Signal detection with spectrum windows.

Spectrum sensing is needed in frequency agile cognitive radio systems, cognitive radar systems as well as in (cognitive) electronic warfare systems. This paper studies window based detectors where window shapes can be matched for the expected spectrum. The provided analytical results for the probabilities of false alarm and detection are valid for arbitrary windows. Based on the window shape mismatch analysis, it was concluded that the simple rectangular window is often sufficient if its bandwidth is properly set. Furthermore, based on the window bandwidth mismatch analysis, it was discussed how many different rectangular windows are needed to cover the instantaneous analysis bandwidth of the receiver without compromising too much on sensitivity. Finally, suitable constant false alarm rate threshold setting schemes were compared. It was observed that almost ideal setting schemes exist.

Large-scale functional networks connect differently for processing words and symbol strings.

Reconfigurations of synchronized large-scale networks are thought to be central neural mechanisms that support cognition and behavior in the human brain. Magnetoencephalography (MEG) recordings together with recent advances in network analysis now allow for sub-second snapshots of such networks. In the present study, we compared frequency-resolved functional connectivity patterns underlying reading of single words and visual recognition of symbol strings. Word reading emphasized coherence in a left-lateralized network with nodes in classical perisylvian language regions, whereas symbol processing recruited a bilateral network, including connections between frontal and parietal regions previously associated with spatial attention and visual working memory. Our results illustrate the flexible nature of functional networks, whereby processing of different form categories, written words vs. symbol strings, leads to the formation of large-scale functional networks that operate at distinct oscillatory frequencies and incorporate task-relevant regions. These results suggest that category-specific processing should be viewed not so much as a local process but as a distributed neural process implemented in signature networks. For words, increased coherence was detected particularly in the alpha (8-13 Hz) and high gamma (60-90 Hz) frequency bands, whereas increased coherence for symbol strings was observed in the high beta (21-29 Hz) and low gamma (30-45 Hz) frequency range. These findings attest to the role of coherence in specific frequency bands as a general mechanism for integrating stimulus-dependent information across brain regions.

Multivariate analysis of correlation between electrophysiological and hemodynamic responses during cognitive processing.

Animal and human studies have frequently shown that in primary sensory and motor regions the BOLD signal correlates positively with high-frequency and negatively with low-frequency neuronal activity. However, recent evidence suggests that this relationship may also vary across cortical areas. Detailed knowledge of the possible spectral diversity between electrophysiological and hemodynamic responses across the human cortex would be essential for neural-level interpretation of fMRI data and for informative multimodal combination of electromagnetic and hemodynamic imaging data, especially in cognitive tasks. We applied multivariate partial least squares correlation analysis to MEG-fMRI data recorded in a reading paradigm to determine the correlation patterns between the data types, at once, across the cortex. Our results revealed heterogeneous patterns of high-frequency correlation between MEG and fMRI responses, with marked dissociation between lower and higher order cortical regions. The low-frequency range showed substantial variance, with negative and positive correlations manifesting at different frequencies across cortical regions. These findings demonstrate the complexity of the neurophysiological counterparts of hemodynamic fluctuations in cognitive processing.

Neural interactions at the core of phonological and semantic priming of written words.

Word processing is often probed with experiments where a target word is primed by preceding semantically or phonologically related words. Behaviorally, priming results in faster reaction times, interpreted as increased efficiency of cognitive processing. At the neural level, priming reduces the level of neural activation, but the actual neural mechanisms that could account for the increased efficiency have remained unclear. We examined whether enhanced information transfer among functionally relevant brain areas could provide such a mechanism. Neural activity was tracked with magnetoencephalography while subjects read lists of semantically or phonologically related words. Increased priming resulted in reduced cortical activation. In contrast, coherence between brain regions was simultaneously enhanced. Furthermore, while the reduced level of activation was detected in the same area and time window (superior temporal cortex [STC] at 250-650 ms) for both phonological and semantic priming, the spatiospectral connectivity patterns appeared distinct for the 2 processes. Causal interactions further indicated a driving role for the left STC in phonological processing. Our results highlight coherence as a neural mechanism of priming and dissociate semantic and phonological processing via their distinct connectivity profiles.

Functional magnetic resonance imaging blood oxygenation level-dependent signal and magnetoencephalography evoked responses yield different neural functionality in reading.

