An investigation of the intrinsic circuitry of the motor cortex of the monkey using intra-cortical microstimulation

The motor cortex contains a distributed map of muscles, with a single muscle represented over a wide cortical area. We have searched for inter-connections between distant sites projecting to common muscles by delivering pairs of 20-μA single-pulse intracortical microstimuli (ICMS) to sites separated by 1.5–2 mm in the hand-area primary motor cortex of two macaque monkeys performing a precision grip task. The facilitation of hand- and forearm-muscle rectified EMG was measured. When stimuli were delivered simultaneously, responses were quantified using a technique to correct for non-linearities inherent in the use of averaged, rectified EMG. A spatial facilitation was seen for such simultaneous stimuli; however, it was of the same magnitude as that occurring when ICMS was paired with stimulation of corticospinal axons in the pyramidal tract (PT), so that it was likely to be spinal in origin. When two such distant sites were stimulated separated by a 10- or 20-ms delay, the second response scaled with the level of background EMG in the same way as a response to the PT stimulus. By contrast, when the same site was stimulated twice with these delays, the second response showed a facilitation compared with a similarly timed PT response. There would therefore appear to be a local facilitation of the cortical output at these intervals, which is not seen between distant sites. Antidromically identified pyramidal-tract neurones (PTNs) were recorded whilst stimuli were delivered to a cortical site, with a distance between stimulating and recording electrodes of also 1.5–2 mm. The most common response was a facilitation followed by a suppression. Six of eleven PTNs showed a facilitation in their discharge following this stimulation (maximum connection strength s=0.19), 8/11 showed a suppression (maximum s=0.16). It is concluded that powerful inter-connections do exist between distributed parts of the motor output and that there is widespread cortical activation after even a single ICMS pulse. However, these inter-connections do not lead to interactions between cortical outputs following stimulation, as assessed from the EMG. It is proposed that this is likely to reflect differences in the summation of output cells to local versus remote stimulation. Received: 10 March 1998 / Accepted: 2 June 1998     

Precise spike synchronization in monkey motor cortex involved in preparation for movement

 It is commonly accepted that perceptually and behaviorally relevant events are reflected in changes of activity in largely distributed neuronal populations. However, it is much less clear how these populations organize dynamically to cope with momentary computational demands. In order to decipher the dynamic organization of cortical ensembles, the activities of up to seven neurons of the primary motor cortex were recorded simultaneously. A monkey was trained to perform a pointing task in six directions. During each trial, two signals were presented consecutively. The first signal provided prior information about the movement direction, whereas the second called for the execution of that movement. Dynamic interactions between the activity of simultaneously recorded neurons were studied by analyzing individual epochs of synchronized firing (”unitary events”). Unitary events were defined as synchronizations which occur significantly more often than expected by chance on the basis of the neurons’ firing rates. The aim of the study was to describe the relationships between synchronization dynamics and changes in activity of the same neurons during the preparation and execution of voluntary movements. The data show that even neurons which were classified, on the basis of the change in their firing rate, to be functionally involved in different processes (e.g., preparation or execution related, different directional tuning) synchronized their spiking activity significantly. These findings indicate that the synchronization of individual action potentials and the modulation of the firing rate may serve different and complementary functions underlying the cortical organization of cognitive motor processes. Received: 6 August 1998 / Accepted: 21 December 1998     

An output zone of the monkey primary motor cortex specialized for bilateral hand movement

Summary We have identified a subregion in the monkey primary precentral motor cortex (MI) that is characterized by its relationship to bilateral or ipsilateral hand movements. The subregion is located between the digit and face representation areas. The majority of single cells in this portion of MI exhibit distinct activity before and during visually triggered key-press movements performed by means of ipsilateral or contralateral digit flexion. Intracortical microstimulation evoked responses of ipsilateral, in addition to contralateral, digit muscles.     

