Lesions of mature barrel field cortex interfere with sensory processing and plasticity in connected areas of the contralateral hemisphere

A latent pool of neurons silenced by sensory-evoked inhibition can be recruited to enhance perception

To investigate which activity patterns in sensory cortex are relevant for perceptual decision-making, we combined two-photon calcium imaging and targeted two-photon optogenetics to interrogate barrel cortex activity during perceptual discrimination. We trained mice to discriminate bilateral whisker deflections and report decisions by licking left or right. Two-photon calcium imaging revealed sparse coding of contralateral and ipsilateral whisker input in layer 2/3, with most neurons remaining silent during the task. Activating pyramidal neurons using two-photon holographic photostimulation evoked a perceptual bias that scaled with the number of neurons photostimulated. This effect was dominated by optogenetic activation of non-coding neurons, which did not show sensory or motor-related activity during task performance. Photostimulation also revealed potent recruitment of cortical inhibition during sensory processing, which strongly and preferentially suppressed non-coding neurons. Our results suggest that a pool of non-coding neurons, selectively suppressed by network inhibition during sensory processing, can be recruited to enhance perception.

Left Barrel Cortical Neurons Activity following Transplantation of Stem Cells into Right Lesioned-Barrel Cortex in Rats

OBJECTIVE

Stem cells (SCs) can improve the functional defects of brain injury. Rodents use their whiskers to get tactile information from their surroundings. The aim of this study was to investigate whether the transplantation of SCs into the lesioned barrel cortex can help neuronal function in the contralateral cortex.

MATERIALS AND METHODS

Sixteen male Wistar rats (200-230 g) were used in this experimental study. We induced a mechanical lesion in the right barrel cortex area of rats by removing this area by a 3 mm skin punch. Four groups containing one intact group of rats: group 1: control, and three lesion groups, group 2: lesion+un-differentiated dental pulp SCs (U-DPSCs), group 3: lesion+differentiated dental pulp SCs (D-DPSCs), and group 4: cell medium (vehicle) that were injected in the lesion area. Three weeks after transplantation of SCs or cell medium, the rats' responses of left barrel cortical neurons to controlled deflections of right whiskers were recorded by using the extracellular single-unit recordings technique.

RESULTS

The results showed that the neural spontaneous activity and response magnitude of intact barrel cortex neurons in the lesion group decreased significantly (P<0.05) compared to the control group while ON and OFF responses were improved in the D-DPSCs (P<0.001) group compared to the vehicle group three weeks after transplantation.

CONCLUSION

Transplantation of dental pulp mesenchymal SCs significantly improved the neural responses of the left barrel cortex that was depressed in the vehicle group.

Functional ultrasound imaging of stroke in awake rats

Anesthesia is a major confounding factor in preclinical stroke research as stroke rarely occurs in sedated patients. Moreover, anesthesia affects both brain functions and the stroke outcome acting as neurotoxic or protective agents. So far, no approaches were well suited to induce stroke while imaging hemodynamics along with simultaneous large-scale recording of brain functions in awake animals. For this reason, the first critical hours following the stroke insult and associated functional alteration remain poorly understood. Here, we present a strategy to investigate both stroke hemodynamics and stroke-induced functional alterations without the confounding effect of anesthesia, i.e., under awake condition. Functional ultrasound (fUS) imaging was used to continuously monitor variations in cerebral blood volume (CBV) in +65 brain regions/hemispheres for up to 3 hr after stroke onset. The focal cortical ischemia was induced using a chemo-thrombotic agent suited for permanent middle cerebral artery occlusion in awake rats and followed by ipsi- and contralesional whiskers stimulation to investigate on the dynamic of the thalamocortical functions. Early (0-3 hr) and delayed (day 5) fUS recording enabled to characterize the features of the ischemia (location, CBV loss), spreading depolarizations (occurrence, amplitude) and functional alteration of the somatosensory thalamocortical circuits. Post-stroke thalamocortical functions were affected at both early and later time points (0-3 hr and 5 days) after stroke. Overall, our procedure facilitates early, continuous, and chronic assessments of hemodynamics and cerebral functions. When integrated with stroke studies or other pathological analyses, this approach seeks to enhance our comprehension of physiopathologies towards the development of pertinent therapeutic interventions.

The functional characterization of callosal connections

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The functional characterization of callosal connections is informed by anatomical data.

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Callosal connections play a conditional driving role depending on the brain state and behavioral demands.

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Callosal connections play a modulatory function, in addition to a driving role.

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The corpus callosum participates in learning and interhemispheric transfer of sensorimotor habits.

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The corpus callosum contributes to language processing and cognitive functions.

The brain operates through the synaptic interaction of distant neurons within flexible, often heterogeneous, distributed systems. Histological studies have detailed the connections between distant neurons, but their functional characterization deserves further exploration. Studies performed on the corpus callosum in animals and humans are unique in that they capitalize on results obtained from several neuroscience disciplines. Such data inspire a new interpretation of the function of callosal connections and delineate a novel road map, thus paving the way toward a general theory of cortico-cortical connectivity. Here we suggest that callosal axons can drive their post-synaptic targets preferentially when coupled to other inputs endowing the cortical network with a high degree of conditionality. This might depend on several factors, such as their pattern of convergence-divergence, the excitatory and inhibitory operation mode, the range of conduction velocities, the variety of homotopic and heterotopic projections and, finally, the state-dependency of their firing. We propose that, in addition to direct stimulation of post-synaptic targets, callosal axons often play a conditional driving or modulatory role, which depends on task contingencies, as documented by several recent studies.

