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, 21 (8), 2784-92

Consistent Features in the Forelimb Representation of Primary Motor Cortex in Rhesus Macaques

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Consistent Features in the Forelimb Representation of Primary Motor Cortex in Rhesus Macaques

M C Park et al. J Neurosci.

Abstract

The purpose of this study was to systematically map the forelimb area of primary motor cortex (M1) in rhesus macaques in an effort to investigate further the organization of motor output to distal and proximal muscles. We used stimulus-triggered averaging (StTAing) of electromyographic activity to map the cortical representation of 24 simultaneously recorded forelimb muscles. StTAs were obtained by applying 15 microA stimuli to M1 sites while the monkey performed a reach and prehension task. Motor output to body regions other than the forelimb (e.g., face, trunk, and hindlimb) was identified using repetitive intracortical microstimulation to evoke movements. Detailed, muscle-based maps of M1 revealed a central core of distal (wrist, digit, and intrinsic hand) muscle representation surrounded by a "horseshoe"-shaped zone of proximal (shoulder and elbow) muscle representation. The core distal and proximal zones were separated by a relatively large region representing combinations of both distal and proximal muscles. On the basis of its size and characteristics, we argue that this zone is not simply the result of stimulus-current spread, but rather a distinct zone within the forelimb representation containing cells that specify functional synergies of distal and proximal muscles. Electrode tracks extending medially from the medial arm of the proximal muscle representation evoked trunk and hindlimb responses. No distal or proximal muscle poststimulus effects were found in this region. These results argue against the existence of a second, major noncontiguous distal or proximal forelimb representation located medially within the macaque M1 representation.

Figures

Fig. 1.
Fig. 1.
Examples of stimulus-triggered averages of rectified EMG activity (15 μA at 15 Hz) illustrating the criteria used for categorizing the strength of poststimulus facilitation (PStF) effects. Time 0 corresponds to the stimulus event used as a trigger for averaging. A pretrigger period of 12.5 msec (baseline) was used to determine mean baseline activity (mean) and SD for each average.A, Peaks <2 SD of baseline and peaks that remained >2 SD for <0.75 msec period were considered insignificant, and the average was categorized as having no effect. B, Weak effects had peaks >2 SD of mean baseline activity but ≤5 SD of mean baseline activity. C, Moderate and strong effects had peaks >5 SD of mean baseline activity. LAT, Latissimus dorsi; FDI, first dorsal interosseus;APB, abductor pollicis brevis. The numberof trigger events is given in parentheses.
Fig. 2.
Fig. 2.
Method for flattening and unfolding cortical layer V in the anterior bank of the central sulcus for two-dimensional representation. A, Parasagittal diagram representing the stimulated cortex, constructed from electrode tracks with the same M-L coordinate (4L) organized in an anterior-to-posterior order. Identification of white matter–gray matter border (thin line), central sulcus (CS), and sensory cortex were based on electrophysiological observations. Then, reference lines (dashed lines), similar to those used by Sato and Tanji (1989), were placed onto the diagram to represent cortical layer V. Ahorizontal reference line(h) was placed at 1.5 mm depth, and avertical reference line (v) was placed at 1.5 mm from the estimated central sulcus to approximate the position of layer V. The intersection of the reference lines (asterisk) was defined to be the convexity of the precentral gyrus. Projecting the selected stimulation sites (●) onto the reference lines, when necessary, flattened the curvature of layer V. In the case of track 2P, stimulation at a depth of 1.0 mm (●) is projected onto the reference line h (○). In this track, the 1.5 mm site was not used because the effects at 1.0 mm were much stronger. B, Flattened and unfolded two-dimensional map of cortical layer V. Layer V was unfolded by rotating the reference line v with respect to the convexity of the precentral gyrus (asterisk). This straightened the two reference lines into one line (dashed line). Completing this manipulation for all tracks yielded a two-dimensional map of M1. C, Tracing of an MR image through the same parasagittal plane as 4L with overlaid electrode tracks and stimulation sites corresponding to layer V (●). A, Anterior;P, posterior; L, lateral;M, medial.
Fig. 3.
Fig. 3.
A, B, Cortical recording chambers implanted over M1 cortex in monkeys D and M, respectively. The coordinate system (5 mm grid) is overlaid in yellow, and locations of electrode tracks are indicated with black-outlined red dots. Thelarge black rectangle overlying each chamber identifies the cortical area represented in maps C–F. In monkey D, a 15 mm incision was made in the dura for visual identification of the central sulcus. CF, Maps of motor cortex for two monkeys represented in two-dimensional coordinates after unfolding the precentral gyrus. C, D, Maps for monkeys D and M, respectively, based on strong and moderate PStF effects together with R-ICMS-evoked movements. E,F, Maps for monkeys D and M, respectively, based on weak, moderate, and strong effects together with R-ICMS-evoked movements.
Fig. 4.
Fig. 4.
Distribution of PStF effects in forelimb muscles from a PDC site. Time 0 corresponds to the stimulus event used for the average. Stimulation was 15 μA at 15 Hz. Moderate and strong PStF effects were observed in both proximal (BIS,BIL, BRA, BR,TLON, PEC) and distal (APB, FDI, FDP,ED23) forelimb muscles. The range of number of trigger events for different channels is given in parentheses.
Fig. 5.
Fig. 5.
Stimulus-triggered averages of rectified EMG activity (15 μA at 15 Hz) of a proximal muscle, biceps shorthead (BIS), for monkey D plotted on a two-dimensional map of M1. Each record is color coded for strength of PStF according to the number of SDs above pre-trigger baseline activity (color bar at top of Figure). The color-shaded motor representation map is taken from Figure 3E.Yellow line, Convexity of the precentral gyrus;black dashed line, fundus.
Fig. 6.
Fig. 6.
Stimulus-triggered averages of rectified EMG activity (15 μA at 15 Hz) of a distal muscle, extensor digitorum communis (EDC), for monkey D plotted on a two-dimensional map of M1. Each record is color coded for strength of PStF according to the number of SDs above pre-trigger baseline activity (color bar at top of Figure). The color-shaded motor representation map is taken from Figure3E. Yellow line, Convexity of the precentral gyrus; black dashed line, fundus.
Fig. 7.
Fig. 7.
Summary of proximal and distal forelimb muscle representation in M1 of rhesus macaques based on the motor output effects in stimulus-triggered averages. Three separable zones are consistent across monkeys, a central zone oriented mediolaterally along the caudal border of M1 representing distal muscle synergies, surrounded by a zone representing synergies involving both distal and proximal muscles, which, in turn, is surrounded by a zone representing only proximal muscle synergies. The latter two zones are not complete circles; rather, they are horseshoe-shaped and open at the 3a/4 border. The proximal muscle forelimb representation in M1 is bordered laterally by a face representation and medially by a trunk and hindlimb representation. We found no evidence supporting the existence of major distal or proximal muscle representations, separate from the core representations. A, Anterior; L, lateral;CS, central sulcus; Fundus, fundus of the central sulcus; P, proximal; D, distal;PDC, proximal–distal cofacilitation.

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