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Searching for a baseline: functional imaging and the resting human brain.: Nat Rev Neurosci, Vol. 2, No. 10. (October 2001), pp. 685-694.Functi onal brain imaging in humans has revealed task-specific increases in brain activity that are associated with various mental activities. In the same studies, mysterious, task-independe nt decreases have also frequently been encountered, especially when the tasks of interest have been compared with a passive state, such as simple fixation or eyes closed. These decreases have raised the possibility that there might be a baseline or resting state of brain function involving a specific set of mental operations. We explore this possibility, including the manner in which we might define a baseline and the implications of such a baseline for our understanding of brain function.DA Gusnard, ME Raichle, ME Raichle

Source: Nat Rev Neurosci, Vol. 2, No. 10. (October 2001), pp. 685-694.
Baseline brain activity fluctuations predict somatosensory perception in humans.: Proc Natl Acad Sci U S A, Vol. 104, No. 29. (17 July 2007), pp. 12187-12192.In perceptual experiments, within-individ ual fluctuations in perception are observed across multiple presentations of the same stimuli, a phenomenon that remains only partially understood. Here, by means of thulium-yttriu m/aluminum-gar net laser and event-related functional MRI, we tested whether variability in perception of identical stimuli relates to differences in prestimulus, baseline brain activity. Results indicate a positive relationship between conscious perception of low-intensity somatosensory stimuli and immediately preceding levels of baseline activity in medial thalamus and the lateral frontoparietal network, respectively, which are thought to relate to vigilance and "external monitoring." Conversely, there was a negative correlation between subsequent reporting of conscious perception and baseline activity in a set of regions encompassing posterior cingulate/prec uneus and temporoparieta l cortices, possibly relating to introspection and self-oriented processes. At nociceptive levels of stimulation, pain-intensity ratings positively correlated with baseline fluctuations in anterior cingulate cortex in an area known to be involved in the affective dimension of pain. These results suggest that baseline brain-activity fluctuations may profoundly modify our conscious perception of the external world.M Boly, E Balteau, C Schnakers, C Degueldre, G Moonen, A Luxen, C Phillips, P Peigneux, P Maquet, S Laureys

Source: Proc Natl Acad Sci U S A, Vol. 104, No. 29. (17 July 2007), pp. 12187-12192.
The effect of respiration variations on independent component analysis results of resting state functional connectivity.: Human brain mapping (25 April 2008)The analysis of functional connectivity in fMRI can be severely affected by cardiac and respiratory fluctuations. While some of these artifactual signal changes can be reduced by physiological noise correction routines, signal fluctuations induced by slower breath-to-brea th changes in the depth and rate of breathing are typically not removed. These slower respiration-in duced signal changes occur at low frequencies and spatial locations similar to the fluctuations used to infer functional connectivity, and have been shown to significantly affect seed-ROI or seed-voxel based functional connectivity analysis, particularly in the default mode network. In this study, we investigate the effect of respiration variations on functional connectivity maps derived from independent component analysis (ICA) of resting-state data. Regions of the default mode network were identified by deactivations during a lexical decision task. Variations in respiration were measured independently and correlated with the MRI time series data. ICA appears to separate the default mode network and the respiration-re lated changes in most cases. In some cases, however, the component automatically identified as the default mode network was the same as the component identified as respiration-re lated. Furthermore, in most cases the time series associated with the default mode network component was still significantly correlated with changes in respiration volume per time, suggesting that current methods of ICA may not completely separate respiration from the default mode network. An independent measure of the respiration provides valuable information to help distinguish the default mode network from respiration-re lated signal changes, and to assess the degree of residual respiration related effects. Hum Brain Mapp 2008. (c) 2008 Wiley-Liss, Inc.Rasmus M Birn, Kevin Murphy, Peter A Bandettini

