Biomedical subjects

Jyrki Ahveninen

Publications and source records attributed to Jyrki Ahveninen.

At least 19 recordsLinked to original sources

PubMed2006

EEG/(f)MRI measurements at 7 Tesla using a new EEG cap ("InkCap").

We aimed at improving the signal-to-noise ratio (SNR) of electroencephalography (EEG) during magnetic resonance imaging (MRI) by introducing a new EEG cap ("InkCap") based on conductive ink technology. The InkCap was tested with temperature measurements on an electrically conductive phantom head and during structural and functional MRI (fMRI) recordings in 11 healthy human volunteers at 7 T. Combined EEG/fMRI measurements were conducted to study the interaction between the two modalities. The EEG recordings with the InkCap demonstrated up to a five-fold average decrease in signal variance during echo-planar imaging, with respect to a cap made of standard carbon fiber leads. During concurrent EEG/fMRI measurements in human volunteers, alpha oscillations were clearly detected at 7 T. Minimal artifacts were present in the T2* and high-resolution structural MR images of the brain parenchyma. Our results show that the InkCap technology considerably improves the quality of both EEG and (f)MRI during concurrent measurements even at 7 T.

Adult↗

PubMed2006

Task-modulated "what" and "where" pathways in human auditory cortex.

Human neuroimaging studies suggest that localization and identification of relevant auditory objects are accomplished via parallel parietal-to-lateral-prefrontal "where" and anterior-temporal-to-inferior-frontal "what" pathways, respectively. Using combined hemodynamic (functional MRI) and electromagnetic (magnetoencephalography) measurements, we investigated whether such dual pathways exist already in the human nonprimary auditory cortex, as suggested by animal models, and whether selective attention facilitates sound localization and identification by modulating these pathways in a feature-specific fashion. We found a double dissociation in response adaptation to sound pairs with phonetic vs. spatial sound changes, demonstrating that the human nonprimary auditory cortex indeed processes speech-sound identity and location in parallel anterior "what" (in anterolateral Heschl's gyrus, anterior superior temporal gyrus, and posterior planum polare) and posterior "where" (in planum temporale and posterior superior temporal gyrus) pathways as early as approximately 70-150 ms from stimulus onset. Our data further show that the "where" pathway is activated approximately 30 ms earlier than the "what" pathway, possibly enabling the brain to use top-down spatial information in auditory object perception. Notably, selectively attending to phonetic content modulated response adaptation in the "what" pathway, whereas attending to sound location produced analogous effects in the "where" pathway. This finding suggests that selective-attention effects are feature-specific in the human nonprimary auditory cortex and that they arise from enhanced tuning of receptive fields of task-relevant neuronal populations.

Adaptation, Physiological↗

PubMed2006

Enhanced magnetic auditory steady-state response in early Alzheimer's disease.

OBJECTIVE: Previous studies have reported abnormalities in both spontaneous and evoked electromagnetic brain activity in Alzheimer's disease (AD). We studied the auditory steady-state response (SSR) which represents the net effect of entrained background activity and superimposed cortical evoked responses, in AD patients and healthy controls. METHODS: Whole-head magnetoencephalography (MEG) was used to measure SSR to monaural 40-Hz stimulation in AD patients and age-matched controls. Equivalent current dipoles (ECD) of the SSR were modeled in each hemisphere, and source amplitudes were compared between the two groups using time-varying ECD models. RESULTS: Our results indicate that the SSR is significantly increased in AD patients with mild to moderate cognitive deterioration in comparison with healthy elderly subjects. CONCLUSIONS: Goal-directed functioning requires optimization of inhibitory and excitatory inputs in the cortex, allowing the adaptation of responsiveness to repetitive stimulation with low relevance. The present results suggest that this balance is impaired in AD, manifesting itself in decreased inhibition in cortical auditory processing and impaired adaptation of the stimulus-locked activity, probably due to abnormalities in cholinergic modulation. SIGNIFICANCE: MEG appears to be a sensitive tool to detect abnormalities of auditory processing already in early stages of AD.

