Somatotopic organization of the processing of muscle and cutaneous pain in the left and right insula cortex: a single-trial fMRI study

Loss of long-term depression in the insular cortex after tail amputation in adult mice

The insular cortex (IC) is an important forebrain structure involved in pain perception and taste memory formation. Using a 64-channel multi-electrode array system, we recently identified and characterized two major forms of synaptic plasticity in the adult mouse IC: long-term potentiation (LTP) and long-term depression (LTD). In this study, we investigate injury-related metaplastic changes in insular synaptic plasticity after distal tail amputation. We found that tail amputation in adult mice produced a selective loss of low frequency stimulation-induced LTD in the IC, without affecting (RS)-3,5-dihydroxyphenylglycine (DHPG)-evoked LTD. The impaired insular LTD could be pharmacologically rescued by priming the IC slices with a lower dose of DHPG application, a form of metaplasticity which involves activation of protein kinase C but not protein kinase A or calcium/calmodulin-dependent protein kinase II. These findings provide important insights into the synaptic mechanisms of cortical changes after peripheral amputation and suggest that restoration of insular LTD may represent a novel therapeutic strategy against the synaptic dysfunctions underlying the pathophysiology of phantom pain.

Topographically organized projection to posterior insular cortex from the posterior portion of the ventral medial nucleus in the long-tailed macaque monkey

Prior anterograde tracing work identified somatotopically organized lamina I trigemino- and spinothalamic terminations in a cytoarchitectonically distinct portion of posterolateral thalamus of the macaque monkey, named the posterior part of the ventral medial nucleus (VMpo; Craig [2004] J. Comp. Neurol. 477:119-148). Microelectrode recordings from clusters of selectively thermoreceptive or nociceptive neurons were used to guide precise microinjections of various tracers in VMpo. A prior report (Craig and Zhang [2006] J. Comp. Neurol. 499:953-964) described retrograde tracing results, which confirmed the selective lamina I input to VMpo and the anteroposterior (head to foot) topography. The present report describes the results of microinjections of anterograde tracers placed at different levels in VMpo, based on the anteroposterior topographic organization of selectively nociceptive units and clusters over nearly the entire extent of VMpo. Each injection produced dense, patchy terminal labeling in a single coherent field within a distinct granular cortical area centered in the fundus of the superior limiting sulcus. The terminations were distributed with a consistent anteroposterior topography over the posterior half of the superior limiting sulcus. These observations demonstrate a specific VMpo projection area in dorsal posterior insular cortex that provides the basis for a somatotopic representation of selectively nociceptive lamina I spinothalamic activity. These results also identify the VMpo terminal area as the posterior half of interoceptive cortex; the anterior half receives input from the vagal-responsive and gustatory neurons in the basal part of the ventral medial nucleus.

Neural and physiological responses to a cold pressor challenge in healthy adolescents

Abnormal autonomic function is common in pediatric diseases. Assessment of central mechanisms underlying autonomic challenges may reveal vulnerabilities antecedent to system failure. Our objective was to characterize central markers and physiological responses to a cold pressor challenge in normal children as a critical step for establishing such screening. We performed functional magnetic resonance imaging (fMRI) and collected physiological measures during cold application to the foot in 24 healthy adolescents (15.5 ± 0.4 years, 13 male). The protocol included a 120-sec baseline, 120-sec right-foot cold water immersion (4°C), and 120-sec recovery. Analyses included heart rate (HR) cross-correlations with fMRI signals. Cold application increased HR 13% 5-7 sec after onset, which remained elevated throughout the challenge. Respiratory rate transiently increased (peak 22%), then declined (nadir 12% below baseline), before normalizing at 75 sec. Cold onset rapidly increased somatosensory cortex and medullary signals, which fell after 25 sec. Right anterior insular cortex signals increased early, followed after 20 sec by the left anterior insula, with HR declining 8 sec later. Amygdalae signals also rose, but signals declined in the posterior cingulate cortex, caudate nucleus, hippocampus, and hypothalamus. Declining signals appeared late in the cerebellar fastigial nuclei (60-120 sec), and in the pons and thalamus. Somatosensory cortex, fastigial nuclei, and hypothalamic responses were principally left-sided, with bilateral responses elsewhere. Late left anterior insula responses likely underlie the HR decline; the late cerebellar pattern may modulate recovery. The laterality, timing, and amplitude of normative responses and rostral response differentiation indicate the complex integration of adolescent autonomic processing and provide indices for pathological comparisons.

Alterations of regional spontaneous neuronal activity and corresponding brain circuit changes during resting state in migraine without aura

Although previous resting-state studies have reported abnormal functional cerebral changes in patients with migraine without aura (MwoA), few have focused on alterations in both regional spontaneous neuronal activity and corresponding brain circuits in MwoA patients during rest. Eighteen MwoA patients and 18 age- and gender-matched healthy controls (HC) were recruited in the current study. Baseline cerebral alterations were investigated using amplitude of low-frequency fluctuation (ALFF) and region of interest (ROI)-based functional connectivity (FC) analyses. Compared with HC, MwoA patients showed decreased ALFF values in the left rostral anterior cingulate cortex (rACC) and bilateral prefrontal cortex (PFC) as well as increased ALFF values in the right thalamus. FC analysis also revealed abnormal FCs associated with these ROIs. In addition, ALFF values of the left rACC correlated with duration of disease in MwoA. Our findings could lead to a better understanding of intrinsic functional architecture of baseline brain activity in MwoA, providing both regional and brain circuit spontaneous neuronal activity properties.

Long-term depression of synaptic transmission in the adult mouse insular cortex in vitro

The insular cortex (IC) is known to play important roles in higher brain functions such as memory and pain. Activity-dependent long-term depression (LTD) is a major form of synaptic plasticity related to memory and chronic pain. Previous studies of LTD have mainly focused on the hippocampus, and no study in the IC has been reported. In this study, using a 64-channel recording system, we show for the first time that repetitive low-frequency stimulation (LFS) can elicit frequency-dependent LTD of glutamate receptor-mediated excitatory synaptic transmission in both superficial and deep layers of the IC of adult mice. The induction of LTD in the IC required activation of the N-methyl-d-aspartate (NMDA) receptor, metabotropic glutamate receptor (mGluR)5, and L-type voltage-gated calcium channel. Protein phosphatase 1/2A and endocannabinoid signaling are also critical for the induction of LTD. In contrast, inhibiting protein kinase C, protein kinase A, protein kinase Mζ or calcium/calmodulin-dependent protein kinase II did not affect LFS-evoked LTD in the IC. Bath application of the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine produced another form of LTD in the IC, which was NMDA receptor-independent and could not be occluded by LFS-induced LTD. Our studies have characterised the basic mechanisms of LTD in the IC at the network level, and suggest that two different forms of LTD may co-exist in the same population of IC synapses.