Source: Human brain mapping (25 April 2008)
Functional connectivity in a baseline resting-state network in autism.: Neuroreport, Vol. 17, No. 16. (6 November 2006), pp. 1687-1690.Brai n activity in people with high-functioni ng autism has been shown to be atypical in a number of ways, including reduced synchronizatio n across areas of activation measured by functional magnetic resonance imaging. This activation atypicality has been observed mostly during the performance of cognitive tasks. This study compares the resting-state network of 57 participants with autism and 57 control participants matched for age and intelligence quotient. The results indicate that both groups have a resting-state network that is very similar both in volume and in organization, but in autism this network is much more loosely connected. This functional underconnectiv ity was observed in the anterior-poste rior connections. The results expand the theory of cortical underconnectiv ity in autism to the resting state of the brain.VL Cherkassky, RK Kana, TA Keller, MA Just

Source: Neuroreport, Vol. 17, No. 16. (6 November 2006), pp. 1687-1690.
How to see what you are looking for in fMRI and PET - or the crucial baseline condition.: J Neurol (18 May 2006)The identification of a baseline or control state is fundamental for the interpretation of task- or stimulation-in duced brain activation patterns. The conscious resting state in darkness is a frequently used, but ill-defined mental state. The mere transition from, for example, lid closed to lid open in darkness causes major changes in brain activity,which can mask or mimic a stimulus-depen dent brain activation. Contradictory results of seemingly identical brain activation studies may be attributed to the choice of different baseline conditions. Therefore, control conditions that are closest to the stimulus or task condition should be used as baseline in most fMRI and PET studies rather than absolute relaxation in darkness and silence (REST).Th Brandt

Source: J Neurol (18 May 2006)
Whole brain analysis of T2* weighted baseline FMRI signal in dementia.: Hum Brain Mapp (8 February 2007)Brain activation in studies using blood oxygenation level dependent (BOLD) FMRI is associated with an increase in T2* weighted signal between baseline and an active condition. This BOLD technique is often applied to study differences in brain activation between patients and healthy controls. However, the baseline T2* signal itself may also be different between groups, as shown in the hippocampus in Alzheimer's disease using the resting oxygen or ROXY approach (Small et al. [2002]: Ann Neurol 51:290-295). In the current study, we analyzed whole brain, voxel-wise T2* weighted signal of averaged baseline scans of a BOLD FMRI experiment in 41 healthy elderly controls and 46 patients with mild cognitive impairment or Alzheimer's disease. In each subject, T2* weighted images were normalized to the CSF signal of the same image. Additionally, gray matter probability maps of high-resolutio n structural scans were also compared between groups to assess atrophy. T2* signal was decreased in dementia in the hippocampus, insula/putamen , posterior and middle cingulate cortex, and parietal cortex. Most of these regions also showed decreased gray matter, except insula/putamen . Hippocampal and posterior cingulate gray matter differences were significantly larger than T2* differences. Therefore, decreased T2* signal in most regions are likely to be caused by gray matter atrophy, although decreased metabolism or perhaps iron deposition are also factors that may contribute. We conclude that in FMRI studies of dementia, not only the dynamic BOLD signal (activation and deactivation) but also the average baseline signal is diminished in certain regions. The method we applied may also be used in task-related BOLD FMRI and add to the understanding of the mechanism of task-related group differences. Hum Brain Mapp, 2007. (c) 2007 Wiley-Liss, Inc.Serge A R B Rombouts, Philip Scheltens, Joost P A Kuijer, Frederik Barkhof

Source: Hum Brain Mapp (8 February 2007)
Adaptive baseline wander removal in the ECG: Comparative analysis with cubic spline technique: Computers in Cardiology 1992. Proceedings. (1992), pp. 143-146.Baseli ne wandering is a classical problem in electrocardiog ram (ECG) records that generally produces artifactual data when measuring ECG parameters. The authors present a cascade adaptive filter for removing the baseline wander and preserving the low-frequency components of the ECG. This cascade adaptive filter works in two stages. The first stage is an adaptive notch filter at zero frequency. The second stage is an adaptive impulse correlated filter that, using a QRS detector, estimates the ECG signal correlated with the QRS occurrence. In this way, all the signal components correlated with the QRS complex are preserved. The authors analyze the frequency response of the filter, showing that the filter can be seen as a comb filter without the DC lobe. The method was applied to ECG signals from the MIT-BIH database and its performance was compared with the cubic spline approach. The method can remove baseline wander in real time without needing to calculate the isoelectric levels, while preserving the low-frequency ECG clinical informationR Jane, P Laguna, NV Thakor, P Caminal

