Response of anterior parietal cortex to different modes of same-site skin stimulation

A thermal nociceptive patch in the S2 cortex of nonhuman primates: a combined functional magnetic resonance imaging and electrophysiology study

ABSTRACT

Human functional magnetic resonance imaging (fMRI) and behavioral studies have established the roles of cortical areas along the Sylvian fissure in sensing subjective pain. Yet, little is known about how sensory aspects of painful information are represented and processed by neurons in these regions and how their electrophysiological activities are related to fMRI signals. The current study aims to partially address this critical knowledge gap by performing fMRI-guided microelectrode mapping and recording studies in the homologous region of the parietal operculum in squirrel monkeys under light anesthesia. In each animal studied (n = 8), we detected mesoscale mini-networks for heat nociception in cortical regions around the lateral sulcus. Within the network, we discovered a ∼1.5 × 1.5-mm2-sized cortical patch that solely contained heat nociceptive neurons that aligned with the heat fMRI activation locus. These neurons responded slowly to thermal (heat and cold) nociceptive stimuli exclusively, continued firing for several seconds after the succession of stimulation, and exhibited multidigit receptive fields and high spontaneous firing rates. Similar to the fMRI responses, increasing temperatures in the nociceptive range led to a nonlinear increase in firing rates. The finding of a clustering of heat nociceptive neurons provides novel insights into the unique functional organization of thermal nociception in the S2 subregion of the primate brain. With fMRI, it supports the existence of a modality-preferred heat nociceptive patch that is spatially separated and intermingled with touch patches containing neurons with comparable receptive fields and the presence of functionally distinct mini-networks in primate opercular cortex.

Brain (re)organisation following amputation: Implications for phantom limb pain

Following arm amputation the region that represented the missing hand in primary somatosensory cortex (S1) becomes deprived of its primary input, resulting in changed boundaries of the S1 body map. This remapping process has been termed 'reorganisation' and has been attributed to multiple mechanisms, including increased expression of previously masked inputs. In a maladaptive plasticity model, such reorganisation has been associated with phantom limb pain (PLP). Brain activity associated with phantom hand movements is also correlated with PLP, suggesting that preserved limb functional representation may serve as a complementary process. Here we review some of the most recent evidence for the potential drivers and consequences of brain (re)organisation following amputation, based on human neuroimaging. We emphasise other perceptual and behavioural factors consequential to arm amputation, such as non-painful phantom sensations, perceived limb ownership, intact hand compensatory behaviour or prosthesis use, which have also been related to both cortical changes and PLP. We also discuss new findings based on interventions designed to alter the brain representation of the phantom limb, including augmented/virtual reality applications and brain computer interfaces. These studies point to a close interaction of sensory changes and alterations in brain regions involved in body representation, pain processing and motor control. Finally, we review recent evidence based on methodological advances such as high field neuroimaging and multivariate techniques that provide new opportunities to interrogate somatosensory representations in the missing hand cortical territory. Collectively, this research highlights the need to consider potential contributions of additional brain mechanisms, beyond S1 remapping, and the dynamic interplay of contextual factors with brain changes for understanding and alleviating PLP.

A nociresponsive specific area of human somatosensory cortex within BA3a: BA3c?

It is well recognized that in primates, including humans, noxious body stimulation evokes a neural response in the posterior bank of the central sulcus, in Brodmann cytoarchitectonic subdivisions 3b and 1 of the primary somatosensory cortex. This response is associated with the 1st/sharp pain and contributes to sensory discriminative aspects of pain perception and spatial localization of the noxious stimulus. However, neurophysiological studies in New World monkeys predict that in humans noxious stimulation also evokes a separate neural response-mediated by C-afferent drive and associated with the 2nd/burning pain-in the depth of the central sulcus in Brodmann area 3a (BA3a) at the transition between the somatosensory and motor cortices. To evoke such a response, it is necessary to use multi-second duration noxious stimulation, rather than brief laser pulses. Given the limited human pain-imaging literature on cortical responses induced by C-nociceptive input specifically within BA3a, here we used high spatial resolution 7T fMRI to study the response to thermonoxious skin stimulation. We observed the predicted response of BA3a in the depth of the central sulcus in five human volunteers. Review of the available evidence suggests that the nociresponsive region in the depth of the central sulcus is a structurally and functionally distinct cortical area that should not be confused with proprioceptive BA3a. It is most likely engaged in interoception and control of the autonomic nervous system, and contributes to the sympathetic response to noxious stimulation, arguably the most intolerable aspect of pain experience. Ablation of this region has been shown to reduce pain sensibility and might offer an effective means of ameliorating some pathological pain conditions.

Mechanisms of Below-Level Pain Following Spinal Cord Injury (SCI)

Mechanisms of below-level pain are discoverable as neural adaptations rostral to spinal injury. Accordingly, the strategy of investigations summarized here has been to characterize behavioral and neural responses to below-level stimulation over time following selective lesions of spinal gray and/or white matter. Assessments of human pain and the pain sensitivity of humans and laboratory animals following spinal injury have revealed common disruptions of pain processing. Interruption of the spinothalamic pathway partially deafferents nocireceptive cerebral neurons, rendering them spontaneously active and hypersensitive to remaining inputs. The spontaneous activity among these neurons is disorganized and unlikely to generate pain. However, activation of these neurons by their remaining inputs can result in pain. Also, injury to spinal gray matter results in a cascade of secondary events, including excitotoxicity, with rostral propagation of excitatory influences that contribute to chronic pain. Establishment and maintenance of below-level pain results from combined influences of injured and spared axons in the spinal white matter and injured neurons in spinal gray matter on processing of nociception by hyperexcitable cerebral neurons that are partially deafferented. A model of spinal stenosis suggests that ischemic injury to the core spinal region can generate below-level pain. Additional questions are raised about demyelination, epileptic discharge, autonomic activation, prolonged activity of C nocireceptive neurons, and thalamocortical plasticity in the generation of below-level pain. PERSPECTIVE: An understanding of mechanisms can direct therapeutic approaches to prevent development of below-level pain or arrest it following spinal cord injury. Among the possibilities covered here are surgical and other means of attenuating gray matter excitotoxicity and ascending propagation of excitatory influences from spinal lesions to thalamocortical systems involved in pain encoding and arousal.

A newly identified nociresponsive region in the transitional zone (TZ) in rat sensorimotor cortex

The primary somatosensory cortex (S1) comprises a number of functionally distinct regions, reflecting the diversity of somatosensory receptor submodalities innervating the body. In particular, two spatially and functionally distinct nociceptive regions have been described in primate S1 (Vierck et al., 2013; Whitsel et al., 2019). One region is located mostly in Brodmann cytoarchitectonic area 1, where a subset of neurons exhibit functional characteristics associated with myelinated Aδ nociceptors and perception of 1st/sharp, discriminative pain. The second region is located at the transition between S1 and primary motor cortex (M1) in area 3a, where neurons exhibit functional characteristics associated with unmyelinated C nociceptors and perception of 2nd/slow, burning pain. To test the hypothesis that in rats the transitional zone (TZ) - which is a dysgranular region at the transition between M1 and S1 - is the functional equivalent of the nociresponsive region of area 3a in primates, extracellular spike discharge activity was recorded from TZ neurons in rats under general isoflurane anesthesia. Thermonoxious stimuli were applied by lowering the contralateral forepaw or hindpaw into a 48-51 °C heated water bath for 5-10 s. Neurons in TZ were found to be minimally affected by non-noxious somatosensory stimuli, but highly responsive to thermonoxious skin stimuli in a slow temporal summation manner closely resembling that of nociresponsive neurons in primate area 3a. Selective inactivation of TZ by topical lidocaine application suppressed or delayed the nociceptive withdrawal reflex, suggesting that TZ exerts a tonic facilitatory influence over spinal cord neurons producing this reflex. In conclusion, TZ appears to be a rat homolog of the nociresponsive part of monkey area 3a. A possibility is considered that this region might be primarily engaged in autonomic aspects of nociception.

