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Biomedical subjects

D Khosla

Publications and source records attributed to D Khosla.

9 recordsLinked to original sources
PubMed2001

Plasticity in the adult human central auditory system: evidence from late-onset profound unilateral deafness.

Experience-related changes in central nervous system (CNS) activity have been observed in the adult brain of many mammalian species, including humans. In humans, late-onset profound unilateral deafness creates an opportunity to study plasticity in the adult CNS consequent to monaural auditory deprivation. CNS activity was assessed by measuring long-latency auditory evoked potentials (AEPs) recorded from teens and adults with late-onset (post-childhood) profound unilateral deafness. Compared to monaurally stimulated normal-hearing subjects, the AEPs recorded from central electrode sites located over auditory cortical areas showed significant increases in inter-hemispheric waveform cross-correlation coefficients, and in inter-hemispheric AEP peak amplitude correlations. These increases provide evidence of substantial changes from the normal pattern of asymmetrical (contralateral > ipsilateral amplitude) and asynchronous (contralateral earlier than ipsilateral) central auditory system activation in the normal-hearing population to a much more symmetrical and synchronous activation in the unilaterally deaf. These cross-sectional analyses of AEP data recorded from the unilaterally deaf also suggest that the changes in cortical activity occur gradually and continue for at least 2 years after the onset of hearing loss. Analyses of peak amplitude correlations suggest that the increased inter-hemispheric symmetry may be a consequence of changes in the generators producing the N (approximately 100 ms peak latency) potential. These experience-related changes in central auditory system activity following late-onset profound unilateral deafness thus provide evidence of the presence and the time course of auditory system plasticity in the adult brain.

Adolescent↗
PubMed1999

Spatial mislocalization of EEG electrodes -- effects on accuracy of dipole estimation.

OBJECTIVE: The estimation of cortical current activity from scalp-recorded potentials is a complicated mathematical problem that requires fairly precise knowledge of the location of the scalp electrodes. It is expected that spatial mislocalization of electrodes will introduce errors in this estimation. The present study uses simulated and real data to quantify these errors for dipole current sources in a spherical head model. METHODS: A 3-dimensional digitizer was used to locate the positions of 31 scalp electrodes placed on the head according to the 10-20 system in 10 normal subjects. Dipole localizations were performed on auditory evoked potentials (AEPs) collected from these subjects. RESULTS: Computer simulations with several dipole source configurations suggest that errors in locations and orientations on the order of 5 mm and 5 degrees, respectively, are possible for electrode mislocalizations of about 5 degrees. In actual experimental settings, digitized electrode positions were typically mislocalized by an average of about 4 degrees from their standard 10-20 positions on a spherical model. These differences in electrode positions translated to mean differences of about 8 mm in dipole locations and 5 degrees in dipole orientations. CONCLUSIONS: Dipole estimation errors due to electrode mislocalizations are within the limits of errors due to other modeling approximations and noise.

Adult↗
PubMed1999

Distributed source imaging of alpha activity using a maximum entropy principle.

OBJECTIVE: We present a method based on the distributed dipole source model to localize sources of spontaneous human brain activity, such as the alpha rhythm. The proposed method relies on the generalized maximum entropy principle and is implemented in frequency-domain. METHODS: Several computer simulation studies of synchronous and asynchronous distributed dipole sources were carried out to test the validity of the method. The method was also applied to spontaneous electroencephalographic (EEG) recordings from human subjects to estimate the sources of alpha activity. The locations of these sources were registered with actual magnetic resonance images for anatomical visualization. RESULTS: The simulation studies suggest the validity of the proposed method and its capability to detect distributed and fairly deep synchronous dipole sources. Results of human studies with 6 subjects suggest that the generators of alpha rhythm are mainly concentrated over the posterior regions of the cortex. CONCLUSION: The proposed distributed source imaging method is a promising technique for localizing rhythmic brain activity.

Adult↗
PubMed1997

Spatio-temporal EEG source localization using simulated annealing.

The estimation of multiple dipole parameters in spatio-temporal source modeling (STSM) of electroencephalographic (EEG) data is a difficult nonlinear optimization problem due to multiple local minima in the cost function. A straightforward iterative optimization approach to such a problem is very susceptible to being trapped in a local minimum, thereby resulting in incorrect estimates of the dipole parameters. In this paper, we present and evaluate a more robust optimization approach based on the simulated annealing algorithm. The complexity of this approach for the STSM problem was reduced by separating the dipole parameters into linear (moment) and nonlinear (location) components. The effectiveness of the proposed method and its superiority over the traditional nonlinear simplex technique in escaping local minima were tested and demonstrated through computer simulations. The annealing algorithm and its implementation for multidipole estimation are also discussed. We found the simulated annealing approach to be 7-31% more effective than the simplex method at converging to the true global minimum for a number of different kinds of three-dipole problems simulated in this work. In addition, the computational cost of the proposed approach was only marginally higher than its simplex counterpart. The annealing method also yielded similar solutions irrespective of the initial guesses used. The proposed simulated annealing method is an attractive alternative to the simplex method that is currently more common in dipole estimation applications.

Algorithms↗
PubMed1992

Stereo pairs of bright-field micrographs via Wiener-type inverse filtering.

In a three-dimensional (3-D) image data set obtained through optical sectioning, each two-dimensional (2-D) segment is blurred by out-of-focus information from neighbouring focal planes superimposed on the in-focus segments from that plane. Instead of attempting to remove this redundant information over the full 3-D data set, we have developed a technique for restoring stereoscopic views. In this paper we describe the implementation of a Wiener-type inverse filtering method for generating stereo pairs of bright-field micrographs. A theoretical optical transfer function valid under certain simplifying approximations has been used in implementing this filtering technique. In developing this method the slice theorem of computed tomography is used. In this way the image reconstruction problem is reduced to one of processing 2-D arrays rather than 3-D arrays and the problem of restoring missing Fourier components within the missing-cone region is circumvented. Limited experimentation with real micrographs shows that the approach provides images that display an effective increased depth of field and 3-D attributes of the specimen, even though some of the underlying assumptions on which this method is based are difficult to verify explicitly. The method can be implemented with a relatively fast execution time on 386-SX computers.

Fourier Analysis↗