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The effects of cooling supplementary motor area and midline cerebral cortex on neuronal responses in area 4 of monkeys
Electroencephalography and clinical Neurophysiotogy, 85 (1992) 61-71
61
© 1992 Elsevier Scientific Publishers Ireland, Ltd. 0924-980X/92/$05.00
ELMOCO 91013
The effects of cooling s u p p l e m e n t a r y m o t o r area and m i d l i n e c e r e b r a l cortex on neuronal responses in area 4 of m o n k e y s E.M. Schmidt, R. Porter * and J.S. Mclntosh Laboratory of Neural Control, National Institute of Neurological Disorders and Stroke, Bethesda, MD 20892 (U.S.A.) (Accepted for publication: 3 June 1991)
Summary
It has been postulated that the supplementary motor area (SMA) is involved in the initiation of movement and in gating of afferent input to motor cortex (MI) from peripheral receptors. We studied the responses of 119 neurons in MI to imposed disturbances of wrist-movement performance generated by the introduction of torque pulses before, during and after localized cooling of the SMA in conscious monkeys. The cooling of SMA did not prevent monkeys from making these simple movements. Eighty-two neurons responded to the wrist perturbations. Only 7 of these neurons changed their responsiveness with unilateral or bilateral cooling of SMA. From the data we have obtained on MI neuronal firing patterns, the SMA does not appear to modulate the long-latency trans-cortical stretch reflex during the periods in a task that we have investigated. Nor does it prevent animals from performing these simple movements to a visual target.
Key words: Cortical neuron; Motor cortex; Macaque monkey; Supplementary motor area (SMA)
The supplementary motor area (SMA) was first described by Penfield and Welch (1949) in man and later in monkey (Penfield and Welch 1951) when it was found that electrical stimulation of this region produced complex movements. The area lies rostral to the precentral motor cortex (MI) and above the cingulate sulcus on the medial wall of the hemisphere. With surface stimulation, Woolsey et al. (1952) mapped out the topographical organization of the SMA which resulted in a simiusculus that is shown in most texts on motor control. More recently, the organization of the SMA has been explored with intracortical microstimulation in the awake primate (Macpherson et al. 1982). Along with projection to the spinal cord (Biber et al. 1978; Murray and Coulter 1981), a close reciprocal relation exists between SMA and MI (Matsumura and Kubota 1979; Muakkassa and Strick 1979). One role postulated for the SMA (Wiesendanger et al. 1975) was to gate afferent input to MI from peripheral receptors. Electrical stimulation of the SMA in an anesthetized monkey could inhibit responses in MI induced by muscle stretch. The study was later ex-
* Robert Porter was a Fogarty Scholar-in-Residence while on leave from the John Curtin School of Medical Research, Canberra, Australia. Present address is: Dean, Faculty of Medicine, Monash University, Clayton, Victoria 3168, Australia.
Correspondence to: Dr. Edward M. Schmidt, Laboratory of Nellral Control, IRP, National Institute of Neurological Disorders and Stroke, Bldg. 36, Room 5A29, Betbesda, MD 20892 (U.S.A.). Tel.: (301) 496-6729; Fax: (301) 496-4276.
tended to awake primates (Hummelsheim et al. 1986), where single unit MI responses to elbow displacements were modulated with electrical stimulation of the SMA. The stretch responses were delayed/reduced in about 50% of the neurons examined after SMA stimulation. In view of the suggestion that the SMA may be anatomically well placed to be a major sensorimotor integration center and the fact that sensory responses in area 4 may be modified following conditioning stimulation of SMA, it should be possible to test its influence on the response of ceils in MI to natural "sensory" stimuli by modifying its function reversibly using local cooling. Imposed disturbances of wrist-movement performance generated by the introduction of torque pulses during movement execution were examined before, during and after localized cooling of the SMA in conscious monkeys. Although some data already exist regarding cooling of SMA and its influence on movement-related neuronal activity recorded from the motor cortex (Tanji et al. 1985), the present experiments extend those observations. In particular, it was possible to examine quantitatively the influence of SMA cooling on each performance of the movement task. Preliminary results have been reported in abstracts (Schmidt et al. 1987; Schmidt and Mclntosh 1988). Methods
Initial training Initially, 3 monkeys (Macaca mulatta) were trained
tO enter and leave a specially built primate chair volun-
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E.M. SCHMIDTET AL.
tarily (S'chmidt et al. 1989). After several days of chairing, they were operantly conditioned to perform a self-paced alternating flexion/extension task of the right wrist for fruit juice rewards. The hand was held in a lightweight aluminum frame that was attached to a torque motor with axis of rotation concentric with the wrist joint. The torque motor was operated as a simulated spring, producing zero force when the wrist was aligned with the forearm and linearly increasing force that opposed either wrist flexion or extension. The spring constant of the torque motor loading system was
2.4 x 10 -3 nm/deg and the target zones were usually 25°+/-5° of wrist flexion and 1 5 ° + / - 5 ° of wrist extension. Targets were indicated by lighted rectangles displayed on a color video monitor. Wrist position, obtained from a potentiometer on the torque motor, was indicated by a small rectangular cursor on the video monitor. At the start of training the random on-target hold time was between 200 and 300 msec. The random hold time was slowly increased as training progressed until the hold time was between 1.0 and 1.25 sec. When the animals could perform between
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Fig. 1. A shows the outline of a coronal section through the cooling chamber and brain. The inset shows a sketch of the lateral view of the left hemisphere. The location of the 11 ram long cooling probe is shown in relation to the central sulcus (CS) and arcuate sulcus (AS). On the final experimental day, temperature measuring probes were inserted into the brain to ascertain cortical temperatures that were obtained during recording sessions. B illustrates the temperatures obtained at the walls of the cooling chamber and at the probe locations. The temperature gradient away from the cooling chamber was calculated to be 2.8 ° C/mm. This chamber and placement were used for bilateral cooling of the SMA in two monkeys.
EFFECTS OF SMA COOLING 1500 and 2500 trials/day at the longest hold times they were implanted for the cooling experiments,
Surgical procedures The monkeys were initially sedated with an intramuscular injection of ketamine HCI (7.5 mg/kg). The saphenous vein was cannulated for the initial injection of pentobarbital sodium to effect (approximately 10 mg/kg) and then supplemented at approximately 30 min intervals throughout surgery. The total dosage of pentobarbital sodium ranged from 29 to 53 mg/kg. All surgical procedures were conducted using strict sterile techniques in a dedicated surgical facility. Two 6-gauge stainless steel head restraint tubes were secured to the skull with no. 22-gauge nichrome wire that was wrapped around no. 2 self-tapping screws and crossed diagonally over each tube. A 25 mm diameter trephine hole was made over the left precentral motor cortex and a chamber for chronic recording was fastened over the opening with 3 stainless steel screws. A small, rectangular, stainless steel chamber, 11 mm long by 8 mm deep by 1 mm thick, was inserted in the midline, adjacent to the supplementary motor area (SMA) of the right hemisphere and separated from the left SMA only by the falx cerebri (Fig. 1A). This chamber had entry and exit ports of 15-gauge stainless steel tubing, These tubes were cemented to a stainless steel plate which was fixed to the skull and through which they projected. (In the second animal the fixation plate, for holding the cooling chamber, was fashioned from a plastic with a low thermal conductivity.) The size of the chamber was chosen to cover the extent of the SMA in rhesus monkeys (see Penfield and Welch 1951). It was placed in the sagittal fissure under direct vision, through an opening in the dura which was then stitched closed and covered with a sheet of Dacron velour. The chamber was positioned to overlay the territory of the SMA as defined in stimulation experiments in this species of animal (see Macpherson et al. 1982). The chamber could later be perfused, during daily recording sessions, with chilled alcohol at a constant flow rate of 170 ml/min. The inlet temperature of the chilled alcohol was maintained at a level that would produce a steady-state outflow temperature of 10 °C as measured with a thermocouple in the outflow fluid stream, The skull region containing the recording chamber, head restraint tubes and cooling chamber was covered with dental acrylic to provide stability for these attachments. Before the acrylic polymerized, a strip of Dacron velour was embedded to form a junction between the acrylic and scalp. A purse-string suture was placed in the skin around the opening to hold the scalp tight to the Dacron velour while the wound healed, A third monkey was initially sedated with an intramuscular injection of ketamine HCI (7.5 mg/kg) and placed in a specially designed head holder for magnetic
63 resonance imaging (MRI) of the brain. Anesthesia was maintained with a mixture of ketamine and Rompun (70/30%) during the MRI procedure. The locations of the central and arcuate sulci were obtained from sagittal and coronal images. Knowing the locations of these sulci, a smaller recording chamber (20 mm diameter) was centered over the h a n d / a r m area of the precentral cortex. The smaller recording chamber allowed placement of the cooling plate next to the mesial surface of the SMA of the same hemisphere from which motor cortex recordings were obtained. The cooling chamber was changed to a 14 mm long by 10 mm deep by 0.5 mm thick fin that was welded to the bottom of a 15-gauge stainless steel U-tube through which chilled alcohol circulated. The fin was covered with a 0.5 mm Silastic sheet on the surface next to the falx cerebri. This modification was undertaken to facilitate unilateral cooling of the SMA. The cooling plate was now separated from the contralateral hemisphere by the Silastic membrane and the falx (Fig. 2A). The overall thickness of the new cooling plate was 1.0 mm. After surgery, the monkeys were continuously monitored until they were sufficiently awake to be placed in a specially designed primate recovery chair. Antibiotics were administered prior to surgery and on alternate days for the first week following surgery. All surgical wounds were inspected daily and any signs of inflammation were treated with topical or systemic antibiotics as prescribed by the attending veterinarian.
