the temporal-frequency tuning was multimodal, with peaks at 0 and 4 Hz. The results demonstrate the use of MEG as a technique for investigating activity from discrete regions of cortex.

Adult↗
PubMed1997

Evaluation of MRI-MEG/EEG co-registration strategies using Monte Carlo simulation.

We present a Monte Carlo analysis method for evaluating MRI-MEG/EEG co-registration techniques. The method estimates the error in co-registration as a function of position within the brain. Using this analysis technique, we demonstrate the limitations of conventional head-based fiducial point methods, and propose a new strategy utilising a dental bite-bar incorporating accurately machined fiducial markers. Results presented demonstrate the improved accuracy of MEG/EEG to MRI co-registration using the bite-bar.

Brain↗
PubMed1997

Topographic mapping of trans-cranial magnetic stimulation data on surface rendered MR images of the brain.

We present a method for the coregistration and topographic mapping of trans-cranial magnetic stimulation (TCMS) data on surface rendered images of the cortex, derived from Magnetic Resonance Images (MRI). We describe the TCMS procedure and the methods used to locate the TCM stimulation sites in the MRI coordinate system, and the algorithms needed to depict the TCMS distribution as a pseudocolour contour map on the cortical surface. The methods are validated using TCMS data from the hand (thenar) and leg (tibialis muscle). The methods used correctly depict the expected motor representations of each of these areas and we therefore propose that this technique may be used as a functional imaging tool in the investigation of cortical function in both normals and patients.

Brain Mapping↗
PubMed1996

Localization and functional analysis of human cortical area V5 using magneto-encephalography.

Using a multi-channel SQUID-based neuromagnetometer, we have determined the location, temporal dynamics and functional response properties of the human homologue of the primate cortical area V5 (MT). We provide evidence that area V5 in humans is located near the occipito-temporal border in a minor sulcus immediately below the superior temporal sulcus. this area is selective for low spatial frequencies ( < or = 4.0 c/deg), responds to a wide range of temporal frequencies ( < or = 35 Hz) and shows response saturation for stimulus contrasts greater than 10%. In addition, we find that this area is not responsive to purely chromatic patterns but is responsive to motion-contrast stimuli. Our results are consistent with the hypothesis that area V5 in humans represents a stage of processing within the magnocellular pathway. We discuss our results in relation to the widespread belief that area V5 in humans is specifically concerned with motion perception.

Adult↗
PubMed1996

The cortical topography of human swallowing musculature in health and disease.

Because no detailed information exists regarding the topographic representation of swallowing musculature on the human cerebral cortex in health or disease, we used transcranial magnetic stimulation to study the cortical topography of human oral, pharyngeal and esophageal musculature in 20 healthy individuals and the topography of pharyngeal musculature in two stroke patients, one with and one without dysphagia. Our results demonstrate that swallowing musculature is discretely and somatotopically represented on the motor and premotor cortex of both hemispheres but displays interhemispheric asymmetry, independent of handedness. Following stroke, dysphagia appeared to be associated with smaller pharyngeal representation on the intact hemisphere, which increases in size with recovery of swallowing.

Adolescent↗
PubMed1996

Prognostic factors in hospitalized children with persistent diarrhea: implications for diet therapy.

A dietary algorithm for management of persistent diarrhea in developing countries, using locally available foods, is yet to be standardized. We identified factors related to poor outcome among 75 malnourished hospitalized male patients aged 3-48 months with persistent diarrhea (> or = 14 days) treated on soy and cereal-based diet (Diet I). The 28 patients with stool output > 60 g/k body weight on the sixth or the seventh treatment day were considered diarrhea treatment failures on Diet I. In the univariate analysis, breast feeding (p < 0.001), carbohydrate malabsorption based on low stool pH or reducing substances > 0.5% (p = 0.03), initial 24-h purge rate (p = 0.001), pneumonia (p = 0.003), or probable septicemia (p = 0.03) were associated with diarrhea treatment failures. Although 16 of these 28 patients responded to systemic antibiotics without dietary modification, all but one of the remaining recovered on a chicken puree, glucose, and oil formulation. Twenty-six children had weight loss after 7 days on Diet I as compared with the postrehydration weight. These children had lower mean age (p = 0.05), lower food intake in the first 24 h (p = 0.05) and during the initial 7 days (p < 0.01), and a higher initial excretion of enteroaggregative Escherichia coli (32 vs. 8%; p = 0.01). In the logistic regression model, significant risk factors for diarrhea treatment failures were initial purge rates, carbohydrate malabsorption, and intercurrent systemic infection; only low food intake was associated with significant risk for weight loss. The significant association of diarrhea treatment failures with carbohydrate malabsorption suggests that in the initial diet itself, part of polysaccharide be substituted with sucrose or glucose to obtain the right balance between osmolarity and energy density. Our data suggest that prompt identification and treatment of systemic infection is critical, as its eradication achieved recovery in more than half of the treatment failures without a dietary change.

Algorithms↗
PubMed1996

Efficacy of milk-based diets in persistent diarrhea: a randomized, controlled trial.

