MI Cortex Research Articles

Quantitative analysis of the bilateral brainstem projections from the whisker and forepaw regions in rat primary motor cortex

The whisker region in rat primary motor (MI) cortex projects to several brainstem regions, but the relative strength of these projections has not been characterized. We recently quantified the MI projections to bilateral targets in the forebrain (Alloway et al. [2009] J Comp Neurol 515:548-564), and the present study extends those findings by quantifying the MI projections to bilateral targets in the brainstem. We found that both the whisker and forepaw regions in MI project most strongly to the basal pons and superior colliculus. While the MI forepaw region projects mainly to the ipsilateral basilar pons, the MI whisker region has significantly more connections with the contralateral side. This bilateral difference suggests that corticopontine projections from the MI whisker region may have a role in coordinating bilateral whisker movements. Anterograde tracer injections in MI did not reveal any direct projections to the facial nucleus, but retrograde tracer injections in the facial nucleus revealed some labeled neurons in MI cortex. The number of retrogradely labeled neurons in MI, however, was dwarfed by a much larger number of labeled neurons in the superior colliculus and other brainstem regions. Together, our anterograde and retrograde tracing results indicate that the superior colliculus provides the most effective route for transmitting information from MI to the facial nucleus.

MI Neuronal Responses to Peripheral Whisker Stimulation: Relationship to Neuronal Activity in SI Barrels and Septa

The whisker region in the rodent primary motor (MI) cortex receives dense projections from neurons aligned with the layer IV septa in the whisker region of the primary somatosensory (SI) cortex. To compare whisker-induced responses in MI with respect to the SI responses in the septa and adjoining barrel regions, we used several experimental approaches in anesthetized rats. Reversible inactivation of SI and the surrounding cortex suppressed the magnitude of whisker-induced responses in the MI whisker region by 80%. Subsequent laminar analysis of MI responses to electrical or mechanical stimulation of the whisker pad revealed that the most responsive MI neurons were located >or=1.0 mm below the pia. When layer IV neurons in SI were recorded simultaneously with deep MI neurons during low-frequency (2-Hz) deflections of the whiskers, the neurons in the SI barrels responded 2-6 ms earlier than those in MI. Barrel neurons displayed similar response latencies at all stimulus frequencies, but the response latencies in MI and the SI septa increased significantly when the whiskers were deflected at frequencies of 8 Hz. Finally, cross-correlation analysis of neuronal activity in SI and MI revealed greater amounts of time-locked coordination among septa-MI neuron pairs than among barrel-MI neuron pairs. These results suggest that the somatosensory corticocortical inputs to MI cortex convey information processed by the SI septal circuits.

Transient Suppression of Ipsilateral Primary Somatosensory Cortex during Tactile Finger Stimulation

The whole human primary somatosensory (SI) cortex is activated by contralateral tactile stimuli, whereas its subarea 2 displays neuronal responses also to ipsilateral stimuli. Here we report on a transient deactivation of area 3b of the ipsilateral SI during long-lasting tactile stimulation. We collected functional magnetic resonance imaging data with a 3 T scanner from 10 healthy adult subjects while tactile pulses were delivered at 1, 4, or 10 Hz in 25 s blocks to three right-hand fingers. In the contralateral SI cortex, activation [positive blood oxygenation level-dependent (BOLD) response] outlasted the stimulus blocks by 20 s, with an average duration of 45 s. In contrast, a transient deactivation (negative BOLD response) occurred in the ipsilateral rolandic cortex with an average duration of 18 s. Additional recordings on 10 subjects confirmed that the deactivation was not limited to the right SI but occurred in the SI cortex ipsilateral to the stimulated hand. Moreover, the primary motor cortex (MI) contained voxels that were phasically deactivated in response to both ipsilateral and contralateral touch. These data indicate that unilateral touch of fingers is associated, in addition to the well known activation of the contralateral SI cortex, with deactivation of the ipsilateral SI cortex and of the MI cortex of both hemispheres. The ipsilateral SI deactivation could result from transcallosal inhibition, whereas intracortical SI-MI connections could be responsible for the MI deactivation. The shorter time course of deactivation than activation would agree with stronger decay of inhibitory than EPSP at the applied stimulus repetition rates.

