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Biomedical subjects

J Virtanen

Publications and source records attributed to J Virtanen.

At least 19 recordsLinked to original sources
PubMed1998

Hemispheric lateralization in preattentive processing of speech sounds.

Event-related magnetoencephalographic (MEG) responses to infrequently presented spoken deviant syllables [di] and [ba] among repetitive standard [da)]syllables were recorded in subjects who either attended to these stimuli in order to discriminate the [ba] syllables or ignored them while attending a silent movie. In both conditions, the deviant syllables elicited a mismatch response (MMNm, the magnetic counterpart of mismatch negativity), which was stronger in the left than in the right auditory cortex, indicating left-hemispheric dominance in speech processing already at a preattentive processing level.

Acoustic Stimulation↗
PubMed1998

Parkinson's disease selectively impairs preattentive auditory processing: an MEG study.

Auditory stimuli elicit auditory evoked magnetic fields (AEFs) called P50m and N100m, which index preconscious auditory processing in human. We investigated with a whole-head magnetometer whether Parkinson's disease (PD) impairs parallel preattentive auditory processing between the hemispheres. Stimulus blocks consisting of standard (80%) and deviant (20%) tones were monaurally presented in a passive condition to 11 PD patients with unilateral motor symptoms and to 11 age-matched healthy controls. The constant interstimulus intervals (ISIs) were 0.5 s and 2.5 s in separate blocks. The interhemispheric latency differences of the P50m and N100m were significantly lengthened in PD patients in the left-ear condition. This might be caused by the basal ganglia dysfunction in PD together with the simultaneous age-related neural degeneration predominant in the left auditory cortex.

Acoustic Stimulation↗
PubMed1998

Reversal of cerebral asymmetry in schizophrenia measured with magnetoencephalography.

It has been suggested that schizophrenic patients fail to develop left-hemisphere dominance because of an early disturbance in neuronal development. This hypothesis has been supported by some post-mortem. CT and magnetic resonance imaging (MRI) studies, while other in-vivo studies have given contradicting results. We used 122-channel whole-head magnetoencephalography and MRI to locate the sources of auditory evoked responses in 19 schizophrenic patients and in 20 healthy controls. Auditory evoked responses were detected in all subjects. The left-right hemisphere asymmetry of cerebral sources for auditory evoked responses was markedly dispersed among patients when compared with controls. The source locations for left auditory cortex were clearly anterior with respect to the right hemisphere in 32% of the patients, while the corresponding prevalence of this abnormal asymmetry was 0% in controls (p = 0.008. Fisher's exact test). The reversed asymmetry appeared to be associated with a shorter anterior-posterior distance between the auditory cortex and the anterior tip of the temporal lobe in the left side when compared with the right side. The reversed asymmetry was associated with higher PANSS general psychopathological score, and especially with higher guilt feelings and motor retardation scores. The large 2.5-fold standard deviation in the inter-hemispheric anterior posterior difference in the location of the auditory cortex among patients (p 0.001 for the difference in the magnitude of variance between controls and patients) clearly reflects the dispersion of the left right asymmetry into both direction, and three of the patients with 'normal asymmetry' had a greater left-right asymmetry than any of the controls. Markedly greater reversal of hemispheric asymmetry among patients implies that regulation of the development of brain asymmetry is disturbed among schizophrenic patients. Abnormality in the cerebral asymmetry may be a crucial factor in the development of schizophrenic disorder in a substantial proportion of patients. The results suggest that the reversed asymmetry is associated with the higher severity of general psychopathological symptoms.

Adult↗
PubMed1998

Replicability of MEG and EEG measures of the auditory N1/N1m-response.

We investigated the replicability of the source location, amplitude and latency measures of the auditory evoked N1 (EEG) and N1m (MEG) responses. Each of the 5 subjects was measured 6 times in two recording sessions. Responses to monaural stimuli were recorded from 122 MEG and 64 EEG channels simultaneously. The EEG data were modeled with a symmetrically-located dipole pair. For the MEG data, one dipole in each hemisphere was located independently using a subset of channels. Standard deviation (SD) was used as a measure for replicability. The average SD of the x, y and z coordinates of the contralateral N1m dipole was about 2 mm, whereas the corresponding figures for the ipsilateral N1m and the contra- and ipsilateral N1 were about twice as large. The SDs of the dipole amplitudes and latencies were almost equal with MEG and EEG. The amplitude and latency measures of the MEG field gradient waveforms were almost as replicable as those of the dipole models. The results suggest that both MEG and EEG can be used for investigating the simultaneous activity of the left and right auditory cortices independently, MEG being superior in certain experimental setups.

Acoustic Stimulation↗
PubMed1998

Combined mapping of human auditory EEG and MEG responses.