Corticaltemperatures during cooling Verification of cortical temperatures during the cooling period was obtained from the first monkey prior to termination of the experiment. Under pentobarbital anesthesia, with the aid of a dental drill, the acrylic and bone on either side of the cooling chamber were removed to allow insertion of temperature measuring probes. Fig. 1A shows the location of the temperature probes in the brain, while Fig. 1B shows the cooling profiles obtained when chilled alcohol was circulated through the chamber. The inflow temperature of the chilled alcohol was maintained at approximately - 10 ° C so that the steady-state outflow temperature from the cooling chamber was 10 ° C. From the temperature probe positions and cooling profiles, the temperature gradient in the region of the cooling chamber was 2.8 ° C / m m . When the cooling chamber was maintained at 10 ° C, the mesial portions of both the ipsilateral and contralateral SMAs were cooled below 24 ° C. Verification of cortical temperatures during the cooling period was also obtained from the third monkey, using the same anesthesia and techniques described above, because of the change from a bilateral cooling chamber to a unilateral cooling fin. Fig. 2A shows the location of the temperature measuring probes in the brain and the cooling fin adjacent to the mesial surface of the
64
E.M. SCHMIDT ET AL.
SMA contralateral to the moving wrist. Temperature profiles, shown in Fig. 2B, were obtained while the temperature of the chilled alcohol was maintained at - 1 0 ° C, which resulted in a steady-state outflow ternperature of approximately - 5 ° C. The deepest part of SMA (about 2 mm below the cortical surface) adjacent to the cooling fin was cooled to at least 20 ° C while the surface o f the opposite SMA was at 23 °C and 2 mm below the surface the temperature was above 30 o C. The mea~sured temperature profiles should have resuited in unilateral disruption of SMA synaptic function. It has been demonstrated that mild cooling of the cortex by a ~fe-,vdegrees, as will have occurred on the contralaterak side of the brain, may cause some in-
crease in excitability but will not suppress the functions of neurons (Brooks 1983). Cooling the cortex below 28 °C has been shown to disrupt neuronal function (Murray et al. 1991).
Experimentalprotocol Glass coated platinum-iridium or tungsten microelectrodes, with tip impedances of approximately 1.5 MD at 1 kHz, were advanced into the convexity of the precentral gyrus through a Silastic membrane and the intact dura, using an electronically controlled mechanical drive unit. At depths of 0.5-2.5 mm, the discharges of individual cortical neurons that were modulated in discharge frequency in association with the animal's
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EFFECTS OF SMA COOLING
65
wrist movements were recorded. Most of the movement-related cellular activity was encountered between 1.0 and 1.5 mm from the cortical surface, When a movement-related neuron was encountered, handle position, EMG activity, cellular activity, and experimental events were formatted with a specially designed hardware device (Sheriff et al. 1986) for offline analysis on a PDP 11/73 computer with the KOFF program (Arrington 1986). The animal flexed or extended the wrist to move a cursor on the video monitor to the alternately displayed flexor or extensor target, and hold it in that position for a random time between 1.0 and 1.25 sec. Half way through the random duration hold period, a brief (50 msec) torque pulse, in either the flexor or extensor direction, was applied to the wrist to perturb the hold phase. The 50 msec torque pulses produced wrist displacements of approximately 2 o. A recording session with an individual neuron commenced with recording 80 repetitions of the movement task with interposed torque perturbations. Once these were accumulated, a valve in the cooling unit was opened to allow chilled fluid to perfuse the midline chamber. After approximately 5 min (this time was chosen on the basis of the outflow temperature curve of Fig. 1B), and while the perfusion continued to hold the temperature of the outflow fluid to 10 o C ( - 5 ° C for the cooling fin in the third animal), a second series of repetitions, including the same torque pulse disturbances, was recorded and analyzed. In a few cases a third series of recordings was made during the cooling phase. Then, usually after the chamber had been maintained at 10 °C ( - 5 °C outflow temperature for the fin) for up to 10 min, the valve was closed. A period of
approximately 5 min was allowed for rewarming of the cortex and then the final 80 repetitions of the task were recorded. To test for significant changes in firing patterns before, during and after cooling, the task was divided into epochs surrounding the movements from flexion to extension and extension to flexion and epochs during the hold phase at either target. The Mann-Whitney U test was employed to determine significance of any observed changes.
Results
Effect of bilateral midline cooling on movement performance When the chamber was cooled to 10 o C and held at that temperature for periods of up to 10 min, the only effect observed on the flexion/extension movements was at times a reduction in their spontaneity. When the animals were satiated with liquid rewards, or had been performing the task repeatedly for long periods, they sometimes stopped flexing and extending their wrist after midline cortex was cooled. But the animals could readily perform the task; it was executed at the same speed as before cooling, and the accuracy of positioning and holding the target were unimpaired. No tremor or drift away from the intended displacement were observed. Fig. 3 shows the 10 fastest wrist movement traces from 40 trials prior to cooling SMA, while Fig. 3B shows the 10 fastest traces of Wrist movements performed during cooling. It is very clear that the animal performed the movements to the target equally well during cooling of SMA.
B
Fig. 3. The 10 fastest of 40 movements were selected for each presentation and are superimposed. A shows the angular wrist movements before cooling as the wrist position was changed from extension to flexion (left side) and then from flexion to extension (right side). The traces are centered on a position half way through the movements. B shows the angular wrist movements during bilateral cooling of the SMA. Bilateral cooling had little effect on the dynamics of movement. The length of each trace is 1 sec. The amplitude markers to the right of the superimposed movement traces are 100 o.
66
E.M. SCHMIDT ET AL.
The first animal often reacted, especially towards the end of a long recording session, by producing a rapid series of ballistic oscillatory movements of the wrist between full flexion and full extension. The occurrence o f t h e s e oscillatory m o v e m e n t s , w h i c h h a d
the juice rewards, or self-initiated movements of the legs, were performed in a normal manner during periods of midline cooling.