Acoustic Stimulation↗

PubMed2006

Inherited auditory-cortical dysfunction in twin pairs discordant for schizophrenia.

BACKGROUND: Information on the inheritance of neurophysiological abnormalities might help elucidate the molecular genetic basis of schizophrenia. We used magnetoencephalography (MEG) and electroencephalography (EEG) to investigate the inheritance of auditory-cortical deficiencies in twin pairs discordant for schizophrenia. METHODS: Auditory EEG/MEG responses to frequent standard and occasional deviant tones were measured in mono- and dizygotic (MZ and DZ) twin pairs discordant for schizophrenia and demographically matched healthy twin pairs, recruited from a total population cohort. The MEG/EEG results were regressed against the genetic resemblance to patients with schizophrenia across the patients' unaffected MZ/DZ co-twins and control subjects (with genetic correlations of 1, .5, and 0 to schizophrenia patients, respectively). RESULTS: The EEG responses P50, N100, and mismatch negativity (MMN), as well as the MEG response P50m, were reduced in the schizophrenic patients. P50 and N100 were significantly decreased also in their unaffected co-twins, as compared with the control subjects. Importantly, the P50 and N100 decrease correlated with the unaffected subjects' genetic resemblance to schizophrenia patients. CONCLUSIONS: Our results suggest inherited abnormalities in cortical auditory processing in schizophrenia, reflected by the decreased P50/P50m and N100 amplitudes, whereas the MMN abnormalities might reflect predominantly state-dependent neurodegeneration.

Brain Mapping↗

PubMed2006

Source estimation of spontaneous MEG oscillations in mild cognitive impairment.

Mild cognitive impairment (MCI) is a memory disorder often preceding Alzheimer's disease (AD). AD has been shown to be associated with abnormal generation of spontaneous electromagnetic activity. We investigated whether the cortical generation of spontaneous brain oscillations in MCI shows changes resembling those observed in AD. A minimum current estimates algorithm was applied to identify cortical sources of magnetoencephalographic (MEG) spontaneous brain oscillations in male MCI patients with a clear memory disorder and in healthy elderly controls. This data was subsequently compared to a male subsample of AD patients from an earlier study. While there were clear oscillatory abnormalities in AD patients, there was no evidence of significant changes in the alpha source distribution in MCI patients as compared to healthy controls. Deficits in the distribution of oscillatory sources in the resting state are thus likely to occur at later stages of cognitive impairment than MCI.

Aged↗

PubMed2005

Altered generation of spontaneous oscillations in Alzheimer's disease.

Slowing of spontaneous alpha oscillations and an anterior shift of a source of alpha activity (8-13 Hz) have been consistently reported in the EEG studies of Alzheimer's disease (AD). It is unknown whether these changes are associated with a gradual shift in location and frequency of existing sources or rather with the involvement of a new set of oscillators. We addressed this question by applying source modeling (minimum current estimates, MCE) to spontaneous alpha activity recorded with a 306-channel MEG system from eleven non-medicated AD patients with mild to moderate cognitive impairment and twelve age-matched controls during the eyes-closed session. AD patients had predominant lower alpha band sources in the temporal regions, whereas in the controls, robust alpha sources were found near the parieto-occipital sulcus. Activation within the parieto-occipital region was significantly weaker, and activation in the right temporal area was significantly enhanced in the AD patients. These results suggest an increased temporal-lobe contribution coinciding with parieto-occipital deficits. We propose that MCE, which provides simultaneous mapping of several oscillatory sources, might be useful for detecting neurophysiological abnormalities associated with AD in combination with other neuropsychological and neurological measures.

Aged↗

PubMed2005

Cholinergic modulation of preattentive auditory processing in aging.