The role of insula-associated brain network in touch

The insula is believed to be associated with touch-evoked effects. In this work, functional MRI was applied to investigate the network model of insula function when 20 normal subjects received tactile stimulation over segregated areas. Data analysis was performed with SPM8 and Conn toolbox. Activations in the contralateral posterior insula were consistently revealed for all stimulation areas, with the overlap located in area Ig2. The area Ig2 was then used as the seed to estimate the insula-associated network. The right insula, left superior parietal lobule, left superior temporal gyrus, and left inferior parietal cortex showed significant functional connectivity with the seed region for all stimulation conditions. Connectivity maps of most stimulation conditions were mainly distributed in the bilateral insula, inferior parietal cortex, and secondary somatosensory cortex. Post hoc ROI-to-ROI analysis and graph theoretical analysis showed that there were higher correlations between the left insula and the right insula, left inferior parietal cortex and right OP1 for all networks and that the global efficiency was more sensitive than the local efficiency to detect differences between notes in a network. These results suggest that the posterior insula serves as a hub to functionally connect other regions in the detected network and may integrate information from these regions.

An investigation into the magnitude of the current window and perception of transcutaneous electrical nerve stimulation (TENS) sensation at various frequencies and body sites in healthy human participants

INTRODUCTION

Strong nonpainful transcutaneous electrical nerve stimulation (TENS) is prerequisite to a successful analgesic outcome although the ease with which this sensation is achieved is likely to depend on the magnitude of current amplitude (mA) between sensory detection threshold (SDT) and pain threshold, that is, the current window.

OBJECTIVES

To measure the current window and participant's perception of the comfort of the TENS sensation at different body sites.

METHODS

A repeated measure cross-over study was conducted using 30 healthy adult volunteers. Current amplitudes (mA) of TENS [2 pulses per second (pps); 30 pps; 80 pps] at SDT, pain threshold, and strong nonpainful intensities were measured at the tibia (bone), knee joint (connective tissue), lower back [paraspinal (skeletal) muscle], volar surface of forearm (nerve) and waist (fat). The amplitude to achieve a strong nonpainful intensity was represented as a percentage of the current window. Data were analyzed using repeated measures analysis of variance.

RESULTS

Effects were detected for body site and frequency for SDT (P<0.001, P=0.018, respectively), current window (P<0.001, P<0.001, respectively), and strong nonpainful TENS as a percentage of the current window (P=0.002, P<0.001, respectively). The current window was larger for the knee joint compared with tibia (difference [95% confidence interval]=12.76 mA [4.25, 21.28]; P=0.001) and forearm (10.33 mA [2.62, 18.40]; P=0.006), and for the lower back compared with tibia (12.10 mA [1.65, 22.52]; P=0.015) and forearm (9.65 mA [1.06, 18.24]; P=0.019). The current window was larger for 2 pps compared with 30 pps (P<0.001) and 80 pps (P<0.001). Participants rated strong nonpainful TENS as most comfortable at the lower back (P<0.001) and least comfortable at the tibia and forearm (P<0.001).

CONCLUSIONS

TENS is most comfortable and easiest to titrate to a strong nonpainful intensity when applied over areas of muscle and soft tissue.

An increase in synaptic NMDA receptors in the insular cortex contributes to neuropathic pain

Neurons in the insular cortex are activated by acute and chronic pain, and inhibition of neuronal activity in the insular cortex has analgesic effects. We found that in a mouse model in which peripheral nerve injury leads to the development of neuropathic pain, the insular cortex showed changes in synaptic plasticity, which were associated with a long-term increase in the amount of synaptic N-methyl-d-aspartate receptors (NMDARs), but not that of extrasynaptic NMDARs. Activation of cyclic adenosine monophosphate (cAMP)-dependent signaling enhanced the amount of synaptic NMDARs in acutely isolated insular cortical slices and increased the surface localization of NMDARs in cultured cortical neurons. We found that the increase in the amount of NMDARs required phosphorylation of the NMDAR subunit GluN2B at Tyr(1472) by a pathway involving adenylyl cyclase subtype 1 (AC1), protein kinase A (PKA), and Src family kinases. Finally, injecting NMDAR or GluN2B-specific antagonists into the insular cortex reduced behavioral responses to normally nonnoxious stimuli in the mouse model of neuropathic pain. Our results suggest that activity-dependent plasticity takes place in the insular cortex after nerve injury and that inhibiting the increase in NMDAR function may help to prevent or treat neuropathic pain.

Using functional magnetic resonance imaging to determine if cerebral hemodynamic responses to pain change following thoracic spine thrust manipulation in healthy individuals

STUDY DESIGN

Case series.

OBJECTIVES

To use blood oxygenation level-dependent functional magnetic resonance imaging (fMRI) to determine if supraspinal activation in response to noxious mechanical stimuli varies pre- and post-thrust manipulation to the thoracic spine.

BACKGROUND

Recent studies have demonstrated the effectiveness of thoracic thrust manipulation in reducing pain and improving function in some individuals with neck and shoulder pain. However, the mechanisms by which manipulation exerts such effects remain largely unexplained. The use of fMRI in the animal model has revealed a decrease in cortical activity in response to noxious stimuli following manual joint mobilization. Supraspinal mediation contributing to hypoalgesia in humans may be triggered following spinal manipulation.

METHODS

Ten healthy volunteers (5 women, 5 men) between the ages of 23 and 48 years (mean, 31.2 years) were recruited. Subjects underwent fMRI scanning while receiving noxious stimuli applied to the cuticle of the index finger at a rate of 1 Hz for periods of 15 seconds, alternating with periods of 15 seconds without stimuli, for a total duration of 5 minutes. Subjects then received a supine thrust manipulation directed to the midthoracic spine and were immediately returned to the scanner for reimaging with a second delivery of noxious stimuli. An 11-point numeric pain rating scale was administered immediately after the application of noxious stimuli, premanipulation and postmanipulation. Blood oxygenation level-dependent fMRI recorded the cerebral hemodynamic response to the painful stimuli premanipulation and postmanipulation.

RESULTS

The data indicated a significant reduction in subjects' perception of pain (P<.01), as well as a reduction in cerebral blood flow as measured by the blood oxygenation level-dependent response following manipulation to areas associated with the pain matrix (P<.05). There was a significant relationship between reduced activation in the insular cortex and decreased subjective pain ratings on the numeric pain rating scale (r = 0.59, P<.05).

