Somatosensory cortex of the neonatal pig: II. Topographic organization of the secondary somatosensory cortex (SII)

Porcine Model of Cerebral Ischemic Stroke Utilizing Intracortical Recordings for the Continuous Monitoring of the Ischemic Area

PURPOSE

Our aim was to use intracortical recording to enable the tracking of ischemic infarct development over the first few critical hours of ischemia with a high time resolution in pigs. We employed electrophysiological measurements to obtain quick feedback on neural function, which might be useful for screening, e.g., for the optimal dosage and timing of agents prior to further pre-clinical evaluation.

METHODS

Micro-electrode arrays containing 16 (animal 1) or 32 electrodes (animal 2-7) were implanted in the primary somatosensory cortex of seven female pigs, and continuous electrical stimulation was applied at 0.2 Hz to a cuff electrode implanted on the ulnar nerve. Ischemic stroke was induced after 30 min of baseline recording by injection of endothelin-1 onto the cortex adjacent to the micro-electrode array. Evoked responses were extracted over a moving window of 180 s and averaged across channels as a measure of cortical excitability.

RESULTS

Across the animals, the cortical excitability was significantly reduced in all seven 30 min segments following endothelin-1 injection, as compared to the 30 min preceding this intervention. This difference was not explained by changes in the anesthesia, ventilation, end-tidal CO2, mean blood pressure, heart rate, blood oxygenation, or core temperature, which all remained stable throughout the experiment.

CONCLUSIONS

The animal model may assist in maturing neuroprotective approaches by testing them in an accessible model of resemblance to human neural and cardiovascular physiology and body size. This would constitute an intermediate step for translating positive results from rodent studies into human application, by more efficiently enabling effective optimization prior to chronic pre-clinical studies in large animals.

Backpack-mediated anti-inflammatory macrophage cell therapy for the treatment of traumatic brain injury

Traumatic brain injury (TBI) is a debilitating disease with no current therapies outside of acute clinical management. While acute, controlled inflammation is important for debris clearance and regeneration after injury, chronic, rampant inflammation plays a significant adverse role in the pathophysiology of secondary brain injury. Immune cell therapies hold unique therapeutic potential for inflammation modulation, due to their active sensing and migration abilities. Macrophages are particularly suited for this task, given the role of macrophages and microglia in the dysregulated inflammatory response after TBI. However, maintaining adoptively transferred macrophages in an anti-inflammatory, wound-healing phenotype against the proinflammatory TBI milieu is essential. To achieve this, we developed discoidal microparticles, termed backpacks, encapsulating anti-inflammatory interleukin-4, and dexamethasone for ex vivo macrophage attachment. Backpacks durably adhered to the surface of macrophages without internalization and maintained an anti-inflammatory phenotype of the carrier macrophage through 7 days in vitro. Backpack-macrophage therapy was scaled up and safely infused into piglets in a cortical impact TBI model. Backpack-macrophages migrated to the brain lesion site and reduced proinflammatory activation of microglia in the lesion penumbra of the rostral gyrus of the cortex and decreased serum concentrations of proinflammatory biomarkers. These immunomodulatory effects elicited a 56% decrease in lesion volume. The results reported here demonstrate, to the best of our knowledge, a potential use of a cell therapy intervention for a large animal model of TBI and highlight the potential of macrophage-based therapy. Further investigation is required to elucidate the neuroprotection mechanisms associated with anti-inflammatory macrophage therapy.

Non-invasive central nervous system assessment of a porcine model of neuropathic pain demonstrates increased latency of somatosensory-evoked potentials

BACKGROUND

The study of chronic pain and its treatments requires a robust animal model with objective and quantifiable metrics. Porcine neuropathic pain models have been assessed with peripheral pain recordings and behavioral responses, but thus far central nervous system electrophysiology has not been investigated. This work aimed to record non-invasive, somatosensory-evoked potentials (SEPs) via electroencephalography in order to quantitatively assess chronic neuropathic pain induced in a porcine model.

NEW METHOD

Peripheral neuritis trauma (PNT) was induced unilaterally in the common peroneal nerve of domestic farm pigs, with the contralateral leg serving as the control for each animal. SEPs were generated by stimulation of the peripheral nerves distal to the PNT and were recorded non-invasively using transcranial electroencephalography (EEG). The P30 wave of the SEP was analyzed for latency changes.

