Pekka Meriläinen

Hemodynamic Responses to Speech and Music in Newborn Infants

We used near‐infrared spectroscopy (NIRS) to study responses to speech and music on the auditory cortices of 13 healthy full‐term newborn infants during natural sleep. The purpose of the study was to investigate the lateralization of speech and music responses at this stage of development. NIRS data was recorded from eight positions on both hemispheres simultaneously with electroencephalography, electrooculography, electrocardiography, pulse oximetry, and inclinometry. In 11 subjects, statistically significant (P < 0.02) oxygenated (HbO2) and total hemoglobin (HbT) responses were recorded. Both stimulus types elicited significant HbO2 and HbT responses on both hemispheres in five subjects. Six of the 11 subjects had positive HbO2 and HbT responses to both stimulus types, whereas one subject had negative responses. Mixed positive and negative responses were observed in four neonates. On both hemispheres, speech and music responses were significantly correlated (r = 0.64; P = 0.018 on the left hemisphere (LH) and r = 0.60; P = 0.029 on the right hemisphere (RH)). On the group level, the average response to the speech stimuli was statistically significantly greater than zero in the LH, whereas responses on the RH or to the music stimuli did not differ significantly from zero. This suggests a more coherent response to speech on the LH. However, significant differences in lateralization of the responses or mean response amplitudes of the two stimulus types were not observed on the group level. Hum Brain Mapp, 2010.

Comparison of principal and independent component analysis in removing extracerebral interference from near-infrared spectroscopy signals

Near-infrared spectroscopy (NIRS) is a method for noninvasive estimation of cerebral hemodynamic changes. Principal component analysis (PCA) and independent component analysis (ICA) can be used for decomposing a set of signals to underlying components. Our objective is to determine whether PCA or ICA is more efficient in identifying and removing scalp blood flow interference from multichannel NIRS signals. Concentration changes of oxygenated (HbO(2)) and deoxygenated (HbR) hemoglobin are measured on the forehead with multichannel NIRS during hyper- and hypocapnia. PCA and ICA are used separately to identify and remove signal contribution from extracerebral tissue, and the resulting estimates of cerebral responses are compared to the expected cerebral responses. Both methods were able to reduce extracerebral contribution to the signals, but PCA typically performs equal to or better than ICA. The improvement in 3-cm signal quality achieved with both methods is comparable to increasing the source-detector separation from 3 to 5 cm. Especially PCA appears to be well suited for use in NIRS applications where the cerebral activation is diffuse, such as monitoring of global cerebral oxygenation and hemodynamics. Performance differences between PCA and ICA could be attributed primarily to different criteria for identifying the surface effect.

IgE sensitisation in relation to flow-independent nitric oxide exchange parameters

BackgroundA positive association between IgE sensitisation and exhaled NO levels has been found in several studies, but there are no reports on the compartment of the lung that is responsible for the increase in exhaled NO levels seen in IgE-sensitised subjects.MethodsThe present study comprised 288 adult subjects from the European Community Respiratory Health Survey II who were investigated in terms of lung function, IgE sensitisation (sum of specific IgE), smoking history and presence of rhinitis and asthma. Mean airway tissue concentration of NO (CawNO), airway transfer factor for NO (DawNO), mean alveolar concentration of NO (CalvNO) and fractional exhaled concentration of NO at a flow rate of 50 mL s-1 (FENO 0.05) were determined using the extended NO analysis.ResultsIgE-sensitised subjects had higher levels (geometric mean) of FENO 0.05 (24.9 vs. 17.3 ppb) (p < 0.001), DawNO (10.5 vs. 8 mL s-1) (p = 0.02) and CawNO (124 vs. 107 ppb) (p < 0.001) and positive correlations were found between the sum of specific IgE and FENO 0.05, CawNO and DawNO levels (p < 0.001 for all correlations). Sensitisation to cat allergen was the major determinant of exhaled NO when adjusting for type of sensitisation. Rhinitis and asthma were not associated with the increase in exhaled NO variables after adjusting for the degree of IgE sensitisation.ConclusionThe presence of IgE sensitisation and the degree of allergic sensitisation were related to the increase in airway NO transfer factor and the increase in NO concentration in the airway wall. Sensitisation to cat allergen was related to the highest increases in exhaled NO parameters. Our data suggest that exhaled NO is more a specific marker of allergic inflammation than a marker of asthma or rhinitis.