Functional circuits mediating sensorimotor integration: Quantitative comparisons of projections from rodent barrel cortex to primary motor cortex, neostriatum, superior colliculus, and the pons

Motor performance depends on somatosensory feedback, and consistent with this finding, primary somatosensory (SI) cortex projects to several regions involved in motor control. Although the pathways mediating sensorimotor integration are known, few studies have compared their projection patterns. Therefore, in each animal, we injected two anterograde tracers into SI barrel cortex and compared the relative density and spatial extent of the labeled projections to the primary motor (MI) cortex, neostriatum, superior colliculus, and basal pons. Quantitative analysis revealed that these projections terminated most extensively in the neostriatum, to a lesser extent in MI cortex, and innervated the least amount of neuropil in the superior colliculus and pontine nuclei. Tracer overlap in the pontine nuclei was significantly higher than in the other three brains regions, and was strongly correlated with overlap in the superior colliculus, presumably because some projections to these two brain regions represent collaterals of the same neurons. The density of labeled varicosities was highest in the pons and lowest in MI. As a proportion of total labeling, densely packed clusters of labeled terminals were most prevalent in the pons, less prevalent in neostriatum and superior colliculus, and least prevalent in MI cortex. These results are consistent with physiological evidence indicating strong coherence between SI barrel cortex and the cerebellum during whisking behavior.

Oscillatory motor cortex–muscle coupling during painful laser and nonpainful tactile stimulation

Noxious stimulation activates–in addition to the brain structures related to sensory, emotional, and cognitive components of pain–also the brain's motor system. Effect of noxious input on the primary motor (MI) cortex remains, however, poorly understood. To characterize this effect in more detail, we quantified the ongoing oscillatory communication between the MI cortex and hand muscles during selectively noxious laser stimulation. The subjects maintained an isometric contraction of finger muscles while receiving the laser stimuli to the dorsum of the hand. Tactile stimuli with well-known effects on the MI cortex reactivity served as control stimuli. Cortex–muscle coherence was computed between magnetoencephalographic (MEG) signals from the contralateral MI and electromyographic (EMG) signals from the hand muscles. Statistically significant coherence at ∼20 Hz was found in 6 out of 7 subjects. The coherence increased phasically after both types of stimuli but significantly later after laser than tactile stimuli (mean ± SEM peak latencies 1.05 ± 0.12 s vs. 0.58 ± 0.06 s; P < 0.05), and the coherence increase lasted longer after laser than tactile stimuli (0.87 ± 0.09 s vs. 0.50 ± 0.06 s, P < 0.05). The observed coherence increase could be related to stabilization of the motor-cortex control after sensory input. Our findings add to the clinically interesting evidence about the cortical pain–motor system interaction.

Septal columns in rodent barrel cortex: Functional circuits for modulating whisking behavior

In rodents, each mystacial whisker is represented in the granular layer of primary somatosensory (SI) cortex by a compact cluster of cells known as a barrel, and barrels are separated from each other by domains that are called septa. Vertical columns of neurons aligned with each barrel act as a functional assembly to process information from a "principal" whisker, but a functional role has not been identified for vertical columns of neurons that are aligned with the septa. To determine whether these septal columns provide the main source of projections to primary motor (MI) cortex, we placed retrograde tracers in MI cortex and analyzed the location of the retrogradely labeled neurons with respect to the septal and barrel compartments of SI barrel cortex. In cases in which SI barrel cortex was sectioned tangentially, retrogradely labeled neurons in the extragranular layers of SI were plotted and superimposed onto reconstructions of the layer IV barrel field. In each of these cases, most labeled neurons were located above or below the septal regions of layer IV. When SI barrel cortex was sectioned coronally, we observed multiple columns of labeled SI neurons that were vertically aligned with the septal zones of layer IV. These results indicate that columns of neurons that are vertically aligned with the septa, or septal columns, are functionally linked by virtue of their projections to MI cortex. We hypothesize that these septal columns represent an interconnected and functionally distinct circuit that transmits information to MI and other brain regions involved in motor control.