Responses of motor cortical cells to short trains of vibration

The response discharges of precentral motor cortical cells to brief trains of vibration applied to the tendon of biceps brachii were analyzed in two alert but passive monkeys. The activity of 20 phasictonic and 6 tonic cells was analyzed. All had functional linkages with flexor muscles during a preceding flexion task and responded to passive movement of the elbow. Taking as a reference the stereotyped reflex response in the stretched muscle, the effect of changes in the amplitude of a constant frequency vibration (4 vibrations at 58 Hz) was quantified statistically in peristimulus histograms of the cortical cell discharges. All cells were transiently influenced by low vibration amplitudes. Most responses (71 %) were excitatory and occurred at a mean latency of 24 ms, which is consistent with cells activated by input from stretch receptors. Excitatory, reproducible responses to the lowest vibration amplitudes were more frequent in phasictonic than in pure tonic cells. Large-amplitude vibrations always excited the motor cortical cells. The sign of the responses to vibration matched that to passive elbow movements for most cells. These findings show that elbowrelated motor cortical cells are very sensitive to proprioceptive input from primary spindle afferents.     

The NMDA receptor participates in respiration-related mitral cell synchrony

The N-methyl-d-aspartate (NMDA)-type glutamate receptor participates in the excitation of olfactory bulb mitral cells and is important in granule-cell-mediated feedback-inhibition. In the present study, extracellular unit recordings were made in vivo to demonstrate that the firing rates of mitral cells are not affected by peripheral administration of the non-competitive NMDA receptor antagonist MK-801. However, while over 50% of odor-driven mitral cell activity is normally correlated with the respiratory cycle, only about 10% of mitral cell activity is correlated with the respiratory cycle 30 min after MK-801 administration. Thus, the NMDA receptor is a participant in normal respiration-related mitral cell activity and may have an important role in the formation of bulb oscillations that encode olfactory information. Furthermore, the NMDA receptor is in a position to mediate activity-dependent changes in the bulb that rely on synchronous activity. Received: 30 March 1997 / Accepted: 15 July 1997     

Role of across-muscle motor unit synchrony for the coordination of forces

Evidence from five-digit grasping studies indicates that grip forces exerted by pairs of digits tend to be synchronized. It has been suggested that motor unit synchronization might be a mechanism responsible for constraining the temporal relationships between grip forces. To evaluate this possibility and quantify the effect of motor unit synchrony on force relationships, we used a motor unit model to simulate force produced by two muscles using three physiological levels of motor unit synchrony across the two muscles. In one condition, motor units in the two muscles discharged independently of one another. In the other two conditions, the timing of randomly selected motor unit discharges in one muscle was adjusted to impose low or high levels of synchrony with motor units in the other muscle. Fast Fourier transform analysis was performed to compute the phase differences between forces from 0.5 to 17 Hz. We used circular statistics to assess whether the phase differences at each frequency were randomly or non-randomly distributed (Rayleigh test). The mean phase difference was then computed on the non-random distributions. We found that the number of significant phase-difference distributions increased markedly with increasing synchronization strength from 18% for no synchrony to 65% and 82% for modest and strong synchrony conditions, respectively. Importantly, most of the mean angles clustered at very small phase difference values (~0 to 10°), indicating a strong tendency for forces to be exerted in a synchronous fashion. These results suggest that motor unit synchronization could play a significant functional role in the coordination of grip forces.     

A multimuscle state analysis of adult motor learning

 We introduce a new EMG state analysis to test two competing hypotheses about the role of muscle coactivity in learning a complex, multijoint reaching movement. Following Bernstein, one hypothesis is that as a task is learned, coactivity should decrease as degrees of freedom are released and limb stiffness is reduced. An alternative hypothesis is that as movement speed increases with learning, muscle coactivity should increase, possibly to stabilize joints against high inertial forces. Three participants performed a vertical reaching movement identical to that used by Schneider et al. We monitored the activity of four arm and shoulder muscles as participants completed 100 practice trials. Each frame of EMG activity was assigned to one of 16 possible combinations of the four monitored muscles based on an on-off activation threshold. This analysis yielded a time-based summary of muscle coactivity during the movement and across practice trials. Results of the state analysis supported the second hypothesis. As participants decreased their movement times over practice, coactivity increased – participants used more three- and four-muscle coactivity states. Changes were especially dramatic during the braking phase of the Up and Down portion of the vertical movement. When participants performed deliberately slow movements after speeded practice, three- and four-muscle coactivity was suppressed. We suggest that increased use of muscle coactivity may serve to counteract unwanted rotational forces generated during fast movements. Received: 12 November 1998 / Accepted: 22 February 1999     