Contributions of Nociresponsive Area 3a to Normal and Abnormal Somatosensory Perception

Traditionally, cytoarchitectonic area 3a of primary somatosensory cortex (SI) has been regarded as a proprioceptive relay to motor cortex. However, neuronal spike-train recordings and optical intrinsic signal imaging, obtained from nonhuman sensorimotor cortex, show that neuronal activity in some of the cortical columns in area 3a can be readily triggered by a C-nociceptor afferent drive. These findings indicate that area 3a is a critical link in cerebral cortical encoding of secondary/slow pain. Also, area 3a contributes to abnormal pain processing in the presence of activity-dependent reversal of gamma-aminobutyric acid A receptor-mediated inhibition. Accordingly, abnormal processing within area 3a may contribute mechanistically to generation of clinical pain conditions. PERSPECTIVE: Optical imaging and neurophysiological mapping of area 3a of SI has revealed substantial driving from unmyelinated cutaneous nociceptors, complementing input to areas 3b and 1 of SI from myelinated nociceptors and non-nociceptors. These and related findings force a reconsideration of mechanisms for SI processing of pain.

Neural Basis of Touch and Proprioception in Primate Cortex

The sense of proprioception allows us to keep track of our limb posture and movements and the sense of touch provides us with information about objects with which we come into contact. In both senses, mechanoreceptors convert the deformation of tissues-skin, muscles, tendons, ligaments, or joints-into neural signals. Tactile and proprioceptive signals are then relayed by the peripheral nerves to the central nervous system, where they are processed to give rise to percepts of objects and of the state of our body. In this review, we first examine briefly the receptors that mediate touch and proprioception, their associated nerve fibers, and pathways they follow to the cerebral cortex. We then provide an overview of the different cortical areas that process tactile and proprioceptive information. Next, we discuss how various features of objects-their shape, motion, and texture, for example-are encoded in the various cortical fields, and the susceptibility of these neural codes to attention and other forms of higher-order modulation. Finally, we summarize recent efforts to restore the senses of touch and proprioception by electrically stimulating somatosensory cortex. © 2018 American Physiological Society. Compr Physiol 8:1575-1602, 2018.

Cortical Representation of Pain and Touch: Evidence from Combined Functional Neuroimaging and Electrophysiology in Non-human Primates

Human functional MRI studies in acute and various chronic pain conditions have revolutionized how we view pain, and have led to a new theory that complex multi-dimensional pain experience (sensory-discriminative, affective/motivational, and cognitive) is represented by concurrent activity in widely-distributed brain regions (termed a network or pain matrix). Despite these breakthrough discoveries, the specific functions proposed for these regions remain elusive, because detailed electrophysiological characterizations of these regions in the primate brain are lacking. To fill in this knowledge gap, we have studied the cortical areas around the central and lateral sulci of the non-human primate brain with combined submillimeter resolution functional imaging (optical imaging and fMRI) and intracranial electrophysiological recording. In this mini-review, I summarize and present data showing that the cortical circuitry engaged in nociceptive processing is much more complex than previously recognized. Electrophysiological evidence supports the engagement of a distinct nociceptive-processing network within SI (i.e., areas 3a, 3b, 1 and 2), SII, and other areas along the lateral sulcus. Deafferentation caused by spinal cord injury profoundly alters the relationships between fMRI and electrophysiological signals. This finding has significant implications for using fMRI to study chronic pain conditions involving deafferentation in humans.

Testing Assumptions in Human Pain Models: Psychophysical Differences Between First and Second Pain

Acute pain arises from activation of myelinated (A delta) and unmyelinated (C) nociceptive afferents, leading to first (A-fiber) or second (C-fiber) pain sensations. The current study sought to investigate first and second pain within glabrous and hairy skin sites in human upper limbs. Fifty healthy adults (25 male/25 female, 18-30 years old, mean = 20.5 ± 1.4 years) participated in a psychophysical study investigating electronically rated, thermal first and second pain sensations within the glabrous skin at the palm and hairy skin of the forearm. Repeated measures analysis of variance indicated that the threshold for first pain was lower (more sensitive) than for second pain (P = .004), for glabrous as well as hairy skin, and thresholds at glabrous skin were higher than for hairy skin (P = .001). Hairy skin presented a steeper slope for testing, whereas there were no differences in slope between first and second pain. The study findings support assumptions associated with mechanistic differences between first and second pain sensations, while offering a novel method for producing first and second pain with the same thermal stimulus. Efforts to understand abnormalities among people with clinical pain and development of new therapeutic agents will benefit from specific psychophysical methods.

PERSPECTIVE

This article presents a novel method for directly comparing first and second pain within the same thermal stimulus. The ability to directly compare first and second pain sensations can aid in understanding pain abnormalities in clinical pain and development of therapeutic aids.

Toward more versatile and intuitive cortical brain-machine interfaces

Brain-machine interfaces have great potential for the development of neuroprosthetic applications to assist patients suffering from brain injury or neurodegenerative disease. One type of brain-machine interface is a cortical motor prosthetic, which is used to assist paralyzed subjects. Motor prosthetics to date have typically used the motor cortex as a source of neural signals for controlling external devices. The review will focus on several new topics in the arena of cortical prosthetics. These include using: recordings from cortical areas outside motor cortex; local field potentials as a source of recorded signals; somatosensory feedback for more dexterous control of robotics; and new decoding methods that work in concert to form an ecology of decode algorithms. These new advances promise to greatly accelerate the applicability and ease of operation of motor prosthetics.

Evidence of heterosynaptic LTD in the human nociceptive system: superficial skin neuromodulation using a matrix electrode reduces deep pain sensitivity

Long term depression (LTD) is a neuronal learning mechanism after low frequency stimulation (LFS). This study compares two types of electrodes (concentric vs. matrix) and stimulation frequencies (4 and 30 Hz) to examine homo- and heterosynaptic effects indirectly depicted from the somatosensory profile of healthy subjects. Both electrodes were compared in a prospective, randomized, controlled cross-over study using 4 Hz as the conditioning LFS compared to 30 Hz (intended sham condition). Quantitative sensory testing (QST) was used to examine 13 thermal and mechanical detection and pain thresholds. Sixteen healthy volunteers (10 women, age 31.0±12.7 years) were examined. Depending on the electrodes and frequencies used a divergent pattern of sensory minus signs occurred. Using LFS the concentric electrode increased thermal thresholds, while the matrix electrode rather increased mechanical including deep pain thresholds. Findings after cutaneous neuromodulation using LFS and a matrix electrode are consistent with the concept of heterosynaptic LTD in the human nociceptive system, where deep pain sensitivity was reduced after superficial stimulation of intraepidermal nerve fibres. Cutaneous neuromodulation using LFS and a matrix electrode may be a useful tool to influence deep pain sensitivity in a variety of chronic pain syndromes.

