Jun Tanji

Gating of motor cortex reflexes by prior instruction

A previous study showed that sensory input can generate reflex motor cortex output in association with learned movement when the sensory input has a strong connection to the motor cortex, e.g., when a stimulus calling for repositioning of the hand consists of a perturbation of hand position. The present study has shown that motor cortex reflexes can be ‘gated’ on or off by the voluntary ‘set’ of the monkey. A monkey was trained to grasp a handle and maintain it in a certain position for 2–4 sec; he was then given an ‘instruction’ as to how he should respond to a forthcoming perturbation of the handle. The ‘instruction’ was a red or green light which appeared between 0.6 and 1.2 sec prior to the handle perturbation. The red light signaled that the monkey should pull toward himself when the perturbation occurred, and the green light meant that he should push away when the perturbation occurred. Two different sorts of perturbation were used, one being a movement of the handle toward the monkey and the other away from the monkey. A given instruction called for a movement synergistic with segmental stretch reflexes for one of the perturbations and antagonistic to these reflexes for the other. Following training, activity of motor cortex neurons was recorded during task performance. Neurons in precentral motor cortex showed changes of activity according to the ‘instruction’ as early as 200 msec following the onset of red or green light. In addition, the short (20 msec) latency motor cortex response evoked by the subsequent perturbing stimuli differed markedly depending upon the prior instruction. The finding that the set and expectancy of the monkey could profoundly modify this ‘reflex’ response indicates that the transcortical serov-loop proposed by Phillips is subject to powerful modulation as a function of learning and volition.

Neuronal activity in the primate supplementary, pre-supplementary and premotor cortex during externally and internally instructed sequential movements.

This study recorded the activity of neurons in the (i) supplementary motor area (SMA), (ii) pre-SMA (the motor area immediately rostral to the SMA), (iii) premotor cortex (PMC) and (iv) primary motor cortex (MI), while the monkey performed a conditional sequential motor task that ensures sequencing of multiple movements to the same manipulandum. This paradigm was chosen in order to prevent the participation of spatial cues in prompting the correct motor sequence. Three different movements (turn-push-pull) were performed under two task conditions: (i) internally determined (I): the monkey had to generate a pre-determined sequence from memory and without visual guidance; (ii) externally triggered (E): the correct sequence of movements was performed by following lights illuminated one after the other. Neuronal activity during the following periods were analyzed: instruction (300 ms following the onset of the auditory instruction signal); delay (interval between the end of the instruction period or the termination of the previous movement and the movement trigger); premovement (interval between the trigger signal and the movement onset); movement (interval between the mechanically-sensed movement onset and the completion of the movement) and reward (500 ms period centered at the time of reward delivery). Pre-SMA neurons were generally more active during the delay and premovement as compared to the movement, instruction and reward periods. Activity in the pre-SMA was more related to E during the pre-movement period, but exhibited a preferential relationship to I in the movement period. SMA neurons were more active when the sequential motor task was internally generated.(ABSTRACT TRUNCATED AT 250 WORDS)

New concepts of the supplementary motor area.

The supplementary motor area, although traditionally defined as a single motor area, is now viewed as including at least three different areas that can be distinguished anatomically and physiologically. The differential use of these three areas for various motor behaviors has been the subject of recent studies that are beginning to provide novel concepts of the functional differentiation of each area.

Selective coding of motor sequence in the supplementary motor area of the monkey cerebral cortex.

SummaryWe describe a property of neurons in the supplementary motor area (SMA) of the cerebral cortex of monkey that is different from those in the primary motor area (MI) in relation to execution of a sequential motor task. A group of SMA neurons was active when the animal remembered and pressed three touch-pads in a predetermined sequence but inactive when the same movement was guided by sequentially presented visual signals. This finding indicates that the SMA is involved in the performance of sequential movements on the basis of the information stored inside the brain.

Neuronal activities in the primate motor fields of the agranular frontal cortex preceding visually triggered and self-paced movement

SummarySingle cell activity was examined in the three motor fields of the monkey frontal cortex with the aim of comparing the neuronal activity preceding movements triggered by a visual signal to that preceding nontriggered (self-paced) movements. The following findings emerged from this study. 1. Neuronal activity changes were observed at two different phases in relation to the movement onset; the shortlead type observed within 480 ms prior to the movement onset and the long-lead type, beginning earlier (typically 1 to 2 s). 2. Neurons in both the supplementary motor area (SMA) and premotor area (PM) exhibited the short-lead activity changes prior to the triggered and self-paced movement. Their magnitudes were similar in 63% of SMA and in 36% of PM neurons, whether the movement was triggered or self-paced. 3. SMA neurons, as a whole, were not less active before the triggered than self-paced movement. 4. On the other hand, as many as 92 PM neurons (61%) were related exclusively or peferentially to the triggered movement. 5. The majority of precentral motor cortex (MC) neurons exhibited similar activity changes before the two modes of movement initiation. 6. The long lead type of activity changes were observed mainly prior to the self-paced and much less frequently before the triggered movement. They were particularly abundant among SMA neurons. These results do not support the simple dichotomy hypothesis that SMA primarily takes part in self-paced movement and PM is only involved in visually triggered movement. However, PM neurons show relatively more prominent responses to the visual trigger signal and SMA neurons are intimately related to a long-lasting process leading to initiation of the self-paced movement.