Extended NO analysis in asthma

The discovery of the flow dependence of exhaled NO made it possible to model NO production in the lung. The linear model provides information about the maximal flux of NO from the airways and the alveolar concentrations of NO. Nonlinear models give additional flow-independent parameters such as airway diffusing capacity and airway wall concentrations of NO. When these models are applied to patients with asthma, a clear-cut increase in NO flux is found, and this is caused by an increase in both airway diffusing capacity and airway wall concentrations of NO. There is no difference in alveolar concentrations of NO compared to healthy subjects, except in severe asthma where an increase has been found. Inhaled corticosteroids are able to reduce the airway wall concentrations but not diffusing capacity or alveolar concentrations. Oral prednisone affects the alveolar concentration, suggesting that in severe asthma there is a systemic component. Steroids distributed by any route do not affect the airway diffusing capacity. Therefore, the airway diffusing capacity should be in focus in testing new drugs or in combination treatment for asthma. Exhaled NO analysis is a promising tool in characterizing asthma in both adults and children. However, there is a strong need to agree on the models and to standardize the flow rates to be used for the modelling in order to perform a systematic and robust analysis of NO production in the lung.

Extended NO analysis in a healthy subgroup of a random sample from a Swedish population

Introduction:  There is an interest in modelling exhaled nitric oxide (NO). Studies have shown that flow‐independent NO parameters i.e. NO of the alveolar region (CANO), airway wall (CawNO), diffusing capacity (DawNO) and flux (JawNO), are altered in several disease states such as asthma, cystic fibrosis, alveolitis and chronic obsmuctive pulmonary disease (COPD). However, values from a healthy population are missing.

Impaired cerebral vasoreactivity may cause cerebral blood volume dip following obstructive sleep apnea termination

Near-infrared spectroscopy (NIRS) is a non-invasive technique for estimating cortical concentration changes of oxy(Δ[HbO2]), deoxy(Δ[HbR]), and total (Δ[HbT]=Δ[HbO2] +Δ[HbR]) hemoglobin [1, 2]. Cortical Δ[HbT] is commonly used as an indicator of cerebral blood volume (CBV) changes. Obstructive sleep apnea (OSA) is characterized by apneas (pause in breathing lasting over 10 s) or hypopneas (reduced respiratory air flow lasting over 10 s, accompanied by blood oxygen desaturation of at least 4% or EEG arousal) during sleep. The resulting oxygen and sleep deprivation can lead to severe health problems ranging from fatigue to coronary artery disease and stroke [3]. In a recent study, transcranial Doppler sonography (TCD) was used to measure cerebral blood flow velocity (CBFV) during sleep in OSA patients [4]. The study concluded that cerebral vasoreactivity decreases during OSA sleep, and apnea termination is followed by a drop in CBFV. Several studies indicate that apnea-induced changes in CBFV should be associated with parallel changes in cerebral blood flow (CBF) and consequently also in CBV [3, 5, 6]. However, in a recent NIRS study, CBV was found to stay relatively constant after apnea termination [2]. Here, we show repeatable NIRS results from a single OSA subject that agree with the TCD results and suggest a hemodynamic response pattern not previously reported in NIRS OSA studies.