Modulation of motor-cortex oscillatory activity by painful Aδ- and C-fiber stimuli

Spontaneous ∼20-Hz oscillations, arising predominantly from the primary motor cortex (MI), are readily observed by magnetoencephalography (MEG). Prior studies have indicated that the level of the ∼20-Hz rhythm reflects the functional state of the MI cortex: increased 20-Hz level is associated with increased inhibition and suppression of the rhythm with excitation of MI. Close interaction is suggested between pain and the motor system by the association of chronic pain with motor dysfunction and by the alleviation of pain by motor-cortex stimulation. We therefore explored the effect of noxious input on motor-cortex functions by recording MEG signals from nine healthy subjects during selective laser stimulation of Aδ- and C-fibers of the hand. The ∼20-Hz level was suppressed in the contralateral MI cortex in all nine subjects after painful Aδ- and C-fiber stimuli ( P < 0.001). The suppression started 180 ± 10 ms (mean ± SEM) after Aδ-fiber stimuli and 820 ± 30 ms after C-fiber stimuli, and peaked 160–170 ms later. Similar, but about 50% weaker, suppression of the ∼20-Hz oscillations occurred in seven out of nine subjects in the ipsilateral MI. These results suggest automatic, lateralized, excitation of the MI cortex by noxious Aδ- and C-fiber input.

Organization of corticostriatal projections from the vibrissal representations in the primary motor and somatosensory cortical areas of rodents.

To characterize corticostriatal projections from rodent sensorimotor cortex, the anterograde tracers biotinylated dextran amine (BDA) and fluororuby (FR) were injected into the whisker representations of the primary motor (MI) and somatosensory (SI) cortices. Reconstructions of labeled terminals and their beaded varicosities in the neostriatum and thalamus were analyzed quantitatively to determine the degree of labeled overlap in both of these subcortical structures. Corticostriatal projections from the vibrissal representation in MI were more extensive than corresponding projections from SI. Both cortical areas sent dense projections to the dorsolateral neostriatum, but the MI vibrissal representation also projected to regions located more rostrally and medially. Despite these differences, both MI and SI projected to overlapping parts of the dorsolateral neostriatum. Tracer injections in both cortical areas also produced dense anterograde and retrograde labeling in the medial sector of the posterior complex of the thalamus (POm). Because POm is somatotopically organized and has reciprocal connections with both SI and MI cortices, the amount of labeled overlap in POm was used to indicate whether the tracers were injected into corresponding whisker representations of MI and SI. We found that the proportion of labeled overlap in the neostriatum was highly correlated with the amount of labeled overlap in POm. These results indicate that the rodent neostriatum receives convergent projections from corresponding regions in MI and SI cortex. Furthermore, the thalamocortical projections of the POm indicate that it may modulate corticostriatal outputs from corresponding representations in MI and SI.

Plasticity and primary motor cortex.

One fundamental function of primary motor cortex (MI) is to control voluntary movements. Recent evidence suggests that this role emerges from distributed networks rather than discrete representations and that in adult mammals these networks are capable of modification. Neuronal recordings and activation patterns revealed with neuroimaging methods have shown considerable plasticity of MI representations and cell properties following pathological or traumatic changes and in relation to everyday experience, including motor-skill learning and cognitive motor actions. The intrinsic horizontal neuronal connections in MI are a strong candidate substrate for map reorganization: They interconnect large regions of MI, they show activity-dependent plasticity, and they modify in association with skill learning. These findings suggest that MI cortex is not simply a static motor control structure. It also contains a dynamic substrate that participates in motor learning and possibly in cognitive events as well.

Input-output organization of the sensorimotor striatum in the squirrel monkey

The basal ganglia receive massive inputs from the neocortex and send outputs that exert both inhibitory and disinhibitory control over parts of the frontal cortex and brainstem. Between these basal ganglia inputs and outputs lies the striatum, which receives most of the cortical afferents and projects to the basal ganglia output nuclei--the globus pallidus and substantia nigra. To analyze this system we conjointly labeled, in squirrel monkeys, sensorimotor cortical inputs to the striatum and striatal outputs to the globus pallidus. Anterograde tracers were injected into the motor (MI) and somatosensory (SI) cortical body maps, at sites determined by electrophysiological stimulation and recording. Retrograde tracers were stereotaxically injected into the external and internal pallidal segments (GPe and GPi). We found that multiple dispersed modules ("matrisomes") in the putamen that all received inputs from single body-part representations in sensorimotor cortex could, in turn, send convergent outputs to single sites in the pallidum. This divergence-reconvergence pattern was found for both GPe and GPi sites, and for inputs from both SI and MI cortex. Thus, information from a single functional region in the cortex can be split up at the striatal stage only to be brought back together in the pallidum. The temporary divergence may increase lateral interactions between sensorimotor matrisomes, as well as between matrisomes and striosomes. One function of striatal modularity may thus be to set up an associative network in the striatum, which might contribute to sensorimotor learning. We also found that some sets of matrisomes did not receive strong sensorimotor inputs, even though they projected to regions of GPe and GPi that are near the sensorimotor-recipient zones described above. Thus, the matrisomal system may sort MI/SI inputs and other inputs before transfer to paired regions of GPe and GPi.

