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The Journal of Neuroscience, November 1, 1998, 18(21):9038-9054
Partial Inactivation of the Primary Motor Cortex Hand Area: Effects on Individuated Finger Movements
Marc H. Schieber1, 2, 3, 4 and Andrew V. Poliakov1 Departments of 1 Neurology and 2 Neurobiology and Anatomy,
Brain and Cognitive Science, and Physical Medicine and Rehabilitation, 3 Center for Visual Science, and 4 Brain Injury Rehabilitation Program at St. Mary's Hospital, University of Rochester School of Medicine and Dentistry, Rochester, New York 14642
ABSTRACT
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Abstract
Introduction
After large lesions of the primary motor cortex (M1), voluntary movements of affected body parts are weak and slow. In addition, the relative independence of moving one body part without others is lost; attempts at individuated movements of a given body part are accompanied by excessive, unintended motion of contiguous body parts. The effects of partial inactivation of the M1 hand area are comparatively unknown, however. If the M1 hand area contains the somatotopically ordered finger representations implied by the classic homunculus or simiusculus, then partial inactivation might produce weakness, slowness, and loss of independence of one or two adjacent digits without affecting other digits. But if control of each finger movement is distributed in the M1 hand area as many studies suggest, then partial inactivation might produce dissociation of weakness, slowness, and relative independence of movement, and which fingers movements are impaired might be unrelated to the location of the inactivation along the central sulcus.
To investigate the motoric deficits resulting from partial inactivation of the M1 hand area, we therefore made single intracortical injections of muscimol as trained monkeys performed visually cued, individuated flexion-extension movements of the fingers and wrist. We found little if any evidence that which finger movements were impaired after each injection was related to the injection location along the central sulcus. Unimpaired fingers could be flanked on both sides by impaired fingers, and the flexion movements of a given finger could be unaffected even though the extension movements were impaired, or vice versa. Partial inactivation also could produce dissociated weakness and slowness versus loss of independence in a given finger movement. These findings suggest that control of each individuated finger movement is distributed widely in the M1 hand area.
Key words: cortex; dexterity; finger; individuation; macaque; motor; muscimol; response time, somatotopy; weakness
INTRODUCTION
The classic homunculus and simiusculus implied that a separate region of the primary motor cortex (M1) moves each digit of the hand (Penfield and Rasmussen, 1950
; Woolsey et al., 1951
either cortical (Nudo and Milliken, 1996
Intracortical injection of muscimol, a long-acting GABAA agonist, recently has been shown to produce reversible inactivation of the M1 hand area, with associated deficits in motor performance (Kubota, 1996
We therefore made single intracortical injections of muscimol in the M1 hand area as monkeys performed individuated finger movements. These injections impaired performance of some finger movements, whereas other finger movements remained unaffected. Which finger movements were impaired after each muscimol injection showed little if any relation to the location of the injection along the central sulcus, however, consistent with distributed control of each finger movement.
MATERIALS AND METHODS
All procedures for the care and use of these purpose-bred monkeys complied with the United States Public Health Service Policy on Humane Care and Use of Laboratory Animals, followed the Public Health Service Guide for the Care and Use of Laboratory Animals and were approved by the appropriate Institutional Animal Care and Use Committee.
Visually cued individuated finger movement task. Three juvenile (~4- to 6-yr-old) rhesus monkeys (Macaca mulatta; K, a 6 kg male; H, a 4 kg female; and A, a 5 kg male) were trained to perform visually cued individuated finger movements. The behavioral paradigm and the finger movements made by monkey K have been described in detail previously (Schieber, 1991
The monkey viewed a display on which each digit (and the wrist) was represented by a row of five light-emitting diodes (LEDs). The middle, yellow LED in a row was illuminated when neither the flexion nor extension switch for that digit was closed. One of two green LEDs on either side of the middle, yellow LED was lit whenever the flexion (leftward green LED) or extension (rightward green LED) switch was closed. When the monkey flexed or extended a digit, closing a microswitch, the middle, yellow LED went out and the leftward or rightward green LED came on. The yellow and green LEDs thus informed the monkey which switches were open and which were closed. Red LEDs at either end of the row were illuminated as cues instructing the monkey to close either the flexion (leftward red LED) or extension (rightward red LED) switch.
The monkey initiated each trial by placing all digits and the wrist in their middle positions, so that no switches were closed and the middle, yellow LED in each row was illuminated. After a pseudorandomly varied initial hold period of 500-750 msec, one red LED was illuminated under microprocessor control, instructing the monkey which switch to close (or to move the wrist). If the monkey closed the instructed switch within the 700 msec allowed response time after illumination of the red LED and held it closed for a final hold period (500 msec for monkeys K and H, 300 msec for monkey A) without closing any other switches, then the trial had been performed correctly, and the monkey received a water reward. After each rewarded trial, the finger movement to be instructed for the next trial was rotated in a pseudorandom order. Consecutive rewarded (correctly performed) trials of a given instructed movement therefore did not occur immediately after one another but instead were separated by trials of other instructed movements. In contrast, if the monkey failed to perform correctly, either by failing to close the instructed switch within the allowed 700 msec response time or by closing another switch before or after the instructed switch, no reward was delivered, and the same instruction was presented again for the next trial. Consecutive failed trials of a given instructed movement therefore did occur immediately after one another and were not separated by trials of other instructed movements. (This protocol was required to prevent the monkey from intentionally passing up trials of difficult movements by failing them and then going on to earn rewards only on trials of easier movements.) After each trial, a minimum intertrial interval (1000 msec for monkeys K and H, 500 msec for monkey A) was required before the monkey could initiate the next trial. Because the monkey had to initiate each trial by actively placing all digits and the wrist in their middle positions, the actual intertrial interval was variable, determined in part by the monkey.