Short-latency peripheral inputs to thalamic neurones projecting to the motor cortex in the monkey

Summary One hundred seventy-five neurones in the n.ventroposterior lateralis (VPL) and n.ventralis lateralis (VL) in the thalamus of anaesthetised monkeys have been tested antidromically for projection to the cortex and for somatosensory input from the contralateral arm.Using bipolar stimulation of the cortical surface, 113 thalamic neurones were successfully identified as antidromically driven from the hand area of the postcentral gyrus (48 neurones) or from the hand area of the precentral gyrus (65 neurones). All but one of these 113 neurones could only be antidromically discharged from the postcentral cortex or from the precentral cortex, and not from both. Most had antidromic latencies between 0.5 and 1.5 ms.Twenty-five/sixty-five precentrally projecting neurones and 45/48 postcentrally projecting neurones were activated by stimulation of the contralateral median or radial nerves. Both groups responded at short latency (4–8 ms) and many were activated by low-threshold shocks (0.8–1.3 T) and had restricted receptive fields on the hand. Precentrally projecting neurones responded most powerfully to joint movement or deep pressure, and some of these neurones were also responsive to cutaneous stimuli.Precentrally projecting neurones with peripheral inputs were all found in the oral subdivision of the VPL (the VPL_o). The properties of these neurones suggest that they may be partly responsible for rapid somatosensory input to the motor cortex.     

Corticostriatal projections from the somatic motor areas of the frontal cortex in the macaque monkey: segregation versus overlap of input zones from the primary motor cortex, the supplementary motor area, and the premotor cortex

It is an important issue to address the mode of information processing in the somatic motor circuit linking the frontal cortex and the basal ganglia. In the present study, we investigated the extent to which corticostriatal input zones from the primary motor cortex (MI), the supplementary motor area (SMA), and the premotor cortex (PM) of the macaque monkey might overlap in the putamen. Intracortical microstimulation was performed to map the MI, SMA, and dorsal (PMd) and ventral (PMv) divisions of the PM. Then, two different anterograde tracers were injected separately into somatotopically corresponding regions of two given areas of the MI, SMA, PMd, and PMv. With respect to the PMd and PMv, tracer injections were centered on their forelimb representations. Corticostriatal input zones from hindlimb, forelimb, and orofacial representations of the MI and SMA were, in this order, arranged from dorsal to ventral within the putamen. Dense input zones from the MI were located predominantly in the lateral aspect of the putamen, whereas those from the SMA were in the medial aspect of the putamen. On the other hand, corticostriatal inputs from forelimb representations of the PMd and PMv were distributed mainly in the dorsomedial sector of the putamen. Thus, the corticostriatal input zones from the MI and SMA were considerably segregated though partly overlapped in the mediolateral central aspect of the putamen, while the corticostriatal input zone from the PM largely overlapped that from the SMA, but not from the MI. Received: 30 June 1997 / Accepted: 2 October 1997     

Context-dependent force coding in motor and premotor cortical areas

In three monkeys trained to finely grade grip force in a visuomotor step-tracking task, the effect of the context on neuronal force correlates was quantitatively assessed. Three trial types, which differed in force range, number, and direction of the force steps, were presented pseudo-randomly and cued with the color of the cursor serving as feedback of the exerted force. Quantitative analyses were made on 85 neurons with similar discharge patterns in the three trial types and significant linear positive (54 cells) or negative (31 cells) correlation coefficients between firing rate and force. An analysis of covariance (ANCOVA) showed that the population slopes for 2-step were steeper than for 3-step trials. Another ANCOVA at the population level, computed on the differences in firing rate and force between force steps, persistently disclosed a significant effect of trial type. For the first two force steps, the differences in firing rate were significantly larger in the 2-step than in the 3-step increase trials. Further analyses revealed that neither the force range nor the number of steps was a unique factor. A small group of neurons was tested in an additional trial series with a uniform cue for all three trials, leading to either a loss of context-dependency or to unexpected changes in firing rate. This demonstrates that the cue color was an important instruction for task performance and neuronal activity. The most important findings are that the context-dependent changes were occurring ”on-line”, and that neurons displaying context-dependency were found in all three lateral premotor cortex hand regions and in the primary motor cortex. Finger muscle activity did not show any context dependency. The context-dependent effect leads to a normalization of the cortical activity. The advantage of normalization is discussed and mechanisms for the gain regulation are proposed. Received: 10 November 1998 / Accepted: 13 March 1999     