CONCLUSION

This study provides preliminary evidence that suggests that supraspinal mechanisms may be associated with thoracic thrust manipulation and hypoalgesia. However, because the study lacked a control group, the results do not allow for the discernment of the causative effects of manipulation, which may also be related to changes in levels of subjects' fear, anxiety, or expectation of successful outcomes with manipulation. Future investigations should strive to elicit more conclusive findings in the form of randomized clinical trials.

Long-term potentiation of synaptic transmission in the adult mouse insular cortex: multielectrode array recordings

The insular cortex (IC) is widely believed to be an important forebrain structure involved in cognitive and sensory processes such as memory and pain. However, little work has been performed at the cellular level to investigate the synaptic basis of IC-related brain functions. To bridge the gap, the present study was designed to characterize the basic synaptic mechanisms for insular long-term potentiation (LTP). Using a 64-channel recording system, we found that an enduring form of late-phase LTP (L-LTP) could be reliably recorded for at least 3 h in different layers of IC slices after theta burst stimulation. The induction of insular LTP is protein synthesis dependent and requires activation of both GluN2A and GluN2B subunits of the NMDA receptor, L-type voltage-gated calcium channels, and metabotropic glutamate receptor 1. The paired-pulse facilitation ratio was unaffected by insular L-LTP induction, and expression of insular L-LTP required the recruitment of postsynaptic calcium-permeable AMPA receptors. Our results provide the first in vitro report of long-term multichannel recordings of L-LTP in the IC in adult mice and suggest its potential important roles in insula-related memory and chronic pain.

Sleep-disordered breathing: effects on brain structure and function

Sleep-disordered breathing is accompanied by neural injury that affects a wide range of physiological systems which include processes for sensing chemoreception and airflow, driving respiratory musculature, timing circuitry for coordination of breathing patterning, and integration of blood pressure mechanisms with respiration. The damage also occurs in regions mediating emotion and mood, as well as areas regulating memory and cognitive functioning, and appears in structures that serve significant glycemic control processes. The injured structures include brain areas involved in hormone release and action of major neurotransmitters, including those playing a role in depression. The injury is reflected in a range of structural magnetic resonance procedures, and also appears as functional distortions of evoked activity in brain areas mediating vital autonomic and breathing functions. The damage is preferentially unilateral, and includes axonal projections; the asymmetry of the injury poses unique concerns for sympathetic discharge and potential consequences for arrhythmia. Sleep-disordered breathing should be viewed as a condition that includes central nervous system injury and impaired function; the processes underlying injury remain unclear.

C-tactile fibers contribute to cutaneous allodynia after eccentric exercise

We recently showed that during acute muscle pain, C-tactile (CT) fibers mediate allodynia in healthy human subjects. In this study, we pursued the following questions: Do CTs contribute to allodynia observed in delayed onset muscle soreness (DOMS)? Is CT-mediated allodynia reproducible in a clinical pain state? In 30 healthy subjects, DOMS was induced in anterior compartment muscles of the leg by repeated eccentric contractions. DOMS was confirmed by mapping the emergence of tender points (decreased pressure pain thresholds). Furthermore, we measured pressure pain thresholds in a clinical subject who presented with activity-triggered heel pain but no resting pain. Cutaneous vibration (sinusoidal; 200 Hz-200 μm)--an otherwise innocuous stimulus--was applied to anterolateral leg before exercise, during DOMS, and following recovery from DOMS. The peripheral origin of allodynia was determined by employing conduction blocks of unmyelinated (intradermal anesthesia) and myelinated (nerve compression) fibers. In DOMS state, there was no resting pain, but vibration reproducibly evoked pain (allodynia). The blockade of cutaneous C fibers abolished this effect, whereas it persisted during blockade of myelinated fibers. In the clinical subject, without exposure to eccentric exercise, vibration (and brushing) produced a cognate expression of CT-mediated allodynia. These observations attest to a broader role of CTs in pain processing.

PERSPECTIVE

This is the first study to demonstrate the contribution of CT fibers to mechanical allodynia in exercise-induced as well as pathological pain states. These findings are of clinical significance, given the crippling effect of sensory impairments on the performance of competing athletes and patients with chronic pain and neurological disorders.

Atypical resting-state functional connectivity of affective pain regions in chronic migraine

OBJECTIVE

Chronic migraineurs (CM) have painful intolerances to somatosensory, visual, olfactory, and auditory stimuli during and between migraine attacks. These intolerances are suggestive of atypical affective responses to potentially noxious stimuli. We hypothesized that atypical resting-state functional connectivity (rs-fc) of affective pain-processing brain regions may associate with these intolerances. This study compared rs-fc of affective pain-processing regions in CM with controls.

METHODS

Twelve minutes of resting-state blood oxygenation level-dependent data were collected from 20 interictal adult CM and 20 controls. Rs-fc between 5 affective regions (anterior cingulate cortex, right/left anterior insula, and right/left amygdala) with the rest of the brain was determined. Functional connections consistently differing between CM and controls were identified using summary analyses. Correlations between number of migraine years and the strengths of functional connections that consistently differed between CM and controls were calculated.

RESULTS

Functional connections with affective pain regions that differed in CM and controls included regions in anterior insula, amygdala, pulvinar, mediodorsal thalamus, middle temporal cortex, and periaqueductal gray. There were significant correlations between the number of years with CM and functional connectivity strength between the anterior insula with mediodorsal thalamus and anterior insula with periaqueductal gray.

CONCLUSION

CM is associated with interictal atypical rs-fc of affective pain regions with pain-facilitating and pain-inhibiting regions that participate in sensory-discriminative, cognitive, and integrative domains of the pain experience. Atypical rs-fc with affective pain regions may relate to aberrant affective pain processing and atypical affective responses to painful stimuli characteristic of CM.

The nature of feelings: evolutionary and neurobiological origins

Feelings are mental experiences of body states. They signify physiological need (for example, hunger), tissue injury (for example, pain), optimal function (for example, well-being), threats to the organism (for example, fear or anger) or specific social interactions (for example, compassion, gratitude or love). Feelings constitute a crucial component of the mechanisms of life regulation, from simple to complex. Their neural substrates can be found at all levels of the nervous system, from individual neurons to subcortical nuclei and cortical regions.