Although the behavioral task was based only on switch closures, in recording from monkeys K and H the position of each finger was transduced continuously via strain gages mounted on the microswitch lever arms, and the position of the wrist was transduced via a potentiometer coupled to the wrist axis (Schieber, 1991
Examination of the finger movements generated by these monkeys showed that in each rewarded trial, the digit the monkey had been instructed to move underwent more movement than any other digit. Moreover, each digit had its greatest excursion when it was the instructed digit. In some movements, particularly when the monkey was instructed to flex the thumb or wrist, other digits remained stationary. In other movements, however, noninstructed digits moved to a greater or lesser degree. Each movement is therefore referred to as an instructed movement of a given digit in a given direction, recognizing that there was often some movement of noninstructed digits. For brevity, each instructed movement is denoted by the number of the instructed digit (1 for the thumb through 5 for the little finger, W for the wrist) and the first letter of the instructed direction (f for flexion, e for extension). Thus "2f" denotes instructed flexion of the index finger. Monkeys K and H were trained to perform 12 or more different finger and wrist movements, although for purposes of exposition only individuated flexion and extension of each digit and of the wrist are analyzed here. Monkey A, in contrast, was trained to perform only six movements, 1f, 2f, 3f, 4f, 2e, and 3e, all with the wrist axis fixed.
Quantifying the independence of the digits. To quantify the degree to which noninstructed digits moved simultaneously with the instructed digit in single trials of each instructed movement, we applied methods previously described in detail (Schieber, 1991
To summarize the extent to which noninstructed digits moved during a given instructed movement, we then computed an Individuation Index as 1
mean of the absolute values of the relative motion coefficients of the noninstructed digits:
where IIjd is the Individuation Index for instructed movement of the jth digit in direction d (flexion or extension); Sijd is the relative motion coefficient of the ith digit during instructed movement of the jth digit in direction d; and n is the number of digits (here n = 6 because the wrist is included). The slope of the instructed digit against itself, which is always 1, was eliminated from the sum by subtracting 1 in the numerator, and the number of noninstructed digits (n
Natural food retrieval task. In addition to these highly overtrained movements, all three monkeys were examined and videotaped retrieving small food morsels (apple pieces ~0.5-1 cc) from two wells machined in a block of clear Lucite: one well large enough to permit entry of the entire hand (89 mm high, 38 mm wide, and 45 mm deep) and another well small enough to permit entry of only one finger (29 mm high, 8 mm wide, and 23 mm deep). An investigator presented these food wells to the monkey one at a time while the monkey sat on the floor (monkey K) in a cage with two openings large enough to permit the hand and forearm to reach through freely on the monkey's right and left (monkey H) or in the primate chair restrained only by the collar (monkey A). The wells were presented at eye level (monkeys K and A) or at floor level (monkey H). To induce the monkey to use the right hand on some trials and the left hand on other trials, the food wells were presented at a comfortable reaching distance to the monkey's right or left, respectively.
Identification of the M1 hand area. Before any muscimol injection sessions, the M1 hand area was identified physiologically in each monkey. Conventional microelectrode techniques were used to record single neuron activity as the monkey performed the visually cued, individuated finger movement task. As previously reported (Schieber and Hibbard, 1993
Experimental sessions. Reversible inactivation of M1 was produced by intracortical injection of the GABAA agonist muscimol (Sigma, St. Louis, MO). Muscimol was prepared for injection in physiological PBS, pH 7.4, at a concentration of 5 µg/µl (for a few injections, concentrations of 1 or 10 µg/µl were used), sterilized by passage through a micropore filter, and kept refrigerated until use. In experimental sessions, the monkey first was videotaped as he retrieved food morsels from the large and small food wells. The monkey then was placed in the primate chair, the recording chamber was opened, and a microelectrode was lowered into the brain at coordinates chosen from previous neuron recording sessions, confirming the depth of the cortical gray matter on the anterior lip and in the anterior bank of the central sulcus. After withdrawing the microelectrode, a 1 µl syringe (Unimetrics, Chicago, IL) loaded with solution for injection was mounted on the same micropositioner, such that its 30 gauge needle passed along the same line as the microelectrode. The needle then was passed through the dural puncture left by the microelectrode and advanced into the brain to the depth of cortex defined from the immediately preceding microelectrode recording. Once the needle was in place, the monkey began performing visually cued, individuated finger movements. After baseline finger movement data had been collected, the muscimol solution was injected by hand in aliquots of 0.1 µl every 30 sec over 5 min. The needle then was left in place while the monkey continued performing the finger movement task as further performance data were collected. In two M1 sessions (31 and 32 in monkey H), however, the needle was removed after the first injection and reloaded with muscimol, and a second injection was made 3 mm deeper than the first. In premotor cortex (PM) sessions, six to nine total injections were made in three different penetration tracks.