Auditory electric and magnetic P50(m), N1(m) and MMN(m) responses to standard, deviant and novel sounds were studied by recording brain electrical activity with 25 EEG electrodes simultaneously with the corresponding magnetic signals measured with 122 MEG gradiometer coils. The sources of these responses were located on the basis of the MEG responses; all were found to be in the supratemporal plane. The goal of the present paper was to investigate to what degree the source locations and orientations determined from the magnetic data account for the measured EEG signals. It was found that the electric P50, N1 and MMN responses can to a considerable degree be explained by the sources of the corresponding magnetic responses. In addition, source-current components not detectable by MEG were shown to contribute to the measured EEG signals.

Acoustic Stimulation↗
PubMed1998

Processing of novel sounds and frequency changes in the human auditory cortex: magnetoencephalographic recordings.

Whole-head magnetoencephalographic (MEG) responses to repeating standard tones and to infrequent slightly higher deviant tones and complex novel sounds were recorded together with event-related brain potentials (ERPs). Deviant tones and novel sounds elicited the mismatch negativity (MMN) component of the ERP and its MEG counterpart (MMNm) both when the auditory stimuli were attended to and when they were ignored. MMNm generators were located bilateral to the superior planes of the temporal lobes where preattentive auditory discrimination appears to occur. A subsequent positive P3a component was elicited by deviant tones and with a larger amplitude by novel sounds even when the sounds were to be ignored. Source localization for the MEG counterpart of P3a (P3am) suggested that the auditory cortex in the superior temporal plane is involved in the neural network of involuntary attention switching to changes in the acoustic environment.

Acoustic Stimulation↗
PubMed1997

Neuronal responses to magnetic stimulation reveal cortical reactivity and connectivity.

Motor and visual cortices of normal volunteers were activated by transcranial magnetic stimulation. The electrical brain activity resulting from the brief electromagnetic pulse was recorded with high-resolution electroencephalography (HR-EEG) and located using inversion algorithms. The stimulation of the left sensorimotor hand area elicited an immediate response at the stimulated site. The activation had spread to adjacent ipsilateral motor areas within 5-10 ms and to homologous regions in the opposite hemisphere within 20 ms. Similar activation patterns were generated by magnetic stimulation of the visual cortex. This new non-invasive method provides direct information about cortical reactivity and area-to-area neuronal connections.

Adult↗
PubMed1997

Biopotential amplifier for simultaneous operation with biomagnetic instruments.

A multichannel biopotential amplifier for simultaneous use with biomagnetic measurements in a magnetically shielded room is designed and evaluated. Particular care is taken to make the amplifier electromagnetically compatible with the biomagnetic instruments over the whole frequency spectrum, from DC to RF. The electromagnetically quiet environment allows the use of high electrode impedances; the preamplifier has been designed accordingly. Special care is taken to analyse the coupling mechanisms of mains interference to the amplifier. Over 170 simultaneous electric and magnetic recordings have been performed using the system in a hospital environment.

Amplifiers, Electronic↗
PubMed1997

Evidence for superlattice arrangements in fluid phosphatidylcholine/phosphatidylethanolamine bilayers.

Recently, evidence for cholesterol and phosphatidylcholine (PC) molecules to adapt superlattice arrangements in fluid lipid bilayers has been presented. Whether superlattice arrangements exist in other biologically relevant lipid membranes, such as phosphatidylethanolamine (PE)/PC, is still speculative. In this study, we have examined the physical properties of fluid 1-palmitoyl-2-oleoyl-PC (POPC) and 1-palmitoyl-2-oleoyl-PE (POPE) binary mixtures as a function of the POPE mole fraction (X(PE)) using fluorescence and Fourier transform infrared spectroscopy. At 30 degrees C, i.e., above the Tm of POPE and POPC, deviations, or dips, as well as local data scattering in the excimer-to-monomer fluorescence intensity ratio of intramolecular excimer forming dipyrenylphosphatidylcholine probe in POPE/POPC mixtures were detected at X(PE) approximately 0.04, 0.11, 0.16, 0.26, 0.33, 0.51, 0.66, 0.75, 0.82, 0.91, and 0.94. The above critical values of X(PE) coincide (within +/-0.03) with the critical mole fractions X(HX,PE) or X(R,PE) predicted by a headgroup superlattice model, which assumes that the lipid headgroups form hexagonal or rectangular superlattice, respectively, in the bilayer. Other spectroscopic data, generalized polarization of Laurdan and infrared carbonyl and phosphate stretching frequency, were also collected. Similar agreements between some of the observed critical values of X(PE) from these data and the X(HX,PE) or X(R,PE) values were also found. However, all techniques yielded critical values of X(PE) (e.g., 0.42 and 0.58) that cannot be explained by the present headgroup superlattice model. The effective cross-sectional area of the PE headgroup is smaller than that of the acyl chains. Hence, the relief of "packing frustration" of PE in the presence of PC (larger headgroup than PE) may be one of the major mechanisms in driving the PE and PC components to superlattice-like lateral distributions in the bilayer. We propose that headgroup superlattices may play a significant role in the regulation of membrane lipid compositions in cells.