Central projection of pain arising from delayed onset muscle soreness (DOMS) in human subjects

Delayed onset muscle soreness (DOMS) is a subacute pain state arising 24–48 hours after a bout of unaccustomed eccentric muscle contractions. Functional magnetic resonance imaging (fMRI) was used to examine the patterns of cortical activation arising during DOMS-related pain in the quadriceps muscle of healthy volunteers evoked by either voluntary contraction or physical stimulation. The painful movement or physical stimulation of the DOMS-affected thigh disclosed widespread activation in the primary somatosensory and motor (S1, M1) cortices, stretching far beyond the corresponding areas somatotopically related to contraction or physical stimulation of the thigh; activation also included a large area within the cingulate cortex encompassing posteroanterior regions and the cingulate motor area. Pain-related activations were also found in premotor (M2) areas, bilateral in the insular cortex and the thalamic nuclei. In contrast, movement of a DOMS-affected limb led also to activation in the ipsilateral anterior cerebellum, while DOMS-related pain evoked by physical stimulation devoid of limb movement did not.

Kainate receptor-mediated synaptic transmissions in the adult rodent insular cortex

Kainate (KA) receptors are expressed widely in the central nervous system and regulate both excitatory and inhibitory synaptic transmission. KA receptors play important roles in fear memory, anxiety, and pain. However, little is known about their function in synaptic transmission in the insular cortex (IC), a critical region for taste, memory, and pain. Using whole cell patch-clamp recordings, we have shown that KA receptors contribute to fast synaptic transmission in neurons in all layers of the IC. In the presence of the GABA(A) receptor antagonist picrotoxin, the NMDA receptor antagonist AP-5, and the selective AMPA receptor antagonist GYKI 53655, KA receptor-mediated excitatory postsynaptic currents (KA EPSCs) were revealed. We found that KA EPSCs are ~5-10% of AMPA/KA EPSCs in all layers of the adult mouse IC. Similar results were found in adult rat IC. KA EPSCs had a significantly slower rise time course and decay time constant compared with AMPA receptor-mediated EPSCs. High-frequency repetitive stimulations at 200 Hz significantly facilitated the summation of KA EPSCs. In addition, genetic deletion of GluK1 or GluK2 subunit partially reduced postsynaptic KA EPSCs, and exposure of GluK2 knockout mice to the selective GluK1 antagonist UBP 302 could significantly reduce the KA EPSCs. These data suggest that both GluK1 and GluK2 play functional roles in the IC. Our study may provide the synaptic basis for the physiology and pathology of KA receptors in the IC-related functions.

Disentangling linear and nonlinear brain responses to evoked deep tissue pain

Pain stimuli evoke widespread responses in the brain. However, our understanding of the physiological significance underlying heterogeneous response within different pain-activated and -deactivated regions is still limited. Using functional magnetic resonance imaging, we evaluated brain responses to a wide range of stimulus intensity levels (1 innocuous, 7 painful) in order to estimate region-specific stimulus-response functions, which we hypothesized could illuminate that region's functional relationship to pain. Linear and nonlinear brain responses to pain were estimated through independent Legendre polynomial transformations of pain ratings within a general linear model. This approach identified at least 5 different, regionally specific activity profiles in the brain. Linearly increasing (eg, primary somatosensory/motor cortex, insulae) and intensity-independent (eg, secondary somatosensory cortex) activation was noted in traditional pain-processing areas, potentially reflecting sensory encoding and all-or-none salience responses, respectively. Multiple activity profiles were seen in areas of the default mode network (DMN): intensity-independent deactivation (eg, posterior cingulate cortex), linearly decreasing (eg, contralateral inferior parietal lobule), and quadratic (U-shaped; eg, medial prefrontal cortex). The latter observation suggests that: (1) different DMN subregions exhibit functional heterogeneity and (2) some DMN subregions respond in a percept-related manner to pain, suggesting closer linkage between the DMN and pain processing than previously thought. Future studies should apply a similar approach using innocuous stimuli of multiple intensities to evaluate whether the response profiles reported here can also be generalized to nonpainful somatosensory processing.

Magnetic resonance spectroscopy in migraine: what have we learned so far?

OBJECTIVE

To summarize and evaluate proton ((1)H) and phosphorus ((31)P) magnetic resonance spectroscopy (MRS) findings in migraine.

METHODS

A thorough review of (1)H and/or (31)P-MRS studies in any form of migraine published up to September 2011.

RESULTS

Some findings were consistent in all studies, such as a lack of ictal/interictal brain pH change and a disturbed energy metabolism, the latter of which is reflected in a drop in phosphocreatine content, both in the resting brain and in muscle following exercise. In a recent interictal study ATP was found to be significantly decreased in the occipital lobe of migraine with aura patients, reinforcing the concept of a mitochondrial component to the migraine threshold, at least in a subgroup of patients. In several studies a correlation between the extent of the energy disturbance and the clinical phenotype severity was apparent. Less consistent but still congruent with a disturbed energy metabolism is an observed lactate increase in the occipital cortex of several migraine subtypes (MwA, migraine with prolonged aura). No increases in brain glutamate levels were found.

CONCLUSION

The combined abnormalities found in MRS studies imply a mitochondrial component in migraine neurobiology. This could be due to a primary mitochondrial dysfunction or be secondary to, for example, alterations in brain excitability. The extent of variation in the data can be attributed to both the variable clinical inclusion criteria used and the variation in applied methodology. Therefore it is necessary to continue to optimize MRS methodology to gain further insights, especially concerning lactate and glutamate.

Real-time imaging of cortical areas involved in the generation of increases in skin sympathetic nerve activity when viewing emotionally charged images

The sympathetic innervation of the skin not only primarily subserves thermoregulation, but has also been commandeered as a means of emotional expression. While the majority of brain imaging studies of emotion have utilised the galvanic skin response as a means of inferring changes in skin sympathetic nerve activity (SSNA), spontaneous fluctuations in the galvanic skin response bear little relation to spontaneous fluctuations in SSNA. To improve our understanding of the central neural processes involved in the generation of autonomic emotional markers, we recorded SSNA concurrently with brain functional magnetic resonance imaging in 13 subjects. Emotional changes were evoked by presentation of positively-charged (erotica) or negatively-charged (mutilation) images from the International Affective Picture System. Positive and negative emotionally-charged images evoked significant increases in total SSNA and signal intensity in the orbital, dorsolateral and ventromedial prefrontal cortices, amygdala, nucleus accumbens and anterior insula. Increases in signal intensity during increases in SSNA occurred in a number of brain regions, including the central and lateral amygdala, dorsolateral pons, thalamus, nucleus accumbens, and cerebellar cortex. Signal intensity decreases during SSNA increases occurred in the left orbitofrontal, frontal and right precuneus cortices. These data reveal for the first time, cortical and subcortical sites involved in generating SSNA changes during emotions.

