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INTRODUCTION |
Among the multiple cortical motor areas identified thus far (Dum and Strick 1991
; He et al. 1993, 1995; Luppino et al. 1991, 1993; Matsuzaka et al. 1992), primary motor cortex (MC) has long been regarded as the motor field with the most direct and robust linkages to the segmental motor apparatus (Biber et al. 1978; He et al. 1993, 1995; Murray and Coulter 1981). Although MC is not the only cortical motor area to send direct projections to spinal levels, it is the only one that shares reciprocal connections with each of the premotor areas that project directly to spinal levels, and with none of those that do not (He et al. 1993, 1995; Luppino et al. 1993). Functional comparisons of MC with other motor fields have consistently shown it to contain higher proportions of neurons whose activity covaries with the purely motor aspects of various behavioral tasks (Alexander and Crutcher 1990b; Johnson et al. 1996; Mushiake et al. 1991; Weinrich and Wise 1982). Thus, although the distributed network of cortical motor fields has a decidedly parallel organization, the weight of evidence suggests that MC may be appropriately regarded as playing a preferential role in purely motor processing, as opposed to information processing related to sensory and/or associative (i.e., context-dependent) functions. On the other hand, some have argued that MC may have a role in various cognitive processes (Georgopoulos 1995; Georgopoulos et al. 1989, 1993), and a role in the processing of somesthetic signals has also been suggested (Mountcastle et al. 1992). Several studies of neuronal activity in awake, behaving monkeys have indicated that MC may participate in either sensory or associative processing of visuospatial information (Alexander and Crutcher 1990b; Georgopoulos et al. 1989; Hocherman and Wise 1991; Lurito et al. 1991).
Visually guided reaching to instructed target locations is a behavioral paradigm that has been used extensively to evaluate the response properties of MC neurons (Caminiti et al. 1990, 1991; Fu et al. 1993; Georgopoulos et al. 1982, 1985; Johnson et al. 1996; Kalaska et al. 1989; Scott and Kalaska 1995, 1997). In the simplest version, that of direct reaching to the site of a visual instruction stimulus (IS), the physical properties of the IS (specifically, its spatial location), the instructed location of the target (i.e., the goal of the movement), and the trajectory of the subject's reach all covary with one another in a stereotypic manner (Caminiti et al. 1991; Georgopoulos et al. 1982, 1985; Schwartz et al. 1988). However, it is also possible for these three factors to be dissociated from one another with the use of appropriate variations on the targeted reaching paradigm.
Viewed as a sensory-to-motor transformation, the simplest model might be that of a chain (or cascade) of information processing leading from purely sensory processing to associative processing to purely motor processing. Neural correlates of these different levels of information processing may be defined operationally in the following way: correlates of sensory processing should covary unconditionally with some physical property of the sensory input, correlates of motor processing should covary unconditionally with some physical property of the motor output, and correlates of associative or context-dependent processing should covary with some aspect of the behavioral context, being neither purely motor nor purely sensory.
There have been numerous studies of the response properties of MC neurons that have dissociated some of the purely motor variables involved in voluntary limb movements, such as hand trajectory, force, direction, amplitude, velocity, and joint configuration (Alexander and Crutcher 1990a; Bauswein et al. 1991; Crutcher and Alexander 1990; Fu et al. 1993; Georgopoulos et al. 1992; Kalaska et al. 1989; Riehle and Requin 1989, 1995; Riehle et al. 1994b; Scott and Kalaska 1995, 1997; Thach 1978; Werner et al. 1991). These studies have provided evidence that MC neurons may participate at several discrete sublevels within the domain of purely motor processing, including those of limb kinematics (Alexander and Crutcher 1990a; Crutcher and Alexander 1990; Fu et al. 1993, 1995; Kalaska and Hyde 1985; Kalaska et al. 1989; Riehle and Requin 1989, 1995) and limb dynamics (Alexander and Crutcher 1990a; Bauswein et al. 1991; Crutcher and Alexander 1990; Georgopoulos et al. 1992; Kalaska and Hyde 1985; Kalaska et al. 1989; Riehle and Requin 1995; Riehle et al. 1994b; Scott and Kalaska 1995, 1997; Thach 1978; Werner et al. 1991).
Despite a wealth of data concerning the participation of MC neurons in purely motor processing, there have been relatively few attempts to study the functional properties of MC neurons with the use of paradigms that dissociated purely motor variables from context-dependent or purely sensory variables. In a study that dissociated limb trajectory from the spatial endpoint of a targeted reaching movement, Hocherman and Wise (1991) found that some MC neurons did discharge exclusively in relation to the spatial endpoint rather than the trajectory of the reach. Lurito et al. (1991) dissociated the location of a visual IS from the trajectory of an instructed reach. They reported that most of the directional, movement-related activity in MC could not be classified as depending exclusively on either the direction of limb movement or the location of the visual IS. Riehl et al. (1994a) dissociated the location of a visual IS from the direction of a targeted wrist movement, and found that some MC neurons did show directional movement-related activity that covaried exclusively with IS location, independent of the direction of wrist movement.
A previous study in our laboratory dissociated the trajectory of a one-dimensional elbow movement from the location of a visual IS, while the latter also served as the target (i.e., goal) of the movement. In that study, the set- and movement-related discharge of some MC neurons was found to covary with IS (i.e., target) location, independent of the limb's trajectory (Alexander and Crutcher 1990b). In the present study we implemented a similar dissociation with the use of a two-dimensional delayed reaching paradigm. The purpose of this study was to provide a further test, extended to two-dimensional arm movements, of the concept that MC neurons participate not only in purely motor processing, but also in associative or purely sensory processing of visuospatial information. Some of these results have been presented elsewhere in abstract form (Shen and Alexander 1994).
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METHODS |
Subjects and experimental apparatus
Two female macaque monkeys [one Macaca mulatta (JA); the other Macaca nemestrina (KO)], each weighing 4-5 kg, were used in these experiments. In all aspects of their care, the monkeys were treated in accordance with the Guiding Principles in the Care and Use of Animals of the American Physiological Society.