Once the monkey had stopped performing the visually cued, individuated movement task, either because of muscimol-induced inability or because of satiation, the needle was withdrawn, and the recording chamber was cleaned and closed. The monkey's hand was removed from the pistol-grip manipulandum and examined clinically for posture, tone, and strength. Then the monkey's postinjection behavior retrieving food morsels from the large and small food wells was evaluated and recorded on videotape. Thereafter, the monkey was returned to its home cage for the night.
Experimental sessions were always separated from one another by at least 1 d. The day after most muscimol injections, the monkey was again videotaped retrieving food morsels from the food wells and again worked at the visually cued, individuated finger movement task. The day after each muscimol injection, performance of both behaviors had returned to baseline levels.
Histology. After the completion of all experiments on each monkey, electrolytic lesions were made by passing DC current (40 µA for 40 sec) through a microelectrode at selected locations. Several days later, the monkey was tranquilized with ketamine (10 mg/kg, i.m.), killed by lethal injection of thiopental (300 mg/kg, i.v.), and perfused transcardially with PBS followed by phosphate-buffered 4% paraformaldehyde. Before removing the brain from the cranium and photographing the cortical surfaces, marking ink tracks were placed at selected locations around the muscimol injection sites via a needle mounted on the same microdrive. Frozen sections of the frontal lobes of both hemispheres were cut in the coronal plane at 30 µm, and every fourth section was mounted and stained for Nissl substance. The location of each muscimol injection was reconstructed based on examination of these sections. When the track of a particular injection could not be identified in the histological material, its location was interpolated based on the coordinates at which the injection was made and the locations of identified tracks, electrolytic lesions, and postmortem inked tracks.
RESULTS
We injected muscimol to reversibly inactivate the left M1 hand area in 20 different sessions in three monkeys: K, H, and A (Table 1). In most of these sessions, ~1 µl of buffered saline containing 5 µg of muscimol was injected at a single site, but in two sessions muscimol was injected at two different depths along the same track, and in two other sessions the concentration of muscimol injected was 1 or 10 µg/µl. To control for the mechanical effects of placing an injection needle in the M1 hand area and injecting 1 µl of fluid volume, in three other sessions we made sham injections of 1 µl of buffered saline in the left M1 hand area. To control for nonspecific effects that might be produced by long-range diffusion of muscimol through the cortex, CSF, or bloodstream and to control for the monkey's potential distraction caused by the investigator performing the injection, in seven other sessions we injected muscimol at distant sites: in the left M1 leg area, in the PM, or in the supplementary motor area (SMA). Many of the sessions in which injections were made outside the M1 hand area entailed injections at multiple depths and in more than one needle track, reaching much larger total doses of muscimol than used in the M1 hand area sessions.
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Table 1. Summary of injection sessions All these sessions are summarized in Table 1, which gives the concentration of muscimol, number of sites injected, volume injected per site, total injected volume, total injected muscimol, and depth of each injection below the hemispheric surface for each session. Figure 1 shows the location of the injection sites for each session in each of the three monkeys. Session numbers in Table 1 and Figure 1 indicate the sequential order in which sessions were performed; omitted numbers represent sessions in which other cortical regions were studied, the results of which will be reported elsewhere.
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Figure 1. Location of injections in each session. For each monkey, K, H, and A, the sites of muscimol injections are shown as points on an enlarged view of the left hemisphere, with the central sulcus to the viewer's right and the arcuate sulcus to the left. A rectangle in the inset of each monkey's left hemisphere shows the region enlarged. Session numbers are shown either next to the point representing the injection site or else connected to the point with a fine dashed line. The extent of the M1 hand area in each monkey, as assessed with single-neuron recording and ICMS, is indicated by a heavy dashed line. Scale bars at the bottom right of each enlargement indicate 1 mm in the rostral and medial directions. In addition to muscimol injections in the M1 hand area, comparison control injections were made in the in the PM just posterior to the arcuate sulcus, in the M1 leg area, and in the SMA. M1 leg area sites and SMA sites were medial to the region shown enlarged (arrows). In PM, multiple sites were injected in the same session (see Table 1).
The heavy dashed lines in Figure 1 indicate the surface projection of the M1 territory in each monkey where neurons were found to discharge in relation to performance of visually cued individuated finger movements and where finger movements were evoked by ICMS. M1 hand area injections were made within this territory, primarily in the lip (Table 1, depths of 2-3 mm below the hemispheric surface) and anterior bank (depths of 3-6 mm) of the central sulcus, where the digits are most heavily represented, rather than on the convexity of the precentral gyrus. Although some studies have distinguished rostral and caudal subdivisions of area 4 (Strick and Preston, 1978
Failure of individuated finger movements after muscimol injection
The success or failure of each individuated finger movement trial in two different sessions is illustrated in Figure 2. For each session, trials have been sorted according to the 12 instructed movements. Successful performance on each trial is indicated by an upward tick mark, and failure is indicated by a downward tick. Before the injection in session 17, the monkey failed occasional trials, particularly of instructed movements 4f and 5e. A sham injection of saline was made over 13 min as the monkey performed trials 334-582 (bar at bottom). After this injection the monkey's performance remained unchanged, however, and he continued to work for a total of 93 min, performing >2200 trials.