Mapping the effects of motor cortex stimulation on somatosensory relay neurons in the rat thalamus: Direct responses and afferent modulation

Single unit recordings were used to map the spatial distribution of motor (MI) cortical influences on thalamic somatosensory relay nuclei in the rat. A total of 215 microelectrode penetrations were made to record single neurons in tracks through the medial and lateral ventroposterior (VPM and VPL), ventrolateral (VL), reticular (nRt), and posterior (Po) thalamic nuclei. Single units were classified according to their: 1) location within the nuclei, 2) receptive fields, and 3) response to standardized microstimulation in deep layers of the forepaw-forelimb areas of MI cortex. For mapping purposes, only short latency (1–7 msec) excitatory neuronal responses to the MI cortex stimulation were considered. Percentages of recorded thalamic neurons responsive to the MI stimulation varied considerably across nuclei: VL: 42.6%, nRt: 23.0%, VPL: 15.7%, VPM: 9.3%, and Po: 3.9%. Within the VPL, most responsive neurons were found in “border” regions, i.e., areas adjacent to the VL, and (to a lesser extent) the nRt and Po thalamic nuclei. The same parameters of MI cortical stimulation were used in studies of corticofugal modulation of afferent transmission through the VPL thalamus. A condition-test (C-T) paradigm was implemented in which the cortical stimulation (C) was delivered at a range of time intervals before test (T) mechanical vibratory stimulation was applied to digit No. 4 of the contralateral forepaw. The time course of MI cortical effects was analyzed by measuring the averaged evoked unit responses of the thalamic neurons to the T stimuli, and plotting them as a function of C-T intervals from 5–50 msec. Of the 30 VPL neurons tested during MI stimulation, the average response to T stimulation was decreased a mean 43%, with the suppression peaking at about 30 msec after the C stimulus. This suppression was more pronounced in the VPL border areas (−52% in areas adjacent to VL and nRt) than in the VPL center (−25%).

Deprived somatosensory‐motor experience in stumptailed monkey neocortex: Dendritic spine density and dendritic branching of layer IIIB pyramidal cells

Infant macaque monkeys (Macaca arctoides) were individually raised to age 6 months in large clear cubes built into one wall of a control colony that allowed them visual access to it but not tactile contact. The two deprivation conditions (Cond 2 and Cond 3) were equal both in physical size and with respect to partial social isolation. They differed in the degree of somatosensory-motor opportunity available during development in that the Cond 2 chamber was empty, whereas Cond 3 contained ladders, a trapeze, and play objects. Four monkeys from each of these conditions were compared with four colony-reared (Cond CR) monkeys. Neuroanatomical changes were evaluated by using light microscopy in Golgi-Cox-stained neocortex. Dendritic spines on the apical shafts of layer IIIB pyramidal cells were counted in primary motor (MI), somatosensory (SI), and visual (area 17, V1) cortical regions. Layer IIIB pyramidal neurons with somas of medium size were selected from each cortical region and the density of apical dendritic spines determined. The basilar dendritic branches of these same neurons were traced, and the dendritic branching complexity was assessed in order to compare the sensitivity of the dendritic spine and branching measures consequent to deprived rearing. The number of apical dendritic spines was significantly reduced in Cond 2 when compared with either Cond 3 or Cond CR (which did not differ from each other). This occurred in both MI and SI cortex, but not in the visual cortex, the region used as a control for a global brain effect. Branching complexity measured on the same pyramidal neurons was reduced only in MI cortex of Cond 2. These results show spine density, a more direct measure of neuronal connectivity, to be the more sensitive measure of early environmental deprivation. Also, the enriched environment provided by Cond 3 relative to Cond 2 offset the effect of partial social isolation such that both morphometric measures had values comparable to Cond CR monkeys.