Each monkey was trained to perform a set of motor behavioral paradigms in which reaching movements were made with the right forelimb to guide a two-dimensional joystick that controlled the position of a cursor on a video display. The display (20-in. diagonal color monitor) was centered at eye level, 30 cm in front of the subject. The subject was seated in a primate chair with the right hand resting on the handle of a vertically oriented joystick and the left arm lightly restrained with foam padding. An opaque neck plate prevented the subject from viewing the working forelimb.
The pivot point of the joystick was located ~15 cm forward and 15 cm lateral to the subject's right hip. With the handle of the joystick centered directly above the pivot point (center position), the upper arm was able to hang vertically (shoulder neither flexed nor extended), with the forearm held in a roughly horizontal orientation (elbow flexed 90°). The joystick was of symmetrical design, with one shaft extending upward for the monkey to grasp, the other extending downward from the pivot point. The two 30-cm shafts were fitted with balanced weights (~300 g each) to increase the dynamic load, and thereby enhance the task-related electromyographic (EMG) signals, while maintaining a net zero static load throughout the workspace (because the center of gravity was located at the pivot point).
The displacement of the joystick handle was measured with two precision potentiometers attached to gimbals (that supported the joystick shafts) whose axes passed through the pivot point. The displacement signal was reflected by the position of a cursor (consisting of a 3 × 3-mm white square) presented on the video display. The joystick and cursor mappings were calibrated so that a 10-cm tangential displacement of the joystick handle (~20° angular displacement of the joystick shaft) from its center position would result in a corresponding 12-cm displacement of the cursor on the video display.
Behavioral paradigms
There were two behavioral paradigms, illustrated in Fig. 1, which differed from each other only with respect to the mappings between joystick and cursor. In one paradigm, designated the nonrotated (or 0° mapping) condition, forward movement of the joystick (with respect to the monkey) moved the cursor upward on the display and rightward movement of the joystick moved the cursor rightward (from the monkey's perspective). In the other paradigm, designated the rotated (or 90° mapping) condition, the mapping between joystick and cursor was rotated by 90°. In the rotated condition, rightward movement of the joystick moved the cursor upward on the display, and backward movement of the joystick moved the cursor rightward.
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| FIG. 1.
Behavioral paradigms employed in this study. Left: sequence of task-related events associated with each trial. During the intertrial interval (ITI), the display remained darkened. Each trial began with the presentation of the center fixation target (CF), which the monkey "captured" with the cursor by moving the joystick in an appropriate direction. With fixation point capture (CFC), all 4 peripheral targets were illuminated. At the end of a preinstruction interval (pre-IS), 1 of the peripheral targets dimmed briefly. This served as the visual instruction stimulus (IS). The postinstruction (delay) interval that ensued was terminated by dimming of the CF. This served as the movement-triggering stimulus (TS), following which the monkey was expected to move the joystick handle along a trajectory that would align the cursor with the target that had been designated by the IS (M). After holding briefly at the correct peripheral target (H), the monkey received a liquid reward. Below the time line are shown drawings of the visual display corresponding to the key events and intervals of a typical behavioral trial. Right: spatial characteristics of the 2 mapping conditions. Solid arrows: cursor movements. Dashed arrows: joystick. Upward arrow: forward movement of the joystick. Downward arrow: backward movement of the joystick. In the nonrotated (0° mapping) condition, forward movement of the joystick moved the cursor upward to capture the top target and rightward movement of the joystick moved the cursor rightward to capture the right target. In the other paradigm, designated the rotated (90° mapping) condition, the mapping between joystick and cursor was rotated by 90°. In the nonrotated condition, rightward movement of the joystick moved the cursor upward to capture the top target and backward movement of the joystick moved the cursor rightward to capture the right target.
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The basic task was a delayed reaching paradigm with indirect visual feedback. Because the subjects were not allowed to view the working limb or joystick, they had to rely on the cursor for visual feedback about their reaching movements. Each trial began with the presentation of a center fixation target (CF), which consisted of a 2 × 2-cm white square, in the center of the display. The subject was required to "capture" the CF by positioning the cursor within its boundaries, at which time the shading of the CF switched from white to gray. Once the CF had been captured, four peripheral targets were displayed simultaneously above, below, and to the right and left of the CF. Each was located 12 cm from the CF (visual angle = 22°), and each consisted of a 2 × 2-cm white square. These peripheral targets remained illuminated for the rest of the trial. After capture of the CF and illumination of the peripheral targets, the subject was required to hold in the center position during a variable (0.6-1.6 s) preinstruction period, during the 400-ms presentation of the visual IS, and during a variable (0.6-1.6 s) postinstruction period (which was timed from IS offset). The IS consisted of the transient dimming of one of the four peripheral targets, selected in a pseudorandom sequence (balanced, but unpredictable). At the end of the postinstruction period, the CF dimmed briefly (200 ms), which served as a nondirectional trigger stimulus (TS). The monkey was then permitted to move the cursor away from the CF and into alignment with the peripheral target whose dimming had served as the IS on that trial. As with CF capture, the shading of the peripheral target switched from white to gray once it was captured. After holding briefly (0.5-1.0 s) at the peripheral target location, the monkey received a liquid reward (0.2 ml of dilute apple juice) delivered through a stainless steel mouthpiece.
If the cursor left the CF before presentation of the TS, the trial was immediately terminated without reward. Minimal constraints were placed on response times following the TS (capture times of up to 5.0 s were permitted), because of the inevitable directional errors and associated corrective movements that occurred early in the adaptation process that accompanied every switch between nonrotated and rotated conditions. Subjects were taught to perform the basic task under both the nonrotated and rotated conditions with an accuracy of >90%. This required ~3-4 mo of training for each subject. Performance was considered correct or accurate on trials in which the initial direction of cursor movement
as the cursor exited the center windowwas appropriate, i.e., within 15° of the correct target direction.