Compensatory motor function of the somatosensory cortex for the motor cortex temporarily impaired by cooling in the monkey

Summary The motor cortex was temporarily impaired by local cooling during repeated execution of visually initiated hand movements in monkeys. The effects of cooling were examined by recording premovement cortical field potentials in the forelimb motor and somatosensory cortices and by measuring reaction time and force exerted by the movement. The cortex was cooled by perfusing cold water (about 1° C) through a metal chamber placed on the cortical epidural surface. Cooling of the forelimb motor area lowered temperature of the cortex under the chamber to 20–29° C within 4–5 min. Recording electrodes for cortical field potentials were implanted chronically on the surface and at 2.5–3.0 mm depth of various cortical areas including that being cooled. Spread of cooling to surrounding cortical areas was prevented by placing chambers perfused with warm water (38–39° C) on the areas.Cooling of the forelimb motor area greatly reduced its premovement cortical field potentials, followed by prolonged reaction times of weakened contralateral wrist muscles. Simultaneous recording from the primary somatosensory cortex revealed an enhancement of its premovement field potentials. All changes were completely reversible by rewarming of the motor cortex. Concomitant cooling of the motor and somatosensory cortices entirely paralysed the contralateral wrist muscles. These results suggest that the motor function of the somatosensory cortex becomes predominant and compensates for dysfunction of the motor cortex when it is temporarily impaired.Supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan     

The effects of muscimol inactivation of small regions of motor and somatosensory cortex on independent finger movements and force control in the precision grip

This study investigated the effects of inactivating small regions of the primary somatosensory (SI) and motor (MI) cortex on the control of finger forces in a precision grip. A monkey was trained to grasp and lift a computer-controlled object between the thumb and index finger and to hold it stationary within a narrow position window for 2 s. The grip force applied perpendicular to the object surface, the lifting or load force applied tangentially in the vertical direction, and the vertical displacement were sampled at 100 Hz. Also, the ability of the monkey to extract small pieces of food from narrow wells of a Klüver board was analyzed from video-tape. Preliminary single-unit recordings and microstimulation studies were used to map the extent of the thumb and index-finger representation within SI and MI. Two local injections of 1 μl each (5 μg/μl) of the GABA_A-agonist muscimol were used to inactivate the thumb and index region of either the pre- or post-central gyrus. The precision grip was differently affected by muscimol injection into either SI or MI. MI injections produced a deficit in the monkey’s ability to perform independent finger movements and a general weakness in the finger muscles. Whole-hand grasping movements were inappropriately performed in an attempt to grasp either the instrumented object or morsels of food. Although the effect seemed strongest on intrinsic hand muscles, a clear deficit in digit extension was also noted. As a result, the monkey was unable to lift and maintain the object within the position window for the required 2 s, and, over time, the grip force decreased progressively until the animal stopped working. Following SI injections, the most obvious effect was a loss of finger coordination. In grasping, the placement of the fingers on the object was often abnormal and the monkey seemed unable to control the application of prehensile and lifting forces. However, the detailed analysis of forces revealed that a substantial increase in the grip force occurred well before any deficit in the coordination of finger movements was noted. This observation suggests that cutaneous feedback to SI is essential for the fine control of grip forces. Received: 05 October 1998 / Accepted: 30 March 1999     

Consequences of altered cerebellar input for the cortical regulation of motor coordination, as reflected in EEG potentials

The cerebellum is certainly involved in fine coordination of movements, but has no efferences of its own to the muscles. Thus, it can exert its influence only via other cerebral areas that have those efferences. This study investigated in patients with cerebellar atrophy how cortical motor areas are affected by dysfunction of the cerebellum. The main question was whether the patients’ slow cortical electroencephalogram (EEG) potentials during key-press preparation and execution would be generally altered or would be specifically altered when fine coordination was needed. In the coordination task, right- and left-hand keys had to be pressed simultaneously with different forces, under visual feedback. Control tasks were to press with both hands equally or with one hand only. The patients indeed had a performance deficit in the coordination task. Their cortical EEG potentials were already drastically reduced in the simple tasks, but were enhanced by the same amount as in healthy subjects when more coordination was needed. These results suggest that the cerebellum is not exclusively active in fine coordination, but is generally involved in any kind of preparatory and executive activity, whereas the motor cortex becomes more active with fine coordination. The role of the cerebellum might be to provide the motor cortex with information needed for coordinating movements. In cerebellar atrophy, this altered input may be sufficient for the motor cortex in controlling simple tasks, but not for complex ones. Received: 4 November 1998 / Accepted: 14 April 1999     