Auditory event-related potential (ERP) components P50 and N100 are thought to index preattentive auditory processing underlying stimulus detection, whereas a subsequent component termed mismatch negativity (MMN) has been proposed to reflect comparison of incoming stimuli to a short-lived sensory memory trace of preceding sounds. Existing evidence suggests impairment of preattentive auditory processing in aging, which appears to be accompanied by decline of cholinergic activity. Previous studies indicate that scopolamine, which is a centrally acting muscarinic receptor antagonist, modulates preattentive auditory processing in young subjects. It has remained elusive, however, to which extent scopolamine affects preattentive auditory processing in aged subjects. We measured auditory responses simultaneously with electroencephalogram (EEG) and magnetoencephalogram (MEG) from nine non-demented elderly subjects after intravenous injection of scopolamine or glycopyrrolate, the latter being a peripherally acting cholinergic antagonist, using a double blind protocol. Scopolamine significantly delayed electric P50, both electric and magnetic N100 responses, whereas subsequent MMN and P200 responses were not altered by scopolamine. Our results indicate that the cholinergic system modulates auditory processing underlying stimulus detection in aging. In addition, auditory evoked responses appear to have different age-related sensitivity to cholinergic modulation. The combined MEG/EEG measurements using particularly auditory N100 response might offer an objective tool to monitor cholinergic activity in aging and Alzheimer's disease (AD).

Aged↗

PubMed2005

Serotonergic modulation of mismatch negativity.

Neurochemical mechanisms mediating the interaction between emotional and cognitive processing are not yet fully understood. Here, we utilized acute tryptophan depletion (ATD) to reduce the brain synthesis of serotonin (5-HT), which is thought to have a central role in regulation of emotions and mood in humans. ATD effects on event-related potentials and magnetic fields were studied using a passive odd-ball paradigm in a randomized, double-blinded, controlled, cross-over design. Auditory responses were recorded simultaneously with high-resolution magnetoencephalography (MEG) and electroencephalography (EEG) in 14 healthy subjects, 5 h after ATD or a control condition. ATD significantly increased depressed mood and lowered plasma tryptophan concentration (total tryptophan decreased by 75%, free tryptophan decreased by 39%). As compared with the control condition, ATD increased the amplitudes of mismatch negativity (MMN) to duration and frequency changes and decreased the latencies of magnetic MMN to frequency changes in the hemisphere ipsilateral to the ear stimulated. Further, ATD modulated N1m latencies and decreased P2m source activity. ATD increased the interhemispheric latency difference of MMNm to frequency changes. No effects on P50 were observed. The present results suggest serotonergic modulation of preattentive auditory change detection, suggested to initiate involuntary attention shifting in the human brain.

Adult↗

PubMed2004

Adaptation of neuromagnetic N1 responses to phonetic stimuli by visual speech in humans.

The technique of 306-channel magnetoencephalogaphy (MEG) was used in eight healthy volunteers to test whether silent lip-reading modulates auditory-cortex processing of phonetic sounds. Auditory test stimuli (either Finnish vowel /ae/ or /ø/) were preceded by a 500 ms lag by either another auditory stimulus (/ae/, /ø/ or the second-formant midpoint between /ae/ and /ø/), or silent movie of a person articulating /ae/ or /ø/. Compared with N1 responses to auditory /ae/ and /ø/ when presented without a preceding stimulus, the amplitudes of left-hemisphere N1 responses to the test stimuli were significantly suppressed both when preceded by auditory and visual stimuli, this effect being significantly stronger with preceding auditory stimuli. This suggests that seeing articulatory gestures of a speaker influences auditory speech perception by modulating the responsiveness of auditory-cortex neurons.

Acoustic Stimulation↗

PubMed2004

Human posterior auditory cortex gates novel sounds to consciousness.