Response to torque pulses
been characteristic of the animal's performance during p r e - i m p l a n t a t i o n training, was c o m p l e t e l y u n a f f e c t e d by c o o l i n g m i d l i n e structures. M o r e o v e r , m o v e m e n t s o f the eyes, t h e m o u t h d u r i n g v o c a l i z a t i o n a n d in licking
M a n y o f t h e n e u r o n s in a r e a 4, w h e t h e r or not t he i r d i s c h a r g e p a t t e r n s i n d i c a t e d a closely c o u p l e d co-variation with t h e l e a r n e d m o v e m e n t task, ex h i b i t e d shortlatency c h a n g e s in firing in r e s p o n s e to b r i e f t o r q u e
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Fig. 4. Responses of a single MI neuron to loading (A) and unloading (B) perturbations during maintained wrist extension in monkey no, 2. A shows the response of a task-related neuron recorded in MI of the contralaterai cortex when wrist perturbations stretched the extensor muscles and shortened the flexor ,muscles, while in B the perturbations were in the opposite direction. The top trace of each row is the average instantaneous firing rate of the neuron (spikes/see) for the 16 individual trials shown in raster form. Row 1 are trials performed before bilateral cooling of~the SMA; row 2 are trials performed during cooling; row 3 are trials performed after the SMA returned to normal temperature. All trials are centered on the perturbation which occurred 100 msec after the start of the trace. Row 4 shows the average wrist position during the perturbation. The length of each trace is 400 msec. The amplitude marker of row 4 is 5 o.
EFFECTS O F SMA C O O L I N G
67
pulses injected in the middle of the hold phase of the task. However, in this study we chose to examine only those neurons that were modulated with the task performance whether or not they exhibited a torque pulse response. If a neuron was only marginally related to the task and did not respond to the torque perturbation, it was not studied during the cooling phase of the experiment. This probably produced a bias in our study in favor of torque pulse responsive neurons. During periods of maintained flexor displacement and also during periods of maintained extensor displacement, either a flexion or an extension disturbance could be imposed. A total of 56 neurons were studied in two monkeys before, during and after cooling of midline
structures including the SMA of both cortices. Thirtysix of the neurons exhibited a response to the brief torque pulses that stretched the extensor or the flexor tendons at the wrist. Fig. 4 illustrates the torque pulse response of a precentral movement-related neuron before, during and after cooling of midline cortex. The firing rate of this neuron increased prior to the flexion to extension wrist movement. The tonic firing rate of the neuron was also higher during the extension hold phase of the task than during the hold in flexion. Small torque perturbations that displaced the wrist less than 2 o produced a short-latency response in the firing pattern of the neuron. The flexor perturbation initially
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Fig. 5. Torque pulse response obtained from a task-related neuron before (row 1), during (row 2) and after (row 3) unilateral cooling of the. SMA. The cooled SMA was contralaterai to the arm where wrist perturbations were applied, and ipsilaterai to the MI cortex where neuronal recordings were obtained. Row 4 shows the average wrist position during the perturbation. Calibrations are the same as Fig. 4.
68
E.M. S C H M I D T E T AL.
stretched the wrist extensor muscles and unloaded the flexor muscles while the extensor perturbation initially unloaded the wrist extensor muscles and stretched the flexor muscles of the wrist. A slight reduction in the torque pulse response is seen during cooling. However, the pulse response remained at the reduced level after the cortex was warmed, Of the 36 torque responsive neurons, only 5 changed their response with bilateral SMA cooling. One neuron had an increased response during cooling, while 4
neurons had a decreased torque pulse response during cooling.
Effects of unilateral cooling of SMA Movement performance. In order to evaluate whether the lack of influence on speed and accuracy of movement performance and no modification of torque pulse with SMA cooling were due to bilateral cooling, a third monkey was implanted with a modified cooling probe that produced enhanced unilateral cooling of the
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ili:!i!'!ii ii! I !iiii!'!:t'i:!i"iiiiiJii !i i'ii!i!i!!,!!il;!i:iii Fig. 6. T h e firing patterns of a task-related area 4 neuron and average wrist positions are shown for 32 m o v e m e n t trials before (A) and during bilateral cooling (B) of the SMA. T h e trials on the left are for m o v e m e n t s from extension to flexion, while those on the right are for movements from flexion to extension. During cooling, movement-related modulation of neuronal activity was reduced. Individual m o v e m e n t trials are shown in Fig. 3. Little change was seen in the dynamics of the movement. T h e length of each trace is 1 sec. T h e amplitude markers to the right of the average m o v e m e n t traces are 100 o.
EFFECTS OF SMA COOLING
69
S M A (see Methods). T h e first few times that the S M A was cooled, the animal stopped performing the flexi o n / e x t e n s i o n wrist m o v e m e n t task. After a few minutes, the animal resumed making the movements with the same range of velocities and positional accuracies as before cooling. After repeated cooling sessions, the animal continued to p e r f o r m the m o v e m e n t task while cooling was turned on and off. The behavior was very similar to that observed with bilateral cooling, Response to torque pulses. A total of 63 movementrelated neurons have been examined for changes in firing patterns when S M A was unilaterally cooled, Forty-six of these neurons showed a response to the torque pulse, however, only two of the neurons exhibited a decrease in their responsiveness to the torque pulses during cooling. Also, torque pulse responses were not revealed in any of the normally non-responsive neurons during SMA cooling. An example of the movement and torque pulse responses of a precentral motor cortex neuron before, during and after unilateral SMA cooling is shown in Fig. 5.
Changes in precentral neuronal activity with SMA cooling Although minimal or no changes were observed in the torque pulse response of task-related precentral cortical neurons during midline cooling, statistically significant changes ( P < 0.05) did occur in the firing patterns accompanying m o v e m e n t performance for 27 neurons (4 neurons had changes in 2 of the categories discussed below). The major change observed in the firing patterns of 12 neurons was a reduction in their modulation during the flexion/extension wrist movement task, while the modulation during movements
increased in 3 neurons. Fig. 6A illustrates the activity of a m o v e m e n t - m o d u l a t e d MI neuron before cooling while Fig. 6B shows the activity during cooling. T h e r e was a reduction in movement-related modulation of neuronal activity during cooling. However, no changes were observed in the velocity and amplitude of movements during S M A cooling. During the hold phase of the movement task, cortical neurons that were examined tended to fire at a constant rate. However, with midline cooling the firing rate of 6 neurons increased while the firing rate of 5 neurons decreased. The other change that was observed in firing patterns of MI neurons during cooling was a change in the timing of neuronal discharge with respect to movement onset. Four neurons were examined that started discharging approximately 50 msec earlier during cooling. One neuron was studied that started firing approximately 75 msec later when midline structures were cooled. A summary of studied neurons and the changes that were seen with unilateral and bilateral cooling is shown in Table I. The studied neurons are also subdivided into the relationship between firing modulation and wrist movement to extension (E), flexion (F) or both (B) directions. The population is biased towards extensor neurons which might be an artifact of our small sample.