Repetitive microstimulation in rat primary somatosensory cortex (SI) strengthens the connection between homotopic sites in the opposite SI and leads to expression of previously ineffective input from the ipsilateral forelimb

The primary somatosensory cortex (SI) receives input from the contralateral forelimb and projects to homotopic sites in the opposite SI. Since homotopic sites in SI are linked by a callosal pathway, we proposed that repetitive intracortical microstimulation (ICMSr) of neurons in layer V of SI forelimb cortex would increase spike firing in the opposite SI cortex thereby strengthening the callosal pathway sufficiently to allow normally ineffective stimuli from the ipsilateral forelimb to excite cells in the ipsilateral SI. The forelimb representation in SI in one hemisphere was mapped using mechanical and electrical stimulation of the contralateral forelimb, a homotopic site was similarly identified in the opposite SI, the presence of ipsilateral peripheral input was tested in both homotopic sites, and ICMS was used to establish an interhemispheric connection between the two homotopic recording sites. The major findings are: (1) each homotopic forelimb site in SI initially received short latency input only from the contralateral forelimb; (2) homotopic sites in layer V in each SI were interconnected by a callosal pathway; (3) ICMSr delivered to layer V of the homotopic SI in one hemisphere generally increased evoked response spike firing in layer V in the opposite homotopic site; (4) increased spike firing was often followed by the expression of a longer latency normally ineffective input from the ipsilateral forelimb; (5) these longer latency ipsilateral responses are consistent with a delay time sufficient to account for travel across the callosal pathway; (6) increased spike firing and the resulting ipsilateral peripheral input were also corroborated using in-vivo intracellular recording; and (7) inactivation of the stimulating site in SI by lidocaine injection or local surface cooling abolished the ipsilateral response, suggesting that the ipsilateral response was very likely relayed across the callosal pathway. These results suggest that repetitive microstimulation can do more than expand receptive fields in the territory adjacent to the stimulating electrode but in addition can also alter receptive fields in homotopic sites in the opposite SI to bring about the expression of previously ineffective input from the ipsilateral forelimb.

Deficits in Behavioral Functions of Intact Barrel Cortex Following Lesions of Homotopic Contralateral Cortex

Focal unilateral injuries to the somatosensory whisker barrel cortex have been shown cause long-lasting deficits in the activity and experience-dependent plasticity of neurons in the intact contralateral barrel cortex. However, the long-term effect of these deficits on behavioral functions of the intact contralesional cortex is not clear. In this study, we used the “Gap-crossing task” a barrel cortex-dependent, whisker-sensitive, tactile behavior to test the hypothesis that unilateral lesions of the somatosensory cortex would affect behavioral functions of the intact somatosensory cortex and degrade the execution of a bilaterally learnt behavior. Adult rats were trained to perform the Gap-crossing task using whiskers on both sides of the face. The barrel cortex was then lesioned unilaterally by subpial aspiration. As observed in other studies, when rats used whiskers that directly projected to the lesioned hemisphere the performance of Gap-crossing was drastically compromised, perhaps due to direct effect of lesion. Significant and persistent deficits were present when the lesioned rats performed Gap-crossing task using whiskers that projected to the intact cortex. The deficits were specific to performance of the task at the highest levels of sensitivity. Comparable deficits were seen when normal, bilaterally trained, rats performed the Gap-crossing task with only the whiskers on one side of the face or when they used only two rows of whiskers (D row and E row) intact on both side of the face. These findings indicate that the prolonged impairment in execution of the learnt task by rats with unilateral lesions of somatosensory cortex could be because sensory inputs from one set of whiskers to the intact cortex is insufficient to provide adequate sensory information at higher thresholds of detection. Our data suggest that optimal performance of somatosensory behavior requires dynamic activity-driven interhemispheric interactions from the entire somatosensory inputs between homotopic areas of the cerebral cortex. These results imply that focal unilateral cortical injuries, including those in humans, are likely to have widespread bilateral effects on information processing including in intact areas of the cortex.

Peripheral Sensory Deprivation Restores Critical-Period-like Plasticity to Adult Somatosensory Thalamocortical Inputs

Recent work has shown that thalamocortical (TC) inputs can be plastic after the developmental critical period has closed, but the mechanism that enables re-establishment of plasticity is unclear. Here, we find that long-term potentiation (LTP) at TC inputs is transiently restored in spared barrel cortex following either a unilateral infra-orbital nerve (ION) lesion, unilateral whisker trimming, or unilateral ablation of the rodent barrel cortex. Restoration of LTP is associated with increased potency at TC input and reactivates anatomical map plasticity induced by whisker follicle ablation. The reactivation of TC LTP is accompanied by reappearance of silent synapses. Both LTP and silent synapse formation are preceded by transient re-expression of synaptic GluN2B-containing N-methyl-D-aspartate (NMDA) receptors, which are required for the reappearance of TC plasticity. These results clearly demonstrate that peripheral sensory deprivation reactivates synaptic plasticity in the mature layer 4 barrel cortex with features similar to the developmental critical period.