EEG frequency tagging to dissociate the cortical responses to nociceptive and nonnociceptive stimuli

Whether the cortical processing of nociceptive input relies on the activity of nociceptive-specific neurons or whether it relies on the activity of neurons also involved in processing nonnociceptive sensory input remains a matter of debate. Here, we combined EEG "frequency tagging" of steady-state evoked potentials (SS-EPs) with an intermodal selective attention paradigm to test whether the cortical processing of nociceptive input relies on nociceptive-specific neuronal populations that can be selectively modulated by top-down attention. Trains of nociceptive and vibrotactile stimuli (Experiment 1) and trains of nociceptive and visual stimuli (Experiment 2) were applied concomitantly to the same hand, thus eliciting nociceptive, vibrotactile, and visual SS-EPs. In each experiment, a target detection task was used to focus attention toward one of the two concurrent streams of sensory input. We found that selectively attending to nociceptive or vibrotactile somatosensory input indistinctly enhances the magnitude of nociceptive and vibrotactile SS-EPs, whereas selectively attending to nociceptive or visual input independently enhances the magnitude of the SS-EP elicited by the attended sensory input. This differential effect indicates that the processing of nociceptive input involves neuronal populations also involved in the processing of touch, but distinct from the neuronal populations involved in vision.

Role of primary somatosensory cortex in the coding of pain

The intensity and submodality of pain are widely attributed to stimulus encoding by peripheral and subcortical spinal/trigeminal portions of the somatosensory nervous system. Consistent with this interpretation are studies of surgically anesthetized animals, demonstrating that relationships between nociceptive stimulation and activation of neurons are similar at subcortical levels of somatosensory projection and within the primary somatosensory cortex (in cytoarchitectural areas 3b and 1 of somatosensory cortex, SI). Such findings have led to characterizations of SI as a network that preserves, rather than transforms, the excitatory drive it receives from subcortical levels. Inconsistent with this perspective are images and neurophysiological recordings of SI neurons in lightly anesthetized primates. These studies demonstrate that an extreme anterior position within SI (area 3a) receives input originating predominantly from unmyelinated nociceptors, distinguishing it from posterior SI (areas 3b and 1), long recognized as receiving input predominantly from myelinated afferents, including nociceptors. Of particular importance, interactions between these subregions during maintained nociceptive stimulation are accompanied by an altered SI response to myelinated and unmyelinated nociceptors. A revised view of pain coding within SI cortex is discussed, and potentially significant clinical implications are emphasized.

Subject-level differences in reported locations of cutaneous tactile and nociceptive stimuli

Recent theoretical advances on the topic of body representations have raised the question whether spatial perception of touch and nociception involve the same representations. Various authors have established that subjective localizations of touch and nociception are displaced in a systematic manner. The relation between veridical stimulus locations and localizations can be described in the form of a perceptual map; these maps differ between subjects. Recently, evidence was found for a common set of body representations to underlie spatial perception of touch and slow and fast pain, which receive information from modality specific primary representations. There are neurophysiological clues that the various cutaneous senses may not share the same primary representation. If this is the case, then differences in primary representations between touch and nociception may cause subject-dependent differences in perceptual maps of these modalities. We studied localization of tactile and nociceptive sensations on the forearm using electrocutaneous stimulation. The perceptual maps of these modalities differed at the group level. When assessed for individual subjects, the differences localization varied in nature between subjects. The agreement of perceptual maps of the two modalities was moderate. These findings are consistent with a common internal body representation underlying spatial perception of touch and nociception. The subject level differences suggest that in addition to these representations other aspects, possibly differences in primary representation and/or the influence of stimulus parameters, lead to differences in perceptual maps in individuals.

Centrally-mediated sensory information processing is impacted with increased alcohol consumption in college-aged individuals

Alcohol consumption can have an impact on a variety of centrally-mediated functions of the nervous system, and some aspects of sensory perception can be altered as a result of long-term alcohol use. In order to assess the potential impact of alcohol intake on sensory information processing, metrics of sensory perception (simple and choice reaction time; static and dynamic threshold detection; amplitude discrimination with and without pre-exposure to conditioning stimulation) were tested in college-aged subjects (18 to 26 years of age) across a broad range of levels of alcohol consumption. The analysis indicated no detectable associations between reaction time and threshold measures with alcohol consumption. However, measures of adaptation to short duration (0.5s) conditioning stimuli were significantly associated with alcohol consumption: the impact of a confounding conditioning stimulus on amplitude discriminative capacity was comparable to values reported in previous studies on healthy controls (28.9±8.6) for light drinkers while the same adaptation metric for heavy drinkers (consuming greater than 60 drinks per month) was significantly reduced (8.9±7.1). The results suggest that while some of the sensory perceptual metrics which are normally impacted in chronic alcoholism (e.g., reaction time and threshold detection) were relatively insensitive to change with increased alcohol consumption in young non-alcoholic individuals, other metrics, which are influenced predominantly by centrally-mediated mechanisms, demonstrate a deviation from normative values with increased consumption. Results of this study suggest that higher levels of alcohol consumption may be associated with alterations in centrally-mediated neural mechanisms in this age group.

Decreased perceptual learning ability in complex regional pain syndrome

Recently, several functional imaging studies have shown that sensorimotor cortical representations may be changed in complex regional pain syndromes (CRPS). Therefore, we investigated tactile performance and tactile learning as indirect markers of cortical changes in patients with CRPS type I and controls. Patients had significant higher spatial discrimination thresholds at CRPS-affected extremities compared to both unaffected sides and control subjects. Furthermore, in order to improve tactile spatial acuity we used a Hebbian stimulation protocol of tactile coactivation. This consistently improved tactile acuity, both in controls and patients. However, the gain of performance was significantly lower on the CRPS-affected side implying an impaired perceptual learning ability. Therefore, we provide further support for an involvement of the CNS in CRPS, which may have implications to future neurorehabilitation strategies for this disease.

A novel device for the study of somatosensory information processing

Current methods for applying multi-site vibratory stimuli to the skin typically involve the use of multiple, individual vibrotactile stimulators. Limitations of such an arrangement include difficulty with both positioning the stimuli as well as ensuring that stimuli are delivered in a synchronized and deliberate manner. Previously, we reported a two-site tactile stimulator that was developed in order to solve these problems (Tannan et al., 2007a). Due to both the success of that novel stimulator and the limitations that were inherent in that device, we designed and fabricated a four-site stimulator that provides a number of advantages over the previous version. First, the device can stimulate four independent skin sites and is primarily designed for stimulating the digit tips. Second, the positioning of the probe tips has been re-designed to provide better ergonomic hand placement. Third, the device is much more portable than the previously reported stimulator. Fourth, the stimulator head has a much smaller footprint on the table or surface where it resides. To demonstrate the capacity of the device for delivering tactile stimulation at four independent sites, a finger agnosia protocol, in the presence and absence of conditioning stimuli, was conducted on seventeen healthy control subjects. The study demonstrated that with increasing amplitudes of vibrotactile conditioning stimuli concurrent with the agnosia test, inaccuracies of digit identification increased, particularly at digits D3 and D4. The results are consistent with prior studies that implicated synchronization of adjacent and near-adjacent cortical ensembles with conditioning stimuli in impacting TOJ performance (Tommerdahl et al., 2007a,b).

