Background and Objectives
This study aimed at investigating the feasibility of functional near‐infrared spectroscopy (fNIRS) to measure changes in cerebral hemodynamics and oxygenation evoked by painful and nonpainful mechanosensory stimulation on the lower back. The main objectives were to investigate whether cortical activity can be (1) detected using functional fNIRS, and (2) if it is possible to distinguish between painful and nonpainful pressure as well as a tactile brushing stimulus based on relative changes in oxy‐ and deoxyhemoglobin ([O2Hb] and [HHb]).
Twenty right‐handed subjects (33.5 ± 10.7 years; range 20–61 years; 8 women) participated in the study. Painful and nonpainful pressure stimulation was exerted with a thumb grip perpendicularly to the spinous process of the lumbar spine. Tactile stimulation was realized by a one‐finger brushing. The supplementary motor area (SMA) and primary somatosensory cortex (S1) were measured bilaterally using a multichannel continuous‐wave fNIRS imaging system.
Characteristic relative changes in [O2Hb] in the SMA and S1 after both pressure stimulations (corrected for multiple comparison) were observed. [HHb] showed only much weaker changes (uncorrected). The brushing stimulus did not reveal any significant changes in [O2Hb] or [HHb].
The results indicate that fNIRS is sensitive enough to detect varying hemodynamic responses to different types of mechanosensory stimulation. The acquired data will serve as a foundation for further investigations in patients with chronic lower back pain. The future aim is to disentangle possible maladaptive neuroplastic changes in sensorimotor areas during painful and nonpainful lower back stimulations based on fNIRS neuroimaging.
back pain; functional near‐infrared spectroscopy; hemodynamic response; sensorimotor cortex
The Forced‐choice Graphics Memory Test (FGMT) is a newly developed measure to assess feigned cognitive impairment. This study investigated the ability and reliability of FGMT for identification of malingering in patients with traumatic brain injury (TBI).
The FGMT was administered to 40 healthy volunteers instructed to respond validly (Healthy Control, H‐C), 40 healthy volunteers instructed to feign cognitive impairment (Healthy Malingering, H‐M), 40 severe TBI patients who responded validly (TBI control, TBI‐C), and 30 severe TBI patients who evidenced invalid performance (TBI malingering, TBI‐M).
Both malingering groups (H‐M and TBI‐M) performed much more poorly than the nonmalingering groups (H‐C and TBI‐C). The FGMT overall total score, score on easy items, and score on hard items differed significantly across the four groups. The total score showed the highest classification accuracy in differentiating malingering from nonmalingering. A cutoff of less than 18 (total items) successfully identified 95% of TBI‐C and 93.3% of TBI‐M participants. The FGMT also demonstrated high test–retest reliability and internal consistency. FGMT scores were not affected by TBI patients' education, gender, age, or intelligence.
Our results suggest that the FGMT can be used as a fast and reliable tool for identification of feigned cognitive impairment in patients with TBI.
malingering; performance validity; response bias
The aim of the present study was to investigate the prevalence and pattern of cognitive dysfunction observed in primary Sjögren's syndrome (PSS) and to examine the relationships between cognitive abilities, depression, fatigue, and quality of life.
Materials and Methods
Thirty‐two subjects with PSS were compared with 19 healthy controls on comprehensive neuropsychological, depression, fatigue, health state, and daily‐life activities tests.
There was low performance in Clock Drawing, COWAT, Paced Auditory Serial Addition Test (PASAT), Colorless Word Reading (Stroop1) and Recognizing Colors (Stroop2) Patterns of STROOP test, SDLT, Auditory–Verbal Learning Test (AVLT), immediate and long‐term verbal memory, Benton Judgment of Line Orientation Test (BJLOT), and in all the patterns of RCFT in PSS patients compared to the healthy control group (p < .05). It was observed an increased depression frequency and fatigue severity, impairment in health condition, and a decreased quality of life in PSS cases compared to the healthy controls (p < .05). All the depression, fatigue severity, and quality of life tests showed a significant positive correlation with each other (p < .05). A significant negative correlation between Clock Drawing and SF‐36‐BP (p = .031, r = −.382) and SF‐36‐GH (p = .027, r = −.392) was observed.