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Figure 2. Success and failure of different instructed movements in two sessions. Horizontal lines for each of the 12 instructed movements are marked with upward ticks for each correctly performed trial and with downward ticks for each incorrectly performed trial of that instructed movement. Dots beneath these lines indicate the mean trial number of 10 consecutive trials of that instructed movement whenever 8 of the 10 were failures. Note that sequential ticks or dots occurring in the same row appear merged into a thickened tick or line segment, respectively, as minor divisions of the horizontal axis represent 10 sequential trials. A sham injection was performed in session 17, and a single muscimol injection was performed in session 21, during the trials indicated by the bar just above the horizontal trial number axis for each session. The sham injection in session 17 did not alter the monkey's performance, whereas the muscimol injection in session 21 was followed by an increased failure rate for several instructed movements but not others. Session 17 spanned 93 min (injection, 13 min); session 21 lasted 102 min (injection, 5 min).
Such was not the case in session 21, during which 5 µg of muscimol in 1 µl of saline was injected at a single site. Before the muscimol injection, the monkey performed the majority of individuated finger movement trials successfully and failed only on occasional trials of instructed movements 4f and 5f. Muscimol then was injected over 5 min as the monkey performed trials 317-431 (Fig. 2, bar at bottom), during which his success changed only in that two failures of movement 5e occurred. Shortly after the injection was complete, the monkey failed a trial of 2e. Although the next two trials of 2e were performed successfully, the monkey thereafter became unable to perform 2e, failing repeatedly. This instructed movement therefore was removed from the rotation for several trials and subsequently was reintroduced on three occasions, but the monkey continued to fail every trial of 2e. Meanwhile, failures on trials of 5e, 5f, and 4f were becoming more frequent, and eventually these movements too had to be removed from the rotation to permit the monkey to continue. Subsequently, intermittent failures of 2f, 1e, 3e, and 4e occurred. Although the monkey continued to be able to perform more 2f trials successfully, movements 1e, 3e, and 4e had to be removed from the rotation. Now with only 1f, 2f, 3f, Wf, and We remaining in the rotation, consecutive correctly performed trials of each of these five movements occurred with fewer other instructed movements in between, reflected by an increased density of upward ticks for these five instructed movements after 1000 trials. Failures of 3f began to appear next. Finally, after a total of 102 min during which he performed <1600 trials, the monkey suddenly stopped performing Wf, presumably because of satiation and/or fatigue. Compared with session 17, in session 21 the monkey performed fewer trials in more time, in part because he waited longer on average between trials and in part because time was required to change the instruction sequence each time an instructed movement had to be removed from the rotation. Throughout session 21, however, few failures occurred on trials of instructed movements 1f or We.
As exemplified by sessions 17 and 21 from monkey K, the visually cued, individuated finger movement task was sufficiently difficult that monkeys failed a number of trials at baseline, and sham injection of saline did not appear to affect this performance. Each monkey's baseline performance, and therefore success and failure rate, on particular instructed movements differed. Nevertheless, several minutes after injection of muscimol in the left M1 hand area, the monkey typically began to fail trials of some instructed movements. The frequency of failures on trials of certain instructed movements gradually increased, and eventually the monkey became unable to perform some of those movements but remained able to perform others.
To examine this impairment more concisely, we used failure of 8 of 10 consecutive trials of a given instructed movement as an empirical criterion of impaired performance of that particular movement. Other criteria, such as failure of 10 consecutive trials, generally were met once the monkey, having recognized his impairment, simply allowed trials of that particular instructed movement to time out rather than attempting to perform them. Still other criteria, based on deviation from the baseline mean error rate for each instructed movement, failed to reflect the fact that some instructed movements were genuinely more difficult than others for the monkey to perform at baseline. Although somewhat arbitrary, the criterion of 8 failures in 10 attempts correlated well with our impression that the monkey, although still motivated to attempt the particular instructed movement, was on the verge of becoming motorically unable to do so successfully.
Points at which this 8 of 10 consecutive failure criterion was met for each instructed movement are indicated in Figure 2 by dots beneath the rows of tick marks. A dot is positioned at the mean trial number of every 10 consecutive trials of a given instructed movement that included 8 failures. Note that because the successfully performed trials of a given instructed movement were followed by trials of other instructed movements, the 10 consecutive trials of one instructed movement initially were not 10 serial trials. The first dot indicating the mean trial number of the 10 trials therefore usually preceded the final series of sequential failures, consistent with our choice of a criterion representing the monkey's verging on being unable to perform the movement. The 8 of 10 consecutive failure criterion was met for only 1 instructed movement in session 17 (4f) but for 8 of the 12 instructed movements in session 21 (2f, 4f, 5f, 1e, 2e, 3e, 4e, and 5e). For brevity, we will refer to instructed movements that met the 8 of 10 consecutive failure criterion as "failed" movements.
[In one situation, however, we did not consider the 8 of 10 criterion to indicate a failed movement. The monkey sometimes ended a session by suddenly not responding on multiple consecutive trials of a particular instructed movement, as happened for movement Wf in session 21 (Fig. 2). Because the monkey may have stopped working for motivational reasons, such as frustration, fatigue, or satiation, we do not consider an instructed movement that met the 8 of 10 criterion at the very end of a session to have been failed motorically.]