Effects of voluntary contraction on descending volleys evoked by transcranial electrical stimulation over the motor cortex hand area in conscious humans

 The spinal volleys evoked by electric anodal and cathodal stimulation over the cerebral motor cortex hand area were recorded from a bipolar electrode inserted into the cervical epidural space of two conscious human subjects. We measured the size of volleys elicited by electric stimulation at active motor threshold and at 3% of maximum stimulator output above this value with subjects at rest and during maximum voluntary contraction of the contralateral first dorsal interosseous muscle. Surface EMG activity was recorded at the same time. Electrical anodal stimulation evoked a single negative wave that we termed D-wave in analogy with data in experimental animals. Cathodal stimulation evoked a single negative wave with a latency of 0.2 ms longer than the D-wave recruited by anodal stimulation. At both intensities tested, voluntary contraction did not modify the amplitude of the descending waves. We conclude that changes in cortical excitability induced by voluntary activity do not modify the corticospinal volley evoked by electric stimulation and that the D-waves evoked by both anodal and cathodal electric stimulation are probably initiated several nodes distant to the cell body. Received: 9 September 1998 / Accepted: 21 October 1998     

Modulation of motor cortex excitability by median nerve and digit stimulation

We investigated the time course of changes in motor cortex excitability after median nerve and digit stimulation. Although previous studies showed periods of increased and decreased corticospinal excitability following nerve stimulation, changes in cortical excitability beyond 200 ms after peripheral nerve stimulation have not been reported. Magnetoencephalographic studies have shown an increase in the 20-Hz rolandic rhythm from 200 to 1000 ms after median nerve stimulation. We tested the hypothesis that this increase is associated with reduced motor cortex excitability. The right or left median nerve was stimulated and transcranial magnetic stimulation (TMS) was applied to left motor cortex at different conditioning-test (C-T) intervals. Motor-evoked potentials (MEPs) were recorded from the right abductor pollicis brevis (APB), first dorsal interosseous (FDI), and extensor carpi radialis (ECR) muscles. Right median nerve stimulation reduced test MEP amplitude at C-T intervals from 400 to 1000 ms for APB, at C-T intervals from 200 to 1000 ms for FDI, and at C-T intervals of 200 and 600 ms for ECR, but had no effect on FDI F-wave amplitude at a C-T interval of 200 ms. Left median nerve (ipsilateral to TMS) stimulation resulted in less inhibition than right median nerve stimulation, but test MEP amplitude was significantly reduced at a C-T interval of 200 ms for all three muscles. Digit stimulation also reduced test MEP amplitude at C-T intervals of 200–600 ms. The time course for decreased motor cortex excitability following median nerve stimulation corresponds well to rebound of the 20-Hz cortical rhythm and supports the hypothesis that this increased power represents cortical deactivation. Received: 11 December 1998 / Accepted: 30 April 1999     