Clock Drawing, PASAT, and AVLT are very useful tests to determine the subclinical and clinical cognitive dysfunction to evaluate attention, information processing speed, executive functions, and short‐term and long‐term verbal memory in PSS patients. Depression and fatigue may not affect the neuropsychological tests performance.
cognitive dysfunction; daily‐life activities; depression; fatigue; primer Sjögren's syndrome
Progressive supranuclear palsy (PSP) is a neurodegenerative disorder that is sometimes confused with Parkinson's disease, multiple system atrophy, and other disorders. The typical clinical features are categorized as Richardson's syndrome (RS), but other clinical subtypes include PSP‐parkinsonism (PSP‐P) and PSP‐pure akinesia with gait freezing (PSP‐PAGF). In this study, we determined the prevalence of PSP in a Japanese rural area compared to our previous 1999 report.
We collected data in Yonago City from 2009 to 2014 using a service‐based study of PSP. We collected case history data from PSP patients in the area from our hospital. The crude prevalence and 95% confidence interval (CI) were calculated using the population demographics on the prevalence day of 1 October 2010. Age‐ and sex‐adjusted prevalence was calculated by direct standardization to the population demographics in Yonago City on the prevalence day of 1 April 1999.
Material and Results
We identified 25 patients: 16 with probable RS, 4 with possible RS, 3 with clinical PSP‐P, and 2 with clinical PSP‐PAGF. The prevalence per 100,000 was 17.90 (male = 18.05; female = 17.76). The prevalence of PSP in Yonago in 2010 increased compared to the measurements from 1999.
The prevalence of PSP in Japan increased from 1999 to 2010.
epidemiology; PSP‐parkinsonism; PSP‐pure akinesia with gait freezing; Richardson's syndrome; tauopathy
Correlated low‐frequency fluctuations of resting‐state functional magnetic resonance imaging (rsfMRI) signals have been widely used for inferring intrinsic brain functional connectivity (FC). In animal studies, accurate estimate of anesthetic effects on rsfMRI signals is demanded for reliable interpretations of FC changes. We have previously shown that inter‐regional FC can reliably delineate local millimeter‐scale circuits within digit representations of primary somatosensory cortex (S1) subregions (areas 3a, 3b, and 1) in monkeys under isoflurane anesthesia. The goals of this study are to determine (1) the general effects of isoflurane on rsfMRI signals in the S1 circuit and (2) whether the effects are functional‐ and regional‐ dependent, by quantifying the relationships between isoflurane levels, power and inter‐regional correlation coefficients in digit and face regions of distinct S1 subregions.
Functional MRI data were collected from male adult squirrel monkeys at three different isoflurane levels (1.25%, 0.875%, and 0.5%). All scans were acquired on a 9.4T magnet with a 3‐cm‐diameter surface transmit‐receive coil centered over the S1 cortex. Power and seed‐based inter‐regional functional connectivity analyses were subsequently performed.
As anesthesia level increased, we observed (1) diminishing amplitudes of signal fluctuations, (2) reduced power of fluctuations in the low‐frequency band used for connectivity measurements, (3) decreased inter‐voxel connectivity around seed regions, and (4) weakened inter‐regional FC across all pairs of regions of interest (digit‐to‐digit). The low‐frequency power measures derived from rsfMRI signals from control muscle regions, however, did not exhibit any isoflurane level‐related changes. Within the isoflurane dosage range we tested, the inter‐regional functional connectivity differences were still detectable, and the effects of isoflurane did not differ across region‐of‐interest (ROI) pairs.
Our data demonstrate that isoflurane induced similar dose‐dependent suppressive effects on the power of rsfMRI signals and local fine‐scale FC across functionally related but distinct S1 subregions.
fMRI; hand; non‐human primates; resting state; somatosensory system
It remains a surgical challenge to treat high‐grade nerve injuries of the upper extremity. Extra‐anatomic reconstructions through the transfer of peripheral nerves have gained clinical importance over the past decades. This contribution outlines the anatomic and histomorphometric basis for the transfer of the superficial branch of the radial nerve (SBRN) to the median nerve (MN) and the superficial branch of the ulnar nerve (SBUN).
The SBRN, MN, and SBUN were identified in 15 specimens and the nerve transfer performed. A favorable site for coaptation was chosen and its location described using relevant anatomical landmarks. Histomorphometric characteristics of donor and target were compared to evaluate the chances of a clinical success.