The failed instructed movements from each session are presented as a separate bar graph in Figure 3. The graph for each session has a bar for every failed movement. The height of each bar indicates the temporal sequence in which different instructed movements failed, from first (tallest bar) to last (shortest bar). In all three monkeys, more instructed movements failed after muscimol injections in the M1 hand area than after control injections. In monkey K, no more than one movement failed in control sessions (either saline injections in the M1 hand area or muscimol injections in other areas), whereas four to eight instructed movements failed after each muscimol injection. A notable exception was session 20, in which a low dose of muscimol (1 µg) was injected, and only one movement subsequently failed. In monkey A, trained to perform only six movements, one to four instructed movements failed in different control sessions, whereas three to six instructed movements failed after muscimol injections in the M1 hand area. Indeed, within a central core of monkey A's M1 hand area
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Figure 3. Failure on 8 of 10 consecutive trials. A bar graph for each session is used to show which instructed movements met the 8 of 10 consecutive failure criterion. Each bar graph has been positioned on the enlarged map of each monkey's left hemispheric surface close to the point at which the injection was made or else connected to it with a fine dashed line. (Three dashed lines indicate the three different locations injected during each PM session.) Each instructed movement that failed (met the 8 of 10 criterion) is shown as a bar positioned along the abscissa to indicate the instructed digit and shaded to indicate the instructed direction: filled, flexion; open, extension. The height of each bar indicates the serial order in which different instructed movements failed within each session from 1st (tallest) to 12th (shortest). The scale at top thus would illustrate an idealized result in which an injection placed laterally in the hand area impaired instructed movements starting with those of the thumb and spreading somatotopically to those of the little finger and wrist, with instructed flexion of each digit failing before instructed extension. Such a result was not obtained, however. Although in each monkey (K, A, and H), muscimol injections in the M1 hand area caused more instructed movement failures than controls, which finger movements failed showed no systematic relationship to the location of the injection along the central sulcus.
Muscimol injection in the M1 hand area thus caused the monkey to fail some, but usually not all, instructed movements. In a given monkey, different instructed movements failed after different injections. Each of the five fingers was affected by some injection in each monkey (except for monkey A, in which digit 5 was not tested), indicating that muscimol injection in the M1 hand area could affect instructed movements of each digit. Little if any relationship was evident, however, between the location of the injection along the central sulcus and which instructed movement(s) failed. This is examined further in Table 2, where each failed movement in each session is indicated by *. Here, the M1 hand area sessions in each monkey have been arranged from top to bottom in order of the injection location from lateral to medial along the central sulcus. Somatotopic representation of the different fingers therefore should be apparent as a diagonal band of effects running from top left (thumb movements affected by lateral injections) to bottom right (little finger and wrist movements affected by medial injections). In monkey K, although thumb flexion (1f) failed only after two lateral injections, thumb extension (1e) failed after all muscimol injections in the M1 hand area, including medial injections. Both little finger movements, 5f and 5e, failed after the three more medial injections, but one or the other also failed after the three more lateral injections. No trends were evident for the other digits. In monkey H, although firm conclusions could not be drawn with only three injection sites, no overt somatotopic segregation of failed movements was evident. No trend whatsoever was seen in monkey A, although digit 5 and the wrist were not tested.
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Table 2. Summary of effects The overall pattern of instructed movement failures after partial inactivation of the M1 hand area thus suggested distributed, rather than somatotopic, control of each individuated finger movement. This suggestion was supported further by instances in which particular instructed movements did not fail. Both in session 19 at the lateral end of monkey K's hand area and in session 16 at the medial end, for example, movements of digits 1, 2, and 5 failed, but movements of digits 3 and 4 did not fail in either of these sessions. If the fingers were controlled via an orderly somatotopic array in the M1 hand area, and if muscimol injection caused failure of movements of digits 1, 2, and 5, then movements of digits 3 and 4 should have failed as well. Additional examples in which neither the flexion nor the extension movement of a particular digit failed despite failure of other digits flanking that digit on both sides can be found in several other sessions as well. Such gaps in the somatotopic order of affected digits are inconsistent with control of the fingers via a somatotopic array in M1, again indicating that control of each individuated finger movement is distributed in the hand area.
Furthermore, the flexion and extension instructed movements of a particular digit did not always fail together. In session 21, for example, instructed movements 2f and 2e failed, 4f and 4e failed, and 3e failed, but 3f did not. If digits 2-4 were controlled via a somatotopic array, and if both the flexion and extension movements of digit 2 and both movements of digit 4 failed, then both movements of digit 3 should have failed as well. Instances in which either the flexion or extension movement (but not both movements) of a particular digit failed can be found in most sessions from each of the three monkeys (although less commonly in monkey A, in which for digits 1 and 4 only flexion movements were examined). These observations suggest that the contributions of M1 to the flexion versus the extension movements of a given digit were to some extent dissociable. Such dissociated impairment of flexion versus extension movement of a given digit, especially when that digit is flanked on both sides by digits with failed movements, again is inconsistent with somatotopic control of the fingers from M1. These observations provide further indications that the contribution of M1 to control of each individuated finger movement is distributed in the hand area.
