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Biomedical subjects

A Kral

Publications and source records attributed to A Kral.

At least 19 recordsLinked to original sources
PubMed2001

Trends of HIV infection among injection drug users in Brazil in the 1990s: the impact of changes in patterns of drug use.

BACKGROUND: Santos Metropolitan Region (SMR), State of Sao Paulo, historically is well known as being one of the areas with the largest number of AIDS cases in Brazil, especially among injection drug users (IDUs). The main objective of this study was to assess the effects of changes in drug-using patterns among IDUs on trends in HIV infection among IDUs in the 1990s. METHODS: During 1991 through 1992 (wave 1; n = 214), 1994 through 1996 (wave 2; n = 135), and 1999 (wave 3; n = 108), we conducted three cross-sectional studies of IDUs. All participants were interviewed and tested for antibodies to HIV. FINDINGS: The overall sample population was 69% male, and 87% of the sample population was under 40 years old. Eighty-four percent of the population had less than 9 years of education. HIV seroprevalence was 63% in wave 1, 65% in wave 2, and 42% in wave 3 ( p <.001). Smoking of crack cocaine increased from 11% in wave 1 to 60% in wave 2 and 67% in wave 3 ( p <.001). The prevalence of frequent injections (>5 per day) decreased from 42% in wave 1 to 30% in wave 2 and 15% in wave 3 ( p <.001). INTERPRETATION: HIV prevalence decreased as injection frequency decreased and crack cocaine use increased. In SMR, patterns of drug use have been affecting the HIV epidemic more than scant public health intervention.

Adult↗
PubMed2001

Overdoses among cocaine users in Brazil.

AIMS: A survey of cocaine users was undertaken to study (i) the frequency of reported drug overdoses among cocaine users; and (ii) the frequency of witnessing drug overdoses in the same population. DESIGN AND SETTING: A cross-sectional study as part of the World Health Organization (WHO) Multi-city Study among injecting drug users (IDUs), phase II, was conducted in Santos Metropolitan Region, State of São Paulo, Brazil, in 1999. PARTICIPANTS: Three hundred and ninety-six exclusive users of cocaine in the Santos Metropolitan Region, São Paulo State, Brazil were surveyed concerning their past experience with drug overdoses. FINDINGS: Eighty (20%) of the cocaine users reported having experienced one or more overdoses, and 50% reported that they knew one or more other cocaine users who had died of an overdose. On multivariate analysis, being female and having spent time in jail were associated with an increased likelihood of having had one or more overdoses. CONCLUSION: Cocaine overdoses are an important and under-recognized health problem in the Santos Metropolitan Region, and possibly in other areas of Brazil.

Adolescent↗
PubMed2000

Temporal code and speech recognition.

The hypothesis that temporal code is mainly responsible for speech recognition is discussed. Data from cochlear implant research demonstrate that single-channel speech processors, supplying only temporal code, enable understanding of spoken language. Physiological findings supporting the observation are presented. A temporal theory of tone pitch perception is discussed.

Cochlea↗
PubMed2000

Congenital auditory deprivation reduces synaptic activity within the auditory cortex in a layer-specific manner.

The present study investigates the functional deficits of naive auditory cortices in adult congenitally deaf cats. For this purpose, their auditory system was stimulated electrically using cochlear implants. Synaptic currents in cortical layers were revealed using current source density analyses. They were compared with synaptic currents found in electrically stimulated hearing cats. The naive auditory cortex showed significant deficits in synaptic activity in infragranular cortical layers. Furthermore, there was also a deficit of synaptic activities at longer latencies (>30 ms). The 'cortical column' was not activated in the well-defined sequence found in normal hearing cats. These results demonstrate functional deficits as a consequence of congenital auditory deprivation. Similar deficits are likely in congenitally deaf children.

Animals↗
PubMed2000

[Determinants of the effectiveness of electric stimulation of the auditory nerve with cochlear implants: II. Configuration of the stimulating electrodes].

The influence of the electrode configuration on the efficacy of the electrical stimulation were investigated in the auditory nerve. For this purpose normal hearing cats were deafened by intrascalar application of neomycin and stimulated by a NUCLEUS-22 implant. Stimulation was performed in monopolar, bipolar and tripolar mode with all electrodes of the implant. The smallest thresholds were determined for monopolar stimulation, followed by tripolar and bipolar stimulation. The most focused stimulation was achieved by tripolar stimulation, followed by bipolar and monopolar stimulation. Tripolar stimulation is therefore suitable for beam forming in electrical stimulation of the auditory nerve.