It is often implicitly assumed that the neural activation patterns revealed by hemodynamic methods, such as functional magnetic resonance imaging (fMRI), and electrophysiological methods, such as magnetoencephalography (MEG) and electroencephalography (EEG), are comparable. In early sensory processing that seems to be the case, but the assumption may not be correct in high-level cognitive tasks. For example, MEG and fMRI literature of single-word reading suggests differences in cortical activation, but direct comparisons are lacking. Here, while the same human participants performed the same reading task, analysis of MEG evoked responses and fMRI blood oxygenation level-dependent (BOLD) signals revealed marked functional and spatial differences in several cortical areas outside the visual cortex. Divergent patterns of activation were observed in the frontal and temporal cortex, in accordance with previous separate MEG and fMRI studies of reading. Furthermore, opposite stimulus effects in the MEG and fMRI measures were detected in the left occipitotemporal cortex: MEG evoked responses were stronger to letter than symbol strings, whereas the fMRI BOLD signal was stronger to symbol than letter strings. The EEG recorded simultaneously during MEG and fMRI did not indicate neurophysiological differences that could explain the observed functional discrepancies between the MEG and fMRI results. Acknowledgment of the complementary nature of hemodynamic and electrophysiological measures, as reported here in a cognitive task using evoked response analysis in MEG and BOLD signal analysis in fMRI, represents an essential step toward an informed use of multimodal imaging that reaches beyond mere combination of location and timing of neural activation.

Spatiotemporal convergence of semantic processing in reading and speech perception.

Retrieval of word meaning from the semantic system and its integration with context are often assumed to be shared by spoken and written words. How is modality-independent semantic processing manifested in the brain, spatially and temporally? Time-sensitive neuroimaging allows tracking of neural activation sequences. Use of semantically related versus unrelated word pairs or sentences ending with a semantically highly or less plausible word, in separate studies of the auditory and visual modality, has associated lexical-semantic analysis with sustained activation at approximately 200-800 ms. Magnetoencephalography (MEG) studies have further identified the superior temporal cortex as a main locus of the semantic effect. Nevertheless, a direct comparison of the spatiotemporal neural correlates of visual and auditory word comprehension in the same brain is lacking. We used MEG to compare lexical-semantic analysis in the visual and auditory domain in the same individuals, and contrasted it with phonological analysis that, according to models of language perception, should occur at a different time with respect to semantic analysis in reading and speech perception. The stimuli were lists of four words that were either semantically or phonologically related, or with the final word unrelated to the preceding context. Superior temporal activation reflecting semantic processing occurred similarly in the two modalities, left-lateralized at 300-450 ms and thereafter bilaterally, generated in close-by areas. Effect of phonology preceded the semantic effect in speech perception but not in reading. The present data indicate involvement of the middle superior temporal cortex in semantic processing from approximately 300 ms onwards, regardless of input modality.

Neural dynamics of reading morphologically complex words.

Despite considerable research interest, it is still an open issue as to how morphologically complex words such as "car+s" are represented and processed in the brain. We studied the neural correlates of the processing of inflected nouns in the morphologically rich Finnish language. Previous behavioral studies in Finnish have yielded a robust inflectional processing cost, i.e., inflected words are harder to recognize than otherwise matched morphologically simple words. Theoretically this effect could stem either from decomposition of inflected words into a stem and a suffix at input level and/or from subsequent recombination at the semantic-syntactic level to arrive at an interpretation of the word. To shed light on this issue, we used magnetoencephalography to reveal the time course and localization of neural effects of morphological structure and frequency of written words. Ten subjects silently read high- and low-frequency Finnish words in inflected and monomorphemic form. Morphological complexity was accompanied by stronger and longer-lasting activation of the left superior temporal cortex from 200 ms onwards. Earlier effects of morphology were not found, supporting the view that the well-established behavioral processing cost for inflected words stems from the semantic-syntactic level rather than from early decomposition. Since the effect of morphology was detected throughout the range of word frequencies employed, the majority of inflected Finnish words appears to be represented in decomposed form and only very high-frequency inflected words may acquire full-form representations.

Fasting plasma total ghrelin concentrations in monozygotic twins discordant for obesity.