Altered central sensitization in subgroups of women with vulvodynia

OBJECTIVE

To investigate the clinical correlates of central nervous system alterations among women with vulvodynia. Altered central sensitization has been linked to dysfunction in central nervous system-inhibitory pathways (e.g., γ-aminobutyric acidergic), and metrics of sensory adaptation, a centrally mediated process that is sensitive to this dysfunction, could potentially be used to identify women at risk of treatment failure using conventional approaches.

METHODS

Twelve women with vulvodynia and 20 age-matched controls participated in this study, which was conducted by sensory testing of the right hand's index and middle fingers. The following sensory precepts were assessed: (1) vibrotactile detection threshold; (2) amplitude discrimination capacity (defined as the ability to detect differences in intensity of simultaneously delivered stimuli to 2 fingers); and (3) a metric of adaptation (determined by the impact that applying conditioning stimuli have on amplitude discriminative capacity).

RESULTS

Participants did not differ on key demographic variables, vibrotactile detection threshold, and amplitude discrimination capacity. However, we found significant differences from controls in adaptation metrics in 1 subgroup of vulvodynia patients. Compared with healthy controls and women with a shorter history of pain [n=5; duration (y) = 3.4 ± 1.3], those with a longer history [n=7; duration (y) = 9.3 ± 1.4)] were found to be less likely to have adaptation metrics similar to control values.

DISCUSSION

Chronic pain is thought to lead to altered central sensitization, and adaptation is a centrally mediated process that is sensitive to this condition. This report suggests that similar alterations exist in a subgroup of vulvodynia patients.

Dipole source analyses of laser evoked potentials obtained from subdural grid recordings from primary somatic sensory cortex

The cortical potentials evoked by cutaneous application of a laser stimulus (laser evoked potentials, LEP) often include potentials in the primary somatic sensory cortex (S1), which may be located within the subdivisions of S1 including Brodmann areas 3A, 3B, 1, and 2. The precise location of the LEP generator may clarify the pattern of activation of human S1 by painful stimuli. We now test the hypothesis that the generators of the LEP are located in human Brodmann area 1 or 3A within S1. Local field potential (LFP) source analysis of the LEP was obtained from subdural grids over sensorimotor cortex in two patients undergoing epilepsy surgery. The relationship of LEP dipoles was compared with dipoles for somatic sensory potentials evoked by median nerve stimulation (SEP) and recorded in area 3B (see Baumgärtner U, Vogel H, Ohara S, Treede RD, Lenz FA. J Neurophysiol 104: 3029-3041, 2010). Both patients had an early radial dipole in S1. The LEP dipole was located medial, anterior, and deep to the SEP dipole, which suggests a nociceptive dipole in area 3A. One patient had a later tangential dipole with positivity posterior, which is opposite to the orientation of the SEP dipole in area 3B. The reversal of orientations between modalities is consistent with the cortical surface negative orientation resulting from superficial termination of thalamocortical neurons that receive inputs from the spinothalamic tract. Therefore, the present results suggest that the LEP may result in a radial dipole consistent with a generator in area 3A and a putative later tangential generator in area 3B.

High-resolution functional magnetic resonance imaging mapping of noxious heat and tactile activations along the central sulcus in New World monkeys

This study mapped the fine-scale functional representation of tactile and noxious heat stimuli in cortical areas around the central sulcus of anesthetized squirrel monkeys by using high-resolution blood oxygen level-dependent (BOLD) fMRI at 9.4T. Noxious heat (47.5°C) stimulation of digits evoked multiple spatially distinct and focal BOLD activations. Consistent activations were observed in areas 3a, 3b, 1, and 2, whereas less frequent activation was present in M1. Compared with tactile activations, thermal nociceptive activations covered more area and formed multiple foci within each functional area. In general, noxious heat activations in area 3b did not colocalize with tactile responses. The spatial relationships of heat and tactile activations in areas 3a and 1/2 varied across animals. Subsequent electrophysiological mapping confirmed that the evoked heat and tactile BOLD signals were somatotopically appropriate. The magnitude and temporal profiles of the BOLD signals to noxious heat stimuli differed across cortical areas. Comparatively late-peaking but stronger signals were observed in areas 3b and 2, whereas earlier-peaking but weaker signals were observed in areas 3a, 1, and M1. In sum, this study not only confirmed the involvement of somatosensory areas of 3a, 3b, and 1, but also identified the engagements of area 2 and M1 in the processing of heat nociceptive inputs. Differential BOLD response profiles of the individual cortical areas along the central sulcus suggest that these areas play different roles in the encoding of nociceptive inputs. Thermal nociceptive and tactile inputs may be processed by different clusters of neurons in different areas. To critically bridge animal and human pain studies, human fMRI was related to primate fMRI and electrophysiology of nociceptive processing, examining the functional role of the primary somatosensory cortex in heat nociception and demonstrating that subregion areas 3a, 3b, 1, 2, and M1 are responsive to noxious heat stimuli.

Attention to painful cutaneous laser stimuli evokes directed functional connectivity between activity recorded directly from human pain-related cortical structures

Our previous studies show that attention to painful cutaneous laser stimuli is associated with functional connectivity between human primary somatosensory cortex (SI), parasylvian cortex (PS), and medial frontal cortex (MF), which may constitute a pain network. However, the direction of functional connections within this network is unknown. We now test the hypothesis that activity recorded from the SI has a driver role, and a causal influence, with respect to activity recorded from PS and MF during attention to a laser. Local field potentials (LFP) were recorded from subdural grid electrodes implanted for the treatment of epilepsy. We estimated causal influences by using the Granger causality (GRC), which was computed while subjects performed either an attention task (counting laser stimuli) or a distraction task (reading for comprehension). Before the laser stimuli, directed attention to the painful stimulus (counting) consistently increased the number of GRC pairs both within the SI cortex and from SI upon PS (SI>PS). After the laser stimulus, attention to a painful stimulus increased the number of GRC pairs from SI>PS, and SI>MF, and within the SI area. LFP at some electrode sites (critical sites) exerted GRC influences upon signals at multiple widespread electrodes, both in other cortical areas and within the area where the critical site was located. Critical sites may bind these areas together into a pain network, and disruption of that network by stimulation at critical sites might be used to treat pain. Electrical activity recorded from the somatosensory cortex drives activity recorded elsewhere in the pain network and may bind the network together; disruption of that network by stimulation at critical sites might be used to treat pain.

