Two-stripe Tuohy, when too deep?
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Analgesia, Epidural↗
Biomedical subjects
S H Scott
Publications and source records attributed to S H Scott.
Dissociation between hand motion and population vectors from neural activity in motor cortex.
The population vector hypothesis was introduced almost twenty years ago to illustrate that a population vector constructed from neural activity in primary motor cortex (MI) of non-human primates could predict the direction of hand movement during reaching. Alternative explanations for this population signal have been suggested but could not be tested experimentally owing to movement complexity in the standard reaching model. We re-examined this issue by recording the activity of neurons in contralateral MI of monkeys while they made reaching movements with their right arms oriented in the horizontal plane-where the mechanics of limb motion are measurable and anisotropic. Here we found systematic biases between the population vector and the direction of hand movement. These errors were attributed to a non-uniform distribution of preferred directions of neurons and the non-uniformity covaried with peak joint power at the shoulder and elbow. These observations contradict the population vector hypothesis and show that non-human primates are capable of generating reaching movements to spatial targets even though population vectors based on MI activity do not point in the direction of hand motion.
Animals↗
Vision to action: new insights from a flip of the wrist.
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Animals↗
Neural activity in primary motor cortex related to mechanical loads applied to the shoulder and elbow during a postural task.
Whole-arm motor tasks performed by nonhuman primates have become a popular paradigm to examine neural activity during motor action, but such studies have traditionally related cell discharge to hand-based variables. We have developed a new robotic device that allows the mechanics of the shoulder and elbow joints to be manipulated independently. This device was used in the present study to examine neural activity in primary motor cortex (MI) in monkeys (Macaca mulatta) actively maintaining their hand at a central target as they compensated for loads applied to the shoulder and/or elbow. Roughly equal numbers of neurons were sensitive to mechanical loads only at the shoulder, only at the elbow, or loads at both joints. Neurons possessed two important properties. First, cell activity during multi-joint loads could be predicted from its activity during single-joint loads as a vector sum in a space defined by orthogonal axes for the shoulder and elbow. Second, most neurons were related to flexor torque at one joint coupled with extensor torque at the other, a distribution that paralleled the observed activity of forelimb muscles. These results illustrate that while MI activity may be described by independent axes representing each mechanical degree-of-freedom, neural activity is also strongly influenced by the specific motor patterns used to perform a given task.
Action Potentials↗
Morphometry of Macaca mulatta forelimb. I. Shoulder and elbow muscles and segment inertial parameters.
The present study examined the morphometric properties of the forelimb, including the inertial properties of the body segments and the morphometric parameters of 21 muscles spanning the shoulder and/or elbow joints of six Macaca mulatta and three M. fascicularis. Five muscle parameters are presented: optimal fascicle length (L(0)(M)), tendon slack length (L(S)(T)), physiological cross-sectional area (PCSA), pennation angle (alpha(0)), and muscle mass (m). Linear regressions indicate that muscle mass, and to a lesser extent PCSA, correlated with total body weight. Segment mass, center-of-mass, and the moment of inertia of the upper arm, forearm, and hand are also presented. Our data indicate that for some segments, radius of gyration (rho) predicts segment moment of inertia better than linear regressions based on total body weight. Key differences between the monkey and human forelimb are highlighted.
Animals↗
Population vectors and motor cortex: neural coding or epiphenomenon?
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Animals↗
Role of motor cortex in coordinating multi-joint movements: is it time for a new paradigm?
Reaching movements to spatial targets require motor patterns at the shoulder to be coordinated carefully with those at the elbow to smoothly move the hand through space. While the motor cortex is involved in this volitional task, considerable debate remains about how this cortical region participates in planning and controlling movement. This article reviews two opposing interpretations of motor cortical function during multi-joint movements. On the one hand, studies performed predominantly on single-joint movement generally support the notion that motor cortical activity is intimately involved in generating motor patterns at a given joint. In contrast, studies on reaching demonstrate correlations between motor cortical activity and features of movement related to the hand, suggesting that the motor cortex may be involved in more global features of the task. Although this latter paradigm involves a multi-joint motor task in which neural activity is correlated with features of movement related to the hand, this neural activity is also correlated to other movement variables. Therefore it is difficult to assess if and how the motor cortex contributes to the coordination of motor patterns at different joints. In particular, present paradigms cannot assess whether motor cortical activity contributes to the control of one joint or multiple joints during whole-arm tasks. The final point discussed in this article is the development of a new experimental device (KINARM) that can both monitor and manipulate the mechanics of the shoulder and elbow independently during multi-joint motor tasks. It is hoped that this new device will provide a new approach for examining how the motor cortex is involved in motor coordination.