Neural mechanisms underlying deafferentation pain: a hypothesis from a neuroimaging perspective

Deafferentation pain following nerve injury annoys patients, and its management is a challenge in clinical practice. Although the mechanisms underlying deafferentation pain remain poorly understood, progress in the development of multidimensional neuroimaging techniques is casting some light on these issues. Deafferentation pain likely results from reorganization of the nervous system after nerve injury via processes that interact with the substrates for pain perception (the pain matrix). Therapeutic effects of motor cortex stimulation on deafferentation pain suggest that the core mechanisms underlying deafferentation pain also interact with the motor system. Therefore, simultaneous neuroimaging and brain stimulation, an emerging neuroimaging technique, was developed to investigate complicated interactions among motor, somatosensory, and pain systems. In healthy participants, parts of the pain matrix (the anterior cingulate cortex, parietal operculum, and thalamus) show activity during both somatosensory stimulation and brain stimulation to the motor cortex. This finding indicates that motor, somatosensory, and pain systems communicate among each other via the neural network. A better understanding of the plastic mechanisms influencing such cross-talk among these systems will help develop therapeutic interventions using brain stimulation and neurofeedback.

Differential responses of the insular cortex gyri to autonomic challenges

Determining insular functional topography is essential for assessing autonomic consequences of neural injury. We examined that topography in the five major insular cortex gyri to three autonomic challenges, the Valsalva, hand grip, and foot cold pressor, using functional magnetic resonance imaging (fMRI) procedures. Fifty-seven healthy subjects (age ± std: 47 ± 9 years) performed four 18 s Valsalva maneuvers (30 mm Hg load pressure), four hand grip challenges (16 s at 80% effort), and a foot cold pressor (60 s, 4°C), with fMRI scans recorded every 2 s. Signal trends were compared across gyri using repeated measures ANOVA. Significantly (P<0.05) higher signals in left anterior versus posterior gyri appeared during Valsalva strain, and in the first 4 s of recovery. The right anterior gyri showed sustained higher signals up to 2 s post-challenge, relative to posterior gyri, with sub-gyral differentiation. Left anterior gyri signals were higher than posterior areas during the hand grip challenge. All right anterior gyri showed increased signals over posterior up to 12 s post-challenge, with decline in the most-anterior gyrus from 10 to 24 s during recovery. The left three anterior gyri showed relatively lower signals only during the 90 s recovery of the cold pressor, while the two most-anterior right gyri signals increased only during the stimulus. More-differentiated representation of autonomic signals appear in the anterior right insula for the Valsalva maneuver, a bilateral, more-posterior signal representation for hand grip, and preferentially right-sided, anterior-posterior representation for the cold pressor. The functional organization of the insular cortex is gyri-specific to unique autonomic challenges.

Altered cortical morphology in sensorimotor processing regions in adolescents and adults with attention-deficit/hyperactivity disorder

Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder that is common in children and frequently persists into adulthood. While ADHD is characterized by developmentally inappropriate, persistent and impairing levels of inattention, impulsiveness and hyperactivity, it is also associated with sensorimotor deficits and altered neural processing of somatosensory stimuli, as well as with executive function deficits. The latter are associated with thinning of frontal lobe structures in ADHD; however, few structural neuroimaging studies have focused on changes in brain morphology in sensorimotor regions in this population. Moreover, little is known about morphological changes that occur in these regions throughout the developmental trajectory into adulthood. In this preliminary cross-sectional study, we examined cortical thickness with a focus on brain regions involved in sensorimotor processing in adolescents and adults with ADHD compared to neurotypical cohorts. Compared to controls, adolescents with ADHD showed significant increased cortical thickness in the pre-supplementary motor area (SMA) and adults with ADHD showed increased thickness in the primary somatosensory cortex (SI). Based on these differences, we collated the data from the adolescents and adults and examined possible age×group interaction effects on cortical thickness. A significant interaction effect was found in SI where healthy participants showed decreased thickness in this region at older ages, whereas the ADHD cohort showed little change. Results suggest that sensorimotor brain regions are altered in ADHD and these changes may not dissipate in adolescence, but rather persist into adulthood.

Localization of pain-related brain activation: a meta-analysis of neuroimaging data

A meta-analysis of 140 neuroimaging studies was performed using the activation-likelihood-estimate (ALE) method to explore the location and extent of activation in the brain in response to noxious stimuli in healthy volunteers. The first analysis involved the creation of a likelihood map illustrating brain activation common across studies using noxious stimuli. The left thalamus, right anterior cingulate cortex (ACC), bilateral anterior insulae, and left dorsal posterior insula had the highest likelihood of being activated. The second analysis contrasted noxious cold with noxious heat stimulation and revealed higher likelihood of activation to noxious cold in the subgenual ACC and the amygdala. The third analysis assessed the implications of using either a warm stimulus or a resting baseline as the control condition to reveal activation attributed to noxious heat. Comparing noxious heat to warm stimulation led to peak ALE values that were restricted to cortical regions with known nociceptive input. The fourth analysis tested for a hemispheric dominance in pain processing and showed the importance of the right hemisphere, with the strongest ALE peaks and clusters found in the right insula and ACC. The fifth analysis compared noxious muscle with cutaneous stimuli and the former type was more likely to evoke activation in the posterior and anterior cingulate cortices, precuneus, dorsolateral prefrontal cortex, and cerebellum. In general, results indicate that some brain regions such as the thalamus, insula and ACC have a significant likelihood of activation regardless of the type of noxious stimuli, while other brain regions show a stimulus-specific likelihood of being activated.

Visualization of painful experiences believed to trigger the activation of affective and emotional brain regions in subjects with low back pain

In the management of clinical low back pain (LBP), actual damage to lower back areas such as muscles, intervertebral discs etc. are normally targeted for therapy. However, LBP may involve not only sensory pain, but also underlying affective pain which may also play an important role overall in painful events. Therefore we hypothesized that visualization of a painful event may trigger painful memories, thus provoking the affective dimension of pain. The present study investigated neural correlates of affect processing in subjects with LBP (n = 11) and subjects without LBP (n = 11) through the use of virtual LBP stimuli. Whole brain functional magnetic resonance imaging (MRI) was performed for all subjects while they were shown a picture of a man carrying luggage in a half-crouching position. All subjects with LBP reported experiencing discomfort and 7 LBP subjects reported experiencing pain. In contrast to subjects without LBP, subjects with LBP displayed activation of the cortical area related to pain and emotions: the insula, supplementary motor area, premotor area, thalamus, pulvinar, posterior cingulate cortex, hippocampus, fusiform, gyrus, and cerebellum. These results suggest that the virtual LBP stimuli caused memory retrieval of unpleasant experiences and therefore may be associated with prolonged chronic LBP conditions.