Reorganization of activity in the supplementary motor area associated with motor learning and functional recovery

SummaryThe supplementary motor area (SMA) of primates has been implicated in the initiation and execution of limb movements. However, when a motor task was extensively overlearned, few SMA neurons, if any, were active before the movement onset. Subsequent lesions of the primary motor cortex gave rise to the appearance of premovement activity changes, indicating usedependent reorganization of the neuronal activity in SMA.

Comparison of cerebellothalamic and pallidothalamic projections in the monkey (Macaca fuscata): A double anterograde labeling study

To address the question of segregated projections from the internal segment of the globus pallidus (GPi) and the cerebellar nuclei (Cb) to the thalamus in the monkey, we employed a double anterograde labeling strategy combining the anterograde transport of horseradish peroxidase conjugated to wheat germ agglutinin (WGA‐HRP) with biotinylated dextran amine (BDA) transport. The tissue was processed sequentially for WGA‐HRP, and then BDA immunohistochemistry using two different chromogens. Since the two labels were easily distinguishable on the same histological section, the interrelationship between the cerebellar and pallidal projection systems could be directly evaluated. We found that both the cerebellothalamic and pallidothalamic label consisted of dense plexuses of labeled fibers and swellings in a patch‐like configuration. The patches or foci of labeling were distributed either as dense single label or as interdigitating patches of double label. We found dense single label in the central portion of the ventral anterior nucleus pars principalis (VApc) and the ventral lateral nucleus pars oralis (VLo) following the GPi injections or in the central portion of the ventral posterior lateral nucleus pars oralis (VPLo) and nucleus X (X) following the cerebellar nuclei injections. Complementary interdigitating patches of WGA‐HRP and BDA labeling were found primarily in transitional border regions between thalamic nuclei. On occasion, we found overlap of both labels. We observed a gradient pattern in the density of the pallidothalamic and cerebellothalamic projections. The pallidothalamic territory included VApc, VLo, and the ventral lateral nucleus pars caudalis (VLc), with the density of these projections decreasing along an anterior to posterior gradient in the thalamus. Occasional patches of pallidal label were found in VPLo and nucleus X. Conversely, the density of cerebellothalamic projections increased along the same gradient, with the cerebellothalamic territory extending anteriorly beyond the cell‐sparse zones of VPLo, X, and VLc to include VLo and VApc also. These data suggest that although the cerebellar and pallidal projections primarily occupy separate thalamic territories, individual thalamic nuclei receive differentially weighted inputs from these sources.

Firing rate of individual motor units in voluntary contraction of abductor digiti minimi muscle in man

Abstract The frequency and variability of discharges of motor units in abductor digiti minimi muscle of 11 human subjects were investigated. In the first series, contraction was performed so as to raise the tension as linearly as possible from 0 to maximum in 2, 3.2, 5, 8, and 10 sec. There was a rise in discharge frequency when the tension was raised; this rise was steeper when the contraction was faster. The frequency at the start of discharge was higher, and the peak value of frequency was also higher at contractions with greater speeds. Irregular fluctuations in the discharge frequency was greater and appeared at earlier stages during the more rapid contractions. In the second series of experiments, the tension was held constant at various levels covering the entire range of contraction. At contractions with larger steady tension the discharge frequency of units was found to be higher and irregularity of discharge was enhanced. The units recruited at smaller tension vary over a greater range of discharge frequencies than those recruited at larger tension.

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

SummaryWe 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.

Recruitment of motor units in voluntary contraction of a finger muscle in man

Abstract The recruitment of motor units during voluntary isometric contraction was studied in the abductor digiti minimi muscle of 13 human subjects trained to increase tension in this muscle almost linearly from zero to maximum. The subjects matched the output of a transducer, measuring the tension in this muscle, to a linear target ramp voltage. The target ramps varied in duration from 2 to 10 sec. When speed of contraction was kept constant there was considerable degree of constancy in the tension at which individual motor units were recruited. The recruitment order of motor units, therefore, appeared to be determined at repeated contractions. However, the order was not rigidly fixed among units whose tension range of recruitment is overlapped, especially among those units recruited at medium to strong contraction. When the contraction was performed more quickly the motor units were recruited at lower tension. Larger population of units were recruited at lower tension range: 45.8% of units were recruited before the tension attained 20% of maximum tension. There is a tendency for motor units with larger action potentials to be recruited at higher tension.