Adaptation of the precentral cortical command to elbow muscle fatigue

The control exerted by individual motor cortical cells on their fatigued target muscles was assessed by analyzing the discharge patterns and electromyographic (EMG) postspike effects of cortical cells in monkeys making repeated forceful, but submaximal, isometric flexions of the elbow to produce fatigue. Two monkeys were trained to perform self-paced isometric contractions (for longer than 2 s) at forces greater than 35% maximal contraction, with three sets of 20 consecutive contractions; the first and last sets were at the same force level. Pairs of EMG electrodes were implanted in the biceps brachii, brachioradialis, and triceps brachii. The cortical cell discharges were modulated with the active and passive movements of the elbow and produced consistent EMG postspike effects during isometric contraction. Muscle fatigue was assessed as a statistically significant (P<0.05) drop in the mean power frequency of the EMG power spectrum in one or both flexors in the last set of contractions. Clear signs of muscular fatigue occurred in 20 different experimental sessions. Before fatigue, cortical cells were classified as phasic-tonic (18), phasicramp (three), or tonic (five). Twenty cells briskly fired to passive elbow extension, and 9 also responded to passive flexion. Only 6 cells showed a decreased discharge to passive extension. A 22–30% increase in the contraction force produced a higher discharge frequency in 13 cells, and a lower frequency in 5 cells. All cells exerted EMG postspike effects in their target muscles: 20 cells facilitated the flexors, and some of these also inhibited (3 cells) or cofacilitated (5 cells) the extensor; the other 6 cells had mixed effects: 5 of them inhibited at least one flexor, and 1 cell only facilitated the extensor. Most cells (24/26) still produced EMG postspike effects in their target muscles during fatigue, and the number of facilitated muscles increased: 21 cells facilitated the flexors, and 12 of them cofacilitated the extensor. Only 3 cells still inhibited the flexors and were tonic cells. The cortical cell firing frequency increased during fatigue in 13 cells and decreased in 8 cells. Increases involved 10 cells excited by passive elbow extension. Fourteen cells showed parallel changes in firing frequency with fatigue and force, and 9 of these cells facilitated both extensors and flexors in fatigue. Increases were found in 8 cells, decreases in 5 cells and no change in 1 cell. As muscle afferents provide substantial information to cortical cells, which in turn establish functional linkages with their target muscles before and during fatigue, the changes in cell firing frequencies during fatigue demonstrate the active participation of the motor cortex in the control of compensation for the peripheral adjustments concomitant with muscle fatigue.     

Role of primary somatic sensory cortex in the categorization of tactile stimuli: effects of lesions

 We lesioned the right primary somatic sensory (SI) cortex in two monkeys trained to categorize the speed of moving tactile stimuli. Animals performed the task by pressing with the right hand one of two target switches to indicate whether the speed of a probe moving across the glabrous skin of the left hand was low or high. Sensory performance was evaluated with psychometric techniques and motor behavior was monitored by measuring the reaction (RT) and movement (MT) times before the experiment and throughout the 60 days after the ablation of SI cortex. After the lesion, there was a slight increase in the RTs but no change in the MTs, indicating that removal of SI cortex did not affect the animals’ capacity to detect the stimuli. However, monkeys lost their ability to categorize the stimulus speeds. This effect was observed from the 1st day after the lesion until the end of the study. We conclude that somatosensory areas outside SI can by themselves process tactile information in a limited way and that the extraction of higher-order features that takes place during the categorization task requires the intervention of SI cortex. Received: 28 October 1996 / Accepted: 27 January 1997     

Prehension movements and motor development in children

 The maturation of manual dexterity and other sensorimotor functions was assessed with various behavioural tests. In healthy children (age 4–5 years) and in adults, the kinematics of reaching and grasping, a bimanual task and fast repetitive tapping movements were analysed. Furthermore a comprehensive motor function score (MOT), probing agility and balance, was evaluated. In the prehension task, the straightness of the reaching trajectories increased with age. Children opened their grip relatively wider than adults, thus grasping with a higher safety margin. The speed of both tapping and bimanual movements increased with age, and higher scores were reached in the MOT. Although the different behavioural tests sensitively indicated maturational changes, their results were generally not correlated, i.e. the outcome of a particular test could not predict the results of other tasks. Hence there is no simple and uniform relationship between different behavioural data describing maturation of sensorimotor functions. Received: 20 July 1998 / Accepted: 11 December 1998     

Differential relation of discharge in primary motor cortex and premotor cortex to movements versus actively maintained postures during a reaching task