Stimulation of the human cortex and the experience of pain: Wilder Penfield's observations revisited

Thanks to the seminal work of Wilder Graves Penfield (1891-1976) at the Montreal Neurological Institute, electrical stimulation is used worldwide to localize the epileptogenic cortex and to map the functionally eloquent areas in the context of epilepsy surgery or lesion resections. In the functional map of elementary and experiential responses he described through >20 years of careful exploration of the human cortex via stimulation of the cortical surface, Penfield did not identify any 'pain cortical area'. We reinvestigated this issue by analysing subjective and videotaped behavioural responses to 4160 cortical stimulations using intracerebral electrodes implanted in all cortical lobes that were carried out over 12 years during the presurgical evaluation of epilepsy in 164 consecutive patients. Pain responses were scarce (1.4%) and concentrated in the medial part of the parietal operculum and neighbouring posterior insula where pain thresholds showed a rostrocaudal decrement. This deep cortical region remained largely inaccessible to the intraoperative stimulation of the cortical surface carried out by Penfield after resection of the parietal operculum. It differs also from primary sensory areas described by Penfield et al. in the sense that, with our stimulation paradigm, pain represented only 10% of responses. Like Penfield et al., we obtained no pain response anywhere else in the cortex, including in regions consistently activated by pain in most functional imaging studies, i.e. the first somatosensory area, the lateral part of the secondary somatosensory area, anterior and mid-cingulate gyri (mid-cingulate cortex), anterior frontal, posterior parietal and supplementary motor areas. The medial parietal operculum and posterior insula are thus the only areas where electrical stimulation is able to trigger activation of the pain cortical network and thus the experience of somatic pain.

Habituation within the somatosensory processing hierarchy

Habituation is a basic process of learning evident in a decrement in neuronal/behavioral responses to repeated sensory stimulation. It is generally accepted that habituation affects all sensory systems in the human brain, including the somatosensory network. However, it is not clear where habituation originates within this hierarchically organized network. In this study, we examined whether habituation effects increase relatively uniformly along the processing hierarchy or rather distinctly at a particular processing stage. We addressed these questions by performing functional magnetic resonance imaging (fMRI) on 43 healthy subjects during unilateral electrical median nerve stimulation using a block design. We found a time-dependent decrease in the positive BOLD response (indicative of habituation) in all areas of the somatosensory network with the exception of Brodmann area (BA) 3b. The increase in habituation within the presumed processing stream was most pronounced between subareas of the primary somatosensory cortex (BA3b, BA1, BA2), and no further increase in habituation effects was observed in the subsequent processing stages within either the secondary somatosensory cortex or the insula. Moreover, we found a relatively strong habituation effect within the thalamus. These findings indicate that the increase in habituation along the processing hierarchy is measurable primarily between subareas of the primary somatosensory cortex, and we hypothesize that this increase originates in thalamocortical interactions early in the processing stream.

Separating brain processing of pain from that of stimulus intensity

Regions of the brain network activated by painful stimuli are also activated by nonpainful and even nonsomatosensory stimuli. We therefore analyzed where the qualitative change from nonpainful to painful perception at the pain thresholds is coded. Noxious stimuli of gaseous carbon dioxide (n = 50) were applied to the nasal mucosa of 24 healthy volunteers at various concentrations from 10% below to 10% above the individual pain threshold. Functional magnetic resonance images showed that these trigeminal stimuli activated brain regions regarded as the "pain matrix." However, most of these activations, including the posterior insula, the primary and secondary somatosensory cortex, the amygdala, and the middle cingulate cortex, were associated with quantitative changes in stimulus intensity and did not exclusively reflect the qualitative change from nonpainful to pain. After subtracting brain activations associated with quantitative changes in the stimuli, the qualitative change, reflecting pain-exclusive activations, could be localized mainly in the posterior insular cortex. This shows that cerebral processing of noxious stimuli focuses predominately on the quantitative properties of stimulus intensity in both their sensory and affective dimensions, whereas the integration of this information into the perception of pain is restricted to a small part of the pain matrix.

Resting-state functional connectivity of the medial superior frontal cortex

The medial superior frontal cortex (SFC), including the supplementary motor area (SMA) and presupplementary motor area (preSMA), is implicated in movement and cognitive control, among other functions central to decision making. Previous studies delineated the anatomical boundaries and functional connectivity of the SMA. However, it is unclear whether the preSMA, which responds to a variety of behavioral tasks, comprises functionally distinct areas. With 24 seed regions systematically demarcated throughout the anterior and posterior medial SFC, we examined here the functional divisions of the medial SFC on the basis of the "correlograms" of resting-state functional magnetic resonance imaging data of 225 adult individuals. In addition to replicating segregation of the SMA and posterior preSMA, the current results elucidated functional connectivities of anterior preSMA-the most anterior part of the medial SFC. In contrast to the caudal medial SFC, the anterior preSMA is connected with most of the prefrontal but not with somatomotor areas. Overall, the SMA is strongly connected to the thalamus and epithalamus, the posterior preSMA to putamen, pallidum, and subthalamic nucleus, and anterior preSMA to the caudate, with the caudate showing significant hemispheric asymmetry. These findings may provide a useful platform for future studies to investigate frontal cortical functions.

Functional deactivations: multiple ipsilateral brain areas engaged in the processing of somatosensory information

Somatosensory signals modulate activity throughout a widespread network in both of the brain hemispheres: the contralateral as well as the ipsilateral side of the brain relative to the stimulated limb. To analyze the ipsilateral somatosensory brain areas that are engaged during limb stimulation, we performed functional magnetic resonance imaging (fMRI) in 12 healthy subjects during electrical median nerve stimulation using both a block- and an event-related fMRI design. Data were analyzed through the use of model-dependent (SPM) and model-independent (ICA) approaches. Beyond the well-known positive blood oxygenation level-dependent (BOLD) responses, negative deflections of the BOLD response were found consistently in several ipsilateral brain areas, including the primary somatosensory cortex, the supplementary motor area, the insula, the dorsal part of the posterior cingulate cortex, and the contralateral cerebellum. Compared to their positive counterparts, the negative hemodynamic responses showed a different time course, with an onset time delay of 2.4 s and a peak delay of 0.7 s. This characteristic delay was observed in all investigated areas and verified by a second (purely tactile) event-related paradigm, suggesting a systematic difference for brain areas involved in the processing of somatosensory information. These findings may indicate that the physiological basis of these deactivations differs from that of the positive BOLD responses. Therefore, an altered model for the negative BOLD response may be beneficial to further model-dependent fMRI analyses.