A suitable location for dissecting the SBRN was identified prior to its first bifurcation. Coaptations were possible near the pronator quadratus muscle, approximately 22 cm distal to the lateral epicondyle of the humerus. The MN and SBUN had to be dissected interfasciculary over 82 ± 5.7 mm and 49 ± 5.5 mm, respectively. Histomorphometric analysis revealed sufficient donor‐to‐recipient axon ratios for both transfers and identified the SBRN as a suitable donor with high axon density.
Our anatomic and histomorphometric results indicate that the SBRN is a suitable donor for the MN and SBUN at wrist level. The measurements show feasibility of this procedure and shall help in planning this sensory nerve transfer. High axon density in the SBRN identifies it or its branches an ideal candidate for sensory reanimation of fingers and thumbs.
hand trauma; histomorphometric nerve analysis; nerve injury; nerve reconstruction; peripheral nerve transfer; reinnervation
In recent years, machine‐learning techniques have gained growing popularity in medical image analysis. Temporal brain‐state classification is one of the major applications of machine‐learning techniques in functional magnetic resonance imaging (fMRI) brain data. This article explores the use of support vector machine (SVM) classification technique with motor‐visual activation paradigm to perform brain‐state classification into activation and rest with an emphasis on different acquisition techniques.
Images were acquired using a recently developed variant of traditional pseudocontinuous arterial spin labeling technique called arterial volume‐weighted arterial spin tagging (AVAST). The classification scheme is also performed on images acquired using blood oxygenation–level dependent (BOLD) and traditional perfusion‐weighted arterial spin labeling (ASL) techniques for comparison.
The AVAST technique outperforms traditional pseudocontinuous ASL, achieving classification accuracy comparable to that of BOLD contrast images.
This study demonstrates that AVAST has superior signal‐to‐noise ratio and improved temporal resolution as compared with traditional perfusion‐weighted ASL and reduced sensitivity to scanner drift as compared with BOLD. Owing to these characteristics, AVAST lends itself as an ideal choice for dynamic fMRI and real‐time neurofeedback experiments with sustained activation periods.
arterial cerebral blood volume; arterial spin labeling; machine learning; support vector machines
Background and Purpose
Clinically, Parkinson's disease (PD) presents with asymmetric motor symptoms. The left nigrostriatal system appears more susceptible to early degeneration than the right, and a left‐lateralized pattern of early neuropathological changes is also described in several neurodegenerative conditions, including Alzheimer's disease, frontotemporal dementia, and Huntington's disease. In this study, we evaluated hemispheric differences in estimated rates of atrophy in a large, well‐characterized cohort of PD patients.
Our cohort included 205 PD patients who underwent clinical assessments and T1‐weighted brain MRI's. Patients were classified into Early (n = 109) and Late stage (n = 96) based on disease duration, defined as greater than or less than 10 years of motor symptoms. Cortical thickness was determined using FreeSurfer, and a bootstrapped linear regression model was used to estimate differences in rates of atrophy between Early and Late patients.
Our results show that patients classified as Early stage exhibit a greater estimated rate of cortical atrophy in left frontal regions, especially the left insula and olfactory sulcus. This pattern was replicated in left‐handed patients, and was not influenced by the degree of motor symptom asymmetry (i.e., left‐sided predominant motor symptoms). Patients classified as Late stage exhibited greater atrophy in the bilateral occipital, and right hemisphere‐predominant cortical areas.
We show that cortical degeneration in PD differs between cerebral hemispheres, and findings suggest a pattern of early left, and late right hemisphere with posterior cortical atrophy. Further investigation is warranted to elucidate the underlying mechanisms of this asymmetry and pathologic implications.
cortex; left hemisphere; MRI; neurodegeneration; Parkinson's disease
Behavioral undercontrol is a well‐established risk factor for substance use disorder, identifiable at an early age well before the onset of substance use. However, the biological mechanistic structure underlying the behavioral undercontrol/substance use relationship is not well understood. The enzyme catechol O‐methyltransferase (COMT) catabolizes dopamine and norepinephrine in the prefrontal cortex and striatum, brain regions involved in behavioral control. The goal of this work was to investigate the association between genetic variation in COMT functioning and fronto‐striatal brain functioning during successful inhibitory control, a critical aspect of behavioral control.