Discussion Tanji et al. (1985) reported both the behavioral effects on a key pressing task and the changes in responses in the motor cortex accompanying cooling of a midline plate adjacent to the SMA contralateral to
TABLE I A summary of the studied neurons in precentral motor cortex when the SMA was bilaterally or unilaterally cooled. Neurons are subdivided in terms of the movement to which they are best related. Extension (E), flexion (F), and both movements (B). Neurons studied Neurons responding to torque pulse (TP)
Bilateral cooling 56 (10B, 28E, 18F)
Unilateral cooling 63 (12B, 34E, 17F)
36 (8B, 17E, llF)
46 (8B, 24E, 14F)
Changes in TP response with cooling Increased response Decreased response
1 (1E) 4 (1B, 2E, 1F)
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Timing changes Earlier Later
2 (1B, 1E) 1 (1E)
2 (2F) 0
Changes during movement Increased modulation Decreased modulation Changes in tonic activity Increased activity Decreased activity
3 (2B, 1E) 11 (1B, 9E, 1F) 5 (1B, 4E) 5 (4E, 1F)
0 1 (1E) 1 (1E) 0
70 the moving hand of a monkey. Most of the SMA throughout the whole cortical depth on the cooled side was below 20 ° C and the cortex ipsilateral to the moving hand was also cooled below 20 o C to a depth of at least 1.5 mm. Hence, significant suppression of neuronal and synaptic function occurred within SMA on both sides of the brain. They examined the neuronal responses of 21 movement-related neurons in area 4 before and during this reversible interference with SMA function. In 15 of the neurons there was a prolongation of the response time to auditory or vibrotactile sensory cues. In 12 of the neurons, the time between the first change in neuronal discharge and the onset of movement was also lengthened. The magnitude of the premovement discharge decreased by more than 20% in 9 neurons but was unchanged in the remaining 12. The duration of peri-movement neuronal activity was unchanged in 14, decreased in 5 and increased in 2 and the spontaneous background discharge of 18 of the 21 neurons was unchanged. It can be seen from this sample of observations that, in general, cooling of the SMA did not cause a great disruption of the relationship between neuronal activities in area 4 and the closely coupled muscular responses involved in a key pressing task (Tanji et al. 1985). A delay in onset, a modest reduction in amount of response and a prolongation of the duration of this response still indicate a close association between discharges in area 4 neurons, the period of the reaction time and, possibly, variability in force development, Whether it was the correctly learned or an inappropriate response, key-pressing still occurred, and the relationship of motor cortex function to this motor performance was not grossly distorted, Our results on the lack of significant effect of SMA cooling on the ability of animals to initiate and perform movements are absolutely consistent with observations made on monkeys with bilateral excision of SMA. Passingham ( 1 9 8 7 ) r e m o v e d SMA bilaterally in monkeys trained to perform simple arm raising motions to obtain food rewards, either self-initiated, "in their own time" or in response to an external cue. As long as an external cue or target was available these monkeys could perform the task normally. Our overtrained animals performed accurate movements of the wrist to achieve target positions of flexion or extension that were indicated by visual cues. Dick et al. (1987) examined the long-latency stretch reflex in a patient with an infarction in the right SMA. Torque perturbations were applied to both wrists, and the onsets of the long-latency E M G responses, which are presumed to be transcortical, were symmetrical in both arms. However, the duration of the long-latency response was considerably prolonged in the arm contralateral to the SMA lesion. In monkeys, we have observed only minor changes
E.M. SCHMIDT ET AL. in the magnitude of the cortical responses produced by brief perturbations of the wrist with unilateral or bilateral reversible lesions of the SMA. One main experimental difference between our study and that of Dick et al. (1987) was that the magnitude of the wrist perturbations were at least 10 times larger in Dick's experiment than those we employed. The larger perturbations may reveal deficits produced by SMA lesions through changes in response properties of MI neurons or other cortical or subcortical structures. Also, the nature of our reversible lesions was probably different than in Dick's patient. The long-latency E M G response to stretch (M2) has been shown to be enhanced or suppressed by the prior instructions to human subjects (Colebatch et al. 1979). In primates, when pulse stretches similar to those we employed in this study were used, there was no modification in the M2 response during cerebellar nuclear dysfunction even though the primate had to respond to the stretch (Vilis and Hore 1980). However, when the primate expected to receive a step perturbation and instead received a pulse, the long-latency response was enhanced (Hore and Vilis 1984). Cooling the lateral cerebellar nuclei decreased or abolished the components of the long-latency response ascribed to expectation or "set." Our lack of modification of the cortical component of the long-latency response with SMA cooling is very similar to that described by Hore and Vilis (1984), with cerebellar nuclei cooling when set was not employed. Use of a task that incorporates set may reveal modulatory effects of SMA on motor cortex. The major modifications in MI neuronal firing patterns that we have observed with bilateral reversible lesions of the SMA have been changes in tonic firing rate and reduction of task-modulated activity of some MI neurons. Movement parameters such as positional accuracy and velocity were unchanged in spite of the changes seen in some MI neurons, indicating that other structures must be compensating for their change or that the balance of changes in similarly associated cells, some of which showed increases and some decreases, was unchanged. From the data we have presented on MI neuronal firing patterns, the SMA does not appear to modulate the long-latency stretch reflex during the periods in a task that we have investigated. Moreover, even when SMA function is seriously depressed bilaterally, animals remain able to initiate and perform movements, guided by a target, as accurately and as rapidly as before intervention. References Arrington, K. An interactive neurophysiologicaldata analysis program, KOFF. Soc. Neurosci. Abst., 1986, 12: 94.2.
EFFECTS OF SMA COOLING Biber, M., Kneisly, L. and LaVail, J. Cortical neurons projecting to the cervical and lumbar enlargements of the spinal cord in young and adult rhesus monkeys. Exp. Neurol., 1978, 59: 492-508. Brooks, V. Study of brain function by local, reversible cooling. Rev. Physiol. Biochem. Pharmacol., 1983, 95: 1-109. Colebatch, J., Gandevia, S., McCloskey, D. and Potter, E. Subject instruction and long latency reflex responses to muscle stretch. J. Physiol. (Lond.), 1979, 292: 527-534. Dick, J., Rothwell, J., Day, B., Wise, R., Benecke, R. and Marsden, C. Modulation of the long-latency reflex to stretch by the supplementary motor area in humans. Neurosci. Lett., 1987, 75: 349-354. Hore, J. and Vilis, T. Loss of set in muscle responses to limb perturbations during cerebellar dysfunction. J. Neurophysiol., 1984, 51: 1137-1148. Hummelsheim, H., Wiesendanger, M. and Bianchetti, M. The supplementary motor area modulates perturbation-evoked discharges of neurones in the precentral motor cortex. Neurosci. Lett., 1986, 67: 119-122. Macpherson, J., Marangoz, C., Miles, T. and Wiesendanger, M. Microstimulation of the supplementary motor area (SMA)in the awake monkey. Exp. Brain Res., 1982, 45: 410-416. Matsumura, M. and Kubota, K. Cortical projection to hand-arm motor area from post-arcuate area in macaque monkeys: a histological study of retrograde transport of horseradish peroxidase, Neurosci. Lett., 1979, 11: 241-246. Muakkassa, K. and Strick, P. Frontal lobe inputs to primate motor cortex: evidence for four somatotopically organized premotor' areas. Brain Res., 1979, 177: 176-182. Murray, E. and Coulter, J. Organization of corticospinal neurons in the monkey. J. Comp. Neurol., 1981, 195: 339-365. Murray, G., Lin, L., Moustafa, E. and Sessle, B. Effects of reversible inactivation by cooling of the primate face motor cortex on the performance of a trained tongue-protrusion task and a trained biting task. J. Neurophysiol., 1991, 65: 511-530. Passingham, R. Two cortical systems for directing movement. In: G.
71 Bock, M. O'Conner and S. Marsh (Eds.), Motor Areas of the Cerebral Cortex. Ciba Foundation Symposium. Wiley, Chichester, 1987. Penfield, W. and Welch, K. The supplementary motor area in the cerebral cortex of man. Trans. Am. Neurol. Ass., 1949, 74: 179-184. Penfield, W. and Welch, K. The supplementary motor area of the cerebral cortex. Arch. Neurol. Psychiat., 1951, 66: 289-317. Schmidt, E. and Mclntosh, J. The effects of bilateral cooling of the supplementary motor area on movement-related neuronal responses in area 4 of conscious monkeys. Soc. Neurosci. Abst., 1988, 14: 329.10. Schmidt, E., Porter, R. and Mclntosh, J. The effects of cooling midline cerebral cortex, including the supplementary motor area, on movement-related neuronal responses in area 4 of conscious monkey. Soc. Neurosci. Abst., 1987, 13: 304.7. Schmidt, E., Dold, G. and Mclntosh, J. A simple transfer and chairing technique for nonhuman primates. Lab. Anim. Sci., 1989, 39: 258-260. Sheriff, W., Schmidt, E. and Christensen, J. PEAT, a computer co-proccessor for correlation of physiological data with behavioral events. Soc. Neurosci. Abst., 1986, 12: 94.3. Tanji, J., Kurata, K. and Okano, K. The effect of cooling of the supplementary motor cortex and adjacent cortical areas. Exp. Brain Res., 1985, 60: 423-426. Vilis, T. and Hore, J. Central neural mechanisms contributing to cerebellar tremor produced by limb perturbations. J. Neurophysiol., 1980, 43: 279-291. Wiesendanger, M., Ruegg, D. and Lucier, G. Why transcortical reflexes? J. Can. Sci. Neurol., 1975, 2: 295-301. Woolsey, C., Settlage, P., Meyer, D., Sencer, W., Pinto-Hamuy, T. and Travis, A. Patterns of localization in precentral and "supplementary" motor areas and their relation to the concept of a premotor area. Res. Pub. Ass. Res. Nerv. Merit. Dis., 1952, 30: 238-264.