Can neuroimaging studies identify pain endophenotypes in humans?

Pain is a complex, multidimensional experience that has defied our understanding for centuries. Through the advent of noninvasive neuroimaging techniques, we have been able to examine the human brain and its response to nociceptive inputs. As a result, our knowledge of which brain regions are critical for generating an acute pain experience has grown, as has our understanding of how cognitive, emotional, contextual and various physiological factors influence the pain experience. Furthermore, we have been able to identify key processes within the brain that underpin the transition to and maintenance of chronic pain states, as well as highlight the dramatic consequences of chronic pain on the brain's structure and neurochemistry. Building upon this knowledge, we are now in a position to consider whether any of these brain imaging 'phenotypes' of acute or chronic pain should be considered as useful endophenotypes; thereby enabling us to relate the complex genetics that underpin everyday pain sensitivity or chronic pain states to intermediate biomarkers. This endophenotypic approach-the focus of this Review-simplifies the connection between genes and behavior and is needed for complex disorders like chronic pain.

Functional neuroanatomy of the insular lobe

The insula is the fifth lobe of the brain and it is the least known. Hidden under the temporal, frontal and parietal opercula, as well as under dense arterial and venous vessels, its accessibility is particularly restricted. Functional data on this region in humans, therefore, are scarce and the existing evidence makes conclusions on its functional and somatotopic organization difficult. 5 patients with intractable epilepsy underwent an invasive presurgical evaluation with implantation of diagnostic invasive-depth electrodes, including insular electrodes that were inserted using a mesiocaudodorsal to laterorostroventral approach. Altogether 113 contacts were found to be in the insula and were stimulated with alternating currents during preoperative monitoring. Different viscerosensitive and somatosensory phenomena were elicited by stimulation of these electrodes. A relatively high density of electrode contacts enabled us to delineate several functionally distinct areas within the insula. We found somatosensory symptoms to be restricted to the posterior insula and a subgroup of warmth or painful sensations in the dorsal posterior insula. Viscerosensory symptoms were elicited by more anterior electrode contacts with a subgroup of gustatory symptoms occurring after stimulation of electrode contacts in the central part of the insula. The anterior insula did not show reproducible responses to stimulation. In line with previous studies, we found evidence for somato- and viscerosensory cortex in the insula. In addition, our results suggest that there is a predominantly posterior and central distribution of these functions in the insular lobe.

Neural aspects of immunomodulation: focus on the vagus nerve

Inflammation and immunity have been implicated in a wide variety of diseases and disorders ranging from Alzheimer's disease to cardiovascular disease to hemorrhagic shock. In this review, we will briefly consider the evidence for the neural concomitants of immunomodulation. First, we will briefly review the anatomy and physiology of neural-immune communication. Evidence for the somatotopic organization of the vagus nerve and for pain processes suggests that such an organization may be relevant for the investigation of the neural concomitants of immunity. Then we will provide an overview of what is known from both animal and human studies including neuroimaging and clinical studies. Finally, we will discuss some of the challenges and opportunities in this exciting area of investigation.

Feeling good: on the role of C fiber mediated touch in interoception

The human skin is innervated by a network of thin, slow-conducting afferent (C and Aδ) fibers, transmitting a diverse range of information. Classically, these fibers are described as thermo-, noci- or chemoreceptive, whereas mechanoreception is attributed exclusively to thick, fast-conducting (Aβ) afferents. A growing body of evidence, however, supports the notion that C tactile afferents comprise a second anatomically and functionally distinct system signaling touch in humans. This review discusses established as well as recent findings which highlight fundamental differences in peripheral and central information coding and processing between Aβ and C mechanoreception. We conclude that from the skin through the brain, C touch shares more characteristics with interoceptive modalities (e.g. pain, temperature, and itch) than exteroceptive Aβ touch, vision or hearing. In this light, we discuss the motivational-affective role of C touch as an integral part of a thin-fiber afferent homeostatic network for the maintenance of physical and social well-being.

Anatomofunctional organization of the insular cortex: a study using intracerebral electrical stimulation in epileptic patients

PURPOSE

Different lines of evidence suggest that the insular cortex has many important functional roles. Direct electrical stimulation (ES) of the human insular cortex during surgical procedures for epilepsy, functional imaging techniques, and lesion studies also occasionally induces clinical responses.

METHODS

In this study, we evaluated 25 patients with drug-refractory focal epilepsy by stereotactically implanting at least one electrode into the insular cortex using an oblique approach (transfrontal or transparietal). One hundred twenty-eight insular sites (each situated between two contiguous contacts within the same electrode) were examined within the gyral substructures. We located each stimulation site by fusing preimplantation three-dimensional (3D) magnetic resonance imaging (MRI) images with the postimplantation 3D computed tomography (CT) scans that revealed the electrode contacts.

RESULTS

Sixty-seven stimulations induced at least one clinical response. Stimulation from within the insular cortex evoked 83 responses, without evidence of afterdischarge in the insular or extrainsular regions. We classified the principal responses as sensory (paresthesias and localized warm sensations), motor, pain, auditory, oropharyngeal, speech disturbances (including speech arrest and reduced voice intensity) and neurovegetative phenomena, such as facial reddening, generalized sensations of warmth or cold, hypogastric sensations, anxiety attacks, respiratory accelerations, sensations of rotation, and nausea.

CONCLUSIONS

These findings may indicate a functional specificity for the insular gyri and show the need for exploring this structure during invasive presurgical evaluation of epileptic patients according to seizure manifestations.

[Nociceptive system : Nociceptors, fiber types, spinal pathways, and projection areas]

In order to transform a nociceptive stimulus into a painful perception, a highly specialized chain of structural and functional elements is necessary. This system comprises nociceptors in the periphery with specific molecular properties for differential coding of noxious submodalities, ascending and descending tracts that can control the input into the dorsal horn of the spinal cord as well as supraspinal processing that regulates the integration of nociceptive information with other sensory modalities and autonomic function. In this article, structures involved in nociceptive signal processing starting from the periphery up to spinal and cerebral structures are discussed in the order of their spatio-temporal activation sequence - as far as these are known. Already from the basis of the different receptor subtypes found on the thinly or unmyelinated nerve fibers in the periphery, the predominant principle of polymodality is demonstrated. Different input to the dorsal horn of the spinal cord by different nerve fiber populations to superficial and deep layers is explained, ascending tracts as well as descending systems capable of either facilitating or inhibiting the upstream flow of nociceptive information, together with their known transmitters. Finally, thalamic relay nuclei for sensory and nociceptive signals, as well as subcortical and cortical projection targets are discussed. To complete the current view of the nociceptive system, information from molecular biology and anatomic tracing studies as well as data from functional electrophysiologic cell recordings in animals and imaging studies in humans are assembled.