Effects of improper source coupling in frequency-domain near-infrared spectroscopy

Currently, there is no widely used method to assess the reliability of contact between optodes and tissue in near-infrared spectroscopy (NIRS). In this study we observe a high linear dependence (R(2) approximately 0.99) of the logarithmic modulation amplitude (ln(I(AC))), average intensity (ln(I(DC))) and phase (phi) on the source-detector distance (SDD) ranging from approximately 20 to 50 mm on human forehead measurements. The regression of phi is clearly reduced in measurements where light leakage occurs, mainly due to insufficient contact between the source optode and tissue. Utilizing this observation, a novel criterion to detect light leakage is developed. The criterion is applied to study the reliability of hemodynamic responses measured on the human forehead when breathing carbon dioxide-enriched air and during hyperventilation. The contrast of the signals is significantly lower in measurements which were adversely affected by light leakage. Furthermore, such unreliable signals at SDDs >or= 50 mm correlate significantly (for [HbO2] p < 0.01 and for [HbR] p < 0.001) better with the signals measured at SDDs < 20 mm. Using this method, poor contact between the source optode and tissue can be detected and corrected before the actual measurement, which enables us to avoid the acquisition of low contrast cortical signals.

Near-infrared spectroscopic imaging of stimulus-related hemodynamic responses on the neonatal auditory cortices.

We have used near-infrared spectroscopy (NIRS) to study hemodynamic auditory evoked responses on 7 full-term neonates. Measurements were done simultaneously above both auditory cortices to study the distribution of speech and music processing between hemispheres using a 16-channel frequency-domain instrument. The stimulation consisted of 5-second samples of music and speech with a 25-second silent interval. In response to stimulation, a significant increase in the concentration of oxygenated hemoglobin ([HbO2]) was detected in 6 out of 7 subjects. The strongest responses in [HbO2] were seen near the measurement location above the ear on both hemispheres. The mean latency of the maximum responses was 9.42±1.51 s. On the left hemisphere (LH), the maximum amplitude of the average [HbO2] response to the music stimuli was 0.76± 0.38 μ M (mean±std.) and to the speech stimuli 1.00± 0.45 μ± μM. On the right hemisphere (RH), the maximum amplitude of the average [HbO2] response was 1.29± 0.85 μM to the music stimuli and 1.23± 0.93 μM to the speech stimuli. The results indicate that auditory information is processed on both auditory cortices, but LH is more concentrated to process speech than music information. No significant differences in the locations and the latencies of the maximum responses relative to the stimulus type were found.

Simultaneous diffuse near-infrared imaging of hemodynamic and oxygenation changes and electroencephalographic measurements of neuronal activity in the human brain

Visually evoked hemodynamic responses and potentials were simultaneously measured using a 16-channel optical imaging instrument and a 60-channel electroencephalography instrument during normo-, hypo- and hypercapnia from three subjects. Flashing and pattern-reversed checkerboard stimuli were used. The study protocol included two counterbalanced measurements during both normo- and hypocapnia and normo- and hypercapnia. Hypocapnia was produced by controlled hyperventilation and hypercapnia by breathing carbon dioxide enriched air. Near-infrared imaging was also used to monitor the concentration changes of oxy- and deoxyhaemoglobin due to hypo- and hypercapnia. Hemodynamic responses and evoked potentials were successfully detected for each subject above the visual cortex. The latencies of the hemodynamic responses during hypocapnia were shorter whereas during hypercapnia they were longer when compared to the latencies during normocapnia. Hypocapnia tended to decrease the latencies of visually evoked potentials compared to those during normocapnia while hypercapnia did not show any consistent effect to the potentials. The developed measurement setup and the study protocol provide the opportunity to investigate the neurovascular coupling and the links between the baseline level of blood flow, electrical activity and hemodynamic responses in the human brain.

Multi-channel near-infrared spectroscopy on the human forehead during hypo- and hypercapnia

Cerebral blood flow and oxygenation changes during different levels of hypo- and hypercapnia were successfully monitored on the human forehead using a multi-channel frequency-domain near-infrared spectroscopy. Optical signals correlate well with simultaneously recorded gas measurements.