The activity of cells in primary motor cortex (MI) and dorsal premotor cortex (PMd) were compared during reaching movements in a reaction-time (RT) task, without prior instructions, which required precise control of limb posture before and after movement. MI neurons typically showed strong, directionally tuned activity prior to and during movement as well as large gradations of tonic activity while holding the limb over different targets. The directionality of their movementand posture-related activity was generally similar. Proximal-arm muscles behaved similarly. This is consistent with a role for MI in the moment-to-moment control of motor output, including both movement and actively maintained postures, and suggests a common functional relation for MI cells to both aspects of motor behavior. In contrast, PMd cells were generally more phasic, frequently emitting only strong bursts of activity confined mainly to the behavioral reaction time before movement onset. PMd tonic activity during different postures was generally weaker than in MI, and showed a much more variable relation with their movement-related directional tuning. These results imply that the major contribution of PMd to this RT task occurred prior to the onset of movement itself, consistent with a role for PMd in the selection and planning of visually guided movements. Furthermore, the nature of the relative contribution of PMd to movement versus actively maintained postures appears to be fundamentally different from that in MI. Finally, there was a continuous gradient of changes in responses across the rostrocaudal extent of the precentral gyrus, with no abrupt transition in response properties between PMd and MI.     

The thalamic connections with medial area 6 (supplementary motor cortex) in the monkey (macaca fascicularis)

Summary 1. The interrelationship of medial area 6 (supplementary motor area) with the thalamus was investigated by means of anterograde and retrograde tracing methods. Nine monkeys were prepared for autoradiography or histochemistry with the marker HRP conjugated to the lectin wheat germ agglutinin. Three of the monkeys received injections into the precentral cortex for comparison. 2. Previous observations were confirmed that the thalamic relays to the motor areas are organized as crescent-shaped lamellae which transgress cytoarchitectonic boundaries. The thalamic VA-VL complex receiving fibres from areas 4 and medial area 6 also sends fibres to these same areas. 3. The thalamic relay to medial area 6 comprised the following subdivisions: VLo, VLc, area X of Olszewski, VLm and, to a smaller extent VA. 4. Labeling (mostly anterograde only) was also prominent in some thalamic compartments outside the motor thalamus: R, CL, CM-Pf, MD, LP, PULo. 5. It was noted that rostral and caudal injections into the medial area 6 resulted in different thalamic labeling: The rostral portion was found to be related mainly with VApc, area X and VLc, the central portion with VLo, and the caudal portion with VLc/VLo. This structural inhomogeneity may reflect also a functional rostro-caudal differentiation of the medial area 6. 6. The thalamic territory projecting to the precentral cortex is separate from the above relay and includes principally VPLo. 7. The present anatomical labeling study is in agreement with the conclusion of Schell and Strick (1984) that the SMA, especially its central portion, is an important target of basal ganglia outflow via the thalamic relay VLo. In addition consistent labeling was also found in thalamic subdivisions (area X, VLc) which had been found to receive cerebellar fibres.Abbreviations AD Nucleus anterior dorsalis - AM Nucleus anterior medialis - AV Nucleus anterior ventralis - ARG Autoradiography - CL Nucleus centralis lateralis - CM Centre median nucleus - Comm. post. Commissura posterior - CLS Nucleus centralis superior lateralis - For Fornix - GM Nucleus geniculatus medialis - In p.c. Nucleus interstitialis of the posterior commissure - LD Nucleus lateralis dorsalis - Li Nucleus limitans - LP Nucleus lateralis posterior - MDmc Nucleus medialis dorsalis, pars magnocellularis - MDmf Nucleus medialis dorsalis, pars multiformis - MDpc Nucleus medialis dorsalis, pars parvocellularis - NRmc Nucleus ruber magnocellularis - NRpc Nucleus ruber parvocellularis - Pcn Nucleus paracentralis - Pf Nucleus parafascicularis - Pul.i. Nucleus pulvinaris inferior - Pul.l. Nucleus pulvinaris lateralis - Pul.m. Nucleus pulvinaris medialis - Pul.o. Nucleus pulvinaris oralis - R Nucleus reticularis thalami - SMA Supplementary motor area - STh Nucleus subthalamicus - VAmc Nucleus ventralis anterior, pars magnocellularis - VApc Nucleus ventralis anterior, pars parvocellularis - VLc Nucleus ventralis lateralis, pars caudalis - VLm Nucleus ventralis lateralis, pars medialis - VLo Nucleus ventralis lateralis, pars oralis - VLps Nucleus ventralis lateralis, pars postrema; - VPI Nucleus ventralis posterior inferior - VPLo Nucleus ventralis posterior lateralis, pars oralis - VPM Nucleus ventralis posterior medialis - WGA-HRP Horseradish peroxidase conjugated to the lectin wheat germ agglutinin; - X Area X - ZI Zona incerta