RESULTS

P30 SEPs were successfully recorded with non-invasive EEG. PNT resulted in significantly longer P30 SEP latencies (p < 0.01 [n = 8]) with a median latency increase of 14.3 [IQR 5.0 - 17.5] ms. Histological results confirmed perineural inflammatory response and nerve damage around the PNT nerves.

COMPARISON WITH EXISTING METHOD(S)

Control P30 SEPs were similar in latency and amplitude to those previously recorded invasively in healthy pigs. Non-invasive recordings have numerous advantages over invasive measures.

CONCLUSIONS

P30 SEP latency can serve as a quantifiable neurological measure that reflects central nervous system processing in a porcine model of chronic pain. Advancing the development of a porcine chronic pain model will facilitate the translation of experimental therapies into human clinical trials.

Feasibility of non-invasive recording of somatosensory evoked potential in pigs

BACKGROUND

Non-invasive measurement of somatosensory-evoked potentials (SEP) in a large animal model is important to translational cognitive research. We sought to develop a methodology for neurophysiological recording via a transcranial electroencephalography (EEG) cap under an effective sedative regimen with dexmedetomidine, midazolam, and butorphanol that will produce sedation instead of anesthesia while not compromising data quality.

RESULTS

Pigs received intramuscular dexmedetomidine, midazolam, and butorphanol for SEP assessment with peroneal nerve stimulation. Semi-quantitative sedation assessment was performed after the animal was sufficiently sedated and 30 min later, during the transcranial SEP recording. SEP data were analyzed with commercial software. Binary qualitative analysis of the recording was categorized by an experienced neurophysiologist. All four animals had adequate surface SEP recordings. Animals received 43 [21-47] mcg/kg of dexmedetomidine, 0.3 [0.2-0.3] mg/kg of midazolam, and 0.3 [0.3-0.3] mg/kg of butorphanol IM. All treatments resulted in moderate to deep sedation (Baseline median sedation score 11.5 [11-12]; median score at 30 min: 11.5 [10.5-12]). Heart rate (median [range]) (55 [49-71] beats per minute), respiratory rate (24 [21-30] breaths per minute), and hemoglobin oxygen saturation (99 [98-100]%) and body temperature (37.7 [37.4-37.9] °C) remained within clinically acceptable ranges. There were no undesirable recovery incidents.

CONCLUSIONS

In this pilot study, we demonstrate the feasibility of SEP recording via a transcranial EEG cap under an effective sedative regimen in pigs. Our approach will expand the use of a large animal model in neurotranslational research.

Perinatal Docosahexaenoic Acid Supplementation Improves Cognition and Alters Brain Functional Organization in Piglets

Epidemiologic studies associate maternal docosahexaenoic acid (DHA)/DHA-containing seafood intake with enhanced cognitive development; although, it should be noted that interventional trials show inconsistent findings. We examined perinatal DHA supplementation on cognitive performance, brain anatomical and functional organization, and the brain monoamine neurotransmitter status of offspring using a piglet model. Sows were fed a control (CON) or a diet containing DHA (DHA) from late gestation throughout lactation. Piglets underwent an open field test (OFT), an object recognition test (ORT), and magnetic resonance imaging (MRI) to acquire anatomical, diffusion tensor imaging (DTI), and resting-state functional MRI (rs-fMRI) at weaning. Piglets from DHA-fed sows spent 95% more time sniffing the walls than CON in OFT and exhibited an elevated interest in the novel object in ORT, while CON piglets demonstrated no preference. Maternal DHA supplementation increased fiber length and tended to increase fractional anisotropy in the hippocampus of offspring than CON. DHA piglets exhibited increased functional connectivity in the cerebellar, visual, and default mode network and decreased activity in executive control and sensorimotor network compared to CON. The brain monoamine neurotransmitter levels did not differ in healthy offspring. Perinatal DHA supplementation may increase exploratory behaviors, improve recognition memory, enhance fiber tract integrity, and alter brain functional organization in offspring at weaning.