61
© 1992 Elsevier Scientific Publishers Ireland, Ltd. 0924-980X/92/$05.00
ELMOCO 91013
The effects of cooling s u p p l e m e n t a r y m o t o r area and m i d l i n e c e r e b r a l cortex on neuronal responses in area 4 of m o n k e y s E.M. Schmidt, R. Porter * and J.S. Mclntosh Laboratory of Neural Control, National Institute of Neurological Disorders and Stroke, Bethesda, MD 20892 (U.S.A.) (Accepted for publication: 3 June 1991)
Summary
It has been postulated that the supplementary motor area (SMA) is involved in the initiation of movement and in gating of afferent input to motor cortex (MI) from peripheral receptors. We studied the responses of 119 neurons in MI to imposed disturbances of wrist-movement performance generated by the introduction of torque pulses before, during and after localized cooling of the SMA in conscious monkeys. The cooling of SMA did not prevent monkeys from making these simple movements. Eighty-two neurons responded to the wrist perturbations. Only 7 of these neurons changed their responsiveness with unilateral or bilateral cooling of SMA. From the data we have obtained on MI neuronal firing patterns, the SMA does not appear to modulate the long-latency trans-cortical stretch reflex during the periods in a task that we have investigated. Nor does it prevent animals from performing these simple movements to a visual target.
Key words: Cortical neuron; Motor cortex; Macaque monkey; Supplementary motor area (SMA)
The supplementary motor area (SMA) was first described by Penfield and Welch (1949) in man and later in monkey (Penfield and Welch 1951) when it was found that electrical stimulation of this region produced complex movements. The area lies rostral to the precentral motor cortex (MI) and above the cingulate sulcus on the medial wall of the hemisphere. With surface stimulation, Woolsey et al. (1952) mapped out the topographical organization of the SMA which resulted in a simiusculus that is shown in most texts on motor control. More recently, the organization of the SMA has been explored with intracortical microstimulation in the awake primate (Macpherson et al. 1982). Along with projection to the spinal cord (Biber et al. 1978; Murray and Coulter 1981), a close reciprocal relation exists between SMA and MI (Matsumura and Kubota 1979; Muakkassa and Strick 1979). One role postulated for the SMA (Wiesendanger et al. 1975) was to gate afferent input to MI from peripheral receptors. Electrical stimulation of the SMA in an anesthetized monkey could inhibit responses in MI induced by muscle stretch. The study was later ex-
* Robert Porter was a Fogarty Scholar-in-Residence while on leave from the John Curtin School of Medical Research, Canberra, Australia. Present address is: Dean, Faculty of Medicine, Monash University, Clayton, Victoria 3168, Australia.
Correspondence to: Dr. Edward M. Schmidt, Laboratory of Nellral Control, IRP, National Institute of Neurological Disorders and Stroke, Bldg. 36, Room 5A29, Betbesda, MD 20892 (U.S.A.). Tel.: (301) 496-6729; Fax: (301) 496-4276.
tended to awake primates (Hummelsheim et al. 1986), where single unit MI responses to elbow displacements were modulated with electrical stimulation of the SMA. The stretch responses were delayed/reduced in about 50% of the neurons examined after SMA stimulation. In view of the suggestion that the SMA may be anatomically well placed to be a major sensorimotor integration center and the fact that sensory responses in area 4 may be modified following conditioning stimulation of SMA, it should be possible to test its influence on the response of ceils in MI to natural "sensory" stimuli by modifying its function reversibly using local cooling. Imposed disturbances of wrist-movement performance generated by the introduction of torque pulses during movement execution were examined before, during and after localized cooling of the SMA in conscious monkeys. Although some data already exist regarding cooling of SMA and its influence on movement-related neuronal activity recorded from the motor cortex (Tanji et al. 1985), the present experiments extend those observations. In particular, it was possible to examine quantitatively the influence of SMA cooling on each performance of the movement task. Preliminary results have been reported in abstracts (Schmidt et al. 1987; Schmidt and Mclntosh 1988). Methods
Initial training Initially, 3 monkeys (Macaca mulatta) were trained
tO enter and leave a specially built primate chair volun-
62
E.M. SCHMIDTET AL.
tarily (S'chmidt et al. 1989). After several days of chairing, they were operantly conditioned to perform a self-paced alternating flexion/extension task of the right wrist for fruit juice rewards. The hand was held in a lightweight aluminum frame that was attached to a torque motor with axis of rotation concentric with the wrist joint. The torque motor was operated as a simulated spring, producing zero force when the wrist was aligned with the forearm and linearly increasing force that opposed either wrist flexion or extension. The spring constant of the torque motor loading system was
2.4 x 10 -3 nm/deg and the target zones were usually 25°+/-5° of wrist flexion and 1 5 ° + / - 5 ° of wrist extension. Targets were indicated by lighted rectangles displayed on a color video monitor. Wrist position, obtained from a potentiometer on the torque motor, was indicated by a small rectangular cursor on the video monitor. At the start of training the random on-target hold time was between 200 and 300 msec. The random hold time was slowly increased as training progressed until the hold time was between 1.0 and 1.25 sec. When the animals could perform between
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Fig. 1. A shows the outline of a coronal section through the cooling chamber and brain. The inset shows a sketch of the lateral view of the left hemisphere. The location of the 11 ram long cooling probe is shown in relation to the central sulcus (CS) and arcuate sulcus (AS). On the final experimental day, temperature measuring probes were inserted into the brain to ascertain cortical temperatures that were obtained during recording sessions. B illustrates the temperatures obtained at the walls of the cooling chamber and at the probe locations. The temperature gradient away from the cooling chamber was calculated to be 2.8 ° C/mm. This chamber and placement were used for bilateral cooling of the SMA in two monkeys.
EFFECTS OF SMA COOLING 1500 and 2500 trials/day at the longest hold times they were implanted for the cooling experiments,
Surgical procedures The monkeys were initially sedated with an intramuscular injection of ketamine HCI (7.5 mg/kg). The saphenous vein was cannulated for the initial injection of pentobarbital sodium to effect (approximately 10 mg/kg) and then supplemented at approximately 30 min intervals throughout surgery. The total dosage of pentobarbital sodium ranged from 29 to 53 mg/kg. All surgical procedures were conducted using strict sterile techniques in a dedicated surgical facility. Two 6-gauge stainless steel head restraint tubes were secured to the skull with no. 22-gauge nichrome wire that was wrapped around no. 2 self-tapping screws and crossed diagonally over each tube. A 25 mm diameter trephine hole was made over the left precentral motor cortex and a chamber for chronic recording was fastened over the opening with 3 stainless steel screws. A small, rectangular, stainless steel chamber, 11 mm long by 8 mm deep by 1 mm thick, was inserted in the midline, adjacent to the supplementary motor area (SMA) of the right hemisphere and separated from the left SMA only by the falx cerebri (Fig. 1A). This chamber had entry and exit ports of 15-gauge stainless steel tubing, These tubes were cemented to a stainless steel plate which was fixed to the skull and through which they projected. (In the second animal the fixation plate, for holding the cooling chamber, was fashioned from a plastic with a low thermal conductivity.) The size of the chamber was chosen to cover the extent of the SMA in rhesus monkeys (see Penfield and Welch 1951). It was placed in the sagittal fissure under direct vision, through an opening in the dura which was then stitched closed and covered with a sheet of Dacron velour. The chamber was positioned to overlay the territory of the SMA as defined in stimulation experiments in this species of animal (see Macpherson et al. 1982). The chamber could later be perfused, during daily recording sessions, with chilled alcohol at a constant flow rate of 170 ml/min. The inlet temperature of the chilled alcohol was maintained at a level that would produce a steady-state outflow temperature of 10 °C as measured with a thermocouple in the outflow fluid stream, The skull region containing the recording chamber, head restraint tubes and cooling chamber was covered with dental acrylic to provide stability for these attachments. Before the acrylic polymerized, a strip of Dacron velour was embedded to form a junction between the acrylic and scalp. A purse-string suture was placed in the skin around the opening to hold the scalp tight to the Dacron velour while the wound healed, A third monkey was initially sedated with an intramuscular injection of ketamine HCI (7.5 mg/kg) and placed in a specially designed head holder for magnetic
63 resonance imaging (MRI) of the brain. Anesthesia was maintained with a mixture of ketamine and Rompun (70/30%) during the MRI procedure. The locations of the central and arcuate sulci were obtained from sagittal and coronal images. Knowing the locations of these sulci, a smaller recording chamber (20 mm diameter) was centered over the h a n d / a r m area of the precentral cortex. The smaller recording chamber allowed placement of the cooling plate next to the mesial surface of the SMA of the same hemisphere from which motor cortex recordings were obtained. The cooling chamber was changed to a 14 mm long by 10 mm deep by 0.5 mm thick fin that was welded to the bottom of a 15-gauge stainless steel U-tube through which chilled alcohol circulated. The fin was covered with a 0.5 mm Silastic sheet on the surface next to the falx cerebri. This modification was undertaken to facilitate unilateral cooling of the SMA. The cooling plate was now separated from the contralateral hemisphere by the Silastic membrane and the falx (Fig. 2A). The overall thickness of the new cooling plate was 1.0 mm. After surgery, the monkeys were continuously monitored until they were sufficiently awake to be placed in a specially designed primate recovery chair. Antibiotics were administered prior to surgery and on alternate days for the first week following surgery. All surgical wounds were inspected daily and any signs of inflammation were treated with topical or systemic antibiotics as prescribed by the attending veterinarian.