Cortical and thalamic contributions to response dynamics across layers of the primary somatosensory cortex during tactile discrimination

Tactile information processing in the rodent primary somatosensory cortex (S1) is layer specific and involves modulations from both thalamocortical and cortico-cortical loops. However, the extent to which these loops influence the dynamics of the primary somatosensory cortex while animals execute tactile discrimination remains largely unknown. Here, we describe neural dynamics of S1 layers across the multiple epochs defining a tactile discrimination task. We observed that neuronal ensembles within different layers of the S1 cortex exhibited significantly distinct neurophysiological properties, which constantly changed across the behavioral states that defined a tactile discrimination. Neural dynamics present in supragranular and granular layers generally matched the patterns observed in the ventral posterior medial nucleus of the thalamus (VPM), whereas the neural dynamics recorded from infragranular layers generally matched the patterns from the posterior nucleus of the thalamus (POM). Selective inactivation of contralateral S1 specifically switched infragranular neural dynamics from POM-like to those resembling VPM neurons. Meanwhile, ipsilateral M1 inactivation profoundly modulated the firing suppression observed in infragranular layers. This latter effect was counterbalanced by contralateral S1 block. Tactile stimulus encoding was layer specific and selectively affected by M1 or contralateral S1 inactivation. Lastly, causal information transfer occurred between all neurons in all S1 layers but was maximal from infragranular to the granular layer. These results suggest that tactile information processing in the S1 of awake behaving rodents is layer specific and state dependent and that its dynamics depend on the asynchronous convergence of modulations originating from ipsilateral M1 and contralateral S1.

Environmental enrichment as a viable neurorehabilitation strategy for experimental traumatic brain injury

Environmental enrichment (EE) emerged as a robust independent variable capable of influencing behavioral outcome in experimental studies after the fortuitous observation by renowned neuropsychologist Donald O. Hebb that rats raised as pets in his home performed markedly better on problem-solving tasks than those kept in the laboratory. In the subsequent years, numerous studies ensued demonstrating that EE was also capable of inducing neuroplasticity in normal (i.e., noninjured) rats. These behavioral and neural alterations provided the impetus for investigating EE as a potential therapy for traumatic brain injury (TBI), which, over the past two decades, has resulted in several reports. Hence, the aim of this review is to integrate the findings and present the current state of EE as a viable neurorehabilitation strategy for TBI. Using the specific key term searches "traumatic brain injury" and "environmental enrichment" or "enriched environment," 30 and 30 experimental TBI articles were identified by PubMed and Scopus, respectively. Of these, 27 articles were common to both search engines. An additional article was found on PubMed using the key terms "enriched environment" and "fluid percussion." A review of the bibliographies in the 34 articles did not yield additional citations. The overwhelming consensus of the 34 publications is that EE benefits behavioral and histological outcome after brain injury produced by various models. Further, the enhancements are observed in male and female as well as adult and pediatric rats and mice. Taken together, these cumulative findings provide strong support for EE as a generalized and robust preclinical model of neurorehabilitation. However, to further enhance the model and to more accurately mimic the clinic, future studies should continue to evaluate EE during more rehabilitation-relevant conditions, such as delayed and shorter time periods, as well as in combination with other therapeutic approaches, as we have been doing for the past few years.

Present status and future challenges of electroencephalography- and magnetic resonance imaging-based monitoring in preclinical models of focal cerebral ischemia

Animal models are useful tools for better understanding the mechanisms underlying neurological deterioration after an ischemic insult as well as subsequent evolution of changes and recovery of functions. In response to the updated requirements for preclinical investigations of stroke to include relevant functional measurement techniques and biomarker endpoints, we here review the state of knowledge on application of some translational electrophysiological and neuroimaging methods, and in particular, electroencephalography monitoring and magnetic resonance imaging in rodent models of ischemic stroke. This may lead to improvement of diagnostic methods and identification of new therapeutic targets, which would considerably advance the translational value of preclinical stroke research.

Physiological slowing and upregulation of inhibition in cortex are correlated with behavioral deficits in protein malnourished rats