The telencephalon of the Göttingen minipig, cytoarchitecture and cortical surface anatomy

During the last 20 years pigs have become increasingly popular in large animal translational neuroscience research as an economical and ethical feasible substitute to non-human primates. The anatomy of the pig telencephalon is, however, not well known. We present, accordingly, a detailed description of the surface anatomy and cytoarchitecture of the Göttingen minipig telencephalon based on macrophotos and consecutive high-power microphotographs of 15 μm thick paraffin embedded Nissl-stained coronal sections. In 1-year-old specimens the formalin perfused brain measures approximately 55 × 47 × 36 mm (length, width, height) and weighs around 69 g. The telencephalic part of the Göttingen minipig cerebrum covers a large surface area, which can be divided into a neocortical gyrencephalic part located dorsal to the rhinal fissure, and a ventral subrhinal part dominated by olfactory, amygdaloid, septal, and hippocampal structures. This part of the telencephalon is named the subrhinal lobe, and based on cytoarchitectural and sulcal anatomy, can be discerned from the remaining dorsally located neocortical perirhinal/insular, pericallosal, frontal, parietal, temporal, and occipital lobes. The inner subcortical structure of the minipig telencephalon is dominated by a prominent ventricular system and large basal ganglia, wherein the putamen and the caudate nucleus posterior and dorsally are separated into two entities by the internal capsule, whereas both structures ventrally fuse into a large accumbens nucleus. The presented anatomical data is accompanied by surface renderings and high-power macrophotographs illustrating the telencephalic sulcal pattern, and the localization of the identified lobes and cytoarchitectonic areas. Additionally, 24 representative Nissl-stained telencephalic coronal sections are presented as supplementary material in atlas form on http://www.cense.dk/minipig_atlas/index.html and referred to as S1-S24 throughout the manuscript.

Effective connectivity maps in the swine somatosensory cortex estimated from electrocorticography and validated with intracortical local field potential measurements

Macroscopic techniques are increasingly being used to estimate functional connectivity in the brain, which provides valuable information about brain networks. In any such endeavors it is important to understand capabilities and limitations of each technique through direct validation, which is often lacking. This study evaluated a multiple dipole source analysis technique based on electrocorticography (ECOG) data in estimating effective connectivity maps and validated the technique with intracortical local field potential (LFP) recordings. The study was carried out in an animal model (swine) with a large brain to avoid complications caused by spreading of the volume current. The evaluation was carried out for the cortical projections from the trigeminal nerve and corticocortical connectivity from the first rostrum area (R1) in the primary somatosensory cortex. Stimulation of the snout and layer IV of the R1 did not activate all projection areas in each animal, although whenever an area was activated in a given animal, its location was consistent with the intracortical LFP. The two types of connectivity maps based on ECOG analysis were consistent with each other and also with those estimated from the intracortical LFP, although there were small discrepancies. The discrepancies in mean latency based on ECOG and LFP were all very small and nonsignificant: snout stimulation, -1.1-2.0 msec (contralateral hemisphere) and 3.9-8.5 msec (ipsilateral hemisphere); R1 stimulation, -1.4-2.2 msec for the ipsilateral and 0.6-1.4 msec for the contralateral hemisphere. Dipole source analysis based on ECOG appears to be quite useful for estimating effective connectivity maps in the brain.

Mapping of sheep sensory cortex with a novel microelectrocorticography grid

Microelectrocorticography (µECoG) provides insights into the cortical organization with high temporal and spatial resolution desirable for better understanding of neural information processing. Here we evaluated the use of µECoG for detailed cortical recording of somatosensory evoked potentials (SEPs) in an ovine model. The approach to the cortex was planned using an MRI-based 3D model of the sheep's brain. We describe a minimally extended surgical procedure allowing placement of two different µECoG grids on the somatosensory cortex. With this small craniotomy, the frontal sinus was kept intact, thus keeping the surgical site sterile and making this approach suitable for chronic implantations. We evaluated the procedure for chronic implantation of an encapsulated µECoG recording system. During acute and chronic recordings, significant SEP responses in the triangle between the ansate, diagonal, and coronal sulcus were identified in all animals. Stimulation of the nose, upper lip, lower lip, and chin caused a somatotopic lateral-to-medial, ipsilateral response pattern. With repetitive recordings of SEPs, this somatotopic pattern was reliably recorded for up to 16 weeks. The findings of this study confirm the previously postulated ipsilateral, somatotopic organization of the sheep's sensory cortex. High gamma band activity was spatially most specific in the comparison of different frequency components of the somatosensory evoked response. This study provides a basis for further acute and chronic investigations of the sheep's sensory cortex by characterizing its exact position, its functional properties, and the surgical approach with respect to macroanatomical landmarks.