Corticaltemperatures during cooling Verification of cortical temperatures during the cooling period was obtained from the first monkey prior to termination of the experiment. Under pentobarbital anesthesia, with the aid of a dental drill, the acrylic and bone on either side of the cooling chamber were removed to allow insertion of temperature measuring probes. Fig. 1A shows the location of the temperature probes in the brain, while Fig. 1B shows the cooling profiles obtained when chilled alcohol was circulated through the chamber. The inflow temperature of the chilled alcohol was maintained at approximately - 10 ° C so that the steady-state outflow temperature from the cooling chamber was 10 ° C. From the temperature probe positions and cooling profiles, the temperature gradient in the region of the cooling chamber was 2.8 ° C / m m . When the cooling chamber was maintained at 10 ° C, the mesial portions of both the ipsilateral and contralateral SMAs were cooled below 24 ° C. Verification of cortical temperatures during the cooling period was also obtained from the third monkey, using the same anesthesia and techniques described above, because of the change from a bilateral cooling chamber to a unilateral cooling fin. Fig. 2A shows the location of the temperature measuring probes in the brain and the cooling fin adjacent to the mesial surface of the
64
E.M. SCHMIDT ET AL.
SMA contralateral to the moving wrist. Temperature profiles, shown in Fig. 2B, were obtained while the temperature of the chilled alcohol was maintained at - 1 0 ° C, which resulted in a steady-state outflow ternperature of approximately - 5 ° C. The deepest part of SMA (about 2 mm below the cortical surface) adjacent to the cooling fin was cooled to at least 20 ° C while the surface o f the opposite SMA was at 23 °C and 2 mm below the surface the temperature was above 30 o C. The mea~sured temperature profiles should have resuited in unilateral disruption of SMA synaptic function. It has been demonstrated that mild cooling of the cortex by a ~fe-,vdegrees, as will have occurred on the contralaterak side of the brain, may cause some in-
crease in excitability but will not suppress the functions of neurons (Brooks 1983). Cooling the cortex below 28 °C has been shown to disrupt neuronal function (Murray et al. 1991).
Experimentalprotocol Glass coated platinum-iridium or tungsten microelectrodes, with tip impedances of approximately 1.5 MD at 1 kHz, were advanced into the convexity of the precentral gyrus through a Silastic membrane and the intact dura, using an electronically controlled mechanical drive unit. At depths of 0.5-2.5 mm, the discharges of individual cortical neurons that were modulated in discharge frequency in association with the animal's
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EFFECTS OF SMA COOLING
65
wrist movements were recorded. Most of the movement-related cellular activity was encountered between 1.0 and 1.5 mm from the cortical surface, When a movement-related neuron was encountered, handle position, EMG activity, cellular activity, and experimental events were formatted with a specially designed hardware device (Sheriff et al. 1986) for offline analysis on a PDP 11/73 computer with the KOFF program (Arrington 1986). The animal flexed or extended the wrist to move a cursor on the video monitor to the alternately displayed flexor or extensor target, and hold it in that position for a random time between 1.0 and 1.25 sec. Half way through the random duration hold period, a brief (50 msec) torque pulse, in either the flexor or extensor direction, was applied to the wrist to perturb the hold phase. The 50 msec torque pulses produced wrist displacements of approximately 2 o. A recording session with an individual neuron commenced with recording 80 repetitions of the movement task with interposed torque perturbations. Once these were accumulated, a valve in the cooling unit was opened to allow chilled fluid to perfuse the midline chamber. After approximately 5 min (this time was chosen on the basis of the outflow temperature curve of Fig. 1B), and while the perfusion continued to hold the temperature of the outflow fluid to 10 o C ( - 5 ° C for the cooling fin in the third animal), a second series of repetitions, including the same torque pulse disturbances, was recorded and analyzed. In a few cases a third series of recordings was made during the cooling phase. Then, usually after the chamber had been maintained at 10 °C ( - 5 °C outflow temperature for the fin) for up to 10 min, the valve was closed. A period of
approximately 5 min was allowed for rewarming of the cortex and then the final 80 repetitions of the task were recorded. To test for significant changes in firing patterns before, during and after cooling, the task was divided into epochs surrounding the movements from flexion to extension and extension to flexion and epochs during the hold phase at either target. The Mann-Whitney U test was employed to determine significance of any observed changes.
Results
Effect of bilateral midline cooling on movement performance When the chamber was cooled to 10 o C and held at that temperature for periods of up to 10 min, the only effect observed on the flexion/extension movements was at times a reduction in their spontaneity. When the animals were satiated with liquid rewards, or had been performing the task repeatedly for long periods, they sometimes stopped flexing and extending their wrist after midline cortex was cooled. But the animals could readily perform the task; it was executed at the same speed as before cooling, and the accuracy of positioning and holding the target were unimpaired. No tremor or drift away from the intended displacement were observed. Fig. 3 shows the 10 fastest wrist movement traces from 40 trials prior to cooling SMA, while Fig. 3B shows the 10 fastest traces of Wrist movements performed during cooling. It is very clear that the animal performed the movements to the target equally well during cooling of SMA.
B
Fig. 3. The 10 fastest of 40 movements were selected for each presentation and are superimposed. A shows the angular wrist movements before cooling as the wrist position was changed from extension to flexion (left side) and then from flexion to extension (right side). The traces are centered on a position half way through the movements. B shows the angular wrist movements during bilateral cooling of the SMA. Bilateral cooling had little effect on the dynamics of movement. The length of each trace is 1 sec. The amplitude markers to the right of the superimposed movement traces are 100 o.
66
E.M. SCHMIDT ET AL.
The first animal often reacted, especially towards the end of a long recording session, by producing a rapid series of ballistic oscillatory movements of the wrist between full flexion and full extension. The occurrence o f t h e s e oscillatory m o v e m e n t s , w h i c h h a d
the juice rewards, or self-initiated movements of the legs, were performed in a normal manner during periods of midline cooling.