It is important to note that from the standpoint of the visual display, the task was identical in both conditions. However, the limb trajectories associated with capturing any one of the peripheral targets differed by 90° in the two conditions. Thus, for example, in the nonrotated condition successful capture of the top target required a forward movement of the hand, whereas in the rotated condition the subject moved the hand to the right to capture the top target. The cursor trajectory on the visual display was similar in both conditions. Across the two conditions, limb (i.e., hand) trajectory and target location were dissociated from one another. The subject made similar reaching movements to capture targets that were 90° apart, and captured identical targets with reaching movements whose trajectories differed by 90°.
Surgical procedures
Once training had been completed, each monkey was surgically prepared for chronic single-cell recording experiments. All surgical procedures were carried out with general anesthesia (induction with ketamine 10 mg/kg im followed by 1.5-3.0% isoflurane gas anesthesia) and standard aseptic technique. A scleral search coil for monitoring eye position was implanted in the left eye of each monkey (Judge et al. 1980). With stereotaxic guidance, a stainless steel recording chamber (29 mm ID) was positioned over a circular opening in the skull that permitted access to the lateral convexity of the left frontal lobe, including motor and premotor cortex. The recording chamber was then cemented to the calvarium with dental acrylic. Stainless steel bolts for immobilization of the head during behavioral performance were also cemented in place with dental acrylic, and the entire assembly was anchored to the calvarium with small stainless steel screws.
Chronic EMG electrodes were also implanted at the time of surgery. These consisted of 11 (monkey JA) or 12 pairs (monkey KO) of bipolar patch electrodes that were sutured to the connective tissue on the surface of each muscle. The leads from these electrodes were tunneled subcutaneously to a connector embedded in the acrylic skull asssembly. Electrodes were implanted in task-related muscles of the working (right) forelimb, including brachialis, biceps, deltoid, infraspinatus, lateral triceps, medial triceps, pectoralis major (2 pairs of electrodes in monkey KO), rhomboid, subscapularis, supraspinatus, and teres major.
Recording procedures
Action potentials from cortical neurons were recorded extracellularly with glass-coated platinum-iridium microelectrodes (impedance 0.5-1.5 M
measured at 1,000 Hz). After appropriate amplification and filtering (0.3-5.0 kHz), the action potentials of individual neurons were discriminated from one another by a computerized spike sorter (Alpha Omega Engineering) that operates on the principle of adaptive template matching. Discriminated, digitized neuronal activity (the spike sorter generated one 0.1-ms standard pulse per action potential) was sampled by the laboratory computer at 1-ms intervals.
Eye position was monitored with the scleral coil-magnetic field technique (Indiran Instruments) (Robinson 1963). EMG activity was recorded differentially from each pair of chronic electrodes. These signals were amplified, filtered (100-500 Hz), and rectified. All analog data, including eye position, joystick position, and EMG activity, were sampled by the laboratory computer at 1,000 Hz and then averaged across five adjacent time bins for an effective sampling rate of 200 Hz.
Data acquisition
During experimental sessions, the monkey's head fixation bolts were attached to a restraining device. Under the control of a hydraulic microdrive (Narishige MO-95), a microelectrode was advanced through the dura and into the precentral cortex. The monkey performed one of the two behavioral paradigms (nonrotated or rotated condition) as task-related neurons were identified and discriminated from one another. Administration of the behavioral paradigms and collection of electrophysiological data were controlled by the laboratory computer (586/66 IBM-compatible PC). On-line rasters of neuronal activity were displayed and continuously updated to permit the rapid identification of neurons with obvious task-related activity, that is, neurons whose activity showed consistent changeseither tonic or phasic, of either polaritypreceding or following either the IS or the TS. When neurons were identified that appeared to show task-related activity, directional or otherwise, an attempt was made to collect complete data files under both rotation conditions. For each rotation condition, a complete data file consisted of 10-15 repetitions of each of the four trial types (corresponding to the 4 peripheral target locations). After the shift from the first to the second rotation condition, we attempted to wait until the monkey had adapted completely to the new rotation condition before collecting the data file for the second condition. Nevertheless, because of the pressures of time associated with experimental recording sessions, the data files occasionally included some trials in which the monkey had not yet managed to adapt completely to the new rotation condition. On trials such as these, the initial direction of limb movement (as the cursor exited the center window) was often incorrect, being more appropriate for the previous rotation condition, even though a midtrajectory correction usually enabled the monkey to reach the correct target and thereby receive a reward. Because of this, our data analysis procedures took into account the actual trajectory of limb movement on each trial, rather than simply assuming that a rewarded response had involved a trajectory that was directed straight toward the appropriate target (see below). In some cases we were able to test a given neuron through two or even three rotation shifts (repeating either the 1st or both the 1st and 2nd rotation conditions), but most cells were tested with only a single rotation shift.
Because of the adaptation process that was required whenever the rotation condition was switched, we generally did not attempt to switch the rotation condition back to the original state after data collection had been completed for a given cell. Thus, for successive neurons along a particular microelectrode track, the order in which the two rotation conditions were presented was generally reversed.
Microstimulation through the recording microelectrode was used to help identify the proximal arm representation in motor cortex, and to characterize the stimulation-evoked motor responses at the cortical recording sites. Currents were limited to 35 µA, delivered in 100-ms trains of balanced bipolar pulse pairs (0.2-ms cathodal pulse, 0.1-ms gap, 0.2-ms anodal pulse) at a frequency of 300 Hz. Microstimulation was carried out at most recording sites, usually as the microelectrode was being withdrawn from each track at the end of the recording session.