A three-dimensional digital segmented and deformable brain atlas of the domestic pig

We used high-magnetic field (4.7 T) magnetic resonance imaging (MRI) to build the first high-resolution (100 microm x 150 microm x 100 microm) three-dimensional (3D) digital atlas in stereotaxic coordinates of the brain of a female domestic pig (Sus scrofa domesticus). This atlas was constructed from one hemisphere which underwent a symmetrical transformation through the midsagittal plane. Concomitant construction of a 3D histological atlas based on the same scheme facilitated control of deep brain structure delimitation and enabled cortical mapping to be achieved. The atlas contains 178 individual cerebral structures including 42 paired and 9 single deep brain structures, 5 ventricular system areas, 6 paired deep cerebellar nuclei, 12 cerebellar lobules and 28 cortical areas per hemisphere. Given the increasing importance of pig brains in medical research, this atlas should be a useful tool for intersubject normalization in anatomical imaging as well as for precisely localizing brain areas in functional MR studies or electrode implantation trials. The atlas can be freely downloaded from our institution's Website.

The use of pigs in neuroscience: Modeling brain disorders

The use of pigs in neuroscience research has increased in the past decade, which has seen broader recognition of the potential of pigs as an animal for experimental modeling of human brain disorders. The volume of available background data concerning pig brain anatomy and neurochemistry has increased considerably in recent years. The pig brain, which is gyrencephalic, resembles the human brain more in anatomy, growth and development than do the brains of commonly used small laboratory animals. The size of the pig brain permits the identification of cortical and subcortical structures by imaging techniques. Furthermore, the pig is an increasingly popular laboratory animal for transgenic manipulations of neural genes. The present paper focuses on evaluating the potential for modeling symptoms, phenomena or constructs of human brain diseases in pigs, the neuropsychiatric disorders in particular. Important practical and ethical aspects of the use of pigs as an experimental animal as pertaining to relevant in vivo experimental brain techniques are reviewed. Finally, current knowledge of aspects of behavioral processes including learning and memory are reviewed so as to complete the summary of the status of pigs as a species suitable for experimental models of diverse human brain disorders.

The prefrontal cortex in the Göttingen minipig brain defined by neural projection criteria and cytoarchitecture

In an attempt to delineate the prefrontal cortex (PFC) in the Göttingen minipig brain the distribution of reciprocal thalamocortical projections was investigated using anterograde and retrograde tracing techniques and evaluated in relation to the specific cytoarchitectonic organization. Tracers were visualized using standard immunohistochemistry or evaluated in vivo using manganese (Mn2+) as an MRI paramagnetic tracer. The in vivo tract tracing turned out to be very sensitive with a high correspondence to the histological labelling. Tracers injected into the mediodorsal thalamus labelled the medial and rostral pole of the frontal lobe as well as the anterior cingulate, anterior insular and dorsomedial frontal cortices. Subsequently, the reciprocity and specificity of these connections were tested from injections into the traced frontal cortices indicating that the PFC has cortical connections to different parts of the MD nucleus. Although the granular layer IV, characteristic of primate PFC could not be identified, both cytoarchitectonic and connectional data suggests that the Göttingen minipig has a structurally divided prefrontal cortex. Stereological estimates of PFC volume showed that the Göttingen minipig PFC constitutes about 24% of the total neocortex volume and 10% of the total brain volume.

Functional magnetic resonance imaging of the primary somatosensory cortex in piglets

OBJECT

The piglet is an excellent model for the developing human brain, and has been used increasingly in various centers for studies of traumatic brain injury and other insults. Unlike rodent or primate models, however, there are few behavioral scales for the piglet, and the available ones are used to test general responsiveness rather than specific functional outcome. The differing behavioral repertoires of animals of different ages provide an additional challenge when age-dependent injury responses are compared. To overcome these experimental limitations of piglets in brain injury research, the authors developed a functional magnetic resonance (fMR) imaging paradigm that can be used to track recovery in the somatosensory cortex over time in anesthetized animals of different ages.