Nociceptive afferent activity alters the SI RA neuron response to mechanical skin stimulation

Procedures that reliably evoke cutaneous pain in humans (i.e., 5-7 s skin contact with a 47-51 °C probe, intradermal algogen injection) are shown to decrease the mean spike firing rate (MFR) and degree to which the rapidly adapting (RA) neurons in areas 3b/1 of squirrel monkey primary somatosensory cortex (SI) entrain to a 25-Hz stimulus to the receptive field center (RF(center)) when stimulus amplitude is "near-threshold" (i.e., 10-50 μm). In contrast, RA neuron MFR and entrainment are either unaffected or enhanced by 47-51 °C contact or intradermal algogen injection when the amplitude of 25-Hz stimulation is 100-200 μm (suprathreshold). The results are attributed to an "activity dependence" of γ-aminobutyric acid (GABA) action on the GABA(A) receptors of RA neurons. The nociceptive afferent drive triggered by skin contact with a 47-51 °C probe or intradermal algogen is proposed to activate nociresponsive neurons in area 3a which, via corticocortical connections, leads to the release of GABA in areas 3b/1. It is hypothesized that GABA is hyperpolarizing/inhibitory and suppresses stimulus-evoked RA neuron MFR and entrainment whenever RA neuron activity is low (as when the RF(center) stimulus is weak/near-threshold) but is depolarizing/excitatory and augments MFR and entrainment when RA neuron activity is high (when the stimulus is strong/suprathreshold).

Temporomandibular disorder modifies cortical response to tactile stimulation

Individuals with temporomandibular disorder (TMD) suffer from persistent facial pain and exhibit abnormal sensitivity to tactile stimulation. To better understand the pathophysiological mechanisms underlying TMD, we investigated cortical correlates of this abnormal sensitivity to touch. Using functional magnetic resonance imaging (fMRI), we recorded cortical responses evoked by low-frequency vibration of the index finger in subjects with TMD and in healthy controls (HC). Distinct subregions of contralateral primary somatosensory cortex (SI), secondary somatosensory cortex (SII), and insular cortex responded maximally for each group. Although the stimulus was inaudible, primary auditory cortex was activated in TMDs. TMDs also showed greater activation bilaterally in anterior cingulate cortex and contralaterally in the amygdala. Differences between TMDs and HCs in responses evoked by innocuous vibrotactile stimulation within SI, SII, and the insula paralleled previously reported differences in responses evoked by noxious and innocuous stimulation, respectively, in healthy individuals. This unexpected result may reflect a disruption of the normal balance between central resources dedicated to processing innocuous and noxious input, manifesting itself as increased readiness of the pain matrix for activation by even innocuous input. Activation of the amygdala in our TMD group could reflect the establishment of aversive associations with tactile stimulation due to the persistence of pain.

PERSPECTIVE

This article presents evidence that central processing of innocuous tactile stimulation is abnormal in TMD. Understanding the complexity of sensory disruption in chronic pain could lead to improved methods for assessing cerebral cortical function in these patients.

Extreme thermal sensitivity and pain-induced sensitization in a fibromyalgia patient

During the course of a psychophysical study of fibromyalgia syndrome (FMS), one of the subjects with a long history of headache and facial pain displayed an extraordinarily severe thermal allodynia. Her stimulus-response function for ratings of cutaneous heat pain revealed a sensitivity clearly beyond that of normal controls and most FMS subjects. Specially designed psychophysical methods showed that heat sensitivity sometimes increased dramatically within a series of stimuli. Prior exposure to moderate heat pain served as a trigger for allodynic ratings of series of normally neutral thermal stimulation. These observations document a case of breakthrough pain sensitivity with implications for mechanisms of FMS pain.

A multi-scale view of skin thermal pain: from nociception to pain sensation

All biological bodies live in a thermal environment, including the human body, where skin is the interface with a protecting function. When the temperature is out of the normal physiological range, skin fails to protect, and the pain sensation is evoked. Furthermore, in medicine, with advances in laser, microwave and similar technologies, various thermal therapeutic methods have been widely used to cure disease/injury involving skin tissue. However, the corresponding problem of pain relief has limited further application and development of these thermal treatments. Skin thermal pain is induced through both direct (i.e. an increase/decrease in temperature) and indirect (e.g. thermomechanical and thermochemical) ways, and is governed by complicated thermomechanical-chemical-neurophysiological responses. However, a complete understanding of the underlying mechanisms is still far from clear. In this article, starting from an engineering perspective, we aim to recast the biological behaviour of skin in engineering system parlance. Then, by coupling the concepts of engineering with established methods in neuroscience, we attempt to establish multi-scale modelling of skin thermal pain through ion channel to pain sensation. The model takes into account skin morphological plausibility, the thermomechanical response of skin tissue and the biophysical and neurological mechanisms of pain sensation.

The impact of non-noxious heat on tactile information processing

A significant number of studies that evaluated tactile-pain interactions employed heat to evoke nociceptive responses. However, relatively few studies have examined the effects of non-noxious thermal stimulation on tactile discriminative capacity. In this study, the impact that non-noxious heat had on three features of tactile information processing capacity was evaluated: vibrotactile threshold, amplitude discriminative capacity, and adaptation. It was found that warming the skin made a significant improvement on a subject's ability to detect a vibrotactile stimulus, and although the subjects' capacities for discriminating between two amplitudes of vibrotactile stimulation did not change with skin heating, the impact that adapting or conditioning stimulation normally had on amplitude discrimination capacity was significantly attenuated by the change in temperature. These results suggested that although the improvements in tactile sensitivity that were observed could have been a result of enhanced peripheral activity, the changes in measures that reflect a decrease in the sensitization to repetitive stimulation are most likely centrally mediated. The authors speculate that these centrally mediated changes could be a reflection of a change in the balance of cortical excitation and inhibition.

Nociceptive laser-evoked brain potentials do not reflect nociceptive-specific neural activity

Brief radiant laser pulses can be used to activate cutaneous Adelta and C nociceptors selectively and elicit a number of transient brain responses [laser-evoked potentials (LEPs)] in the ongoing EEG. LEPs have been used extensively in the past 30 years to gain knowledge about the cortical mechanisms underlying nociception and pain in humans, by assuming that they reflect at least neural activities uniquely or preferentially involved in processing nociceptive input. Here, by applying a novel blind source separation algorithm (probabilistic independent component analysis) to 124-channel event-related potentials elicited by a random sequence of nociceptive and non-nociceptive somatosensory, auditory, and visual stimuli, we provide compelling evidence that this assumption is incorrect: LEPs do not reflect nociceptive-specific neural activity. Indeed, our results indicate that LEPs can be entirely explained by a combination of multimodal neural activities (i.e., activities also elicited by stimuli of other sensory modalities) and somatosensory-specific, but not nociceptive-specific, neural activities (i.e., activities elicited by both nociceptive and non-nociceptive somatosensory stimuli). Regardless of the sensory modality of the eliciting stimulus, the magnitude of multimodal activities correlated with the subjective rating of saliency, suggesting that these multimodal activities are involved in stimulus-triggered mechanisms of arousal or attentional reorientation.