Participants were 65 (22 female) 7–12 year olds who were genotyped for the functional COMT Val158Met (rs4680) single‐nucleotide polymorphism and underwent functional magnetic resonance imaging while performing a go/no‐go task. The majority of the sample (80%) had at least one parent with a history of alcohol use disorder and were thus at heightened risk for substance use disorders.
There was a significant main effect of genotype on brain activation in left and right putamen during successful versus failed inhibition and in right inferior frontal gyrus/insula during successful inhibition versus baseline. Follow‐up tests revealed that Met homozygotes had greater activation in each region relative to Val homozygotes.
These results are relevant for understanding how specific genes influence brain functioning related to underlying risk factors for substance use disorders and other disinhibitory psychopathologies.
catechol O‐methyltransferase; children; dopamine; functional magnetic resonance imaging; go/no‐go; inferior frontal gyrus; insula; putamen; response inhibition; substance use disorder risk
Disturbed phosphate homeostasis in early Parkinson′s disease (PD) may originate from a stress‐related condition and nutritional status among other risk factors, age, and gender.
Risk of malnutrition using Mini‐nutritional assessment (MNA score) and plasma levels of protein markers and daytime cortisol at the time of diagnosis in PD (n = 75) were compared with a control group (n = 24). Cognition was assessed using the Mini‐Mental State Examination (MMSE score) and motor function using Unified Parkinson′s Disease Rating Scale (UPDRS‐part III scale).
The patients with PD had significantly lower MNA score than controls which correlated with plasma phosphate levels. The logistic regression revealed that increasing MNA protected from low plasma phosphate, final score (OR = 0.399; 95% CI = 0.196–0.816; p = .012) and total score (OR = 0.656; 95% CI = 0.422–1.018; p = .060). Phosphate correlated with albumin (r = .315; p < .006), transferrin (r = .331; p < .004) and cortisol (r = −0.355; p < .002) confirmed by logistic regressions. Increasing albumin protects from low phosphate after adjusting in logistic regression (OR = 0.806; 95% CI = 0.682–0.952; p = .011) and after including variables from Table 1 in backwards elimination, final step (OR = 0.800; 95% CI = 0.660–0.969; p = .022). MNA total score and cortisol correlated inversely, confirmed in logistic regression for MNA total score (OR = 0.786; 95% CI = 0.627–0.985; p = .037) and for MNA initial score (OR = 0.650; 95% CI = 0.453–0.930; p = .020).
This study highlights the importance of phosphate for optimal nutritional status by association with MNA score and albumin in plasma. An inverse relationship between phosphate and cortisol indicate, in addition, that low phosphate levels may affect cognition and motor function in PD.
albumin; cognition; cortisol; Mini‐Nutritional Assessment; Parkinson's disease; phosphate; stress; transferrin
Navigation is a fundamental and multidimensional cognitive function that individuals rely on to move around the environment. In this study, we investigated the neural basis of human spatial navigation ability.
A large cohort of participants (N > 200) was examined on their navigation ability behaviorally and structural and functional magnetic resonance imaging (MRI) were then used to explore the corresponding neural basis of spatial navigation.
The gray matter volume (GMV) of the bilateral parahippocampus (PHG), retrosplenial complex (RSC), entorhinal cortex (EC), hippocampus (HPC), and thalamus (THAL) was correlated with the participants’ self‐reported navigational ability in general, and their sense of direction in particular. Further fMRI studies showed that the PHG, RSC, and EC selectively responded to visually presented scenes, whereas the HPC and THAL showed no selectivity, suggesting a functional division of labor among these regions in spatial navigation. The resting‐state functional connectivity analysis further revealed a hierarchical neural network for navigation constituted by these regions, which can be further categorized into three relatively independent components (i.e., scene recognition component, cognitive map component, and the component of heading direction for locomotion, respectively).
Our study combined multi‐modality imaging data to illustrate that multiple brain regions may work collaboratively to extract, integrate, store, and orientate spatial information to guide navigation behaviors.
cognitive map; heading direction; hierarchical brain network; scene perception; spatial navigation; voxel‐based morphometry
Although mild traumatic brain injury (mTBI) comprises 80% of all TBI, the morphological examination of the orbitofrontal cortex (OFC) in relation to clinical symptoms such as aggression, anxiety and depression in a strictly mTBI sample has never before been performed.