Because muscimol diffuses to some extent through the cortex over time (Martin, 1991
Prolongation of response time
Permanent lesions or reversible inactivations of the entire M1 hand area (Travis, 1955
To determine whether such slowing occurred after the present muscimol injections and whether slowing affected all instructed movements or selectively affected some movements but not others, we measured the total response time for each correctly performed trial and sorted the response times according to the instructed movement. Total response time was defined as the time from the onset of the red LED instructional cue until closure of the instructed switch and thus included both the premovement reaction time and the movement time itself. We excluded from this analysis any trials immediately preceded by a failed trial, because in this circumstance the response time could be shortened by the monkey's knowledge that the same movement would be cued again after the failed trial.
Figure 4 shows a plot of response times on the remaining correctly performed trials of each instructed movement versus trial number in session 50. Before the muscimol injection in this session, monkey A closed the 4f switch 250-450 msec after the cue appeared. After muscimol was injected, however, 4f response times gradually increased. As the monkey became unable to perform 4f successfully, response times approached the 700 msec limit imposed by the behavioral task. Although this trend was not necessarily linear, we calculated the slope of the best-fit line of response time versus trial number, starting from the beginning of the muscimol injection, as an empirical measure of the rate at which response times increased. For trials of 4f, the response time increased at an average rate of 0.40 msec/trial after the muscimol injection began until the monkey failed 4e (a slope significantly different from zero at p < 0.05 after Bonferroni correction for six tests). Similarly in trials of 2e, response times increased at 0.32 msec/trial after the muscimol injection began until the monkey failed 2e. In trials of 3e, response times also increased significantly but more slowly (0.09 msec/trial), and the monkey remained able to perform 3e throughout the session. Response times did not change significantly for movements 1f, 2f, and 3f, however, although the monkey failed 2f near the end of the session.
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Figure 4. Prolongation of response times. The response times, from cue onset to switch closure, on successful trials have been sorted according to instructed movement (1f, 2f, 3f, 4f, 2e, and 3e) and plotted as a function of sequential trial number in session 50. Muscimol was injected as monkey A performed trials 317-462 (black bar beneath each plot). Subsequently, response times became progressively longer in trials of 4f, 2e, and 3e, and linear regression showed slopes significantly different from zero, whereas response times in trials of 1f, 2f, and 3f remained stable. Shortly before 1000 trials, monkey A became unable to perform 4f and 2e, and these instructed movements were removed from the rotation, making subsequent trials of 1f, 2f, 3f, and 3e relatively more frequent. The total elapsed time in session 50 was 72 min, and the injection was performed over 5 min.
Figure 5 graphically displays such changes in total response times after each injection in monkey A, using a bar graph for each session to show the magnitude of significant slopes. The graph for each session has a bar for every instructed movement that showed a response time trend with slope significantly different from zero. The height of each bar indicates the regression slope of the trend in milliseconds per trial. Each muscimol injection in monkey A's M1 hand area was followed by significant prolongation of response times for some instructed movements but not others, except for session 53, in which response times increased slightly but significantly for all six instructed movements. In contrast, neither the sham injections in M1 nor the larger and more extensive muscimol injections in PM significantly prolonged any response times in monkey A. In fact, response times of some movements shortened over the session, resulting in a significant negative slope.
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Figure 5. Response time changes in each session in Monkey A. A bar graph for each session in monkey A is used to show which instructed movements had progressive changes in response time with slope significantly different from zero. Bar graphs are positioned and formatted as in Figure 3, but here the height of each bar indicates the slope of the best-fit line of response time versus trial number in units of milliseconds per trial. The scale at top thus would illustrate an idealized result in which an injection placed medially in the hand area prolonged response times most dramatically for wrist and little finger movements and less for ring and middle finger movements, whereas response times of thumb movements shortened. Such a result was not obtained, however. Rather, different muscimol injections prolonged the response times of some instructed movements but not others in any given session. Which movements were affected showed little relation to the location of the injection along the central sulcus.
Table 2 summarizes the instructed movements for which response times were significantly prolonged in each session in all three monkeys. Instructed movements in which the response times became prolonged after muscimol injections are indicated by $. Prolonged response times were especially frequent in monkey A, occurring after every M1 hand area muscimol injection. Nevertheless, in all three monkeys, more instructed movements showed prolonged response times after muscimol injections in the M1 hand area than after control injections. In monkey K, four of the seven M1 hand area muscimol injections prolonged the response times of 1-3 of the 12 instructed movements performed. In contrast, of the five control injections in monkey K
In monkey A, Figure 5 reveals little if any relationship between the location of the injection along the central sulcus and which response times of the digit were prolonged. Even focusing on the instructed movements with the most rapid prolongations of response times (tallest bars) did not reveal somatotopic trends. In Table 2, where the sessions in each monkey have been arranged such that a somatotopically ordered representation of the different fingers should be apparent as a diagonal band of significant effects running from top left (thumb movements affected by lateral injections) to bottom right (little finger and wrist movements affected by medial injections), no such trend was evident in the response time prolongations for any of the three monkeys.