Area-specific representation of mechanical nociceptive stimuli within SI cortex of squirrel monkeys

While functional imaging studies in humans have consistently reported activation of primary somatosensory cortex (SI) with painful stimuli, the specific roles of subdivisions of areas 3a, 3b, and 1 within SI during pain perception are largely unknown, particularly in the representation of mechanical evoked pain. In this study, we investigated how modality, location, and intensity of nociceptive stimuli are represented within SI by using high-spatial resolution optical imaging of intrinsic signals in Pentothal-anesthetized squirrel monkeys. Perceptually comparable mechanical nociceptive and innocuous tactile stimuli were delivered by indenting the glabrous skin of the distal finger pads with 0.2 and 2mm diameter probes, respectively. Within each of areas 3a, 3b, and 1, activations to mechanical nociceptive stimulation of individual distal finger pads were spatially distinct and somatotopically organized. We observed differential cortical activation patterns. Areas 3a, 3b, and 1 were all activated during mechanical nociceptive stimulation and were modulated by nociceptive stimulus intensity. However, with innocuous tactile stimulation, mainly areas 3b and 1 exhibited response modulation with different levels of stimulation. In summary, mechanical nociceptive inputs are area-specific and topographically represented within SI. We propose that all areas of SI are implicated in encoding the features of mechanical nociception, where areas 3a and 3b are distinctively involved in coding nociceptive and pressure sensation components of stimulation.

Area 3a neuron response to skin nociceptor afferent drive

Area 3a neurons are identified that respond weakly or not at all to skin contact with a 25-38 degrees C probe, but vigorously to skin contact with the probe at > or =49 degrees C. Maximal rate of spike firing associated with 1- to 7-s contact at > or =49 degrees C occurs 1-2 s after probe removal from the skin. The activity evoked by 5-s contact with the probe at 51 degrees C remains above-background for approximately 20 s after probe retraction. After 1-s contact at 55-56 degrees C activity remains above-background for approximately 4 s. Magnitude of spike firing associated with 5-s contact increases linearly as probe temperature is increased from 49-51 degrees C. Intradermal capsaicin injection elicits a larger (approximately 2.5x) and longer-lasting (100x) increase in area 3a neuron firing rate than 5-s contact at 51 degrees C. Area 3a neurons exhibit enhanced or novel responsivity to 25-38 degrees C contact for a prolonged time after intradermal injection of capsaicin or alpha, beta methylene adenosine triphosphate. Their 1) delayed and persisting increase in spike firing in response to contact at > or =49 degrees C, 2) vigorous and prolonged response to intradermal capsaicin, and 3) enhanced and frequently novel response to 25-38 degrees C contact following intradermal algogen injection or noxious skin heating suggest that the area 3a neurons identified in this study contribute to second pain and mechanical hyperalgesia/allodynia.

Functional imaging of sensory decline and gain induced by differential noxious stimulation

It is increasingly recognized that pain-induced plasticity may provoke secondary sensory decline, i.e. centrally-mediated hypoesthesia and hypoalgesia. We investigated perceptual changes induced by conditioning electrical stimulation of C-nociceptors differing in stimulation frequencies and duty cycles provoking either sensory gain (i.e. mechanical hyperalgesia; Stim1) or sensory decline (i.e. hypoesthesia and hypoalgesia; Stim2). Underlying brain processing was investigated using functional magnetic resonance imaging. Before conditioning stimuli, tactile stimulation and pin-prick stimuli led to differential activations of primary and secondary somatosensory cortices (S1, S2), insula and prefrontal cortices (PFC). After induction of mechanical hyperalgesia (Stim1), increased activations were detected in somatosensory/pain-related areas (S1, S2, insula, cingulate cortex) and networks involved in attentional and cognitive processing (parieto-frontal, parieto-cingulate and frontal circuits). In contrast, after induction of hypoesthesia and hypoalgesia (Stim2) the degree of sensory decline for touch and mechanical pain was directly correlated with deactivations within S1, whereas networks associated with attentional and cognitive processing showed increased activation. Therefore, our results demonstrate that brain processing underlying pain-induced sensory gain substantially differs from pain-induced sensory decline. A potential neurobiological mechanism of secondary CNS-mediated hypoesthesia and hypoalgesia may involve modification of local inhibitory networks within somatosensory cortices.

Diffuse optical tomography of pain and tactile stimulation: activation in cortical sensory and emotional systems

Using diffuse optical tomography (DOT), we detected activation in the somatosensory cortex and frontal brain areas following tactile (brush) and noxious heat stimulation. Healthy volunteers received stimulation to the dorsum of the right hand. In the somatosensory cortex area, tactile stimulation produced a robust, contralateral to the stimulus, hemodynamic response with a weaker activation on the ipsilateral side. For the same region, noxious thermal stimuli produced bilateral activation of similar intensity that had a prolonged activation with a double peak similar to results that have been reported with functional MRI. Bilateral activation was observed in the frontal areas, oxyhemoglobin changes were positive for brush stimulation while they were initially negative (contralateral) for heat stimulation. These results suggest that based on the temporal and spatial characteristics of the response in the sensory cortex, it is possible to discern painful from mechanical stimulation using DOT. Such ability might have potential applications in a clinical setting in which pain needs to be assessed objectively (e.g., analgesic efficacy, pain responses during surgery).

Whole-hand sensorimotor area: cortical stimulation localization and correlation with functional magnetic resonance imaging

OBJECT

The pli de passage moyen (PPM) is an omega-shaped cortical landmark bulging into the central sulcus. There has been considerable interest in the PPM given that hand motor and sensory tasks have been found on functional magnetic resonance (fMR) imaging to activate the structure. Note, however, that the cortical function subserved by the PPM is not completely understood. Finger and thumb function are somatotopically organized over the central area and encompass a larger cortical surface than the anatomical PPM. Therefore, a sensory or motor hand area within the PPM would be redundant with the somatotopically organized digit function in the primary sensorimotor cortex. In this study the authors aimed to clarify the function subserved by the PPM and further evaluate hand area function in the primary sensorimotor cortex.

METHODS

To further elucidate the function subserved by the PPM, patients underwent cortical stimulation in the region of the PPM as well as fMR imaging-demonstrated activation of the hand area. Two separate analytical methods were used to correlate hand area functional imaging with whole-hand sensory and motor responses induced by cortical stimulation.

RESULTS

A relationship of the anatomical PPM with cortical stimulation responses as well as hand fMR imaging activation was observed.

CONCLUSIONS

A strong relationship was identified between the PPM, whole-hand sensory and motor stimulation responses, and fMR imaging hand activation. Whole-hand motor and whole-hand sensory cortical regions were identified in the primary sensorimotor cortex. It was localized to the PPM and exists in addition to the somatotopically organized finger and thumb sensory and motor areas.

High-resolution fMRI maps of cortical activation in nonhuman primates: correlation with intrinsic signal optical images

One of the most widely used functional brain mapping tools is blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI). This method has contributed to new understandings of the functional roles of different areas in the human brain. However, its ability to map cerebral cortex at high spatial (submillimeter) resolution is still unknown. Other methods such as single- and multiunit electrophysiology and intrinsic signal optical imaging have revealed submillimeter resolution of sensory topography and cortical columnar activations. However, they are limited either by spatial scale (electrophysiology characterizes only local groups of neurons) or by the inability to monitor deep structures in the brain (i.e., cortical regions buried in sulci or subcortical structures). A method that could monitor all regions of the brain at high spatial resolution would be ideal. This capacity would open the doors to investigating, for example, how networks of cerebral cortical columns relate to or produce behavior. In this article we demonstrate that, without benefit of contrast agents, at a magnetic field strength of 9.4 tesla, BOLD fMRI can reveal millimeter-sized topographic maps of digit representation in the somatosensory cortex of the anesthetized squirrel monkey. Furthermore, by mapping the "funneling illusion," it is possible to detect even submillimeter shifts in activation in the cortex. Our data suggest that at high magnetic field strength, the positive BOLD signal can be used to reveal high spatial resolution maps of brain activity, a finding that weakens previous notions about the ultimate spatial specificity of the positive BOLD signal.