Source: Computers in Cardiology 1992. Proceedings. (1992), pp. 143-146.
Energetics of neuronal signaling and fMRI activity: Proceedings of the National Academy of Sciences, Vol. 104, No. 51. (18 December 2007), pp. 20546-20551.En ergetics of resting and evoked fMRI signals were related to localized ensemble firing rates (nu) measured by electrophysiol ogy in rats. Two different unstimulated, or baseline, states were established by anesthesia. Halothane and alpha-chloralo se established baseline states of high and low energy, respectively, in which forepaw stimulation excited the contralateral primary somatosensory cortex (S1). With alpha-chloralo se, forepaw stimulation induced strong and reproducible fMRI activations in the contralateral S1, where the ensemble firing was dominated by slow signaling neurons (SSN; nu range of 113 Hz). Under halothane, weaker and less reproducible fMRI activations were observed in the contralateral S1 and elsewhere in the cortex, but ensemble activity in S1 was dominated by rapid signaling neurons (RSN; nu range of 1340 Hz). For both baseline states, the RSN activity (i.e., higher frequencies, including the gamma band) did not vary upon stimulation, whereas the SSN activity (i.e., alpha band and lower frequencies) did change. In the high energy baseline state, a large majority of total oxidative energy [cerebral metabolic rate of oxygen consumption (CMRO2)] was devoted to RSN activity, whereas in the low energy baseline state, it was roughly divided between SSN and RSN activities. We hypothesize that in the high energy baseline state, the evoked changes in fMRI activation in areas beyond S1 are supported by rich intracortical interactions represented by RSN. We discuss implications for interpreting fMRI data where stimulus-speci fic DeltaCMRO2 is generally small compared with baseline CMRO2. 10.1073/pnas.0 709515104Natas ja Maandag, Daniel Coman, Basavaraju Sanganahalli, Peter Herman, Arien Smith, Hal Blumenfeld, Robert Shulman, Fahmeed Hyder

Source: Proceedings of the National Academy of Sciences, Vol. 104, No. 51. (18 December 2007), pp. 20546-20551.
Minds at rest? Social cognition as the default mode of cognizing and its putative relationship to the "default system" of the brain.: Consciousness and cognition (21 April 2008)The "default system" of the brain has been described as a set of regions which are 'activated' during rest and 'deactivated' during cognitively effortful tasks. To investigate the reliability of task-related deactivations, we performed a meta-analysis across 12 fMRI studies. Our results replicate previous findings by implicating medial frontal and parietal brain regions as part of the "default system". However, the cognitive correlates of these deactivations remain unclear. In light of the importance of social cognitive abilities for human beings and their propensity to engage in such activities, we relate our results to findings from neuroimaging studies of social cognition. This demonstrates a remarkable overlap between the brain regions typically involved in social cognitive processes and the "default system". We, henceforth, suggest that the physiological 'baseline' of the brain is intimately linked to a psychological 'baseline': human beings have a predisposition for social cognition as the default mode of cognizing which is implemented in the robust pattern of intrinsic brain activity known as the "default system".L Schilbach, S B Eickhoff, A Rotarska-Jagie la, G R Fink, K Vogeley

Source: Consciousness and cognition (21 April 2008)
Baseline resistance cancellation circuit for high resolution thiolate-monol ayer-protected gold nanoparticle vapor sensor arrays: Circuits and Systems, 2008. ISCAS 2008. IEEE International Symposium on (2008), pp. 2002-2005.Chem iresistive (CR) sensors and sensor arrays coated with thiolate-monol ayer-protected gold nanoparticle (MPN) interfaces show great promise for high-sensitivi ty multi-vapor analysis but suffer from process variation and drift in baseline values. This paper describes a new readout circuit that cancels baseline resistance and compensates for baseline drift to achieve ppm resolution. Requiring only 5100 mum in a 0.5 mum CMOS process, the circuit is well suited for high density on-chip CR sensor arrays. The resulting CR array microsystem introduces a valuable tool for monitoring environmental hazards including explosive compounds.D Rairigh, A Mason, MP Rowe, ET Zellers

Source: Circuits and Systems, 2008. ISCAS 2008. IEEE International Symposium on (2008), pp. 2002-2005.

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