Response to torque pulses
been characteristic of the animal's performance during p r e - i m p l a n t a t i o n training, was c o m p l e t e l y u n a f f e c t e d by c o o l i n g m i d l i n e structures. M o r e o v e r , m o v e m e n t s o f the eyes, t h e m o u t h d u r i n g v o c a l i z a t i o n a n d in licking
M a n y o f t h e n e u r o n s in a r e a 4, w h e t h e r or not t he i r d i s c h a r g e p a t t e r n s i n d i c a t e d a closely c o u p l e d co-variation with t h e l e a r n e d m o v e m e n t task, ex h i b i t e d shortlatency c h a n g e s in firing in r e s p o n s e to b r i e f t o r q u e
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Fig. 4. Responses of a single MI neuron to loading (A) and unloading (B) perturbations during maintained wrist extension in monkey no, 2. A shows the response of a task-related neuron recorded in MI of the contralaterai cortex when wrist perturbations stretched the extensor muscles and shortened the flexor ,muscles, while in B the perturbations were in the opposite direction. The top trace of each row is the average instantaneous firing rate of the neuron (spikes/see) for the 16 individual trials shown in raster form. Row 1 are trials performed before bilateral cooling of~the SMA; row 2 are trials performed during cooling; row 3 are trials performed after the SMA returned to normal temperature. All trials are centered on the perturbation which occurred 100 msec after the start of the trace. Row 4 shows the average wrist position during the perturbation. The length of each trace is 400 msec. The amplitude marker of row 4 is 5 o.
EFFECTS O F SMA C O O L I N G
67
pulses injected in the middle of the hold phase of the task. However, in this study we chose to examine only those neurons that were modulated with the task performance whether or not they exhibited a torque pulse response. If a neuron was only marginally related to the task and did not respond to the torque perturbation, it was not studied during the cooling phase of the experiment. This probably produced a bias in our study in favor of torque pulse responsive neurons. During periods of maintained flexor displacement and also during periods of maintained extensor displacement, either a flexion or an extension disturbance could be imposed. A total of 56 neurons were studied in two monkeys before, during and after cooling of midline
structures including the SMA of both cortices. Thirtysix of the neurons exhibited a response to the brief torque pulses that stretched the extensor or the flexor tendons at the wrist. Fig. 4 illustrates the torque pulse response of a precentral movement-related neuron before, during and after cooling of midline cortex. The firing rate of this neuron increased prior to the flexion to extension wrist movement. The tonic firing rate of the neuron was also higher during the extension hold phase of the task than during the hold in flexion. Small torque perturbations that displaced the wrist less than 2 o produced a short-latency response in the firing pattern of the neuron. The flexor perturbation initially
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Fig. 5. Torque pulse response obtained from a task-related neuron before (row 1), during (row 2) and after (row 3) unilateral cooling of the. SMA. The cooled SMA was contralaterai to the arm where wrist perturbations were applied, and ipsilaterai to the MI cortex where neuronal recordings were obtained. Row 4 shows the average wrist position during the perturbation. Calibrations are the same as Fig. 4.
68
E.M. S C H M I D T E T AL.
stretched the wrist extensor muscles and unloaded the flexor muscles while the extensor perturbation initially unloaded the wrist extensor muscles and stretched the flexor muscles of the wrist. A slight reduction in the torque pulse response is seen during cooling. However, the pulse response remained at the reduced level after the cortex was warmed, Of the 36 torque responsive neurons, only 5 changed their response with bilateral SMA cooling. One neuron had an increased response during cooling, while 4
neurons had a decreased torque pulse response during cooling.
Effects of unilateral cooling of SMA Movement performance. In order to evaluate whether the lack of influence on speed and accuracy of movement performance and no modification of torque pulse with SMA cooling were due to bilateral cooling, a third monkey was implanted with a modified cooling probe that produced enhanced unilateral cooling of the
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ili:!i!'!ii ii! I !iiii!'!:t'i:!i"iiiiiJii !i i'ii!i!i!!,!!il;!i:iii Fig. 6. T h e firing patterns of a task-related area 4 neuron and average wrist positions are shown for 32 m o v e m e n t trials before (A) and during bilateral cooling (B) of the SMA. T h e trials on the left are for m o v e m e n t s from extension to flexion, while those on the right are for movements from flexion to extension. During cooling, movement-related modulation of neuronal activity was reduced. Individual m o v e m e n t trials are shown in Fig. 3. Little change was seen in the dynamics of the movement. T h e length of each trace is 1 sec. T h e amplitude markers to the right of the average m o v e m e n t traces are 100 o.
EFFECTS OF SMA COOLING
69
S M A (see Methods). T h e first few times that the S M A was cooled, the animal stopped performing the flexi o n / e x t e n s i o n wrist m o v e m e n t task. After a few minutes, the animal resumed making the movements with the same range of velocities and positional accuracies as before cooling. After repeated cooling sessions, the animal continued to p e r f o r m the m o v e m e n t task while cooling was turned on and off. The behavior was very similar to that observed with bilateral cooling, Response to torque pulses. A total of 63 movementrelated neurons have been examined for changes in firing patterns when S M A was unilaterally cooled, Forty-six of these neurons showed a response to the torque pulse, however, only two of the neurons exhibited a decrease in their responsiveness to the torque pulses during cooling. Also, torque pulse responses were not revealed in any of the normally non-responsive neurons during SMA cooling. An example of the movement and torque pulse responses of a precentral motor cortex neuron before, during and after unilateral SMA cooling is shown in Fig. 5.
Changes in precentral neuronal activity with SMA cooling Although minimal or no changes were observed in the torque pulse response of task-related precentral cortical neurons during midline cooling, statistically significant changes ( P < 0.05) did occur in the firing patterns accompanying m o v e m e n t performance for 27 neurons (4 neurons had changes in 2 of the categories discussed below). The major change observed in the firing patterns of 12 neurons was a reduction in their modulation during the flexion/extension wrist movement task, while the modulation during movements
increased in 3 neurons. Fig. 6A illustrates the activity of a m o v e m e n t - m o d u l a t e d MI neuron before cooling while Fig. 6B shows the activity during cooling. T h e r e was a reduction in movement-related modulation of neuronal activity during cooling. However, no changes were observed in the velocity and amplitude of movements during S M A cooling. During the hold phase of the movement task, cortical neurons that were examined tended to fire at a constant rate. However, with midline cooling the firing rate of 6 neurons increased while the firing rate of 5 neurons decreased. The other change that was observed in firing patterns of MI neurons during cooling was a change in the timing of neuronal discharge with respect to movement onset. Four neurons were examined that started discharging approximately 50 msec earlier during cooling. One neuron was studied that started firing approximately 75 msec later when midline structures were cooled. A summary of studied neurons and the changes that were seen with unilateral and bilateral cooling is shown in Table I. The studied neurons are also subdivided into the relationship between firing modulation and wrist movement to extension (E), flexion (F) or both (B) directions. The population is biased towards extensor neurons which might be an artifact of our small sample.