Decrease of individuation
Loss of dexterity in fine finger movements is a well known result of lesions affecting the M1 hand area or corticospinal tract. In part, this may result from slowing of movements when speed is critical or weakness when strength is critical. In addition to slowness and weakness, voluntary movement of an affected body part often is accompanied by unintended simultaneous movement of other body parts. For example, in retrieving food morsels from small wells, instead of the precision pinch they normally use, monkeys with M1 lesions (Passingham et al., 1983
To determine whether noninstructed digits moved excessively after muscimol injections, we computed previously described (Schieber, 1991
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Figure 6. Changes in individuation in a single session. A, B, Plots of normalized finger positions (0, position at extension switch closure; 100, position at flexion switch closure) from two successful trials of instructed movement 1f, one performed before (A) and the other after (B) the muscimol injection in session 15. In these two panels, the normalized position of each digit and of the wrist has been plotted as a function of the simultaneous normalized position of the instructed digit, digit 1. Whereas the noninstructed digits did not move as the monkey flexed his thumb before the muscimol injection (A), digits 2 and 4 extended as the monkey flexed his thumb after the injection (B). C, Relative motion coefficients (ordinate) derived from the slopes of such plots for each digit in each correctly performed trial of 1f in session 15. The single trials shown in A and B are indicated here by arrowheads. After the muscimol injection (bar at bottom), coefficients for noninstructed digits diverged from their near-zero baselines. D, Individuation Index calculated from these coefficients for each successful 1f trial. E, Individuation Index for trials of 4e in the same session. The Individuation Index of 1f trials decreased progressively after the muscimol injection, whereas the Individuation Index for 4e trials remained stable. The total elapsed time in session 15 was 100 min (injection, 8 min).
Figure 6C plots these relative motion coefficients for each correctly performed trial of 1f in session 15. The two trials shown in Figure 6, A and B, are indicated with arrowheads. The coefficient for digit 1, the instructed digit, by definition was unity for all 1f trials. Before the muscimol injection, the coefficients for the other, noninstructed digits all were close to 0, indicating little if any motion of other digits as the thumb flexed. After the injection, however, the coefficients of noninstructed digits deviated progressively away from 0, reflecting increasing amounts of movement of these noninstructed digits in the same (positive values) or opposite (negative values) direction as the flexing thumb.
An overall Individuation Index for each correctly performed instructed movement then was computed as 1
To determine whether the Individuation Index decreased systematically after muscimol injections in the M1 hand area, we performed a separate linear regression of Individuation Index versus trial number for each instructed movement, from the first trial performed during the injection through the last correctly performed trial of each instructed movement. Although these trends again were not necessarily linear, we used the probability of the slope being significantly different from zero (p < 0.05 after Bonferroni correction) as an indicator that the trend was significant, and we used the slope of the best-fit line as a measure of the rate at which the Individuation Index decreased. Because changes in individuation to some degree might occur as the monkey became satiated or fatigued over a long session, we performed the same analysis on the control sessions in each monkey.
Figure 7 displays as a bar graph the significant changes in Individuation Index from each muscimol injection session in monkey K. The graph for each session has a bar for every instructed movement that showed an Individuation Index trend with slope significantly different from zero. The height of each bar indicates the regression slope of the trend in Individuation Index units per 1000 trials. In monkey K, the fastest rate of Individuation Index decrease observed in control sessions was
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Figure 7. Individuation Index changes in each session in monkey K. A bar graph for each session in monkey K is used to show which instructed movements had progressive changes in Individuation Index with slope significantly different from zero. Bar graphs are positioned and formatted as in Figure 3, but here the height of each bar indicates the slope of the best-fit line of Individuation Index versus trial number, in Individuation Index units per 1000 trials. The scale at top thus would illustrate an idealized result in which an injection placed centrally in the hand area caused the greatest Individuation Index decrease for movements of the middle finger and less for index and ring finger movements, whereas Individuation Indexes of thumb and wrist movements increased. Such a result was not obtained, however. Rather, different muscimol injections impaired individuation of some instructed movements but not others in any given session. Which movements were affected showed little relation to the location of the injection along the central sulcus.
No relationship was evident, however, between the location of the muscimol injection along the central sulcus and the instructed movements that showed a decrease in individuation (Table 2, &). In monkey K, although the individuation of 1f was most profoundly affected after the three most lateral injections, 1e was even more profoundly affected after a relatively medial injection. In monkey H, although 3f was affected by the most medial injection, 1f was affected by the other two injections, separated by 4-5 mm along the central sulcus.
Combinations of effects
Table 2 summarizes the observed effects in each session: which instructed movements failed, which showed significant prolongation of response time, and which showed significant decreases in Individuation Index beyond the
Use of the fingers in retrieving food morsels
Once the monkey had become either unable to perform any instructed movement or else satiated with water rewards, its hand was removed from the manipulandum and examined clinically. After muscimol injections that produced impairment in the visually cued finger movement task, this examination typically revealed qualitatively decreased tone in the fingers, weakness of power grip, and some degree of finger and wrist drop, none of which was found after control injections. The monkey then was removed from the primate chair, and its ability to retrieve food morsels from the large and small food wells was examined.
In retrieving food morsels before muscimol injections, the monkey's hand typically preshaped, with extension of the index finger and flexion of the middle, ring, and little fingers, as the arm reached to either well. All fingers then entered the larger well, and a precision pinch was used to pick up the food morsel. Only the index entered the smaller well, and flexion of the index finger then was used to pull the food morsel to the lip of the well, where it was grasped in a precision pinch against the thumb. Similar patterns were observed in the right hand after the control injections and in the left hand after all injections.