Protein malnutrition during early development has been correlated with cognitive and learning disabilities in children, but the neuronal deficits caused by long-term protein deficiency are not well understood. We exposed rats from gestation up to adulthood to a protein-deficient (PD) diet, to emulate chronic protein malnutrition in humans. The offspring exhibited significantly impaired performance on the 'Gap-crossing' (GC) task after reaching maturity, a behavior that has been shown to depend on normal functioning of the somatosensory cortex. The physiological state of the somatosensory cortex was examined to determine neuronal correlates of the deficits in behavior. Extracellular multi-unit recording from layer 4 (L4) neurons that receive direct thalamocortical inputs and layers 2/3 (L2/3) neurons that are dominated by intracortical connections in the whisker-barrel cortex of PD rats exhibited significantly low spontaneous activity and depressed responses to whisker stimulation. L4 neurons were more severely affected than L2/3 neurons. The response onset was significantly delayed in L4 cells. The peak response latency of L4 and L2/3 neurons was delayed significantly. In L2/3 and L4 of the barrel cortex there was a substantial increase in GAD65 (112% over controls) and much smaller increase in NMDAR1 (12-20%), suggesting enhanced inhibition in the PD cortex. These results show that chronic protein deficiency negatively affects both thalamo-cortical and cortico-cortical transmission during somatosensory information processing. The findings support the interpretation that sustained protein deficiency interferes with features of cortical sensory processing that are likely to underlie the cognitive impairments reported in humans who have suffered from prolonged protein deficiency.

Effects of core auditory cortex deactivation on neuronal response to simple and complex acoustic signals in the contralateral anterior auditory field

Interhemispheric communication has been implicated in various functions of sensory signal processing and perception. Despite ample evidence demonstrating this phenomenon in the visual and somatosensory systems, to date, limited functional assessment of transcallosal transmission during periods of acoustic signal exposure has hindered our understanding of the role of interhemispheric connections between auditory cortical fields. Consequently, the present investigation examines the impact of core auditory cortical field deactivation on response properties of contralateral anterior auditory field (AAF) neurons in the felis catus. Single-unit responses to simple and complex acoustic signals were measured across AAF before, during, and after individual and combined cooling deactivation of contralateral primary auditory cortex (A1) and AAF neurons. Data analyses revealed that on average: 1) interhemispheric projections from core auditory areas to contralateral AAF neurons are predominantly excitatory, 2) changes in response strength vary based on acoustic features, 3) A1 and AAF projections can modulate AAF activity differently, 4) decreases in response strength are not specific to particular cortical laminae, and 5) contralateral inputs modulate AAF neuronal response thresholds. Collectively, these observations demonstrate that A1 and AAF neurons predominantly modulate AAF response properties via excitatory projections.

Cortical reorganization after experimental traumatic brain injury: a functional autoradiography study

Cortical sensorimotor (SM) maps are a useful readout for providing a global view of the underlying status of evoked brain function, as well as a gross overview of ongoing mechanisms of plasticity. Recent evidence in the rat controlled cortical impact (CCI) injury model shows that the ipsilesional (injured) hemisphere is temporarily permissive for axon sprouting. This would predict that size and spatial alterations in cortical maps may occur much earlier than previously tested and that they might be useful as potential markers of the postinjury plasticity period as well as indicators of outcome. We investigated the evolution of changes in brain activation evoked by affected hindlimb electrical stimulation at 4, 7, and 30 days following CCI or sham injury over the hindlimb cortical region of adult rats. [(14)C]-iodoantipyrine autoradiography was used to quantitatively examine the local cerebral blood flow changes in response to hindlimb stimulation as a marker for neuronal activity. The results show that although ipsilesional hindlimb SM activity was persistently depressed from 4 days, additional novel regions of ipsilesional activity appeared concurrently within SM barrel and S2 regions as well as posterior auditory cortex. Simultaneously with this was the appearance of evoked activity within the intact, contralesional cortex that was maximal at 4 and 7 days, compared to stimulated sham-injured rats, where activation was solely unilateral. By 30 days, however, contralesional activation had greatly subsided and existing ipsilesional activity was enhanced within the same novel cortical regions that were identified acutely. These data indicate that significant reorganization of the cortical SM maps occurs after injury that evolves with a particular postinjury time course. We discuss these data in terms of the known mechanisms of plasticity that are likely to underlie these map changes, with particular reference to the differences and similarities that exist between rodent models of stroke and traumatic brain injury.

Distinct developmental principles underlie the formation of ipsilateral and contralateral whisker-related axonal patterns of layer 2/3 neurons in the barrel cortex

Axonal organizations with specific patterns underlie the functioning of local intracortical circuitry, but their precise anatomy and development still remain elusive. Here, we selectively visualized layer 2/3 neurons using in utero electroporation and examined their axonal organization in the barrel cortex contralateral to the electroporated side. We found that callosal axons run preferentially in septal regions of layer 4 and showed a whisker-related pattern in the contralateral barrel cortex in rats and mice. In addition, presynaptic marker proteins were found in this whisker-related axonal organization. Although the whisker-related patterns were observed in both the ipsilateral and contralateral barrel cortex, we found a difference in their developmental processes. While the formation of the whisker-related pattern in the ipsilateral cortex consisted of two distinct steps, that in the contralateral cortex did not have the 1st step, in which the axons were diffusely distributed without preference to septal or barrel regions. We also found that these more diffuse axons ran close to radial glial fibers. Together, our results uncovered a whisker-related axonal pattern of callosal axons and two independent developmental processes involved in the formation of the axonal trajectories of layer 2/3 neurons.

Decoding emotional prosody: resolving differences in functional neuroanatomy from fMRI and lesion studies using TMS

BACKGROUND

Prosody conveys information about the emotional state and intention of others. Lesion studies have shown that damage to the right posterior temporal region is associated with prosody decoding deficits. Dissimilarly to findings from lesion studies, neuroimaging data show substantial bilateral peri-Sylvian activation.