The primary objective of the study was to determine if mTBI patients would show morphological differences in the OFC and if the morphology of this region would relate to clinical symptoms.
Using structural images acquired in a 3T MRI machine, the cortical thickness and cortical volume (corrected for total brain volume) of the OFC was collected for healthy control (N = 27) subjects and chronic mTBI (N = 55) patients at least one year post injury. Also, during clinical interviews, measures quantifying the severity of clinical symptoms, including aggression, anxiety, and depression, were collected.
MTBI subjects displayed increased aggression, anxiety, and depression, and anxiety and depression measures showed a relationship with the number of mTBI in which the subject lost consciousness. The cortical thickness of the right lateral OFC displayed evidence of thinning in the mTBI group; however, after correction for multiple comparisons, this difference was no longer significant. Clinical measures were not significantly related with OFC morphometry.
This study found increased aggression, anxiety, and depression, in the mTBI group as well as evidence of cortical thinning in the right lateral OFC. The association between clinical symptoms and the number of mTBI with loss of consciousness suggests the number and severity of mTBI may influence clinical symptoms long after injury. Future studies examining other brain regions involved in the production and regulation of affective processes and inclusion of subjects with well‐characterized mood disorders could further elucidate the relationship between mTBI, brain morphology, and clinical symptoms.
aggression; mild traumatic brain injury; morphometry; orbitofrontal cortex
Recent evidence suggests that attention‐deficit hyperactivity disorder (ADHD) is associated with brain functional connectivity (FC) abnormalities.
In this study, we use steady‐state visually evoked potential event‐related partial coherence as a measure of brain FC to examine functional connectivity differences between a typically developing (TD) group of 25 boys and an age/IQ‐matched group of 42 drug naive boys newly diagnosed with ADHD (ADHD group). Functional connectivity was estimated while both groups performed a low‐demand reference task and the A‐X version of the continuous performance task (CPT A‐X).
While the TD and ADHD groups exhibited similar prefrontal FC increases prior to the appearance of the target in the reference task, these groups demonstrated significant FC differences in the interval preceding the appearance of the target in the CPT A‐X task. Specifically, the ADHD group exhibited robust prefrontal and parieto‐frontal FC increases that were not apparent in the TD group.
The FC differences observed in the ADHD group are discussed in the context of inadequate suppression of cortical networks that may interfere with task performance.
brain functional connectivity; default mode network; steady state visually evoked potential
Recent evidence suggests that attention deficit hyperactivity disorder (ADHD) is associated with a range of brain functional connectivity abnormalities, with one of the most prominent being reduced inhibition of the default mode network (DMN) while performing a cognitive task. In this study, we examine the effects of a methylphenidate dose on brain functional connectivity in boys diagnosed with ADHD while they performed a cognitive task.
Brain functional connectivity was estimated using steady‐state visual evoked potential partial coherence before and 90 min after the administration of a methylphenidate dose to 42 stimulant drug‐naïve boys newly diagnosed with ADHD while they performed the A‐X version of the continuous performance task (CPT A‐X).
Methylphenidate robustly reversed the transient functional connectivity increase in the A‐X interval seen premedication to a postmedication decrease during this interval. In addition, methylphenidate‐induced reductions in individual reaction time were correlated with corresponding reductions in functional connectivity.
These findings suggest that methylphenidate suppresses the increased functional connectivity observed in ADHD and that such suppression is associated with improved performance. Our findings support the suggestion that the increased functional connectivity we have observed in ADHD is associated with abnormal DMN activity. In addition, we comment on the significance of specific frequency channels mediating top‐down communication within the cortex and the extent to which our findings are selectively sensitive to top‐down intracortical communication.
brain functional connectivity; default mode network; dopamine; methylphenidate; steady‐state visually evoked potential
The association between multiple sclerosis (MS) and hereditary and sporadic demyelinating disorders of the peripheral nervous system is extremely rare. We herein report a case of Charcot‐Marie‐Tooth disease type 1B with p.Val102fs mutation in the MPZ gene that developed relapsing remitting MS.
Tonic immobility (TI) is a state of physical immobility associated with extreme stress and the development of posttraumatic stress disorder (PTSD). However, it is unknown whether TI is associated with a distinct actual stress response, i.e., objective immobility measured by a stabilometric platform. This study made a first step in exploring this as well as differences in body sway responses between PTSD patients and healthy controls. We hypothesized that PTSD would be related to increased body sway under stress, whereas TI would be related to decreased body sway under stress.