Distribution of somatic sensory and active-movement neuronal discharge properties in the MI-SI cortical border area in the rat

The rat somatosensory (SI) cortex contains a precise map of the cutaneous periphery, yet its rostromedial edge, which includes part of the fore- and hind paw representation, has been reported to functionally overlap with the electrically excitable primary motor (MI) cortex. Thus, the MI cortex in the rat contains two subregions: (i) rostrally, the “MI-agranular” cortex (i.e., “typical” MI cortex), and (ii) caudally, the “MI-SI-granular” cortex (i.e., the MI-SI overlap). The aim of this study was to assess the degree of overlap in the physiologic properties of single neurons recorded across the MI-SI boundary zone in awake, freely moving rats. Computer techniques were used to characterize both somatosensory receptive fields (cutaneous or “passive joint-manipulation”) and discharge correlates of active limb movement in these MI-agranular and MI-SI-granular subregions of the MI. “Active-movement” units were defined as those which discharged strongly in correlation with specific limb movements, but in a manner which could not be predicted by their observable somatosensory properties. Of 92 completely analyzed cells in the MI-SI-granular forepaw region, 86 exhibited specific cutaneous receptive fields on the palmar surface of the forepaw. By contrast with the similarity of these neurons' responses to passive stimulation, they varied markedly in their discharge during active limb movements. For example, many did not respond when their forepaw receptive fields touched the ground during stepping. Furthermore, 31 (of 92) neurons in this region were identified as active-movement, firing in correlation with reaching movements of the forelimb. Seven of these were completely unresponsive to any sensory stimuli, but 24 exhibited an apparent convergence of cutaneous and active-movement properties. Of 86 units recorded in the MI-agranular subregion, 46 responded strongly to passive joint manipulation, but only three responded exclusively to cutaneous stimulation. Twenty-eight (of 86) cells were defined as active-movement, discharging mainly in correlation with forelimb reaching movements. Thus, the active-movement properties of neurons in these two subregions were quite similar, whereas the somatosensory properties were markedly different.

Bilateral [14C]2-deoxyglucose uptake by motor pathways after unilateral neonatal cortex lesions in the rat.

The left sensorimotor (SM) cortex was removed in 1-day-old and 30-day-old rats. At 90 days of age [14C]2-deoxyglucose (2DG) was injected and the right motor (MI) cortex was electrically stimulated to produce predominantly left vibrissae movements. Animals were sacrificed and their brains autoradiographed. Right motor cortex (MI) stimulation activated subcortical structures bilaterally in 3 of 5 neonatally lesioned rats. Right MI stimulation activated most subcortical structures unilaterally in 30-day lesioned rats. This implies that a critical period for SM cortex injury may occur between 1 and 30 days of age. We suggest that removal of left SM cortex at 1 day of age occurs before or during the critical period and this induces right MI cortex to form bilateral subcortical connections. The bilateral 2DG uptake increases imply the bilateral connections are functional. The unilateral subcortical activation in 2 of 5 neonatally lesioned animals may have been due to differences in size of the lesion, parameters of stimulation, or perhaps due to the time of the lesion being near the critical period. We also suggest that removal of left SM cortex at 30 days of age occurs after the critical period, but that this does not affect the predominantly ipsilateral subcortical connections from MI cortex which have previously formed during normal development.

The motor cortex of the rat: cytoarchitecture and microstimulation mapping.

The first motor (MI) cortex of the rat was identified as the region from which movements could be evoked by the lowest intensity of electrical stimulation. The location of this region was correlated with cytoarchitecture in the frontal and parietal cortex. Two frontal areas can be discerned in Nissl-stained sections: (1) the medial agranular field, marked by a pale-staining layer III and a compact layer II, and (2) the lateral agranular field, which has more homogeneous superficial layers and a broad layer V containing large, densely staining cells. Both of these regions project to the spinal cord and can therefore be included in the somatic sensorimotor cortex. MI in the rat coincides with the lateral agranular field but also overlaps with part of the adjacent granular cortex of the first somatic sensory (SI) representation. We conclude that the rat MI cortex can be identified by microstimulation techniques and by cytoarchitecture in the rat.

Regional (14C) 2-deoxyglucose uptake during vibrissae movements evoked by rat motor cortex stimulation.