OBJECTIVE

This study aimed to investigate the involvement of the left and right superior temporal gyrus (STG) in prosodic and semantic processing using transcranial magnetic stimulation (TMS). These two regions of interest were chosen for their correspondence to Wernicke's area in the left hemisphere and its analog in the right.

METHODS

Offline TMS with a stimulation frequency of 1 Hz and intensity of 60% of stimulator output (approximately 1.1 Tesla) with one pulse applied per second for 10 minutes (600 pulses) was performed. Directly after TMS on the right STG, the left STG or sham-stimulation, participants completed a prosody decoding or a semantic judgment task (whether the tone/meaning was happy or sad).

RESULTS

Reaction times (RT) for the prosodic task were significantly slower when TMS was applied in the right STG in comparison to left STG and sham conditions. TMS over both right and left STG delayed RT in the semantic task, significantly when the tone of voice was incongruent with the meaning.

CONCLUSIONS

Our data strongly suggests that left temporal regions are not crucial to the basic task of prosody decoding per se; however, the analogous region on the right is. Hence, involvement of the left STG in prosodic decoding revealed in previous imaging data is incidental.

Optogenetic-guided cortical plasticity after nerve injury

Peripheral nerve injury causes sensory dysfunctions that are thought to be attributable to changes in neuronal activity occurring in somatosensory cortices both contralateral and ipsilateral to the injury. Recent studies suggest that distorted functional response observed in deprived primary somatosensory cortex (S1) may be the result of an increase in inhibitory interneuron activity and is mediated by the transcallosal pathway. The goal of this study was to develop a strategy to manipulate and control the transcallosal activity to facilitate appropriate plasticity by guiding the cortical reorganization in a rat model of sensory deprivation. Since transcallosal fibers originate mainly from excitatory pyramidal neurons somata situated in laminae III and V, the excitatory neurons in rat S1 were engineered to express halorhodopsin, a light-sensitive chloride pump that triggers neuronal hyperpolarization. Results from electrophysiology, optical imaging, and functional MRI measurements are concordant with that within the deprived S1, activity in response to intact forepaw electrical stimulation was significantly increased by concurrent illumination of halorhodopsin over the healthy S1. Optogenetic manipulations effectively decreased the adverse inhibition of deprived cortex and revealed the major contribution of the transcallosal projections, showing interhemispheric neuroplasticity and thus, setting a foundation to develop improved rehabilitation strategies to restore cortical functions.

Shaping plasticity to enhance recovery after injury

The past decade of neuroscience research has provided considerable evidence that the adult brain can undergo substantial reorganization following injury. For example, following an ischemic lesion, such as occurs following a stroke, there is a cascade of molecular, genetic, physiological and anatomical events that allows the remaining structures in the brain to reorganize. Often, these events are associated with recovery, suggesting that they contribute to it. Indeed, the term plasticity in stroke research has had a positive connotation historically. But more recently, efforts have been made to differentiate beneficial from detrimental changes. These notions are timely now that neurorehabilitative research is developing novel treatments to modulate, increase, or inhibit plasticity in targeted brain regions. We will review basic principles of plasticity and some of the new and exciting approaches that are currently being investigated to shape plasticity following injury in the central nervous system.

Reorganization of motor cortex after controlled cortical impact in rats and implications for functional recovery

We report the results of controlled cortical impact (CCI) centered on the caudal forelimb area (CFA) of rat motor cortex to determine the feasibility of examining cortical plasticity in a spared cortical motor area (rostral forelimb area, RFA). We compared the effects of three CCI parameter sets (groups CCI-1, CCI-2, and CCI-3) that differed in impactor surface shape, size, and location, on behavioral recovery and RFA structural and functional integrity. Forelimb deficits in the limb contralateral to the injury were evident in all three CCI groups assessed by skilled reach and footfault tasks that persisted throughout the 35-day post-CCI assessment period. Nissl-stained coronal sections revealed that the RFA was structurally intact. Intracortical microstimulation experiments conducted at 7 weeks post-CCI demonstrated that RFA was functionally viable. However, the size of the forelimb representation decreased significantly in CCI-1 compared to the control group. Subdivided into component movement categories, there was a significant group effect for proximal forelimb movements. The RFA area reduction and reorganization are discussed in relation to possible diaschisis, and to compensatory functional behavior, respectively. Also, an inverse correlation between the anterior extent of the lesion and the size of the RFA was identified and is discussed in relation to corticocortical connectivity. The results suggest that CCI can be applied to rat CFA while sparing RFA. This CCI model can contribute to our understanding of neural plasticity in premotor cortex as a substrate for functional motor recovery.

Long-range plasticity between intact hemispheres after contralateral cervical nerve transfer in humans

OBJECT

Peripheral nerve injury in a limb usually causes intrahemispheric functional reorganization of the contralateral motor cortex. Recently, evidence has been emerging for significant interhemispheric cortical plasticity in humans, mostly from studies of direct cortical damage. However, in this study, a long-range interhemispheric plasticity was demonstrated in adults with brachial plexus avulsion injury (BPAI) who had received a contralateral cervical nerve transfer, and this plasticity reversed the BPAI-induced intrahemispheric cortical reorganization.