Ghrelin is a hormone that is involved in the regulation of food intake. Neuronal, endocrine, and genetic factors have been shown to regulate plasma ghrelin levels; but the determinants of fasting ghrelin concentrations are not yet fully understood. The main aim was to explore the roles of adiposity and genetic differences in determining fasting plasma total ghrelin levels. We measured total ghrelin levels in a population of 23 monozygotic twin pairs discordant for obesity. In addition, 2 variants of ghrelin gene, namely, Arg51Gln and Leu72Met, were genotyped in 3 populations of monozygotic twin pairs: 23 obesity-discordant, 43 lean-concordant, and 46 obesity-concordant twin pairs. In discordant twins, lean co-twins had higher fasting plasma total ghrelin levels (950 pg/mL, SD = 328 pg/mL) than obese twins (720 pg/mL, SD = 143 pg/mL; P = .003). Arg51Gln-polymorphism of the ghrelin gene was equally distributed between the twin groups. However, there were significant differences in genotype frequencies at the Leu72Met polymorphism between the discordant and obese-concordant groups (P = .003) and between the discordant and lean-concordant groups (P = .011), but not between the 2 concordant groups. In the discordant group, there were fewer Met carriers (4%) than among the obese (17%) or the lean-concordant groups (15%). Plasma total ghrelin levels are affected by acquired obesity independent of genetic background. The Leu72 allele is particularly common among monozygotic twins discordant for obesity, suggesting that this ghrelin allele is more permissive in the regulation of energy balance. The ghrelin gene may thus play a role in the regulation of variability of body weight, such that Leu72 allele carriers are more prone to weight variability in response to environmental factors.

Sequencing analysis of ghrelin gene 5' flanking region: relations between the sequence variants, fasting plasma total ghrelin concentrations, and body mass index.

Ghrelin is a 28-amino-acid peptide with several functions linked to energy metabolism. Low ghrelin plasma concentrations are associated with obesity, hypertension, and type 2 diabetes mellitus, whereas high concentrations reflect states of negative energy balance. Several studies addressing the hormonal and neural regulation of ghrelin gene expression have been carried out, but the role of genetic factors in the regulation of ghrelin plasma levels remains unclear. To elucidate the role of genetic factors in the regulation of ghrelin expression, we screened 1657 nucleotides of the ghrelin gene 5' flanking region (promoter and possible regulatory sites) for new sequential variations from patient samples with low (n = 50) and high (n = 50) fasting plasma total ghrelin concentrations (low- and high-ghrelin groups). Eleven single nucleotide polymorphisms (SNPs), 3 of which were rare variants (allelic frequency less than 1%) were found in our population. The genotype distribution patterns of the SNPs did not differ between the study groups, except for SNP-501A>C (P = .039). In addition, the SNP-01A>C was associated with body mass index (BMI) (P = .018). This variant was studied further in our large and well-defined Oulu Project Elucidating Risk for Atherosclerosis (OPERA) cohort (n = 1045) by the restriction fragment length polymorphism (RFLP) technique. No significant association of SNP-501A>C genotypes with fasting ghrelin plasma concentrations was found in the whole OPERA population. However, the association of this SNP with BMI and with waist circumference reached statistical significance in OPERA (P = .047 and .049, respectively), remaining of borderline significance for BMI after adjustments (P = .055). The results indicate that factors other than the 11 SNPs found in this study in the 5' flanking region of ghrelin gene are the main determinants of ghrelin plasma levels. However, SNP-501 A>C genotype distribution seems to be different in subjects having the highest compared with those with the lowest ghrelin levels, and the SNP may be associated with BMI and waist circumference.

Sequencing analysis of the ghrelin receptor (growth hormone secretagogue receptor type 1a) gene.

OBJECTIVES AND DESIGN: Ghrelin is a novel 28 amino acid peptide which is reported to have several endocrine and non-endocrine actions. It possesses strong growth hormone (GH)-releasing activity, which is mediated via the GH secretagogue receptor type 1a (GHS-R1a). We hypothesised that there might be functional sequential variations in the GHS-R1a gene affecting phenotypes linked to the GH/insulin-like growth factor-I (IGF-I)-axis. METHODS: To test our hypothesis we chose patients from our OPERA (Oulu Project Elucidating Risk of Atherosclerosis) study with low (n=96) and high (n=96) IGF-I levels, sequenced their GHS-R1a gene exons and performed association studies. RESULTS: We found five single-nucleotide polymorphisms (SNPs) which did not change the amino acid sequence. We were unable to detect associations between the SNPs and the IGF-I plasma concentrations, but instead we showed that SNP 171C>T was associated with the values of the area under the insulin curve (AUCIN) in an oral glucose tolerance test and with IGF-binding protein-1 (IGFBP-1) concentrations (P<0.05). SNP 477G>A was associated with the low density lipoprotein and very low density lipoprotein cholesterol plasma levels and AUCIN values (P<0.05). CONCLUSIONS: This study was the first genomic screening of the GHS-R1a gene in a population. It suggests that genetic variations in the GHS-R1a gene are not the main regulators of IGF-I levels. However, the variants may be associated with IGFBP-1 concentrations and insulin metabolism.