Repetitive left mystacial vibrissae movements were produced by electrical stimulation of right motor cortex (MI) were a bipolar electrode in the alert, unanesthetized rat. Regional increases of (14C) 2-deoxyglucose (2DG) uptake were mapped autoradiographically during these left vibrissae movements. Uptake of 2DG increased in a 2-4-mm-diameter area about the stimulating electrode in right MI and in a smaller region in left MI cortex. Columnar increases of 2DG uptake occurred bilaterally in somatosensory cortex in the face region of somatosensory cortex (SI). Bilateral increases of 2DG uptake occurred subcortically in dorsolateral caudate-putamen (CP) and subthalamic nucleus. Primary right-sided increases of 2DG uptake occurred in other basal ganglia structures including dorsal globus pallidus (GP), posterior, entopeduncular nucleus (EPN), ventrolateral substantia nigra pars reticulata (SNr), and anterolateral substantia nigra pars compacta (SNc). Uptake of 2DG increased on the right side of the following thalamic regions: much of the ventrolateral (VL) nucleus, particularly dorsally; the anterodorsal reticular nucleus; dorsolateral posteromedial (POm) nucleus; the ventromedial nucleus; and dorsolateral parafasicular nucleus. The anterior and ventral posterior portions of VL were not activated. Caudal to thalamus right-sided 2DG uptake increased in the medial, ventral, and lateral pontine nuclei, deep layers of superior colliculus, lateral deep mesencephalic nucleus (DMN), and nucleus cuneiformis (NCU). UPtake of 2DG increased in right rostral parvocellular and red nucleus in a few animals. Discrete portions of the right internal capsule and right medial pyramidal tract increased 2DG uptake during MI stimulation. Uptake of 2DG increased on the left side of the brain during right MI stimulation in the left lateral nucleus (NL) of cerebellum and in several discrete regions of left cerebellar hemisphere granule cells including anterior paravermis, lobulus simplex, crus II, and the paramedian lobule. Uptake of 2DG increased in left nucleus of the spinal tract of the trigeminal nerve (ntV) ventrally in subnuclei interpolaris and oralis. Left lateral portions of the facial nucleus were activated in a few animals. The lateral portions of the facial nucleus are known to project to vibrissae musculature. All of the above structures may be involved in the motor-sensory processing responsible for vibrissae movements. Regions not previously suggested to play a major role in vibrissae movements include DMN and NCU. Though NCU has been called the "locomotor center" it may play a role in facial movements as well. Polysynaptic activation of GP, EPN, NL, and cerebellar hemisphere occurred since no connections between MI and these regions exist. A pathway from ntV to POm to MI and SI is suggested to provide proprioceptive input to motor-sensory cortex from the moving vibrissae since neither the principal trigeminal sensory nucleus nor the ventrobasal nucleus of the thalamus increased 2DG uptake during MI stimulation.

The organization of thalamic projections to the parietal cortex of the Virginia opossum.

The thalamic projections to somatic sensory-motor (SSM) cortex and adjacent cortical areas of the Virginia opossum were studied using anterograde and retrograde axoplasmic transport techniques. Large injections of horseradish peroxidase and/or tritiated amino acids were made in the parietal cortex to identify all of the thalamic nuclei that are interconnected with this large cortical area. Very restricted injections were then made in physiologically identified subdivisions of SSM cortex, in the remaining posterior portion of parietal cortex, and in the anteriorly adjacent postorbital cortex. The results show that the parietal cortex is reciprocally connected with a number of thalamic nuclei. Different combinations of these thalamic areas project to specific subregions within the parietal field. All parts of the SSM cortex, which occupies the anterior four-fifths of parietal cortex, receive input from the ventrobasal complex (VB), the ventrolateral complex (VL), the central intralaminar nucleus (CIN), the central lateral nucleus (CL), and the ventromedial nucleus (VM). We could detect no segregation of VL and VB inputs in any part of SSM cortex. Projections from all of these thalamic nuclei, except VM, show at least some degree of topographic organization. Anterior-posterior strips of SSM cortex receive input from clusters of thalamic neurons that extend dorsoventrally and rostrocaudally through VB and VL. The posterior one-fifth of the parietal cortex (the posterior parietal area) receives input from VL, the posterior nuclear complex, and the lateral complex, as well as input from CL, CIN, and VM. Postorbital cortex receives input mainly from intralaminar, midline, and medial thalamic nuclei. We conclude that the projection field of VB in the parietal cortex coincides precisely with the first somatic sensory area (SI) as defined by single unit studies (Pubols et al., '76). The VB projection field also delineates the area of the first motor (MI) representation. Thus, there is no separation of SI and MI cortex in the opossum. The posterior parietal area lies outside of SSM cortex and has thalamic connections similar to the posterior parts of parietal cortex in other mammals.