Animals↗
An alternative interpretation of population vector rotation in macaque motor cortex.
Neural recordings from the primary motor cortex of monkeys performing movements at an angle to a cue stimulus have yielded two main results: (A) the population vector rotates from the direction of the cue to the direction of movement, and (B) cells with intermediate preferred directions are recruited during the middle of this rotation. These results have been interpreted as the neural correlates of a process of 'mental rotation'. Here we propose that results A and B are also consistent with an alternate hypothesis of 'response substitution', given four well known features of cortical neurophysiology.
Animals↗
Apparatus for measuring and perturbing shoulder and elbow joint positions and torques during reaching.
Visually guided reaching movements by monkeys has become an important paradigm for examining the function of various sensory and motor areas of the brain. However, a major problem with interpreting neural discharge during this motor task has been the difficulty to quantify and manipulate the mechanics of movement. To address this problem, a new experimental facility has been developed to allow neural recordings in a monkey while it makes movements with a mechanical linkage attached to its arm. The device (KINARM) has hinge joints aligned with the monkey's shoulder and elbow and allows the monkey to make arm movements in the horizontal plane. Custom-made fibreglass braces attach the linkage to the monkey's forearm and arm. Motors attached to the mechanical linkage provide angular position of the joints and apply torques either to the shoulder or elbow, or both. The KINARM is used in concert with a computer projection system that provides virtual targets in the plane of the arm. Preliminary results illustrate the ability of a monkey to perform a variety of multi-joint motor tasks under various static and dynamic loads.
Animals↗
The perceived impact of extraction and nonextraction treatments on matched samples of African American patients.
Orthodontic patients of African descent often seek some measure of reduction in profile protrusion. Lip retraction, however, seems to imply a need for premolar extraction. But in a nonextraction era, what does orthodontics have to offer the bimaxillary protrusion patient? To a considerable extent, it depends on what the patient wants and what treatment can provide. The present study was designed to address these questions by comparing the esthetic impact of extraction and nonextraction therapy on two morphologically similar samples of African American patients with Class I and II malocclusions. In addition to the patients, four panels of judges (black and white orthodontists and black and white laypersons) were asked to compare the pretreatment and posttreatment profile tracings and to quantify their perceptions of the esthetic impact of the change that occurred during treatment. Although there was a strong correlation among the various groups' ratings, there were a number of statistically significant differences. Premolar extraction led to a modest reduction in dental and soft tissue protrusion; nonextraction, by way of contrast, produced an increase. Presumably in response to these changes, all panels, independent of race or education, tended to prefer the posttreatment profiles, especially the posttreatment extraction profiles. There was, however, a significant interaction between race and treatment. The perceived effects of nonextraction treatment were modest and largely unrelated to the pretreatment profile, whereas the impact of premolar extraction was a highly significant function of initial protrusion: convex profiles were improved, but relatively straight profiles were made worse. The various panels, however, differed in their estimate of the point at which a profile might benefit from extraction (ie, profile reduction). For our white panelists, that point was reached when the lower lip was about 2 mm in front of Ricketts' E-plane. For our black panelists, the equilibrium point came at +4 mm. These findings imply that there is significant interaction among the type of treatment, the race of the patient, and both the race and professional background of the observer. Facial beauty, it would seem, is in the hand of the orthodontist, as well as in the eye of the beholder and the face of the beheld.
Black or African American↗
Hand and joint paths during reaching movements with and without vision.
This study examines whether the kinematics of pointing movements are altered by the sensory systems used to select spatial targets and to guide movement. Hand and joint paths of visually guided reaching movements of human subjects were compared with two non-visual conditions where only proprioception was available: (1) movements of the same subjects with blindfolds, and (2) movements by congenitally blind subjects. While hand-path curvatures were overall quite small, sighted subjects wearing a blindfold showed a statistical increase in hand-path curvature compared with their visually guided movements. Blindfolded subjects also showed greater hand-path curvature than blind subjects. These increases in hand-path curvature for blindfolded subjects did not always lead to a decrease in joint-path curvature. While there were differences between blind subjects and sighted subjects using vision for some movement directions, there was no systematic difference between these two groups. The magnitude of joint-path curvature showed much greater variation than hand-path curvature across the movement directions. We found variation in joint-path curvature to be correlated to two factors, one spatial and one geometrical. For all subject groups, joint-path curvature tended to be smaller for sagittal-plane movements than for transverse or diagonal movements. As well, we found that the magnitude of joint-path curvature was also related to the relative motion at each joint. Joint-path curvature tended to increase when movements predominantly involved changes in shoulder angle and was minimal when movements predominantly involved elbow motion. The consistently small curvatures of hand trajectory across blind and sighted subjects emphasize the powerful tendency of the motor system to generate goal-directed reaching movements with relatively straight hand trajectories, even when deprived of visual feedback from very early in life.