Interference of tactile and pain stimuli on thalamocortical signal processing in humans revealed by median nerve SEPs

OBJECTIVE

We investigated the interference of tactile and painful stimuli on human early somatosensory evoked potentials (SEPs) including high frequency oscillations (HFOs) to further study thalamocortical processing of somatosensory information.

METHODS

Multi-channel median nerve SEPs were recorded during (1) no interference, (2) sensory interference by tactile stimulation to digits 2 and 3, and (3) application of pain to the same digits. Spatio-temporal source analysis separated brain stem (S1), thalamic (S2) and two cortical sources (S3, S4), which were evaluated for the low (20-450 Hz) and high (450-750 Hz) frequency portion of the signal.

RESULTS

Low frequency SEPs showed a decrease of activity at cortical source S3 during both conditions, while thalamic source S2 was significantly increased during pain interference. HFOs showed an increase of cortical source S3 and in trend of thalamic source S2 and cortical source S4 during both kinds of interference.

CONCLUSIONS

Although the painful stimulus might not be specific for the nociceptive afferents, the present data affirm that at this early stage of sensory information processing within the primary sensory cortex (area 3b, area 1) pain is handled similar to sensory interference.

SIGNIFICANCE

HFOs might represent an intrinsic "somatosensory alerting" system which reacts to both interference stimuli in a similar way, therefore indicating an interference without a qualitative evaluation.

A portable tactile sensory diagnostic device

Current methods for applying multi-site vibratory stimuli to the skin typically involve the use of two separate vibrotactile stimulators, which can lead to difficulty with positioning of stimuli and in ensuring that stimuli are delivered perfectly in phase at the same amplitude and frequency. Previously, we reported a two-point stimulator (TPS) that was developed in order to solve the problem of delivering two-point stimuli to the skin at variable distances between the sites of stimulation. Because of the success of the TPS, we designed and fabricated a new stimulator with four significant improvements over our original device. First, the device is portable, lightweight and can be used in a variety of non-laboratory settings. Second, the device consists of two independently controlled stimulators which allow delivery of stimuli simultaneously to two distinct skin sites with different amplitude, frequency and/or phase. Third, the device automatically detects the skin surface and thus allows for much better automated control of stimulus delivery. Fourth, the device is designed for rapid manufacture and, therefore, can be made readily available to other research (non-laboratory) settings. To demonstrate the device, a modified Bekesy tracking method was used to evaluate the simultaneous amplitude discrimination capacity of 20 subjects.

Endogenous and exogenous modulators of potentials evoked by a painful cutaneous laser (LEPs)

Little is known about the specific functions of the human cortical structures receiving nociceptive input, their relationship to various dimensions of pain, and the modulation of these inputs by attention. We now review studies demonstrating the subdural potentials evoked by a cutaneous laser stimulus which produces a pure pain sensation by selective activation of cutaneous nociceptors (LEPs). These LEPs were localized over human anterior and middle cingulate (A & MCC), somatosensory (S1) and parasylvian (PS) cortices. LEP, lesion and imaging data define pain-related elements within each of these structures: insula and parietal operculum within PS, anterior and middle cingulate cortex, and possibly Brodman's areas 3a, 3b and 1 within SI. LEPs recorded over each of these areas is modulated with laser intensity and evoked pain. Attention to the painful laser produces an increase in the amplitude of LEPs over all three cortical areas and emergence of a late positive potential over ACC alone. These studies provide clear evidence of human cortical structures receiving nociceptive input and the modulation of that input by exogenous (e.g. laser intensity) and endogenous factors (e.g. directed attention).

Brain activity related to temporal summation of C-fiber evoked pain

Temporal summation of "second pain" (TSSP) is considered to be the result of C-fiber-evoked responses of dorsal horn neurons, termed 'windup'. This phenomenon is dependent on stimulus frequency (0.33 Hz) and relevant for central sensitization and chronic pain. Previous brain imaging studies have only been used to characterize neural correlates of second pain but not its temporal summation. We utilized functional magnetic resonance imaging (fMRI) in healthy volunteers to measure brain responses associated with TSSP. Region of interest analysis was used to assess TSSP related brain activation. Eleven pain-free normal subjects underwent fMRI scanning during repetitive heat pulses to the right foot at 0.33 and 0.17 Hz. Stimulus intensities were adjusted to each individual's heat sensitivity to achieve comparable TSSP ratings of moderate pain in all subjects. As predicted, experimental pain ratings showed robust TSSP during 0.33 Hz but not 0.17 Hz stimuli. fMRI statistical maps identified several brain regions with stimulus and frequency dependent activation consistent with TSSP, including contralateral thalamus (THAL), S1, bilateral S2, anterior and posterior insula (INS), mid-anterior cingulate cortex (ACC), and supplemental motor areas (SMA). TSSP ratings were significantly correlated with brain activation in somatosensory areas (THAL, S1, left S2), anterior INS, and ACC. These results show that neural responses related to TSSP are evoked in somatosensory processing areas (THAL, S2), as well as in multiple areas that serve other functions related to pain, such as cognition (ACC, PFC), affect (INS, ACC, PAG), pre-motor activity (SMA, cerebellum), and pain modulation (rostral ACC).

Long-term optical imaging of intrinsic signals in anesthetized and awake monkeys

Some exciting new efforts to use intrinsic signal optical imaging methods for long-term studies in anesthetized and awake monkeys are reviewed. The development of such methodologies opens the door for studying behavioral states such as attention, motivation, memory, emotion, and other higher-order cognitive functions. Long-term imaging is also ideal for studying changes in the brain that accompany development, plasticity, and learning. Although intrinsic imaging lacks the temporal resolution offered by dyes, it is a high spatial resolution imaging method that does not require application of any external agents to the brain. The bulk of procedures described here have been developed in the monkey but can be applied to the study of surface structures in any in vivo preparation.

Differentiating hemodynamic responses in rat primary somatosensory cortex during non-noxious and noxious electrical stimulation by optical imaging

Nociception in the primary somatosensory (S1) cortex remains in need of further elucidation. The spatiotemporal comparison on changes of the cerebral blood volume evoked by graded peripheral electrical stimulation was performed in rat contralateral somatosensory cortex with optical intrinsic signal imaging (OISI, optical reflectance at 550 nm). Non-noxious electrical stimulus was applied with 5 Hz pulses (0.5 ms peak duration) for 2 s at the threshold current for muscle twitch, while noxious stimulus was delivered at currents of 10x and 20x amplitude of the predetermined threshold. Although the dimensions of peak response defined in the spatial domain (cerebral blood volume increase) in the S1 cortex presented no significant difference under non-/noxious stimuli, its early response component (about 1 s after stimulation onset) revealed by OISI technique was suggested to differentiate the loci of activated cortical region due to different stimulation in this study. The magnitude and duration of the optical intrinsic signal (OIS) response was found increasing with the varying stimulus intensity. Regions activated by the delivery of a noxious stimulus were surrounded by a ring of inverted optical intrinsic signal, the amplitude of that was inversely proportional to the strength of the optical signal attributable to activation. Intense stimuli significantly augmented the inverted optical signal in magnitude and spatial extent. These results indicated that noxious stimulation evoked different response patterns in the contralateral S1 cortex. The magnitude-dependent inverted optical signal might contribute to the differentiation of nociceptive input in the S1 cortex.