Life or death in hostile environments depends crucially on one's ability to detect and gate novel sounds to awareness, such as that of a twig cracking under the paw of a stalking predator in a noisy jungle. Two distinct auditory cortex processes have been thought to underlie this phenomenon: (i) attenuation of the so-called N1 response with repeated stimulation and (ii) elicitation of a mismatch negativity response (MMN) by changes in repetitive aspects of auditory stimulation. This division has been based on previous studies suggesting that, unlike for the N1, repetitive "standard" stimuli preceding a physically different "novel" stimulus constitute a prerequisite to MMN elicitation, and that the source loci of MMN and N1 are different. Contradicting these findings, our combined electromagnetic, hemodynamic, and psychophysical data indicate that the MMN is generated as a result of differential adaptation of anterior and posterior auditory cortex N1 sources by preceding auditory stimulation. Early ( approximately 85 ms) neural activity within posterior auditory cortex is adapted as sound novelty decreases. This alters the center of gravity of electromagnetic N1 source activity, creating an illusory difference between N1 and MMN source loci when estimated by using equivalent current dipole fits. Further, our electroencephalography data show a robust MMN after a single standard event when the interval between two consecutive novel sounds is kept invariant. Our converging findings suggest that transient adaptation of feature-specific neurons within human posterior auditory cortex filters superfluous sounds from entering one's awareness.

Adult↗

PubMed2003

Effects of an acute D2-dopaminergic blockade on the somatosensory cortical responses in healthy humans: evidence from evoked magnetic fields.

We tested the possible role of dopaminergic activity in the processing of somatosensory afferent information in healthy humans. Somatosensory evoked magnetic fields (SEFs) were recorded in seven subjects in response to left median nerve stimulation. SEFs were obtained in all subjects after oral administration of 2 mg haloperidol, an antagonist to dopaminergic D2 receptors, and placebo, which were given in a randomized, double-blind cross-over design. SEFs were analyzed using a multiple equivalent current dipole (ECD) model, with one dipole at the right primary somatosensory cortex (SI) and at both left and right secondary somatosensory cortices (SII). The earliest responses from SI, peaking at about 20 ms (N20m) and 35 ms (P35m), were not affected by haloperidol. A later deflection peaking at about 75 ms (P60m), however, was slightly reduced (p < 0.05). Responses arising from SII were not significantly changed. The results suggest that dopaminergic activity may be involved in modulating somatosensory processing after the initial stages of cortical activation.

Adult↗

PubMed2003

Acute tryptophan depletion does not change somatosensory evoked magnetic fields.

RATIONALE: Serotonin is shown to regulate the activity of primary auditory cortex, but little is known about serotonin modulation of other sensory cortices. METHODS: We investigated somatosensory evoked magnetic fields (SEF) to left median nerve stimulation in eight healthy subjects in a double-blind, controlled, cross-over design study after acute tryptophan depletion (ATD) and control mixture. SEFs were recorded with the whole-head magnetoencephalography 6 h after ingestion of mixtures. The SEF sources and strength were estimated by a least-squares fit of a single equivalent current dipole. RESULTS: ATD decreased the total and free TPR levels by 75 and 48% and control mixture increased them by 98% and 44%. ATD had no effect on the amplitudes or latencies of SEF components. The source locations of the responses were not significantly affected by ATD. CONCLUSION: Serotonin does not affect stimuli processing in the primary somatosensory cortex.

Adult↗

PubMed2003

Auditory selective attention modulated by tryptophan depletion in humans.

To elucidate serotonin modulation of selective attention, 13 volunteers (21-30 years) were studied in two sessions, 5 h after either acute tryptophan depletion (ATD) that decreases brain serotonin synthesis, or control-mixture ingestion (randomized, double-blind, cross-over design). Simultaneous electroencephalogram and magnetoencephalogram were measured during dichotic listening of two concurrent trains of standard and deviant tones. Subjects counted the deviants presented to one ear and ignored those presented to the other ear. ATD lowered plasma total tryptophan by 75% and free tryptophan by 39%. ATD suppressed the amplitude enhancement of P50 and N1 to selectively attended tones, but did not affect the later aspects of processing negativity. The P50 latencies were increased after ATD, irrespective of attention. In conclusion, serotonin may regulate attentional modulation of early cortical stimulus processing.