After termination of the recording sessions, each monkey was deeply anesthetized with pentobarbital sodium and perfused transcardially with aldehyde fixatives. Immediately before the perfusion, three stainless steel pins were inserted into the brain of each monkey at known coordinates within the recording chamber, with the use of the same microdrive that was used for the single-cell recordings. The pins remained in place during the perfusion, after which the calvarium, recording chamber, and dura were removed and the frontal lobe over which the chamber had been placed was photographed with pins still in place. The brain was then removed from the skull and placed briefly (1-2 days) in additional fixative, following which the pins were removed. The brain was then blocked in the coronal plane, frozen, and sectioned at 40-µm intervals on a freezing microtome. Every fifth section was stained with cresyl violet. With the use of the records of chamber coordinates for each microelectrode track, the site of surface penetration for each microelectrode track was reconstructed with reference to the chamber coordinates of the pin tracks, which were referenced in turn to sulcal landmarks on the basis of the surface photographs and the histological material. We did not attempt to identify each microelectrode track within the histological material, or to reconstruct the precise depth of each of the neuronal recording sites.
Data analysis
All data files containing task-related activity were subjected to statistical analysis. Movement-aligned and stimulus-aligned (CF, IS, TS) rasters and perievent histograms of each cell's activity were inspected and evaluated, but the final classification of task-related responses was based on a formal statistical analysis. The categorical analysis of task-related activity was based on two orthogonal classification schemes. One scheme involved a temporal classification according to the task-related events with which the response was associated, such as presentation of the IS, onset of movement, etc. This was based on the temporal dissociation of events that was built into the experimental paradigms. The other scheme was based on the level of information processing with which the response was associated, as determined operationally by the effects of the spatial dissociation between target location and limb trajectory that was built into the paradigms.
We defined four temporal epochs within each trial. Each epoch was a 200-ms window whose boundaries were defined with respect to a specific intratask event. In addition, an epoch for characterizing baseline discharge rates was defined within the intertrial interval (ITI). The four intratrial epochs and the single intertrial epoch are described in Table 1.
Neuronal activity (spike discharge rate) was classified in terms of its dependence on both the temporal and spatial features of the two behavioral tasks. To be considered task related, neuronal activity occurring during one of the four intratrial epochs was required to show either 1) significant dependence on the spatial features of the tasks, as measured by the target and limb analyses of variance (ANOVAs) described below or 2) significant deviation from the baseline discharge rate as measured during the ITI. The latter determination was made across all trials by the use of a paired Student's t-test (2-tailed, P < 0.001), which was applied if and only if both the target and limb ANOVAs failed to demonstrate a significant directional effect.
On the basis of the epoch in which it occurred, task-related activity was categorized (Table 1) as stimulus related, set related, or movement related (early or late). The following analysis was used to further classify epoch-specific, task-related activity in terms of its spatial dependencies. Neural data from each trial were appropriately coded with respect to 1) rotation condition (0° mapping, 90° mapping); 2) location of the target (right = 0°, top = 90°,left = 180°, bottom = 270°); and 3) initial direction of limb movement (rightward = 0°, forward = 90°, leftward = 180°, backward = 270°). The initial direction of limb (joystick) movement was measured directly, for each trial, at the instant the cursor exited the CF window. This measured direction was then coded in terms of the nearest cardinal direction of joystick movement (i.e., 0, 90, 180, or 270°). Thus it was the actual initial direction of limb movement, rather than an assumed ideal direction, that was used in the assessment of spatial dependencies. Each epoch-specific neural response was subjected to two separate ANOVAs, one in which the effect of target location was assessed across rotation conditions (target ANOVA) and another in which the effect of limb trajectory was assessed across rotation conditions (limb ANOVA). For the two-way target ANOVA, the factors were target location and rotation condition. For the two-way limb ANOVA, the factors were initial direction of limb movement and rotation condition.
After a preliminary, exploratory analysis of the neural data, the significance level for both the target and the limb ANOVAs was set at 
= 0.001. With this significance level, we found close agreement between the ANOVA assessments and the apparent task relatedness of cells as judged by visual inspections of the rasters and perievent histograms.
Table 2 shows how epoch-specific neuronal activity was categorized according to its spatial dependencies. Target-dependent activity covaried exclusively with target location across both conditions. Activity was classified as target dependent if the target ANOVA showed a main effect for target location and no location × condition interaction, provided that the limb ANOVA showed no main effect for limb direction or showed a direction × condition interaction. Limb-dependent activity covaried exclusively with limb trajectory. Activity was classified as limb dependent if the limbANOVA showed a main effect for limb direction and no direction × condition interaction, provided that the target ANOVA showed no main effect for target location or showed a location × condition interaction.
It is important to emphasize that, for a cell's activity to be categorized as either target dependent or limb dependent, there must have been an adequate dissociation between target and limb directions across rotation conditions. Had there not been an adequate dissociation between these two variables, directional activity that showed dependence on one variable (say, target dependence) would necessarily have shown dependence on the other variable as well, and that was prohibited in the definitions of both categories. And conversely, neither target-dependent nor limb-dependent activity could be attributable to a simple failure to dissociate target and limb directions across rotation conditions (i.e., a failure of the monkey to adapt sufficiently to the second rotation condition). For the limb ANOVA was based on the actual, measured directions of initial limb movement: any substantial failure of the monkey to adapt to the second rotation condition, so that target and limb directions were not dissociated across rotation conditions, would lead to results for the limb ANOVA that were similar to those of the target ANOVA, and that would be incompatible with the definitions of both target dependence and limb dependence.
Complex activity, although directional, did not show consistent covariation with either target location or limb direction across both rotation conditions. If a directional response did not meet the strict criteria for either target dependence or limb dependence (i.e., a main effect for either target or limb direction without a corresponding direction × condition interaction) because of significant direction × condition interactions, it was classified as complex. Complex activity was subclassified according to whether it showed a main effect only for target direction (complex-T), only for limb direction (complex-L), or for both target and limb direction (complex-I, "intermediate").
Neuronal activity that met the specific criteria for being either target dependent, limb dependent, or complex was considered to be directionally classifiable. However, if activity within a given epoch met the criteria for both target and limb dependence, it was considered to be directionally unclassifiable. This is because, in principle, if target and limb directions had been adequately dissociated across rotation conditions, it would have been logically impossible for a neuron's activity to simultaneously fulfill the criteria for both target and limb dependence. In fact, instances of directionally unclassifiable activity were generally explained by a monkey's failure to adapt sufficiently to the second rotation condition before the recordings from a given neuron had been suspended.