Vibrotactile adaptation enhances spatial localization

A two-interval forced choice tracking procedure was used to evaluate the effects of a pre-exposure to vibrotactile stimulation ("adaptation") on the capacity of human subjects to spatially localize a subsequent tactile stimulus. A 25 Hz flutter adapting stimulus was presented at a randomly selected position within a 20 mm linear array oriented transversely on the hand dorsum. Two flutter stimuli delivered subsequently were applied to different sites along the linear array; one to the same locus that received the adapting stimulation (the "standard" stimulus), the other to a distant site (the "test" stimulus). Following each trial, subjects were queried as to which of the two stimuli was delivered to the same skin site that received adapting stimulation. A correct response resulted in a reduced distance between the sites contacted by the standard and test stimuli in the following trial. Four subjects participated in 10 sessions each. A session consisted of two sets of 20 trials (one set at 0.5 s and another at 5 s adapting stimulus duration). For every subject, 5 s adaptation resulted in an approximately 2-fold improvement in spatial discrimination performance over that achieved following 0.5 s adaptation. It is proposed that the improved human vibrotactile spatial localization performance following 5 s of 25 Hz stimulation is due to enhanced spatial funneling of the global neuronal population response of primary somatosensory cortex (SI) that has been demonstrated to accompany increases in duration of 25 Hz flutter stimuli delivered to the skin.

Optical imaging of SI topography in anesthetized and awake squirrel monkeys

Orderly topographic maps in the primary somatosensory cortex (SI) serve as an anchor for our understanding of somatosensory cortical organization. However, this view is mostly based on data collected in the anesthetized animal. Less is known about these topographies in the awake primate. Even less is known about the relative activations of different subdivisions of SI (areas 3a, 3b, 1, and 2). Toward the goal of understanding the functional activation of SI, we conducted intrinsic signal optical imaging of areas 3b and 1 in awake squirrel monkeys. Monkeys were imaged repeatedly for a period of >2 years in awake and anesthetized states in response to vibrotactile and electrocutaneous stimuli presented to individual fingerpads. During this period, we found stable somatotopic maps in both the anesthetized and awake states, consistent with electrophysiologically recorded maps in areas 3b and 1 in the anesthetized state. In the awake animal, signal sizes were larger, but variability was greater, leading to decreased signal-to-noise ratios. Topographic activations were larger (in both area and amplitude) in the awake animal, suggesting either a less precise topography and/or more complex integration. This brings into question the role of a precise topographic map during behavior. In addition, whereas in the anesthetized animal strongest imaging signals were obtained from area 3b, in the awake animal, area 1 activation dominated over that in area 3b. Differences in relative dominance of area 3b versus area 1 suggest that inter-areal interactions in the alert animal differ substantially from that in the anesthetized animal.

Activation of cat SII cortex by flutter stimulation of contralateral vs. ipsilateral forepaws

A distinguishing feature of SII cortex is that it receives substantial input from skin mechanoreceptors located on both sides of the body. It remains uncertain, however, if integration of bilateral inputs occurs mainly in those regions of SII that represent near-midline body regions or also occurs to a significant extent in those regions of SII that represent the distal extremities. This issue was addressed using extracellular microelectrode recordings in cat SII in combination with the method of optical intrinsic signal (OIS) imaging. Stimulation of the central pad of either the contra- or ipsilateral forepaw with a 25-Hz sinusoidal vertical skin displacement ("skin flutter") stimulus evoked a prominent OIS response ("activation") in an extensive anteroposterior sector of SII. In the anteriorly located SII region that yielded the maximal OIS response to stimulation of the contralateral central pad, neurons consistently possessed receptive fields that included the stimulated skin site. This "forepaw" SII region also exhibited significant although 75% weaker OIS activation in response to stimulation of the ipsilateral central pad. Stimulation of the central pads of either contra- or ipsilateral forepaws also evoked OIS activation in the posteriorly located 'hindlimb' region of SII--defined as the SII region comprised of neurons with receptive fields on the contralateral hindlimb. The OIS response to ipsilateral central pad stimulation was strongest in the posterior SII region that borders the suprasylvian fringe--a region in which neurons have very large, and frequently bilateral, cutaneous receptive fields. The results indicate that widespread regions within cat SII receive cutaneous inputs from the ipsilateral distal forelimb. It is suggested that the functional role of these ipsilateral inputs may be different in different SII regions.

Cortical reorganization in the aging brain

Aging exerts major reorganization and remodeling at all levels of brain structure and function. Studies in aged animals and in human elderly individuals demonstrate that sensorimotor cortical representational maps undergo significant alterations. Because cortical reorganization is paralleled by a decline in perceptual and behavioral performance, this type of cortical remodeling differs from the plastic reorganization observed during learning processes in young individuals where map changes are associated with a gain in performance. It is now clear that brain plasticity is operational into old age; therefore, protocols for interventions such as training, exercising, practicing, and stimulation, which make use of neuroplasticity principles, are effective to ameliorate some forms of cortical and behavioral age-related changes, indicating that aging effects are not irreversible but treatable. However, old individuals cannot be rejuvenated, but restoration of function is possible through the emergence of new processing strategies. This implies that cortical reorganization in the aging brain occurs twice: during aging, and during treatment of age-related changes.

Stimulus-dependent spatial patterns of response in SI cortex

BACKGROUND

Recently we reported that vibrotactile flutter stimulation of a skin locus at different amplitudes evokes an optical response confined to the same local region of the primary somatosensory cortex (SI), where its overall magnitude varies proportionally to the flutter amplitude. In this report, we characterize the impact of the flutter amplitude on the spatial patterns of activity evoked within the responding SI region.

RESULTS

In order to characterize the spatial pattern of activity within the responding SI region, images of the flutter-evoked SI optical response were segmented and analyzed with spatial frequency analysis. The analysis revealed that: (1) dominant spatial frequencies in the optical intrinsic signal emerge within the responding SI region within 3-5 sec of stimulus onset; (2) the stimulus-evoked activity is spatially organized in a form of several roughly parallel, anterior-posteriorly extended waves, spaced 0.4-0.5 mm apart; (3) the waves themselves exhibit spatial periodicities along their long axis; and (4) depending on the flutter stimulus amplitude, these periodicities can range from fine 0.15 mm "ripples" at 50 microm amplitude to well-developed 0.5 mm fluctuations at the amplitude of 400 microm.

CONCLUSION

The observed spatiointensive fractionation on a sub-macrocolumnar scale of the SI response to skin stimulation might be the product of local competitive interactions within the stimulus-activated SI region and may be a feature that could yield novel insights into the functional interactions that take place in SI cortex.