Acoustic Stimulation↗

PubMed2003

Effects of scopolamine on MEG spectral power and coherence in elderly subjects.

OBJECTIVE: Scopolamine, a muscarinic receptor antagonist, can produce temporary cognitive impairments as well as electroencephalographic changes that partially resemble those observed in Alzheimer's disease. In order to test the sensitivity of spectral power and hemispheric coherence to changes in cholinergic transmission, we evaluated quantitative magnetoencephalogram (MEG) after intravenous injection of scopolamine. METHODS: MEG of 8 elderly healthy subjects (59-80 years) were measured with a whole-head magnetometer after intravenous injection of scopolamine. An injection of glycopyrrolate, a peripheral muscarinic antagonist, was used as the placebo in a double-blind, randomized, cross-over design. Spectral power and coherence were computed over 7 brain regions in 3 frequency bands. RESULTS: Scopolamine administration increased theta activity (4-8 Hz) and resulted in the abnormal pattern of MEG desynchronization in eyes-open vs. eyes-closed conditions in the alpha band (8-13 Hz). These effects were most prominent over the posterior regions. Interhemispheric and left intrahemispheric coherence was significantly decreased in the theta band (4-8 Hz). CONCLUSIONS: Spontaneous cortical activity at the theta and alpha range and functional coupling in the theta band are modulated by the cholinergic system. MEG may provide a tool for monitoring brain dynamics in neurological disorders associated with cholinergic abnormalities.

Aged↗

PubMed2002

Acute tryptophan depletion decreases intensity dependence of auditory evoked magnetic N1/P2 dipole source activity.

RATIONALE: Intensity dependence of the N1/P2 components may be regulated by serotonergic neurons in the primary auditory cortex, where low activity leads to a high intensity dependence and vice versa. Depletion of tryptophan (TRP), a precursor for serotonin has been described to reduce serotonin content in brain of animals and humans. OBJECTIVE: We investigated the intensity dependence of magnetic and electric N1/P2 components in ten subjects in a double-blind, controlled, cross-over design study after oral mixture of amino-acids leading to acute tryptophan depletion (ATD) and control. METHODS: Auditory evoked magnetic fields (AEF) and potentials (AEP) were recorded with 122-channel magnetoencephalography simultaneously with 64-channel EEG 5 h after ingestion of mixtures. The AEF sources and strength were estimated by a least-squares fit of a single equivalent current dipole. The amplitudes and latencies of N1 and P2 recorded with EEG were analyzed at frontal electrode site. RESULTS: TRP depletion decreased the total and free TRP levels by 76 and 45% and control mixture increased it by 48 and 28%. ANOVA showed that ATD had a significant main effect on the N1m/P2m dipole moments at the contralateral ( P=0.02), but failed significantly to influence the ipsilateral responses. A significant mixture ingestion-by-stimulus intensity interaction was observed on the N1m/P2m dipole moments at the contralateral hemisphere ( P=0.01). The N1/P2 slope for intensity dependence function was decreased following ATD compared with the control experiment ( P=0.01) at the contralateral hemisphere. For EEG, a significant mixture ingestion-by-stimulus intensity interaction on the N1 latencies at the Fz electrode position was observed ( P=0.01). CONCLUSION: ATD decreased the intensity dependence of N1m/P2m source dipole moments in the primary auditory cortex at the hemisphere contralateral to the ear stimulated. These results suggest that serotonin participates in the regulation of intensity of auditory stimulation.

Adult↗

PubMed2002

Memory-based comparison process not attenuated by haloperidol: a combined MEG and EEG study.