If there were no main effect either for target location or for limb direction, yet the epoch-specific activity differed significantly from baseline levels (paired, 2-tailed t-test, < 0.001), the activity was categorized as nondirectional.
This classification scheme, based on spatial dependencies, was inherently conservative in the designation of directional responses as either target or limb dependent, because of the stipulation that neural activity assigned to either of these categories was required to show no corresponding direction × condition interaction. Another important feature of this classification scheme is that it effectively precluded the misclassification of truly limb-dependent activity as target dependent, and vice versa. This was due to two factors: 1) the use of actual limb trajectories in computing the limb ANOVAs and 2) the fact that the definitions of target and limb dependence were mutually exclusive. In combination, these two factors guaranteed that even if the monkey failed to adapt to the second rotation condition, in which case the initial hand trajectories would maintain the same target mappings as in the previous rotation condition, truly limb-dependent activity (or truly target-dependent activity) would have met the criteria for both limb and target dependence, in which case it would have been categorized as "unclassifiable." This is demonstrated in the APPENDIX, which describes the results of a computer simulation of how trajectory errors, including those due to incomplete adaptation to the second rotation condition, may have influenced our spatial classification scheme.
Categorical comparisons of the frequencies of neuronal response types across epochs were made with the use of Pearson's 
2 test.
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RESULTS |
Task performance
Even after each subject had been fully trained and was performing both paradigms at >90% correct, both monkeys required a brief period, consisting of ~80-120 trials, to adapt to the new joystick/cursor mapping each time the condition was changed. The same adaptation period was required whether the task condition changed from nonrotated to rotated or from rotated to nonrotated. Immediately after the condition was changed, the monkey would continue to generate initial limb trajectories that were appropriate for the previous mapping condition, so the initial cursor trajectory would be directed ~90° away from the correct target location. Corrective movements would be required to bring the cursor into alignment with the target, and the result would be a curvilinear hand trajectory early in the course of adaptation. As adaptation progressed, the trajectories to each of the peripheral targets would gradually straighten. When the mapping was changed again, the process would be repeated. Even after each monkey had been performing both tasks for >1 yr, an adaptation process of approximately the same length was required every time the joystick/cursor mapping was changed.
For both subjects, the reaction times (RTs) were comparable across trials of differing target locations and mapping conditions. For monkey JA, the mean RT across all target locations was 349 ± 65 (SD) ms during the 0° mapping condition and 351 ± 72 ms during the 90° mapping condition. For monkey KO, the mean RT values were 385 ± 84 ms and 369 ± 75 ms, respectively.
During the adaptation process, the average movement time (MT) increased temporarily because of the curvilinear trajectories that were associated with the error correction process. Once the subject had adapted to the new rotation condition, however, the MTs in the new adapted state were comparable with those recorded before the change in rotation condition. The average MT for monkey JA was 285 ± 38 ms when the monkey was fully adapted to the 0° mapping condition, and 278 ± 45 ms when adapted to the 90° mapping condition. For monkey KO, the comparable MT values were 296 ± 75 ms and 290 ± 87 ms.
All of the sampled muscles in both monkeys showed directional, movement-related activity patterns during task performance. All of these movement-related responses were limb (direction) dependent. This is illustrated in Fig. 2, which shows the task-related EMG patterns for two muscles, deltoid and brachialis, sampled from monkey JA. In each case, the movement-related activity pattern can be seen to covary with the direction of limb movement rather than with target location. It should also be noted that both muscles were silent during the postinstruction period. None of the muscles sampled from either monkey showed significant directional activation during the postinstruction period.
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| FIG. 2.
Task-related electromyographic (EMG) activity recorded from 2 of the forelimb muscles, deltoid and brachialis, that were monitored throughout the experiments in monkey JA. The data sample presented in this figure was obtained in the course of testing a single primary motor cortex (MC) neuron for its task relatedness. A: data recorded during the 0° mapping condition. B: data from the 90° mapping condition. Each of the 4 rows of illustrations represents the EMG and kinematic data obtained from trials in which the same peripheral target served as the locus for the visual IS: in clockwise order of targets, the 1st (top) row contains data from top target trials, the 2nd row for right target trials, the 3rd row for bottom target trials, and the 4th row for left target trials. Within each rotation condition, the 2 leftmost columns show the EMG data from the deltoid and brachialis muscles, and the column to their immediate right shows the corresponding kinematic data. The EMG records for trials of a given type (based on target location and rotation condition) are organized in raster form. EMG records are aligned with TS (trigger) onset. In the columns of kinematic data, the target-capturing hand trajectories from all trials of a given type are superimposed and presented on a facsimile of the monkey's behavioral display, with the designated target for each trial type indicated by shading. Trajectories shown as upward in the illustration represent forward limb movements, whereas those shown as downward represent backward limb movements. The units for the horizontal scale under each kinematic display represent hand displacement (cm); this same scale is applicable to the vertical axis as well. The time scale for the EMG records is in ms. Both muscles showed directional, movement-related responses that covaried with the trajectory of limb movement rather than with target location. Under both rotation conditions, the deltoid was maximally activated by rightward movements of the joystick, and the brachialis by backward movements. Neither muscle showed significant directional activity during the postinstruction (delay) interval.
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Subjects were not required to maintain fixation during performance of the behavioral paradigms. However, once the IS had been delivered, both subjects did maintain fixation on the CF throughout the postinstruction period until the TS appeared, at which point a saccade was made to the appropriate peripheral target in rough synchrony with the onset of limb movement. A sample of task-related eye movement recordings from monkey JA is presented in Fig. 3.
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| FIG. 3.