OBJECTIVE: Previous studies have shown increased stool output when children with persistent diarrhea (PD) received milk as the predominant source of nutrition. METHODS: We evaluated the efficacy of milk given in modest amounts as a part of a mixed diet in children with PD. One hundred sixteen children 3 to 24 months of age with diarrhea for between 14 days and 12 weeks were allocated to milk-based (n = 60) or milk-free (n = 56) cereal dietary regimens. The two diets were isocaloric (86.9 calories/100 g for < or = 9 months; 95.6 cal/100 g for > 9 months) consisting of puffed rice cereal, sugar, and oil differing in only their source of protein, which was either milk or egg white, respectively. An average of 30% of the calories were constituted by milk in the milk-cereal diet. Both diets were offered at the rate of 150 kcal/kg per day. Children receiving milk-cereal consumed an average of 1.9 g/kg lactose per day. RESULTS: The baseline characteristics in the two groups were similar. Comparable amounts of diet were consumed in both groups. The milk-cereal group did not have higher median (range) stool output (g/kg/h) compared with the milk-free group during a 0- to 48-hour (milk-cereal, 1.7 [0.2 to 8.7]; milk-free, 1.5 [0.1 to 6.6]) or 0- to 120-hour (milk-cereal, 1.6 [0.4 to 7.2]; milk-free, 1.3 [0.1 to 7.6]) period. The percentage of weight gain was similar in the two groups, and there were no significant differences in the duration of diarrhea. Overall, 23 children had treatment failures, 10 (17%) in the milk-cereal and 13 (23.6%) in the milk-free groups. CONCLUSIONS: Our findings suggest that modest intakes of milk are well tolerated as a part of mixed diet during PD.

Animals↗
PubMed1995

Functional imaging of the brain using superconducting magnetometry.

Magnetoencephalography (MEG) is the measurement of the weak magnetic fields generated by neuronal activity in the human brain. By measuring the magnetic field evoked when specific sensory stimuli are presented to a subject, a map of the functional organization of the brain can be deduced with a subcentimetre spatial resolution and a millisecond temporal resolution. As well as a tool for fundamental study of the brain, MEG can also be used in clinical studies and assessment of patients with specific neurological disorders.

Brain↗
PubMed1994

Distributed current analyses of bi-hemispheric magnetic N1m responses to ipsi/contralateral monaural stimuli from a single subject.

Magnetoencephalographic (MEG) responses of both auditory cortices to simple auditory stimuli presented monaurally to either ear were recorded from a single subject. A distributed current model and a current dipole model were used to analyse the responses at the latency of the dominant N1m complex. At the N1m the current density was localised to a single area and was consequently well modelled by a single current dipole close to the peak current density. In the left hemisphere, the contralateral response (as identified by the peak current density) preceded the ipsilateral response by 3 msec. This value was 7 msec for the right hemisphere. Evidence was found in the right hemisphere of a posterior-anterior movement along the sylvian fissure. Also, the left hemisphere N1m sources were all represented more posterior than the right hemisphere N1m sources.

Acoustic Stimulation↗
PubMed1993

Comparison of single current dipole and magnetic field tomography analyses of the cortical response to auditory stimuli.

Measurements of the magnetic field elicited by a 50 ms long auditory stimulus, from three normal subjects and one head injured subject, are used to estimate the three dimensional distribution of generators in the brain. The resulting images are compared with point source solutions obtained with the usual single current dipole fitting procedures, over a latency range which includes the extrema in the (average) measured signal. In all cases considered, 100 or so epochs time-locked to the stimulus were magnetically recorded. These were averaged, and then analyzed using two techniques; a new distributed current model known as Magnetic Field Tomography (MFT), and the standard single current dipole (SCD) model. Both methods provide estimates of the current generators in the brain. In two of the normal subjects, the MFT solutions are super-imposed onto Magnetic Resonance Images (MRI) of the relevant cortical area. The results show that when the SCD model provides a reasonable description of the data, the MFT estimate shows one dominant localized region in agreement with the current dipole position. In the MFT sequence of solutions the activity evolves smoothly; multiple areas of activity often arise as the focal activity in one region declines while focal activity in another region grows. In contrast the SCD solutions during these intermediate periods fit the data poorly, and may move erratically from one locale to another. We conclude that MFT seems to provide a reasonable description of the activity through cortical and subcortical regions. The evolution of activity, as derived from the average signal, can be traced continuously from the onset of the stimulus, not just at the peaks.(ABSTRACT TRUNCATED AT 250 WORDS)

Acoustic Stimulation↗
PubMed1992

Primary pyogenic abscess of the psoas muscle.

Nine children who were less than nine years old and had a primary pyogenic abscess of the psoas muscle were treated in a five-year period. This rare lesion often simulated septic arthritis of the hip. Ultrasonography was found to be the most reliable and easily performed diagnostic study. Because of the rarity of the lesion, the diagnosis requires a high index of suspicion. The best treatment is early operative drainage and administration of systemic antibiotics.

Child↗
PubMed1991

Magnetic field tomography of coherent thalamocortical 40-Hz oscillations in humans.

This paper introduces the use of magnetic field tomography (MFT), a noninvasive technique based on distributed source analysis of magnetoencephalography data, which makes possible the three-dimensional reconstruction of dynamic brain activity in humans. MFT has a temporal resolution better than 1 msec and a spatial accuracy of 2-5 mm at the cortical level, which deteriorates to 1-3 cm at depths of 6 cm or more. MFT is used here to visualize the origin of a spatiotemporally organized pattern of coherent 40-Hz electrical activity. This coherence, initially observed during auditory input, was proposed to be generated by recurrent corticothalamic oscillation. In support of this hypothesis, we illustrate well-defined 40-Hz coherence between cortical-subcortical sites with a time shift that is consistent with thalamocortical conduction times. Studies on Alzheimer patients indicate that, while a similar activity pattern is present, the cortical component is reduced in these subjects.

Adult↗