Auditory P50 and N100 responses reflect preattentive processing, whereas subsequent mismatch negativity (MMN) response indexes memory-based comparison process. Divergent ERP responses have been found in schizophrenia and in Parkinson's disease (PD), which have abnormalities in cerebral dopamine activity. We used simultaneously magnetoencephalography and electroencephalography to investigate, whether a single dose of haloperidol, a dopamine D2-receptor antagonist, modulates preattentive auditory processing using a randomized, double-blind, placebo-controlled crossover design. Our results showed that haloperidol did not alter MMN to frequency and duration changes, whereas the magnetic MMN to frequency change was significantly accelerated. The amplitude and latency changes of the electric and magnetic P50 and N100 were insignificant. Our results indicate that memory-based sound comparison and preceding cortical processing underlying stimulus detection are not attenuated by haloperidol, whereas haloperidol appears to accelerate preattentive sound comparison.

Adult↗

PubMed2002

Aging and cholinergic modulation of the transient magnetic 40-Hz auditory response.

Cholinergic blockade by scopolamine, a central muscarinic receptor antagonist, may produce transient memory impairment in healthy subjects, and it has been used as a neurochemical model of cognitive degeneration in aged individuals. To observe the muscarinic modulation of memory and cortical auditory processing, nine cognitively intact elderly subjects (59-80 years) were studied using neuropsychological tests and 122-channel magnetoencephalography (MEG) after an administration of scopolamine hydrobromide (0.3 mg, i.v.) or glycopyrrolate (0.2 mg, i.v.), a peripheral muscarinic antagonist. A double-blind randomized crossover design was used in two sessions separated by at least 1 week. Scopolamine, but not glycopyrrolate, produced a transient impairment of verbal memory performance in the elderly subjects. MEG indicated that the auditory-evoked 40-Hz magnetic response was significantly larger after scopolamine than after glycopyrrolate administration. Furthermore, reanalysis of our earlier results in younger subjects (20-31 years), basically supporting the present MEG findings, tentatively suggests that the scopolamine effects on the 40-Hz response may be slightly pronounced with aging. In sum, the transient magnetic 40-Hz auditory response may be useful in studies on brain cholinergic deficits in elderly subjects.

Aged↗

PubMed2002

Tryptophan depletion effects on EEG and MEG responses suggest serotonergic modulation of auditory involuntary attention in humans.

Involuntary attention shifting, i.e., detecting and orienting to unexpected stimulus changes, may be altered at low brain serotonin (5-hydroxytryptamine; 5-HT) levels. This was studied in 13 healthy subjects (21-30 years old; 6 females) by using a dietary challenge, acute tryptophan depletion (ATD), which decreases 5-HT synthesis in the brain. Five hours after ingestion of either ATD or control mixture (randomized, double-blinded, crossover design), brain responses indexing involuntary attention were measured with simultaneous 64-channel electroencephalography (EEG) and 122-channel magnetoencephalography (MEG). During the measurement, the subjects were instructed to discriminate equiprobable 200- and 400-ms tones by pressing one of two buttons rapidly. Occasionally, the frequency of the tones changed (10% increase/decrease), causing involuntary attention shifting. ATD significantly lowered plasma tryptophan concentrations (total tryptophan decreased by 75%, free tryptophan decreased by 35%). As compared to the control condition, ATD reduced the amplitude of the deviant-tone N2 wave, including the overlapping mismatch negativity (MMN) and N2b subcomponents, which are suggested to reflect change detection in the brain. The EEG results were accompanied by a significant increase in the peak latency of the magnetic counterpart of MMN. However, no ATD effects were observed in P3 to task-irrelevant frequency change. Reaction time (RT) to deviants per se was not significantly affected, but RT in trials succeeding the deviant-frequency tones was increased by ATD, which suggested impaired reorienting to the task-relevant activity. In conclusion, the results suggest that decreased level of central 5-HT function after ATD may decrease involuntary attention shifting to task-irrelevant sound changes and thus modulate resource allocation to the task-relevant activity.

Acoustic Stimulation↗