Task-related eye movements. Samples of eye position recordings from monkey JA, illustrating the pattern of eye movements that was associated with task performance. The layout is similar to that of Fig. 2. Each row contains data from trials with the same target location. For clarity, only data from a single example of each of the 8 types of behavioral trials are shown. The examples chosen for this illustration were selected randomly, as the first instances of each trial type to occur within a block of 0° mapping trials and a block of 90° mapping trials collected during a single recording session. Solid lines: horizontal eye position signals (right is up, left is down). Broken lines: vertical signals. The times of occurrence of capture of CF (f), IS or cue onset (c), TS or trigger onset (t), onset of limb movement (m), and end of trial (e) are indicated on each record. These records demonstrate a pattern of task-related eye movements that was common to both monkeys. During the preinstruction period, after the CF had been captured, there was a tendency for the subject to maintain central fixation of the CF, but this was frequently interrupted by random saccades. Once the IS had been presented, however, fixation of the CF was rarely broken again until the postinstruction period was terminated by delivery of the TS. At this point, a saccade would be made to the correct target, followed shortly thereafter by the onset of the target-capturing limb movement. Gaze was then maintained at the correct peripheral target until the end of the trial, when all targets were extinguished. The vertical scale to the left of each record represents gaze angle (°); the horizontal scale beneath each record represents time (ms).
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Data base
A total of 180 MC neurons (108 from monkey JA, 72 from monkey KO) showed task-related activity, that is, activity that showed significant directional effects or that differed significantly from baseline levels during at least one of the task-defined epochs. The final data base included only those task-related neurons that had been shown by local microstimulation to lie within a zone where proximal arm movements were represented (as evidenced by shoulder and/or elbow movements evoked with currents of 
35 µA), and whose activity had been tested with both rotation conditions. The final data base comprised 119 MC neurons.
Overview of task-related activity
For the sample as a whole, the most prominent task-related responses that we observed among MC neurons were movement-related and set-related. The distribution and functional categorization of task-related responses across the various epochs is summarized in Table 3. Every neuron in the sample showed phasic movement-related activity changes. Most of the neurons in the sample also showed tonic, set-related activity during the delay period, after delivery of the directional IS. Many MC neurons also showed phasic, stimulus-related activity immediately following the IS. The large majorities of both the set- and movement-related responses were directional, and 
50% of the directional activity within each epoch was found to be directionally classifiable.
Phasic, stimulus-related activity following the IS
Three quarters of the sample of MC neurons (76%, 91 of 119) showed brief, phasic responses to the IS (Table 3), but the majority of these were nondirectional. Approximately one quarter of the entire sample showed phasic responses to the IS that were directionally selective (24%, 29 of 119). Nearly all of the stimulus-related responses that were directionally classifiable were target-dependent (94%, 15 of 16), whereas only one was found to be limb-dependent (6%, 1 of 16). None were judged to be complex. An example of a stimulus-related response that was classified as target-dependent is illustrated in Fig. 4. In both the nonrotated and the rotated conditions, this cell showed a weak, phasic burst of activity that began shortly after IS onset at the right target. There was no response when the IS was presented at any of the other three targets. The response was independent of the direction of the instructed limb movement.
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| FIG. 4.
Target-dependent, stimulus-related response recorded from an MC neuron. Each of the 4 rows of illustrations represents the neural and kinematic data obtained from trials in which the same peripheral target served as the locus for the visual IS. Data from both 0° and 90° mapping conditions are presented. In this and subsequent figures that illustrate neuronal responses, data from the 2 rotation conditions are ordered from left (A) to right (B) in the sequence in which they were administered. The neural data are shown as corresponding raster displays (top) and perievent histograms (bottom). Each dot in the raster displays represents a single action potential, and each line of dots contains the data from a single trial. The time base, in ms, is the same for rasters and histograms, both of which are aligned on the same behavioral event. The alignment point for the rasters is indicated by a single caret mark at the bottom of each raster display. Binwidth for histograms: 25 ms. Vertical scale: spikes/s. Trials are aligned with cue onset. The kinematic data are shown to the right of each corresponding set of neural data, and are presented in the same format described in Fig. 2. This neuron responded to the presentation of the instructional cue with a weak, phasic discharge whenever the IS was located at the right target, irrespective of rotation condition.
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Tonic, set-related activity preceding the TS
Approximately three fifths of the sample of MC neurons (61%, 72 of 119) showed tonic, directionally selective set-related activity during the delay (i.e., postinstruction) epoch between IS and TS (Table 3). Of the set-related responses that were directionally classifiable, 37% (16 of 43) were target-dependent, 35% (15 of 43) were limb-dependent, and 28% (12 of 43) were complex. All MC neurons with directional set-related activity showed movement-related responses as well.
An example of target-dependent, set-related activity is illustrated in Fig. 5. In both rotation conditions, this cell showed a sustained increase in discharge rate throughout the delay period on trials in which the IS had indicated that the left target was to be captured following the TS. The cell also showed a target-dependent, movement-related response with the same directionality.
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| FIG. 5.
Target-dependent set-related response in combination with early target-dependent movement-related respose. Conventions similar to those of Fig. 4, except that extra columns of neural data aligned with TS (trigger) onset have been added for both rotation conditions. Within the TS-aligned rasters, heavy dots designate the times of movement onset. On left target trials under both rotation conditions, this MC neuron showed tonic set-related activity during the postinstruction (delay) interval before TS onset, and phasic early movement-related activity immediately following the TS. Both types of target-dependent activity were unrelated to the direction of limb movement.
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An example of limb-dependent, set-related activity is illustrated in Fig. 6. In both rotation conditions, this cell showed a sustained increase in activity during the delay period on trials in which the instructed motor response involved a rightward limb movement. This set-related activity was independent of the target that had been signified by the IS. The cell also showed strong movement-related activity that was limb-dependent (maximal with forward or rightward limb movements), and weak (but statistically significant) stimulus-related activity that was target-dependent (right target).
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| FIG. 6.