METHODS

In this study, 8 adult male patients with BPAI were studied using PET scanning.

RESULTS

The results indicated that the right somatomotor cortices, which may contribute to the control of the injured limb before brachial plexus deafferentation, still played an important role when patients with BPAI tried to move their affected limbs, despite the fact that the contralateral C-7 nerve transfer had been performed and the peripheral output had changed dramatically. Such findings are consistent with the results of the authors' previous animal study.

CONCLUSIONS

The brain may try to restore the control of an injured limb to its original cortex area, and a complicated change of peripheral pathway also can induce long-range interhemispheric cortical reorganization in human motor cortex.

Single unit oscillations in rat trigeminal nuclei and their control by the sensorimotor cortex

Oscillatory activity at both the single and multiunit levels has been reported in most central nervous system structures, and is postulated as a key factor in information processing and coding. Rats provide an excellent model for oscillation-based information processing, since tactile perception of the environment is achieved by rhythmic movements of their whiskers and information-related rhythmic activity has been identified in the thalamus and cortex. However, rhythmic activity related to information processing has never been reported in the sensory trigeminal complex (STC), the first brain stem relay station for whisker-related tactile information. In the present work, we demonstrate the existence of neural oscillations in the vibrissae-related neurons of the nuclei principalis (Pr5), oralis (Sp5o), interpolaris (Sp5i) and caudalis (Sp5c). Rhythmic activity was associated with the main task of each nucleus, prominent in nuclei responsible for tactile vibrissae information processing (up to 17% oscillating neurons in Pr5 and 26% in Sp5i) and less conspicuous in those concerned with pain (8% oscillating neurons in Sp5o and in Sp5c). The higher percentage of oscillating neurons and higher frequencies in Sp5i than in Pr5 suggests an active role for rhythmic activity in integrating multivibrissa inputs. Oscillations are generated within the brainstem; data obtained from decorticated animals suggest the existence of a differential cortical control of the rhythmic processes in STC nuclei. Corticofugal activity modifies oscillation frequency and synchronization strength of the rhythmic activity mainly during tactile stimulation of the vibrissae.

Early frequency-dependent information processing and cortical control in the whisker pathway of the rat: electrophysiological study of brainstem nuclei principalis and interpolaris

The rat facial whiskers form a high-resolution sensory apparatus for tactile information coding and are used by these animals for the exploration and perception of their environment. Previous work on the rat vibrissae system obtained evidence for vibration-based feature extraction by the whiskers, texture classification by the cortical neurons, and "low-pass", "high-pass", and "band-pass" filtering properties in both thalamic and cortical neurons. However, no data are available for frequency-dependent information processing in the brainstem sensory trigeminal complex (STC), the first relay station of the vibrissae pathway. In the present paper, we studied the frequency-dependent processing characteristics of the STC nuclei that mainly project to the thalamus, nuclei principalis, and interpolaris. This is the first time that STC nuclei have been studied together via a wide range of stimulation frequencies (1-40 Hz), four different and complementary metrics, and the same experimental protocol. Moreover, the role of corticofugal projection to these nuclei as well as the influence of input from the whiskers has been analyzed. We show that both nuclei perform frequency-dependent coding of tactile information: low pass and band-pass filtering occurs for the spiking rate in short post-stimuli time intervals, high-pass and band-pass filtering occurs for the spiking rate in long trains of stimuli, and an increase of response latencies and low pass filtering occurs for phase-locked stimuli. These information-processing characteristics are neither imposed by the sensorimotor cortex nor introduced by the afferent fibres. The sensorimotor cortex exerts a distinct modulatory effect on each nucleus.

Suppression of activity in the forelimb motor cortex temporarily enlarges forelimb representation in the homotopic cortex in adult rats

After forelimb motor cortex (FMC) damage, the unaffected homotopic motor cortex showed plastic changes. The present experiments were designed to clarify the electrophysiological nature of these interhemispheric effects. To this end, the output reorganization of the FMC was investigated after homotopic area activity was suppressed in adult rats. FMC output was compared after lidocaine-induced inactivation (L-group) or quinolinic acid-induced lesion (Q-group) of the contralateral homotopic cortex. In the Q-group of animals, FMC mapping was performed, respectively, 3 days (Q3D group) and 2 weeks (Q2W group) after cortical lesion. In each animal, FMC output was assessed by mapping movements induced by intracortical microstimulation (ICMS) in both hemispheres (hemisphere ipsilateral and contralateral to injections). The findings demonstrated that in the L-group, the size of forelimb representation was 42.2% higher than in the control group (P < 0.0001). The percentage of dual forelimb-vibrissa movement sites significantly increased over the controls (P < 0.0005). The dual-movement sites occupied a strip of the map along the rostrocaudal border between the forelimb and vibrissa representations. This form of interhemispheric diaschisis had completely reversed, with the recovery of the baseline map, 3 days after the lesion in the contralateral FMC. This restored forelimb map showed no ICMS-induced changes 2 weeks after the lesion in the contralateral FMC. The present results suggest that the FMCs in the two hemispheres interact continuously through predominantly inhibitory influences that preserve the forelimb representation and the border vs. vibrissa representation.