METHODS

Fifteen fMR imaging studies in eight piglets were performed before and after scaled cortical impact injury to the primary somatosensory cortex subserving snout sensation. Specific anesthetic and imaging protocols enabled visualization of cortical activation, and comparison with somatosensory evoked potentials obtained before and after injury was obtained. A piglet brain template for group-level analysis of these data was constructed, similar to the fMR imaging techniques used in humans, to allow for group comparisons and longitudinal change analysis over time.

CONCLUSIONS

Loss of function in a specifically traumatized cortical region and its subsequent recovery over time can now be demonstrated visually by fMR imaging in the piglet. Besides its value in understanding intrinsic recovery mechanisms and plasticity at different ages, this functional outcome measure will enable the use of the piglet model in treatment trials specifically designed for the immature brain.

Origins of the somatic N20 and high-frequency oscillations evoked by trigeminal stimulation in the piglets

OBJECTIVE

In humans, the somatic evoked potentials (SEPs) and magnetic fields (SEFs) elicited by peripheral nerve stimulation contain high-frequency oscillations (HFOs) around 600 Hz superimposed on the initial cortical response N20. Responses elicited by snout stimulation in the swine also contain similar HFOs during the rising phase of the porcine N20. This study examined the generators of the N20 and HFOs in the swine.

METHODS

We recorded intracortical SEPs and multi-unit activities in the sulcal area of the primary somatosensory cortex (SI) simultaneously with SEFs. The laminar profiles of the potential and current-source-density (CSD) were analyzed.

RESULTS

The CSD analysis revealed that the N20 was produced by two dipolar generators, both directed toward the cortical surface. After the arrival of the initial thalamocortical volley in layer IV, the sink of the first generator shifted toward shallower layers II-III with a velocity of 0.109+/-0.038 m/s (mean+/-SD). The sink of the second generator moved to layer V. The initial thalamocortical axonal component of the HFO was produced by repolarizing current with the sink in layer IV. The CSD laminar profile of the postsynaptic component was very similar to the profile of intracortical N20. The current sink within each cycle of HFO propagated upward with a velocity of 0.633+/-0.189 m/s, indicating backpropagation.

CONCLUSIONS

We propose that the N20 is generated by two sets of excitatory neurons which also produce the HFOs. Although the loci of synaptic inputs are unknown, these neurons appear to fire initially in the soma and produce backpropagating spikes toward distal apical dendrites.

SIGNIFICANCE

These conclusions relate the N20 to the HFO and provide a new explanation of how the current underlying the N20 is invariantly directed toward superficial layers across species.

Stable synchronized high-frequency signals from the main sensory and spinal nuclei of the pig activated by Abeta fibers of the maxillary nerve innervating the snout

The primary somatosensory cortex of various species including man, monkey, pig and rat is capable of producing high-frequency signals in the 600 Hz range and above with very little latency jitter. We have recently observed such cortical signals for the trigeminal system of the swine. This study determined the projection of the maxillary nerve innervating the snout to the sensory trigeminal nuclear complex in the brain stem and stability of outputs of each nucleus receiving the projection. The snout stimulation activated large-caliber Abeta fibers in the trigeminal nerve with a mean velocity of 64.4+/-2.7 m/s (mean+/-1 S.E.M., six animals) comparable in velocity to the tooth pulp Abeta fibers (57.9+/-3.4 m/s) obtained from the same animals. These afferents activated the main sensory nucleus, and subnuclei oralis, interpolaris and caudalis of the spinal nucleus, as judged by evoked field potential maps superimposed on the histological maps of the trigeminal nuclei from the same animals. Inputs from these fast afferents arrived at all the four trigeminal nuclei almost simultaneously within a span of 0.7+/-0.2 ms (mean+/-1 S.D., seven animals). Evoked high-frequency signals were reproducible with a latency jitter of less than 0.2 ms during the first 4 ms of postsynaptic activity for each of main sensory and spinal nuclei. These results indicate that the snout stimulation activates fast-conducting peripheral afferents which project to all the sensory trigeminal nuclei and produces highly reproducible initial responses nearly simultaneously across the multiple trigeminal nuclei. These outputs from the trigeminal nuclei may play an important role in triggering the stable high-frequency signals in the cortex.

MR perfusion, diffusion and BOLD imaging of methotrexate-exposed swine brain

PURPOSE

To evaluate the methotrexate (MTX)-exposed swine brain, functional magnetic resonance imaging (MRI), including perfusion, diffusion, and blood-oxygen-level-dependent (BOLD) contrast imaging, was used.