In contrast, after muscimol injection in the left M1 hand area, the right hand failed to preshape. When retrieving food morsels from the larger well, all fingers still were able to enter in a partially flexed posture, and a raking motion was used to remove the food morsel, which might be held in the palm by flexing all the fingers. Often, however, the fingers failed to hold the food morsel, which fell to the ground after reaching the edge of the well. Occasionally the food morsel lodged between the lateral surfaces of two adjacent fingers and was held successfully. Retrieving food morsels from the small well was much more difficult after muscimol injection. Because the hand failed to preshape, the middle, ring, and little fingers remained extended and bumped into the Lucite surface next to the well, preventing the index finger from entering. The deficits produced by the present muscimol injections thus were not limited to the overtrained performance in the visually cued, individuated finger movement task. The present injections also impaired performance of a natural use of the fingers, producing deficits qualitatively similar to those described in previous studies involving more extensive permanent lesions, reversible inactivation of M1, or lesions of the corticospinal tract. Moreover, similar deficits in retrieving food morsels were observed after all muscimol injections in the M1 hand area, regardless of mediolateral location along the central sulcus.
DISCUSSION
Partial inactivation of the M1 hand area
We made single injections of muscimol that produced only partial inactivation of the M1 hand area. Although each injection impaired the monkey's ability to perform some individuated finger and wrist movements, others remained unimpaired. In different sessions, however, all the instructed movements were affected in one way or another. Furthermore, although each injection also impaired the monkey's performance in retrieving food morsels, this impairment appeared to be quantitatively less profound than that described after total structural ablation of the M1 hand area (Fulton and Kennard, 1932
Comparing the extent of the macaque M1 hand area with the extent of muscimol diffusion measured in rats suggests that the injected muscimol diffused over only part of the M1 hand area. The region of M1 containing neurons that discharge in relation to finger movements, and from which ICMS evoked finger movements, extended ~6-9 mm along the central sulcus in each of our monkeys. In comparison, 1 µg of muscimol in 1 µl injected in rat cortex has been shown to spread in 120 min (longer than our experimental sessions) over an average radius of <2 mm and to decrease glucose utilization over only a 3 mm radius (Martin, 1991
Motor abnormalities resulting from M1 lesions
Motor abnormalities resulting from lesions of M1 or of the corticospinal tract are well known to neurological clinicians. The ability to use fine finger movements in performing tasks such as buttoning buttons or tying shoelaces is first lost and last recovered after small lesions. Performance of such tasks lacks both strength and dexterity. Voluntary attempts to move a single finger often result in excessive motion of several digits, the wrist, and even more proximal parts of the arm. Larger lesions produce overt weakness of the hand and fingers.
Similar experimental lesions in subhuman primates impair the use of relatively independent finger movements in retrieving food morsels from small wells (Lawrence and Kuypers, 1968
The weakness and slowness that result from ablation or inactivation of M1 correlate well with physiological properties of M1 neurons. The discharge frequencies of M1 neurons in a variety of experimental paradigms correlate with the force exerted in a voluntary movement and the rate of change of force, as well as with other kinematic parameters such as joint position and velocity (Evarts, 1968
Weakness and/or slowness, however, cannot explain the progressive decreases observed in individuation. Decreases in the Individuation Index reflect increased active movement of noninstructed digits occurring while the monkey attempted to move the instructed digit. Increased movement of nearby fingers also contributed to the deficit observed in the natural performance of retrieving food morsels from the small food well. Normally the monkey's index finger extended alone as the hand preshaped in approaching the food wells. After muscimol injection, however, the middle, ring, and little fingers extended along with the index finger as the hand preshaped, precluding smooth entry of the index finger into the smaller well. This increased active movement of nearby body parts, which contributes significantly to the loss of dexterity that follows M1 ablation, suggests that one aspect of the normal function of M1 may be to individuate movements by actively minimizing the motion of noninstructed digits. M1 normally might minimize unwanted motion in part through inhibition mediated by horizontal intracortical connections (Huntley and Jones, 1991
The present findings also suggest that decrease in individuation is dissociable from weakness and slowness. The deficits in a given instructed movement produced by M1 inactivation often occurred together. After muscimol injection in session 15, for example, thumb flexions (1f) showed (1) decreased individuation, (2) prolonged response times, and eventually (3) failure in 8 of 10 consecutive trials. But at least as often a given instructed movement showed only one or two of these three abnormalities. Failing 8 of 10 consecutive trials without antecedent changes in response time or individuation in theory could have resulted if the latter two abnormalities developed so rapidly that too few successful trials occurred for trends to attain significance before complete failure set in. Conversely, mild prolongation of response time and mild decrease in individuation could have occurred in parallel, although never becoming severe enough to cause failure. If enough successful trials were available to demonstrate a significant prolongation of response time, however, enough successful trials should have been available to demonstrate a decrease in individuation as well. The finding that response time for a particular instructed movement could be prolonged without a simultaneous decrease in individuation or, vice versa, that individuation could decrease without concurrent prolongation of response time, suggests that these two abnormalities were to some extent dissociable. If we