Physiology and morphology of callosal projection neurons in mouse

In the mammalian neocortex, the corpus callosum serves as the major source of interhemispheric communication, composed of axons from callosal neurons located in supragranular (II/III) and infragranular (V/VI) layers. We sought to characterize the physiology and morphology of supragranular and infragranular callosal neurons in mice using retrograde tracers and whole-cell patch clamp recordings. Whole-cell patch clamp recordings were made from retrogradely labeled callosal neurons following unilateral injection of fluorescent latex microspheres in the contralateral sensory-motor cortex. Following recordings and biocytin dialysis, labeled neurons were reconstructed using computer-assisted camera lucida (Neurolucida) for morphological analyses. Whole-cell recordings revealed that callosal neurons in both supra- and infragranular layers display very similar intrinsic membrane properties and are characteristic regular-spiking neurons. Morphological features examined from biocytin-filled reconstructions as well as retrogradely BDA labeled cells did not reveal any differences. Analysis of spontaneous postsynaptic potentials from callosal neurons did reveal several differences including average amplitude, frequency, and decay time. These findings suggest that callosal neurons in both supra- and infragranular layers have similar phenotypes though belong to different local, intracortical networks.

Functional MRI detection of bilateral cortical reorganization in the rodent brain following peripheral nerve deafferentation

Evidence is emerging for significant inter-hemispheric cortical plasticity in humans, opening important questions about the significance and mechanism for this long range plasticity. In this work, peripheral nerve deafferentation was performed on both the rat forepaw and hindpaw and cortical reorganization was assessed using functional MRI (fMRI). Sensory stimulation of the forepaw or the hindpaw in rats that experienced only partial denervation resulted in activation in only the appropriate, contralateral, primary somatosensory cortex (SI). However, 2-3 weeks following complete denervation of the rats' forepaw or hindpaw, stimulation of the intact paw resulted in fMRI activation of ipsilateral as well as contralateral SI. To address whether inter-cortical communication is required for this cortical reorganization, the healthy hindpaw SI representation was stereotaxically lesioned in rats which had the other hindpaw denervated. No fMRI activation was detected in the ipsilateral SI cortex after lesioning of the contralateral cortex. These results indicate that extensive inter-hemispheric cortical-cortical reorganization can occur in the rodent brain after peripheral nerve deafferentation and that cortical-cortical connections play a role in mediating this inter-hemispheric cortical reorganization.

Early-phase of learning enhances communication between brain hemispheres

In the somatosensory system, inputs from one side of the body are only transmitted to the contralateral primary somatosensory cortex, but both sides of the body representation can interact via interhemispheric connections. These interactions depend on the behavioural requirements of the animal and its level of arousal. During the process of learning, alertness and attention may modify the responsiveness of neuronal pathways. We functionally mapped the brains of mice by using [14C]2-deoxyglucose (2DG) autoradiography during the first and the third session of a classical conditioning paradigm, involving whiskers stimulation on one side of the muzzle paired with an aversive or appetitive unconditioned stimulus. During the first pairing session, an increased 2DG uptake was seen in the barrel cortex of both hemispheres, independently of the type of applied unconditioned stimulus. In the third session of the sensory pairing, activation of the barrel cortex was solely contralateral, as expected after unilateral whisker stimulation. Thus, sensory stimulation directed to one cerebral hemisphere during the initial stages of Pavlovian conditioning activates the primary sensory area in both hemispheres. These results suggest that during the early phase of conditioning, when alertness is presumably strongest, the interhemispheric interactions are enhanced.

Balancing bilateral sensory activity: callosal processing modulates sensory transmission through the contralateral thalamus by altering the response threshold

Rats tactually explore a nearly spherical space field around their heads with their whiskers. The information sampled by the two sets of whiskers is integrated bilaterally at the cortical level in an activity dependent manner via the corpus callosum. We have recently shown that sensory activity in one barrel field cortex (BFC) modulates the processing of incoming sensory information to the other BFC. Whether interhemispheric integration is dynamically linked with corticothalamic modulation of incoming sensory activity is an important hypothesis to test, since subcortical relay neurons are directly modulated by cortical neurons through top-down processes. In the present study, we compared the direct sensory responses of single thalamic relay neurons under urethane anesthesia before and after inactivating the BFC contralateral to a thalamic neuron. The data show that silencing one BFC reduces response magnitude in contralateral thalamic relay neurons, significantly and reversibly, in response to test stimuli applied to the principal whisker at two times response threshold (2T) intensity for each unit. Neurons in the ventral posterior medial (VPM) nucleus and the medial division of the posterior nucleus (POm) react in a similar manner, although POm neurons are more profoundly depressed by inactivation of the contralateral BFC than VPM neurons. The results support the novel idea that the subcortical relay of sensory information to one hemisphere is strongly modulated by activity levels in the contralateral as well as in the ipsilateral SI cortex. The mechanism of the modulation appears to be based on shifting the stimulus-response curves of thalamic neurons, thereby rendering them more or less sensitive to sensory stimuli. We conclude that global sensory processing is created by combining activity in each cerebral hemisphere and continually balancing the flow of information to cortex by adjusting the responsiveness of ascending sensory pathways.