Limb-dependent set-related activity in combination with limb-dependent movement-related activity. Conventions are the same as in Fig. 5. Under both rotation conditions, this MC neuron showed tonic, set-related activity during the postinstruction interval before TS onset on trials that required rightward limb movements, regardless of target location. The cell also showed early movement-related activity that was limb dependent: under both rotation conditions, activity was maximal with forward or rightward movements of the limb, intermediate with backward movements, and minimal with leftward movements. Note that even the temporal pattern of the movement-related response covaried with limb direction across both rotation conditions, independent of target location: for example, with forward movements the activity was relatively tonic and sustained, whereas with rightward movements the activity was much more phasic. There was also a weak stimulus-related response that was target dependent, being selective for right target trials.
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Phasic, movement-related activity following the TS
Movement-related responses following the TS were considered to be early or late, depending on whether they preceded or followed the onset of limb movement (Table 1). The large majority of MC neurons (86%, 102 of 119) showed early movement-related activity that was directionally tuned (Table 3). Of the early movement-related responses that were directionally classifiable, there were only one third as many target-dependent responses (14%, 11 of 79) as limb-dependent responses (43%, 34 of 79), with the remainder being complex (43%, 34 of 79). There was also a large majority of MC neurons that showed late movement-related activity that was directionally tuned (84%, 100 of 119). And of the late movement-related responses that were directionally classifiable, there were only one ninth as many target-dependent responses (4%, 4 of 88) as there were limb-dependent responses (41%, 36 of 88), with the remainder being complex (55%, 48 of 88).
Fig. 7 shows an example of early movement-related activity that was classified as limb-dependent. This cell discharged maximally before the onset of rightward or backward limb movements, irrespective of the rotation condition.
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| FIG. 7.
Limb-dependent movement-related activity in MC. Conventions are the same as in Fig. 4, except that the neural data are aligned with movement onset. Heavy dots: TS onsets. Under both rotation conditions, this neuron showed early movement-related activity that was directionally tuned. Beginning before movement onset, the phasic discharge was maximal with rightward limb movements and only slightly less prominent with backward movements. The response was independent of target location.
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Many MC neurons showed movement-related activity that had more than one component. For example, the cell whose activity is illustrated in Fig. 8 showed early movement-related activity that was target-dependent, and late movement-related activity that was limb-dependent. The early, target-dependent activity preceded the onset of limb movements to capture the right target, regardless of rotation condition. The late, limb-dependent activity was associated with forward or leftward limb movements, irrespective of rotation condition. We also encountered several examples in which two distinct limb-dependent responses could be discerned (1 early, 1 late) because of differences in their respective preferred directions (PDs). However, there were no examples of limb-dependent components preceding target-dependent components.
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| FIG. 8.
MC neuron with 2-component movement-related response: the early response was target dependent, and the late response was limb dependent. Conventions similar to Fig. 7, except the neural data are aligned with TS (trigger) onset. The times of movement onset, indicated by heavy dots in the raster displays, serve to demarcate the early (premovement) and late components. The early component was characteristed by a burst of activity before the onset of limb movements associated with capturing the right target, regardless of the direction of the limb movement itself. The late component was characterized by a burst of activity that began with movement onset on trials in which the required limb movement was either forward or leftward, regardless of target location.
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Combinations of set- and movement-related activity
The cell whose activity is illustrated in Fig. 9 showed set-related activity that was target dependent (maximal for the bottom target and minimal for the left target), and late movement-related activity that was limb dependent (maximal after the onset of movements to the left). The late movement-related activity was preceded by early movement-related activity that was target dependent (present for all targets but the left). Of the four possible combinations of target- and limb dependencies, only one was not represented in our sample of MC neurons with combined set- and movement-related activity: we encountered no neurons that combined limb-dependent set-related activity with target-dependent movement-related activity.
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| FIG. 9.
Target-dependent set-related activity in combination with 2 early movement-related responses, the latter of which was limb dependent. Conventions are the same as in Fig. 6. Under both rotation conditions, this MC neuron showed tonic set-related activity (during the postinstruction interval, before trigger onset) that covaried with the location of the target independent of the direction of limb movement, being maximal for bottom target trials and absent on left target trials. Accompanying the set-related activity was an early and very brief movement-related response that was also target dependent, with the same directionality as the set-related activity. A 2nd movement-related response was also seen: it began after the 1st component but before the onset of limb-movement. This 2nd movement-related response was limb dependent in that it was characterized by a large, phasic discharge that was selective for trials that required a leftward limb movement, independent of target location.
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In some of the cells that combined both set- and movement-related activity that was target dependent, such as the example illustrated in Fig. 10, the movement-related activity was so early and so brief as to suggest that it might have been appropriate to classify it as stimulus-related activity (related to the visual TS). It is important to emphasize, however, that the TS itself contained no spatial information; it served only to trigger the delayed motor response whose spatial target had been instructed by the preceding IS.
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| FIG. 10.
Target-dependent set- and movement-related activity. Conventions are the same as in Fig. 6. Under both rotation conditions, this MC neuron showed tonic set-related activity and phasic early movement-related activity only on trials in which either the top or left target was specified by the visual IS. There was sustained suppression of neural activity throughout the postinstruction and movement intervals on trials in which either the right or bottom target was specified. Although the movement-related activity was so early and so brief that it might have been construed as a sensory response to the visual TS, it is important to note that the TS itself contained no directional information.
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Complex activity
Some of the directional set- and movement-related activity observed in this study did not meet the criteria for classification as either target or limb dependent, because of complex interactions between direction and rotation condition. An example of complex set-related activity is illustrated in Fig. 11. In the 0° mapping condition, this cell showed maximal set-related activity before backward movements of the joystick that were associated with capturing the bottom target. In the 90° mapping condition, the directionality of the set-related activity shifted so that the activity was now comparable for trials in which the movement was either leftward, to capture the bottom target, or backward, to capture the right target. (The cell also showed late movement-related activity that was clearly limb dependent, associated with backward movements of the joystick.) This set-related activity might have been characterized, equivalently, as being either target dependent or limb dependent with its PD shifting across rotation conditions. Because of this ambiguity, it was classified as complex-I (intermediate), because both the target ANOVA and the limb ANOVA showed significant direction effects as well as significant direction × condition interactions (Table 2).