Biophysical Phenomena↗
PubMed1997

Somatosensory evoked magnetic fields to median nerve stimulation: interhemispheric differences in a normal population.

The objective of the present study was to evaluate the normal interhemispheric variability of the locations and activation strengths of the somatosensory cortices. Somatosensory evoked magnetic fields (SEFs) were recorded with a 122-channel magnetometer in 23 healthy subjects (mean age 57 years) to stimulation of left and right median nerves. Equivalent current dipole (ECD) strengths and locations were determined for the main SEF deflections at the contralateral primary sensorimotor (SMI) and secondary somatosensory (SIIc) cortices. In a Cartesian co-ordinate system, defined by the preauricular points and the nasion, the SMI sources were slightly but significantly more laterally and anteriorly located in the right than in the left hemisphere. No systematic co-ordinate asymmetries were found for the SIIc sources. In individual subjects, the interhemispheric differences in the ECD co-ordinates averaged less than 6 mm at both SMI and SIIc. The group means of the source strengths did not differ between the hemispheres, but individual differences were on average 20% for the SMI and 65% for the SIIc sources. We conclude that at the individual level, the median nerve SEFs from SMI can be used to detect abnormally large interhemispheric asymmetries of source locations in the centimetre scale.

Adult↗
PubMed1996

Alzheimer's disease affects parallel processing between the auditory cortices.

Auditory evoked magnetic fields (AEFs) were recorded from 11 patients with Alzheimer's disease (AD) and 11 age-matched controls using the 122-channel whole-head magnetometer. Auditory stimuli were monaurally presented with interstimulus intervals (ISI) of 0.5 and 2.5 s in different blocks. The peak latencies of P50m and N100m responses were significantly longer in AD patients than in controls over the ipsilateral but not over the contralateral auditory cortex with respect to the ear stimulated. This finding suggests that parallel auditory processing is impaired between the auditory cortices in AD patients. The present MEG measurement might provide an objective index to evaluate auditory dysfunction in AD.

Acoustic Stimulation↗
PubMed1996

MEG-compatible multichannel EEG electrode array.

We describe an EEG electrode array, which is designed to facilitate simultaneous multichannel EEG and MEG recordings. The special electrode design allows shorter preparation times and more reliable contacts than commercially available solutions. The electrode array is magnetically compatible with MEG and the low-profile electrodes consume minimal space inside the magnetometer.

Electrodes↗
PubMed1996

Effects of interstimulus interval on somatosensory evoked magnetic fields (SEFs): a hypothesis concerning SEF generation at the primary sensorimotor cortex.

Cerebral responses evoked by peripheral stimuli are known to depend critically on the interstimulus interval (ISI). Here we report on the effects of ISI on somatosensory evoked magnetic fields (SEFs) to right median nerve stimulation, obtained in 9 healthy adults with ISIs of 0.15 0.3, 1,3 and 5 s. At the contralateral (left) primary sensorimotor cortex (SMI), the first cortical response, N20m, was stable between the ISIs 0.3 and 5 s, but slightly attenuated at the shortest ISI of 0.15 s. In contrast, the P35m and P60m deflections were very sensitive to changes of the ISI, declining steadily with shortening of the ISI throughout the entire range. These deflections were frequently undetectable at the shortest ISI of 0.15 s. Concomitant with the reductions of P35m and P60m, an N45m deflection was enhanced toward the short ISIs. Responses from second somatosensory cortex (SII) and posterior parietal cortex (PPC) were seen only with ISIs of 1 s or greater, being strongest at the 5 s ISI. Based on known effects of the ISI on intracellular evoked potentials, we present the following tentative model for the generation mechanism of the SMI response: N20m represents early excitatory postsynaptic potentials (EPSPs), P35m early inhibitory postsynaptic potentials (IPSPs), N45m secondary EPSPs and P60m late IPSPs in pyramidal neurones of area 3b. For practical purposes, SEFs from SMI can be obtained with short ISIs, while responses from SII and PPC require an ISI of at least 1 s.

Adult↗
PubMed1995

Age-related functional differences between auditory cortices: a whole-head MEG study.