MATERIAL AND METHODS

Juvenile pigs received either 2 x 5 g/m(2), or 5 x 2 g/m(2) MTX intravenously within one month. MRI was performed (sedative: propofol) before (14-17 kg, N = 6) and after (21-27 kg, N = 4) the MTX exposure. Also, age-matched controls (22-27 kg, N = 4) were imaged.

RESULTS

After the MTX exposure, reduced (from 2%-4% to 0%-1%) or negative (-2% to -3%) BOLD responses were detected; apparent diffusion coefficient (ADC) or relative perfusion values did not change.

CONCLUSION

This study suggests that MTX-related changes in the brain may be detected as changes in flow-metabolism coupling as reduced or negative response (for somatosensory activation) in the BOLD contrast MRI. The contrast agent perfusion MRI, without absolute quantification, may not show global damage in brain perfusion related to the MTX exposure in the swine model used. ADC (in one direction) may not indicate MTX-related changes in the brain.

Analysis of MEG signals of spreading cortical depression with propagation constrained to a rectangular cortical strip. II. Gyrencephalic swine model

Currents produced during spreading cortical depression (SCD) in a gyrencephalic species (swine) were studied with magnetoencephalography (MEG) and electrocorticography (ECoG). SCD, initiated using electrical stimulation of the cortex, was constrained to propagate within a rectangular cortical strip in order to simplify the interpretation of the underlying currents. The ECoG signals monitored along the strip revealed that SCD propagated from an initiation site on the gyrus at a rate of 7.9+/-3.2 mm/min (n=23), entered the deep coronal sulcus and in most cases emerged from the other side of the sulcus, continuing to propagate across the next gyrus at a rate of 5.9+/-2.7 mm/min (n=22). The apparent propagation velocity within the sulcus was reduced to 1.7+/-0.8 mm/min (n=21). Strong MEG signals were observed as SCD entered the sulcus. The direction of magnetic field was opposite for SCD's on opposite banks of the sulcus. The currents were directed from a superficial layer to deeper layers of the cortex. The characteristics of SCD and associated MEG patterns from a gyrencephalic species may be similar to those in human patients during migraine aura.

Comparison of MEG and EEG on the basis of somatic evoked responses elicited by stimulation of the snout in the juvenile swine

OBJECTIVE

Some basic characteristics of magnetoencephalographic (MEG) and electroencephalographic (EEG) signals were studied by comparing somatic evoked fields (SEFs) and potentials (SEPs) elicited by electrical stimulations of different areas of the snout in piglets.

METHODS

SEFs were measured with and without an intact skull, whereas SEPs were measured on the skull and cortex (Electrocorticograms - ECoG) and within the cortex of the same animal.

RESULTS

The SEFs above the skull and dura were very similar to each other in temporal waveform and spatial topography, indicating small effects of the skull. They both revealed very similar somatotopic projections of the snout. The SEPs on the skull and cortex were, in contrast, clearly different in their amplitudes as well as temporal and spatial morphologies, indicating significant effects of the skull. However, an early component of the SEP on the skull revealed a somatotopic representation of the snout, indicating that EEG can be also useful for inferring cortical projection areas. Discrepancies in their maps were due to predominance of the potentials produced by currents in the gyral cortex. The projection sites inferred from SEFs were quite accurate in comparison to those inferred from ECoGs and intracortical SEPs.

CONCLUSION

The similarities and differences clearly point out the complementary nature of MEG and EEG.

Patterns of lateral sensory cortical activation determined using functional magnetic resonance imaging

OBJECT

Functional magnetic resonance (fMR) imaging was performed in human volunteers to determine the lateral perisylvian cortical areas activated by innocuous cutaneous stimulation.

METHODS

Eight volunteers who underwent 53 separate experiments form the basis of this report. Eight contiguous coronal slices were obtained using echoplanar fMR imaging techniques while participants were at rest and while somatosensory activation stimuli consisting of vibration or air puffs were delivered to various body areas. The data were analyzed using Student's t-test and cluster analysis to determine significant differences between the resting and activated states. The findings were as follows: the areas in the lateral cortex activated by the stimuli were the primary sensory cortex (SI), the second somatosensory area (SII), the insula, the superior parietal lobule, and the retroinsular parietal operculum (RIPO). Somatotopy was demonstrable in SI but not in the other areas identified. There was a surprisingly low correlation between the amount of cortex activated in the various areas, which could mean separate inputs and functions for the areas identified. The highest correlation was found between activity in SII and RIPO (0.69).