Reducing Contralateral SI Activity Reveals Hindlimb Receptive Fields in the SI Forelimb-Stump Representation of Neonatally Amputated Rats

In adult rats that sustained forelimb amputation on the day of birth, >30% of multiunit recording sites in the forelimb-stump representation of primary somatosensory cortex (SI) also respond to cutaneous hindlimb stimulation when cortical GABAA+B receptors are blocked (GRB). This study examined whether hindlimb receptive fields could also be revealed in forelimb-stump sites by reducing one known source of excitatory input to SI GABAergic neurons, the contralateral SI cortex. Corpus callosum projection neurons connect homotopic SI regions, making excitatory contacts onto pyramidal cells and interneurons. Thus in addition to providing monosynaptic excitation in SI, callosal fibers can produce disynaptic inhibition through excitatory synapses with inhibitory interneurons. Based on the latter of these connections, we hypothesized that inactivating the contralateral (intact) SI forelimb region would “unmask” normally suppressed hindlimb responses by reducing the activity of SI GABAergic neurons. The SI forelimb-stump representation was first mapped under normal conditions and then during GRB to identify stump/hindlimb responsive sites. After GRB had dissipated, the contralateral (intact) SI forelimb region was mapped and reversibly inactivated with injections of 4% lidocaine, and selected forelimb-stump sites were retested. Contralateral SI inactivation revealed hindlimb responses in ∼60% of sites that were stump/hindlimb responsive during GRB. These findings indicate that activity in the contralateral SI contributes to the suppression of reorganized hindlimb receptive fields in neonatally amputated rats.

Chronic suppression of activity in barrel field cortex downregulates sensory responses in contralateral barrel field cortex

Numerous lines of evidence indicate that neural information is exchanged between the cerebral hemispheres via the corpus callosum. Unilateral ablation lesions of barrel field cortex (BFC) in adult rats induce strong suppression of background and evoked activity in the contralateral barrel cortex and significantly delay the onset of experience-dependent plasticity. The present experiments were designed to clarify the basis for these interhemispheric effects. One possibility is that degenerative events, triggered by the lesion, degrade contralateral cortical function. Another hypothesis, alone or in combination with degeneration, is that the absence of interhemispheric activity after the lesion suppresses contralateral responsiveness. The latter hypothesis was tested by placing an Alzet minipump subcutaneously and connecting it via a delivery tube to a cannula implanted over BFC. The minipump released muscimol, a GABA(A) receptor agonist at a rate of 1 mul/h, onto one barrel field cortex for 7 days. Then with the pump still in place, single cells were recorded in the contralateral BFC under urethan anesthesia. The data show a approximately 50% reduction in principal whisker responses (D2) compared with controls, with similar reductions in responses to the D1 and D3 surround whiskers. Despite these reductions, spontaneous firing is unaffected. Fast spiking units are more sensitive to muscimol application than regular spiking units in both the response magnitude and the center/surround ratio. Effects of muscimol are also layer specific. Layer II/III and layer IV neurons decrease their responses significantly, unlike layer V neurons that fail to show significant deficits. The results indicate that reduced activity in one hemisphere alters cortical excitability in the other hemisphere in a complex manner. Surprisingly, a prominent response decrement occurs in the short-latency (3-10 ms) component of principal whisker responses, suggesting that suppression may spread to neurons dominated by thalamocortical inputs after interhemispheric connections are inactivated. Bilateral neurological impairments have been described after unilateral stroke lesions in the clinical literature.

Heterogeneous integration of bilateral whisker signals by neurons in primary somatosensory cortex of awake rats

Bilateral single-unit recordings in primary somatosensory cortex (S1) of anesthetized rats have revealed substantial cross talk between cortical hemispheres, suggesting the possibility that behaviorally relevant bilateral integration could occur in S1. To determine the extent of bilateral neural responses in awake animals, we recorded S1 multi- and single-unit activity in head-immobilized rats while stimulating groups of 4 whiskers from the same column on both sides of the head. Results from these experiments confirm the widespread presence of single units responding to tactile stimuli on either side of the face in S1 of awake animals. Quantification of bilateral integration by multiunits revealed both facilitative and suppressive integration of bilateral inputs. Varying the interval between left and right whisker stimuli between 0 and 120 ms showed the temporal integration of bilateral stimuli to be dominated on average by suppression at intervals around 30 ms, in agreement with comparable recordings in anesthetized animals. Contrary to the anesthetized data, in the awake animals we observed a high level of heterogeneity of bilateral responses and a strong interaction between synchronous bilateral stimuli. The results challenge the traditional conception of highly segregated hemispheric processing channels in the rat S1 cortex, and support the hypothesis that callosal cross-projections between the two hemispheres mediate rats' known ability to integrate bilateral whisker signals.