Eye closure was selected as a PTSD‐relevant stress induction procedure. Body sway and heart rate (HR) were measured in 12 PTSD patients and 12 healthy controls in four conditions: (1) maintaining a stable stance with eyes open, (2) with eyes closed, (3) during a mental arithmetic task with eyes open, and (4) with eyes closed.
As predicted, PTSD patients showed increased body sway from eyes open to eyes closed compared to controls and this effect was eliminated by executing the arithmetic task. Most importantly, retrospective self‐reported TI was associated with lower body sway increases in PTSD and higher body sway decreases in controls from eyes‐open to eyes‐closed conditions.
These preliminary findings suggest that eye closure has a different effect on PTSD patients than controls and that high self‐reported TI might indicate a distinct stress response pattern, i.e., a proneness for immobility. It may be relevant to take such individual differences in stress‐response into account in PTSD treatment.
behavioral activation; life events; PTSD; stress; trauma
Previous prospective studies on ischemic stroke patients have shown conflicting results concerning the association between cholesterol level and patient outcome. We aimed to investigate the relation between cholesterol level and prior ischemic stroke. We hypothesized that acute ischemic stroke patients with increased cholesterol on admission more frequently had experienced prior ischemic stroke.
All consecutive patients with acute ischemic stroke (the index stroke) admitted to the Stroke Unit, Department of Neurology, Haukeland University Hospital between February 2006 and October 2013 were prospectively registered in The Bergen NORSTROKE Registry. On admission, cholesterol, low‐density lipoprotein, and high‐density lipoprotein levels were measured and prior ischemic stroke, risk factors, and medication were registered. Patients with prior versus no prior ischemic stroke were compared regarding risk factors, cholesterol levels, and use of statins on admission for the index stroke. Only patients with available cholesterol values measured on admission were included in the analyses.
Of the 2,514 included patients admitted with acute ischemic stroke, 429 (17%) patients had prior ischemic stroke. We found a U‐curve relationship between the relative frequency of prior ischemic stroke and cholesterol level. Lower frequency of prior ischemic stroke was associated with high cholesterol level on admission up to 5.5 mmol/L. For cholesterol levels higher than this, the opposite was true. These associations included all patients and statin‐naive patients. For patients using statin there was a declining relative frequency of prior ischemic stroke from low to high cholesterol levels.
Our hypothesis was falsified. The association between lower cholesterol levels and higher frequency of prior ischemic stroke in patients with cholesterol <5.5 mmol/L cannot be solely an effect of aggressive statin treatment in patients with prior ischemic stroke, as the association pertained also to patients who did not use statin.
cholesterol; ischemic stroke; reverse epidemiology; statin; stroke recurrence; vascular risk factors
The purpose of this study was to investigate the local spatiotemporal consistency of spontaneous brain activity in patients with frontal lobe epilepsy (FLE).
Eyes closed resting‐state functional magnetic resonance imaging (fMRI) data were collected from 19 FLE patients and 19 age‐ and gender‐matched healthy controls. A novel measure, named FOur‐dimensional (spatiotemporal) Consistency of local neural Activities (FOCA) was used to assess the spatiotemporal consistency of local spontaneous activity (emphasizing both local temporal homogeneity and regional stability of brain activity states). Then, two‐sample t test was performed to detect the FOCA differences between two groups. Partial correlations between the FOCA values and durations of epilepsy were further analyzed.
Compared with controls, FLE patients demonstrated increased FOCA in distant brain regions including the frontal and parietal cortices, as well as the basal ganglia. The decreased FOCA was located in the temporal cortex, posterior default model regions, and cerebellum. In addition, the FOCA measure was linked to the duration of epilepsy in basal ganglia.
Our study suggested that alterations of local spontaneous activity in frontoparietal cortex and basal ganglia was associated with the pathophysiology of FLE; and the abnormality in frontal and default model regions might account for the potential cognitive impairment in FLE. We also presumed that the FOCA measure had potential to provide important insights into understanding epilepsy such as FLE.
FOur‐dimensional (spatiotemporal) Consistency of local neural Activities; frontal lobe epilepsy; functional magnetic resonance imaging; resting‐state