Neuronal responses in sensorimotor cortex to ramp displacements and maintained positions imposed on hindlimb of the unanesthetized monkey

1. Neuronal responses to passively imposed displacements of the foot were examined in area 3a, the precentral motor cortex (MI), and the first somatic sensory cortex (SI) of unanesthetized monkeys. The foot displacements, in the form of "ramps" and maintained displacements, were applied at several different velocities and achieved a number of maintained positions. The response in relation to the ramp was termed a "dynamic" response, while that in association with the maintained displacement was termed a "static" response. 2. Units in the rostral part of MI (MI/r) and in area 3a usually responded in relation both the ramps and to maintained displacements. The activity of the majority of area 3a and MI/r units was strongly modulated only during ramps of one direction. Such response properties contrast with those observed in the caudal part of MI (MI/c) and areas 3b and 1 of SI, where most units responded in relation to the ramp phase of the displacement in both of two ramp directions and not to the maintained displacements. 3. Virtually all units, including those in MI, displayed a dynamic response. However, the dynamic response amplitude of area 3a neurons greatly exceeded that of MI units for equal velocities of displacement. 4. For area 3a units, peak neuronal activity was well correlated with ramp velocity over the range examined. Such units were a minority in areas 3b and 1 of SI (excluding area 3a) and MI. 5. Most area 3a units and almost half of MI/r units displayed, in addition to a dynamic response, a static response that reflected foot position. MI units were more sensitive than area 3a units to equal degrees of maintained displacement. 6. These results demonstrate that in the unanesthetized, intact animal, responses of most neurons in area 3a and MI fall into two categories: a) mixed dynamic-static responses resembling those of muscle spindle afferents, and b) purely dynamic responses. These data are consistent with the hypothesis that afferent input concerning the velocity and position of a segment of a limb may have a role in cortically mediated regulation of movement and posture, and that both areas 3a and the MI cortex may be directly involved in this regulation. 7. The cytoarchitectonic characteristics of area 3a, as defined in single-unit studies, are discussed. It is argued that the salient characteristic of area 3a in the primate is a thinned internal granular layer (layer IV) that contrasts with the heavily granular area 3b and the agranular area 4.

Intra- and interhemispheric projections of the precentral, premotor and arcuate areas in the rhesus monkey

Summary The intra- and interhemispheric connections of various parts of the precentral gyrus and rostrally adjacent frontal areas are studied with silver impregnation techniques in the rhesus monkey. The subdivisions of the precentral motor area (MI) are found to be connected ipsilaterally in a topographic manner with the premotor area, MII, primary somatosensory area (SI) and the second sensory area (SII). The areas in MI representing the hand and foot do not project to the opposite hemisphere whereas face-, trunk-, and limb-girdle areas do so, and their projections are directed to both homotopical and non-homotopical areas, including topographic projections to MII. These non-homotopical projections form an arch anterior to the projection-free hand and foot areas representing the dorsal aspect of the body from face to occiput and from thoracolumbar to sacral segment of the trunk. The premotor and arcuate regions can be differentiated on the basis of their intrahemispheric connectivity. The premotor area sends projections to its immediate vicinity (the prefrontal cortex MI and MII), to the superior parietal lobule including the adjacent upper bank of the intraparietal sulcus and to the rostral part of the inferior parietal lobule. In contrast, the projections of areas within the concavity of the arcuate sulcus are directed to the prefrontal cortex, to the caudal part of the inferior parietal lobule including the adjacent lower bank of the intraparietal sulcus and to the cingulate gyrus. Likewise, the interhemispheric connections of the premotor areas and the arcuate areas are mainly found in the homotopic regions except for a small area above the caudal tip of the principal sulcus. The non-homotopic projection of the premotor areas are directed to MI, MII, and prefrontal cortex, whereas those from the arcuate areas are observed in the prefrontal cortex only. The possible significance of these projections is discussed.