Discussion Tanji et al. (1985) reported both the behavioral effects on a key pressing task and the changes in responses in the motor cortex accompanying cooling of a midline plate adjacent to the SMA contralateral to
TABLE I A summary of the studied neurons in precentral motor cortex when the SMA was bilaterally or unilaterally cooled. Neurons are subdivided in terms of the movement to which they are best related. Extension (E), flexion (F), and both movements (B). Neurons studied Neurons responding to torque pulse (TP)
Bilateral cooling 56 (10B, 28E, 18F)
Unilateral cooling 63 (12B, 34E, 17F)
36 (8B, 17E, llF)
46 (8B, 24E, 14F)
Changes in TP response with cooling Increased response Decreased response
1 (1E) 4 (1B, 2E, 1F)
0 2 (1E, 1F)
Timing changes Earlier Later
2 (1B, 1E) 1 (1E)
2 (2F) 0
Changes during movement Increased modulation Decreased modulation Changes in tonic activity Increased activity Decreased activity
3 (2B, 1E) 11 (1B, 9E, 1F) 5 (1B, 4E) 5 (4E, 1F)
0 1 (1E) 1 (1E) 0
70 the moving hand of a monkey. Most of the SMA throughout the whole cortical depth on the cooled side was below 20 ° C and the cortex ipsilateral to the moving hand was also cooled below 20 o C to a depth of at least 1.5 mm. Hence, significant suppression of neuronal and synaptic function occurred within SMA on both sides of the brain. They examined the neuronal responses of 21 movement-related neurons in area 4 before and during this reversible interference with SMA function. In 15 of the neurons there was a prolongation of the response time to auditory or vibrotactile sensory cues. In 12 of the neurons, the time between the first change in neuronal discharge and the onset of movement was also lengthened. The magnitude of the premovement discharge decreased by more than 20% in 9 neurons but was unchanged in the remaining 12. The duration of peri-movement neuronal activity was unchanged in 14, decreased in 5 and increased in 2 and the spontaneous background discharge of 18 of the 21 neurons was unchanged. It can be seen from this sample of observations that, in general, cooling of the SMA did not cause a great disruption of the relationship between neuronal activities in area 4 and the closely coupled muscular responses involved in a key pressing task (Tanji et al. 1985). A delay in onset, a modest reduction in amount of response and a prolongation of the duration of this response still indicate a close association between discharges in area 4 neurons, the period of the reaction time and, possibly, variability in force development, Whether it was the correctly learned or an inappropriate response, key-pressing still occurred, and the relationship of motor cortex function to this motor performance was not grossly distorted, Our results on the lack of significant effect of SMA cooling on the ability of animals to initiate and perform movements are absolutely consistent with observations made on monkeys with bilateral excision of SMA. Passingham ( 1 9 8 7 ) r e m o v e d SMA bilaterally in monkeys trained to perform simple arm raising motions to obtain food rewards, either self-initiated, "in their own time" or in response to an external cue. As long as an external cue or target was available these monkeys could perform the task normally. Our overtrained animals performed accurate movements of the wrist to achieve target positions of flexion or extension that were indicated by visual cues. Dick et al. (1987) examined the long-latency stretch reflex in a patient with an infarction in the right SMA. Torque perturbations were applied to both wrists, and the onsets of the long-latency E M G responses, which are presumed to be transcortical, were symmetrical in both arms. However, the duration of the long-latency response was considerably prolonged in the arm contralateral to the SMA lesion. In monkeys, we have observed only minor changes
E.M. SCHMIDT ET AL. in the magnitude of the cortical responses produced by brief perturbations of the wrist with unilateral or bilateral reversible lesions of the SMA. One main experimental difference between our study and that of Dick et al. (1987) was that the magnitude of the wrist perturbations were at least 10 times larger in Dick's experiment than those we employed. The larger perturbations may reveal deficits produced by SMA lesions through changes in response properties of MI neurons or other cortical or subcortical structures. Also, the nature of our reversible lesions was probably different than in Dick's patient. The long-latency E M G response to stretch (M2) has been shown to be enhanced or suppressed by the prior instructions to human subjects (Colebatch et al. 1979). In primates, when pulse stretches similar to those we employed in this study were used, there was no modification in the M2 response during cerebellar nuclear dysfunction even though the primate had to respond to the stretch (Vilis and Hore 1980). However, when the primate expected to receive a step perturbation and instead received a pulse, the long-latency response was enhanced (Hore and Vilis 1984). Cooling the lateral cerebellar nuclei decreased or abolished the components of the long-latency response ascribed to expectation or "set." Our lack of modification of the cortical component of the long-latency response with SMA cooling is very similar to that described by Hore and Vilis (1984), with cerebellar nuclei cooling when set was not employed. Use of a task that incorporates set may reveal modulatory effects of SMA on motor cortex. The major modifications in MI neuronal firing patterns that we have observed with bilateral reversible lesions of the SMA have been changes in tonic firing rate and reduction of task-modulated activity of some MI neurons. Movement parameters such as positional accuracy and velocity were unchanged in spite of the changes seen in some MI neurons, indicating that other structures must be compensating for their change or that the balance of changes in similarly associated cells, some of which showed increases and some decreases, was unchanged. From the data we have presented on MI neuronal firing patterns, the SMA does not appear to modulate the long-latency stretch reflex during the periods in a task that we have investigated. Moreover, even when SMA function is seriously depressed bilaterally, animals remain able to initiate and perform movements, guided by a target, as accurately and as rapidly as before intervention. References Arrington, K. An interactive neurophysiologicaldata analysis program, KOFF. Soc. Neurosci. Abst., 1986, 12: 94.2.
EFFECTS OF SMA COOLING Biber, M., Kneisly, L. and LaVail, J. Cortical neurons projecting to the cervical and lumbar enlargements of the spinal cord in young and adult rhesus monkeys. Exp. Neurol., 1978, 59: 492-508. Brooks, V. Study of brain function by local, reversible cooling. Rev. Physiol. Biochem. Pharmacol., 1983, 95: 1-109. Colebatch, J., Gandevia, S., McCloskey, D. and Potter, E. Subject instruction and long latency reflex responses to muscle stretch. J. Physiol. (Lond.), 1979, 292: 527-534. Dick, J., Rothwell, J., Day, B., Wise, R., Benecke, R. and Marsden, C. Modulation of the long-latency reflex to stretch by the supplementary motor area in humans. Neurosci. Lett., 1987, 75: 349-354. Hore, J. and Vilis, T. Loss of set in muscle responses to limb perturbations during cerebellar dysfunction. J. Neurophysiol., 1984, 51: 1137-1148. Hummelsheim, H., Wiesendanger, M. and Bianchetti, M. The supplementary motor area modulates perturbation-evoked discharges of neurones in the precentral motor cortex. Neurosci. Lett., 1986, 67: 119-122. Macpherson, J., Marangoz, C., Miles, T. and Wiesendanger, M. Microstimulation of the supplementary motor area (SMA)in the awake monkey. Exp. Brain Res., 1982, 45: 410-416. Matsumura, M. and Kubota, K. Cortical projection to hand-arm motor area from post-arcuate area in macaque monkeys: a histological study of retrograde transport of horseradish peroxidase, Neurosci. Lett., 1979, 11: 241-246. Muakkassa, K. and Strick, P. Frontal lobe inputs to primate motor cortex: evidence for four somatotopically organized premotor' areas. Brain Res., 1979, 177: 176-182. Murray, E. and Coulter, J. Organization of corticospinal neurons in the monkey. J. Comp. Neurol., 1981, 195: 339-365. Murray, G., Lin, L., Moustafa, E. and Sessle, B. Effects of reversible inactivation by cooling of the primate face motor cortex on the performance of a trained tongue-protrusion task and a trained biting task. J. Neurophysiol., 1991, 65: 511-530. Passingham, R. Two cortical systems for directing movement. In: G.
71 Bock, M. O'Conner and S. Marsh (Eds.), Motor Areas of the Cerebral Cortex. Ciba Foundation Symposium. Wiley, Chichester, 1987. Penfield, W. and Welch, K. The supplementary motor area in the cerebral cortex of man. Trans. Am. Neurol. Ass., 1949, 74: 179-184. Penfield, W. and Welch, K. The supplementary motor area of the cerebral cortex. Arch. Neurol. Psychiat., 1951, 66: 289-317. Schmidt, E. and Mclntosh, J. The effects of bilateral cooling of the supplementary motor area on movement-related neuronal responses in area 4 of conscious monkeys. Soc. Neurosci. Abst., 1988, 14: 329.10. Schmidt, E., Porter, R. and Mclntosh, J. The effects of cooling midline cerebral cortex, including the supplementary motor area, on movement-related neuronal responses in area 4 of conscious monkey. Soc. Neurosci. Abst., 1987, 13: 304.7. Schmidt, E., Dold, G. and Mclntosh, J. A simple transfer and chairing technique for nonhuman primates. Lab. Anim. Sci., 1989, 39: 258-260. Sheriff, W., Schmidt, E. and Christensen, J. PEAT, a computer co-proccessor for correlation of physiological data with behavioral events. Soc. Neurosci. Abst., 1986, 12: 94.3. Tanji, J., Kurata, K. and Okano, K. The effect of cooling of the supplementary motor cortex and adjacent cortical areas. Exp. Brain Res., 1985, 60: 423-426. Vilis, T. and Hore, J. Central neural mechanisms contributing to cerebellar tremor produced by limb perturbations. J. Neurophysiol., 1980, 43: 279-291. Wiesendanger, M., Ruegg, D. and Lucier, G. Why transcortical reflexes? J. Can. Sci. Neurol., 1975, 2: 295-301. Woolsey, C., Settlage, P., Meyer, D., Sencer, W., Pinto-Hamuy, T. and Travis, A. Patterns of localization in precentral and "supplementary" motor areas and their relation to the concept of a premotor area. Res. Pub. Ass. Res. Nerv. Merit. Dis., 1952, 30: 238-264.