Animals↗
PubMed2000

[Determinants of the effectiveness of cochlear implants: I. Placement in the cochlea].

The present paper introduces a registration method for the electrical field produced by a cochlear implant in the scala tympani in vitro. It was possible to determine the electrical potentials with high spatial resolution using an additional recording electrode. The results indicate that the efficacy of the stimulation by the more apical electrodes is higher than by the more basal ones. The reason is the decreasing cross-sectional area of the scala tympani, which represents a decreasing shunt between the stimulation electrodes.

Animals↗
PubMed1999

Recruitment of the auditory cortex in congenitally deaf cats by long-term cochlear electrostimulation.

In congenitally deaf cats, the central auditory system is deprived of acoustic input because of degeneration of the organ of Corti before the onset of hearing. Primary auditory afferents survive and can be stimulated electrically. By means of an intracochlear implant and an accompanying sound processor, congenitally deaf kittens were exposed to sounds and conditioned to respond to tones. After months of exposure to meaningful stimuli, the cortical activity in chronically implanted cats produced field potentials of higher amplitudes, expanded in area, developed long latency responses indicative of intracortical information processing, and showed more synaptic efficacy than in naïve, unstimulated deaf cats. The activity established by auditory experience resembles activity in hearing animals.

Acoustic Stimulation↗
PubMed1999

Monitoring of anaesthesia in neurophysiological experiments.

Cortical activity can be substantially changed by the type of anaesthetic used, and by its dose level. For easy monitoring of depth of anaesthesia we describe the changes in electroencephalogram and electrocardiogram accompanying changes in depth of anaesthesia in the cat. Anaesthesia was induced by the volatile anaesthetic isoflurane. The high-frequency components (around 30 Hz) in the electroencephalogram disappear in deep anaesthesia. The electrocardiogram also shows substantial changes in contamination due to muscle fasciculations with anaesthesia level. Fasciculations appear as noise in the electrocardiogram. The amplitude of the electrical muscle activity contaminating the ECG can be easily used for the maintainance of a constant level of anaesthesia during a neurophysiological experiment.

Anesthesia↗
PubMed1999

[Lateral inhibition and its role in the physiology of sensory organs].

Lateral inhibition represents the neural mechanism stabilizing the activity in neural networks. The present paper introduces the different types of lateral inhibition and presents the neurophysiological data demonstrating its existence in the nervous system. Lateral inhibition is mainly discussed in sensory organs. However, its manifestation in psychophysics are ambiguous. Some psychophysical phenomena do support the concept of lateral inhibition, other contradict. A possible explanation is a covering of the existent lateral inhibition network by excitatory synapses. Lateral inhibition therefore does not dominate the behavior of the whole system, even though it is important in stabilizing the excitatory activity in it.

Animals↗
PubMed1998

Spatial resolution of cochlear implants: the electrical field and excitation of auditory afferents.

This paper investigates the spatial resolution of electrical intracochlear stimulation in order to enable further refinement of cochlear implants. For this purpose electrical potential distributions around a conventional human intracochlear electrode (NUCLEUS-22) were measured in a tank, in cat cadaver cochleae and in living cat cochleae. Potential gradients were calculated where of importance. The values were compared to spatial tuning curves from cat primary auditory afferents in electrical mono-, bi-, and various tripolar stimulation modes. Finally, a lumped element model was developed to elucidate the single fiber data. Tank potential measurements show the principal features of the different stimulation modes but are not sufficient to explain all the features of experimental data from single fibers. Intracochlear potential measurements indicate an increase in spatial resolution in an apical direction. The single fiber data also confirm that a tripolar stimulus configuration provides significantly better spatial resolution than any other stimulation mode presently in use.

Animals↗
PubMed1996

On lateral inhibition in the auditory system.

Suga (1994) has presented arguments supporting the existence of lateral inhibition in the auditory system. We developed a computational model of a lateral inhibition neural network possibly taking part in auditory stimulus processing. The behavior of the model under several hypothetical auditory stimuli matches experimental results. We have shown that lateral inhibition can be the cause of tinnitus in some conditions. 95% of subjects have phantom perceptions in the absence of auditory stimulation (acoustically shielded chambers). We suggest that the spontaneous activity in the auditory nerve (a Poisson-like neural noise), processed by lateral inhibition, is responsible for this phenomenon. Activity is generated in the output layer of the network under stimulation with a Poisson-like noise. In the presence of acoustic stimulation this neural noise is masked by the stimulus or the ambient wideband acoustic noise. The wideband noise is effectively suppressed in lateral inhibition. The shape of the probability density function of the noise determines whether it will be effectively suppressed. The edge effect has been demonstrated to be also a possible consequence of lateral inhibition.