Auditory evoked magnetic fields (AEFs) were recorded from 10 healthy younger and 10 older subjects using a whole-head magnetometer. Two blocks of tone pips were presented to the left ear with constant inter-stimulus intervals (ISIs) of 0.5 and 2.5 s. The amplitude of P50m, unlike that of N100m, was larger in the older subjects. In both groups, the peak latencies of P50m and N100m responses were significantly shorter over the contralateral than ipsilateral cortex. The interhemispheric latency difference of N100m was significantly increased with age. These findings suggest that ageing delays signal processing in the ipsilateral auditory cortex and that ageing affects consecutive AEFs in a different manner.

Acoustic Stimulation↗
PubMed1995

Transversal distribution of acyl-linked pyrene moieties in liquid-crystalline phosphatidylcholine bilayers. A fluorescence quenching study.

Quenching of the fluorescence of pyrene-labeled phospholipids by dibromolipids was used to determine the chain length dependence of the bilayer depths of the pyrenyl moieties. Six 1-palmitoyl-2-(pyrenyl-n-acyl)-phosphatidylcholines (PyrnPC) were examined, with end-labeled pyrenyl chains varying in length, n, from 4 to 14 carbons. These lipids were incorporated, at a concentration of 0.3 mol%, into bilayers composed of various mixtures of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) and of one of three 1-palmitoyl-2-(x,y-dibromostearoyl)phosphatidylcholine quencher lipids (Brx,yPC; x,y = 6,7; 9,10; or 11,12). Parallel experiments were carried out with bilayers containing 50 mol % cholesterol. Quenching in these systems is dynamic, as demonstrated by the identical dependence of steady-state fluorescence intensities and excited state lifetimes of Pyr8PC on the mole fraction of Br6,7PC. Stern--Volmer analysis of the Brx,PC mole fraction dependence of PyrnPC fluorescence yielded apparent quenching constants, KSV, which show a systematic relation with both the length of the pyrenyl acyl chain and the position of the bromine atoms. The quenching data were further analyzed by plotting KSV as a function of n (defined above), or b (the average of the two bromine positions for each PyrnPC), or n--b (the separation between pyrenes and bromines). In all cases, the data were fit by Gaussian functions yielding estimates of the centers and the apparent 1/e half-widths of the transversal distributions of the pyrenyl moieties in methylene units (mu). Both in the absence and in the presence of cholesterol, the position of each PyrnPC Gaussian center is equal to the sum of n plus a constant d approximately 2.5 mu, corresponding to the distance from the effective center of the pyrenyl moiety to its point of attachment to the acyl chain.(ABSTRACT TRUNCATED AT 250 WORDS)

Crystallization↗
PubMed1988

Intracavernous self-injection for erectile failure.

Thirty-three patients with erectile failure were taught to self-inject papaverine intracavernosally. The dose was from 15 to 80 mg. Phentolamine was added if 80 mg was not sufficient. The patients kept a diary on the effects of the regimen, and also filled out a questionnaire after a follow-up of 4-16 months. The results showed that 55% were satisfied with the method. However, technical difficulties were common. Sexual stimulation turned out to be very important resulting in varying erections on consecutive occasions with the same papaverine dose. Prolonged erection occurred once in 5 patients and was easily handled conservatively in all. Fibrous plaques developed in 2 patients. Twelve patients (36%) stopped the injections for various reasons. When failure occurred the disappointment was usually severe. Thus, the selection of patients for self-injection is important.

Adult↗
PubMed1986

Toxic catheters and diminished urethral blood circulation in the induction of urethral strictures.

Local effects of indwelling urinary catheters are poorly characterized. Latex catheter brands of various degrees of tissue toxicity were implanted into the urethra of 27 male piglets. The systemic hemodynamic states varied from normal to hypovolemia, where the circulation changes simulated the extracorporeal perfusion used in open-heart surgery. The urethral epithelial changes caused by the catheters were studied by light ad scanning electron microscopy (SEM). The blood circulation in paraurethral tissue reacted strongly to systemic hemodynamic changes. A reduced local blood flow induced an accumulation of polymorphonuclear leucocytes in the urethral epithelium and subepithelial space in connection with implanted latex urinary catheter strips. This was not seen in normovolemic piglets. Non-toxic wholly silicone control catheter implants did not induce changes. Epithelial cell damage correlated with the tissue toxicity values of implanted catheter strips. In SEM analysis destruction of microvilli and cell membranes was also discovered in connection with a latex catheter brand regarded as non-toxic. The wholly silicone catheters did not cause any detectable changes. The observed cell damage is induced by toxic chemicals dissolving from latex catheters. Local ischemia exacerbates these toxic effects and activates complement cascade. This induces polymorphonuclear granulocyte accumulation in the damaged urethral epithelium leading to demolition and urethral fibrotization. The present findings explain the etiology of the long urethral stricture epidemics seen in open-heart surgery in the early 1980s by showing the interference between indwelling latex catheters and reduced local blood flow.

Animals↗