CONCLUSIONS

The authors maintain that fMR imaging can be used to identify multiple lateral somatosensory areas in humans. Somatotopy is demonstrated in SI but not in the other lateral cortical sensory areas. The correlations between the amounts of cortex activated in the different lateral sensory areas are low. Recognition of the multiple lateral sensory areas is important both for understanding sensory cortical function and for safe interpretation of studies designed to identify the central sulcus by activating SI.

Organization of somatosensory cortex and distribution of corticospinal neurons in the eastern mole (Scalopus aquaticus)

The somatotopic organization of somatosensory cortex of the eastern mole (Scalopus aquaticus) was explored with multiunit microelectrode recordings from middle layers of cortex. The recordings revealed the presence of at least parts of two systematic representations of the body surface in the lateral cortex. One of the representations appears to be primary somatosensory cortex (S1), and it contained cytochrome oxidase dark regions, separated by light septa that formed isomorphs with some body parts. The rostral portion of this presumptive S1 cortex contained a face representation with a series of barrel-like cytochrome oxidase dark ovals that corresponded to the vibrissae on the snout. In caudolateral S1, light septa outline the palm and digits of the forepaw. Cortex caudal to S1, in the expected region of auditory cortex, responded to vibration, suggesting a modification of auditory cortex. Injections of wheat germ agglutinin-horseradish peroxidase into the cervical enlargement of the spinal cord revealed two dense foci of cortical cells that project to the spinal cord. The focus medial to the face region in S1 may correspond to primary motor cortex (M1). The second focus was coextensive with the somatosensory representation of the forelimb and the trunk in S1. The dense corticospinal projections from the forelimb representation of S1 and motor cortex may reflect sensorimotor specializations related to digging behaviors in moles.

Neuronal activities from a deep subcortical structure can be detected magnetically outside the brain in the porcine preparation

We assessed whether subcortical structures can generate magnetic fields detectable outside the brain by first measuring the somatic evoked magnetic fields (SEFs) from a decorticated porcine preparation and then from an intact preparation. Strong SEFs were detected a few millimeters above the corpus callosum after electrical stimulations of the snout. The waveforms consisted of a large spike ( < or = 6 pT) with a peak latency of 11-18 ms depending on the age of the animal, followed by a slow wave. The waveform and latency of the SEF spike were virtually identical to those of the field potential within the brain. The SEF topography indicated that the underlying generator of the spike was in a region contralateral to the stimulation and inferior to the thalamus. The subcortical SEF was strong enough to be detectable even above the intact brain, after the cortically generated SEF was removed by ablation of the primary cortical area. The results indicate that a structure deep in the brain can produce remarkably strong magnetic fields detectable outside the brain.

Somatosensory cortex of the neonatal pig: I. Topographic organization of the primary somatosensory cortex (SI)

The cerebral cortex of adult mammals contains several somatotopic representations of the body surface. Although the organization of the various somatosensory cortices of numerous species of adult animals has been elucidated, data on the somatosensory representations of fetal and neonatal animals are limited. As part of an investigation into the perinatal development of the somatosensory cortices, it was necessary to delineate the organization of the somatosensory cortices of the perinatal pig. This study presents the topographical organization of the primary somatosensory cortex (SI) of the perinatal pig. Multiunit microelectrode mapping methods were used to produce topographic maps of SI from barbiturate anesthetized pigs ranging in age from 7 days preterm to 2 months postpartum. It was demonstrated that the overall organization of this region of cortex was similar to that of other mammals: a somatotopic projection of predominantly the contralateral body surface was delineated in which the hindlimb is represented medially and the face laterally across the cortex. A disproportionately enlarged rostrum representation was mapped in detail, and multiple representations of the rostrum, face, and mouth were found. Several of these representations exhibited bilateral and ipsilateral input. The SI trunk and hindlimb representations were located on the medial wall of the hemisphere; these representations were small but their presence refutes speculation that ungulates do not have a complete body representation in SI.