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| FIG. 11.
Complex set-related activity. Conventions are the same as in Fig. 6. In the 0° mapping condition, this MC neuron showed maximal set-related discharge (during the delay period preceding TS trigger onset) before backward movements of the joystick that were associated with capturing the bottom target. In the 90° mapping condition, the directionality of the set-related response shifted so that the response was now comparable for trials in which the movement was either leftward, to capture the bottom target, or backward, to capture the right target. There was also a late movement-related response that was limb dependent, being associated with backward movements of the joystick across both rotation conditions.
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Most of the complex activity observed in MC was of this complex-I variety (90%, 85 of 94; Table 4). However, there were also small numbers of neurons whose movement-related activity was classified as complex-T (3 cells) or complex-L (6 cells) because of main direction effects plus direction × condition interactions that were seen exclusively with either the target ANOVA or the limb ANOVA, respectively (Table 2).
Temporal distribution of directionally classifiable activity
Figure 12 shows the temporal distribution of target-dependent versus limb-dependent versus complex activity across the task-defined epochs that followed the IS onset. It is evident that over the extended interval between IS and motor response, there was an initial predominance followed by a gradual decline in the proportion of MC neurons showing target-dependent activity, and a gradual increase in the proportion of neurons showing limb-dependent activity. There was also a gradual increase in the proportion of neurons showing complex activity. An overall 2 analysis of the relative frequencies of these three categories of directionally classifiable activity across the four task epochs showed that the apparent temporal changes were highly significant (2 = 78.01, df = 6, P = 9.21 × 10
15).
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| FIG. 12.
Bar plot showing numbers of MC neurons with target-dependent, limb-dependent, or complex activity in each of the 4 task-defined epochs that followed IS onset. Stimulus-related activity occurred during the IS epoch, set-related activity during the delay epoch, early movement-related activity during the reaction time (RT) epoch, and late movement-related activity during the movement time (MT) epoch (see Table 1).
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To clarify the relationships between directional categories and task epoch, we used additional 2 tests to partition the variance associated with the overall 2 analysis (Snedecor and Cochran 1989). Those results are presented in Table 4. Because of the apparent similarity in proportions of limb-dependent activity and complex activity across epochs (Table 3), we compared the frequencies of these two categories across epochs and found that they did not differ significantly (Table 4, row b: 2 = 2.70, df = 3, P = 0.44). Consequently, these two categories were combined, and their combined frequency was then compared with the frequency of target-dependent activity across epochs. The result (Table 4, row c: 2 = 76.28, df = 3, P = 2.22 × 1016) showed that nearly all of the variance in the overall 2 analysis could be accounted for by the difference across epochs in the frequencies of target-dependent versus limb-dependent plus complex activity.
To determine whether the trend toward declining proportions of target-dependent activity was significant for each successive epoch, we made those comparisons as well and found that the trend was significant across each pair of adjacent epochs (Table 4, rows d-f). However, the sum of 2 values for these last three comparisons (2 = 28.19, df = 3) was substantially less than the 2 for the comparable comparison across all epochs (Table 4, row c), because of the fact that the latter comparison also accounted for variance across nonadjacent epochs. This last point was demonstrated by combining the corresponding frequencies of the first two epochs (IS and delay), and those of the last two epochs (RT and MT), and comparing the resulting frequencies across the two composite epochs. When the 2 resulting from this comparison (Table 4, row g: 2 = 51.03, df = 1, P = 9.10 × 1013) was added to those obtained from the independent comparisons made across the first and last two epochs (Table 4, rows d and f, respectively), the sum of 2 values (2 = 73.16, df = 3) approximated that of the 2 for the comparable comparison across all epochs (Table 4, row c: 2 = 76.28, df = 3). The small residual difference is attributable to slight, but unavoidable, algebraic differences between the overall and partitioned analyses (Snedecor and Cochran 1989).
Locations of neurons with task-related activity
The surface penetration sites of microelectrode tracks that contributed to the final data base are indicated in Fig. 13. All of these tracks were located within zones of proximal arm representation in MC, as evidenced by contralateral shoulder and/or elbow movements evoked by local microstimulation at currents of 35 µA.
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| FIG. 13.
Surface diagrams of the MC region explored in each monkey. Center of each circle marks the penetration site for each microelectrode track that contributed task-related responses to the final data base. Area of each circle is proportional to the number of neurons that contributed to the final data base along that track. The smallest circles represent 1 neuron per track. In monkey JA, the largest circle represents 7 neurons for that track. In monkey KO, the largest circles represent 8 neurons per track. The X- and Y-axes of each map (with scales in mm) correspond to the axes of the microdrive stage. AS, arcuate sulcus; CS, central sulcus; SPS, superior precentral sulcus.
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We made and examined plots of stimulus-related, set-related, and early and late movement-related activity, and of target-dependent, limb-dependent, and complex activity, and found no apparent differences in the spatial distributions of these different types of task-related activity within the MC recording area of either monkey; nor were any such differences apparent when the data from both monkeys were pooled and projected onto composite maps.
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DISCUSSION |
The primary purpose of this study was to determine whether neurons in MC participate in sensory and/or associative (context-dependent) processing of spatial information relevant to visually guided reaching movements. We found that substantial proportions of MC neurons did show behavior-correlated discharge, termed target dependent, that depended on the visuospatial target of the monkey's instructed reach, irrespective of the limb trajectory that was used to acquire the target. There were also substantial proportions of neurons with limb-dependent activity, discharging selectively in relation to the direction of limb movement, irrespective of target location. The relative frequencies of target-dependent versus limb-dependent activity varied considerably across the various task epochs. There was more target-dependent activity in the early stages of the
Abstract
Introduction
Abstract