Acoustic Stimulation↗
PubMed1996

Neural networks simulating the frequency discrimination of hearing for non-stationary short tone stimuli.

This paper addresses the question of frequency discrimination of hearing for non-stationary (short) tone stimuli (duration < or = 125 ms). Shortening of the stimulus duration leads to widening of the frequency spectrum of the tone. It can be shown that for hearing no acoustical uncertainty relation holds and thus some nonlinear elements must be present in hearing physiology. We present neurophysiological and psychoacoustical findings supporting the hypothesis that frequency discrimination of non-stationary short tone stimuli is performed in neural networks of the auditory system. Neural network architectures that could process the temporal and place excitation patterns originating in the cochlea are suggested. We show how these networks (temporal coincidence network processing the temporal code and lateral inhibition network processing the place code) can be combined to show performance consistent with auditory physiology. They might explain the frequency discrimination of hearing for non-stationary short tone stimuli. We show the fitting of psychophysical relations based on these networks with the experimentally determined data.

Acoustic Stimulation↗
PubMed1993

Sharpening of input excitation curves in lateral inhibition.

Various lateral inhibition neural networks are studied by means of computer simulation regarding their ability to sharpen the input excitation curves. To quantify this ability we introduced two new entropy-like quantities (called the iteration entropy and the rate of convergence) which represent suitable measures for the sharpening of the input excitation curves in a certain lateral inhibition neural network. Using these quantities we quantitatively described the sharpening ability of different lateral inhibition networks.

Neural Inhibition↗
PubMed1993

Negative growth selection against rodent fibroblasts targeted for genetic inhibition of farnesyl transferase.

The Ras oncoprotein must be modified by farnesyl transferase (FTase) for biological activity. Therefore, inhibition of FTase may offer a means to block ras induced cell transformation. To address this hypothesis, we have introduced antisense and dominant inhibitory FTase expression plasmids into a panel of normal, mutant ras-, and mos- transformed rodent fibroblasts in an effort to genetically suppress FTase activity. Antisense FTase constructs reduced colony formation efficiency approximately 29% in normal and approximately 41% in ras-transformed cells relative to control plasmids. In contrast, antisense FTase plasmids did not exhibit a statistically significant effect on colony formation efficiency in mos-transformed transfectants. FTase alpha N199K is a mutant form of the alpha subunit of FTase that exhibits dominant inhibitory activity versus native FTase. Only mos-transformed transfectants exhibited expression of alpha N199K RNA in 15 of 16 fibroblast lines that were randomly selected and characterized. Our data suggest that genetic inhibition of FTase may result in a selection against animal cell growth.

Alkyl and Aryl Transferases↗
PubMed1990

Isolation and characterization of a zinc finger polypeptide gene at the human chromosome 11 Wilms' tumor locus.

We have isolated a series of genomic and cDNA clones mapping within the boundaries of constitutional and tumor deletions that define the Wilms' tumor locus on human chromosome 11 (band p13). The transcription unit corresponding to these clones spans approximately 50 kb and encodes an mRNA approximately 3 kb long. This mRNA is expressed in a limited range of cell types, predominantly in the kidney and a subset of hematopoietic cells. The polypeptide encoded by this locus has a number of features suggesting a potential role in transcriptional regulation. These include the presence of four zinc finger domains and a region rich in proline and glutamine. The amino acid sequence of the predicted polypeptide shows significant homology to two growth regulated mammalian polypeptides, EGR1 and EGR2. The genetic localization of this gene, its tissue-specific expression, and the function predicted from its sequence lead us to suggest that it represents the 11p13 Wilms' tumor gene.

Amino Acid Sequence↗
PubMedDate not supplied

Plastic changes in the auditory cortex of congenitally deaf cats following cochlear implantation.

Congenitally deaf cats were used as a model for human inborn deafness and auditory deprivation. The deaf cats were supplied with a cochlear implant, chronically exposed to an acoustic environment and conditioned to acoustic stimuli. In case of early implantation the cats learned to make use of the newly gained auditory channel behaviourally. Neurophysiological and fMRI data showed that the central auditory system was recruited, if implantation took place within a sensitive period of <6 months.

Animals↗