Within-limb somatotopic representation of acute muscle pain in the human contralateral dorsal posterior insula

It is well established that the insular cortex processes noxious information. We have previously shown that noxious inputs from the arm and leg are coarsely organized somatotopically within the dorsal posterior insula. The same has been shown for inputs from C tactile afferents, which mediate affective touch, and it has been suggested that the insula may be responsible for the localization of some somatosensory stimuli. Knowing the degree of spatial detail may have significant implications for the potential role of the dorsal posterior insula in the processing of noxious stimuli. Using high-resolution functional magnetic resonance imaging (fMRI), we compared insula activation patterns in 13 subjects during muscle pain induced by injection of hypertonic saline (5%) into three muscles within the same limb: shoulder (deltoid), forearm (flexor carpi radialis), and hand (first dorsal interosseous). Mapping the maximally activated voxels within the contralateral dorsal posterior insula in each individual subject during each pain stimulus revealed a clear somatotopy of activation within the contralateral dorsal posterior insula. Shoulder pain was represented anterior to forearm pain and medial to hand pain. This fine somatotopic organization may be crucial for pain localization or other aspects of the pain experience that differ depending on stimulation site.

Multiple somatotopic representations of heat and mechanical pain in the operculo-insular cortex: a high-resolution fMRI study

Whereas studies of somatotopic representation of touch have been useful to distinguish multiple somatosensory areas within primary (SI) and secondary (SII) somatosensory cortex regions, no such analysis exists for the representation of pain across nociceptive modalities. Here we investigated somatotopy in the operculo-insular cortex with noxious heat and pinprick stimuli in 11 healthy subjects using high-resolution (2 × 2 × 4 mm) 3T functional magnetic resonance imaging (fMRI). Heat stimuli (delivered using a laser) and pinprick stimuli (delivered using a punctate probe) were directed to the dorsum of the right hand and foot in a balanced design. Locations of the peak fMRI responses were compared between stimulation sites (hand vs. foot) and modalities (heat vs. pinprick) within four bilateral regions of interest: anterior and posterior insula and frontal and parietal operculum. Importantly, all analyses were performed on individual, non-normalized fMRI images. For heat stimuli, we found hand-foot somatotopy in the contralateral anterior and posterior insula [hand, 9 ± 10 (SD) mm anterior to foot, P < 0.05] and in the contralateral parietal operculum (SII; hand, 7 ± 10 mm lateral to foot, P < 0.05). For pinprick stimuli, we also found somatotopy in the contralateral posterior insula (hand, 9 ± 10 mm anterior to foot, P < 0.05). Furthermore, the response to heat stimulation of the hand was 11 ± 12 mm anterior to the response to pinprick stimulation of the hand in the contralateral (left) anterior insula (P < 0.05). These results indicate the existence of multiple somatotopic representations for pain within the operculo-insular region in humans, possibly reflecting its importance as a sensory-integration site that directs emotional responses and behavior appropriately depending on the body site being injured.

Cortical and brain stem changes in neural activity during static handgrip and postexercise ischemia in humans

Static isometric exercise increases muscle sympathetic nerve activity (MSNA) and mean arterial pressure, both of which can be maintained at the conclusion of the exercise by occlusion of the arterial supply [postexercise ischemia (PEI)]. To identify the cortical and subcortical sites involved, and to differentiate between central command and reflex inputs, we used blood oxygen level-dependent (BOLD) functional MRI (fMRI) of the whole brain (3 T). Subjects performed submaximal static handgrip exercise for 2 min followed by 6 min of PEI; MSNA was recorded on a separate day. During the contraction phase, parallel increases in BOLD signal intensity occurred in the contralateral primary motor cortex and cerebellar nuclei and cortex; these matched the effort profile and ceased at the conclusion of the contraction. Progressive increases in the contralateral insula and primary and secondary somatosensory cortices, with progressive decreases in the perigenual anterior cingulate and midcingulate cortices, were sustained during the period of PEI and thus did not depend on central command. Discrete bilateral activation of the medial and lateral dorsal medulla was also observed during the contraction and PEI; we believe that these represent the nucleus tracts solitarius (NTS) and rostral ventrolateral medulla (RVLM), respectively. Given that metaboreceptor afferents are known to project to the NTS and that the RVLM is the primary output nucleus for MSNA, our data support that the metaboreflex is mediated by the medulla, whereas the somatosensory, insular, and anterior cingulate cortices are involved in the sensory and affective components of the maneuver.

A comparison of physiologic and physical discomfort responses between exercise modalities

To determine differences in physiologic responses and perceived discomfort during maximal and submaximal exercise with 2 common exercise modalities, the elliptical cross-trainer (ECT) and the arc trainer (ARC) were used, which undergoes an excursion of motion using an arc pathway. Eighteen subjects (10 male and 8 female; age = 24.7 +/- 2.6 yr, height = 172.2 +/- 10.3 cm, mass = 69.8 +/- 14.9 kg, %fat = 22.5 +/- 8.1%; mean +/- SD) performed maximal exercise protocols on a treadmill (TML), the ECT, and ARC. Subjects also performed 3 10-minute submaximal exercise bouts on the ECT and ARC at 55%, 65%, and 75% of TML maximal oxygen uptake (VO2max) in which heart rate (HR), VO2, ventilation, respiratory exchange ratio, and hip, knee, and low-back discomfort were monitored. All testing on the ECT and ARC were performed in a counterbalanced order. The VO2max was greater during TML exercise compared with ECT (p = 0.007) but similar to the ARC. Both ECT and ARC elicited lower maximal HR values compared with maximal TML exercise (p = 0.0001). No difference was observed between ECT and ARC for VO2 during the submaximal exercise bouts (p > 0.05). However, HR was greater during submaximal ECT exercise (p < 0.0001). Perception of discomfort was not different between ECT and ARC for knees (p > 0.05) and lower back (p > 0.05) but different for the hips (p = 0.02). Similar VO2max values were observed with ARC and TML modalities. Greater perceptions of discomfort were observed with ECT compared with ARC at similar submaximal exercise intensities, suggesting individuals with, or at risk for, lower-extremity joint pathology may benefit from exercise with modalities other than the ECT.