Adult↗
Cortical control of reaching movements.
Recent studies provide further support for the hypothesis that spatial representations of limb position, target locations, and potential motor actions are expressed in the neuronal activity in parietal cortex. In contrast, precentral cortical activity more strongly expresses processes involved in the selection and execution of motor actions. As a general conceptual framework, these processes may be interpreted in terms of such formalisms as sensorimotor transformations and 'internal models'.
Animals↗
Comparison of onset time and magnitude of activity for proximal arm muscles and motor cortical cells before reaching movements.
The activity of motor cortical cells and proximal arm muscles during the initiation of planar reaching movements were analyzed to identify whether features of coordinated motor patterns of muscles spanning the elbow and shoulder were evident in the discharge patterns of motor cortical cells. Shoulder and elbow muscles were divided into four groups, flexors and extensors at each joint. Features of the initial agonist activity, onset time and magnitude, at the shoulder and elbow were compared for movements in different spatial directions. As observed for human movements, differences in the onset time and the relative magnitude of electromyographic activity (EMG) of muscles acting about the shoulder and elbow were dependent on the direction of movement. Motor cortical cells were categorized as elbow or shoulder related on the basis of their response to passive movement of the joints. Differences in the onset time and the relative magnitude of activity of cells related to the shoulder and elbow were both dependent on the direction of movement and were similar to those observed for muscles spanning these joints. There was a modest, but significant correlation between the onset time and magnitude of EMG for individual muscles. A similar magnitude-time coupling was observed for individual motor cortical cells. Variations in the discharge pattern of motor cortical cells before movement that mirror those observed for muscles spanning the shoulder and elbow support the potential role of primary motor cortex in the selection, timing, and magnitude of agonist motor patterns at the shoulder and elbow to initiate reaching movements.
Animals↗
Reaching movements with similar hand paths but different arm orientations. I. Activity of individual cells in motor cortex.
This study shows that the discharge of many motor cortical cells is strongly influenced by attributes of movement related to the geometry and mechanics of the arm and not only by spatial attributes of the hand trajectory. The activity of 619 directionally tuned cells was recorded from the motor cortex of two monkeys during reaching movements with the use of similar hand paths but two different arm orientations, in the natural parasagittal plane and abducted into the horizontal plane. Nearly all cells (588 of 619, 95%) showed statistically significant changes in activity between the two arm orientations [analysis of variance (ANOVA). P < 0.01]. A majority of cells showed a significant change in their overall level of activity (ANOVA, main effect of task, P < 0.01) between arm orientations before, during, and after movement. Many cells (433 of 619, 70%) also showed a significant change in the relation of their discharge with movement direction (ANOVA, task x direction interaction term, P < 0.01) during movement, including changes in the dynamic range of discharge with movement and changes in the directional preference of cells that were directionally tuned in both arm orientations. Similar effects were seen for the discharge of cells while the monkey maintained constant arm postures over the different peripheral targets with the use of different arm orientations. Repeated data files from the same cell with the use of the same arm orientation showed only small changes in the level of discharge or in directional tuning, suggesting that changes in cell discharge between arm orientations cannot be explained by random temporal variations in cell activity. The distribution of movement-related preferred directions of the whole sample differed between arm orientations, and also differed strongly between cells receiving passive input predominantly from the shoulder or elbow. The electromyographic activity of most prime mover muscles at the shoulder and elbow was also strongly affected by arm orientation, resulting in changes in overall level of activity and/or directional tuning that often resembled those of the proximal arm-related motor cortical cells. A mathematical model that represented movements in terms of movement direction centered on the hand could not account for any of the arm-orientation-related response changes seen in this task, whereas models in intrinsic parameter spaces of joint kinematics and joint torques predicted many of the effects.
Analysis of Variance↗
Reaching movements with similar hand paths but different arm orientations. II. Activity of individual cells in dorsal premotor cortex and parietal area 5.
Reaching movements with similar hand paths but different arm orientations. II. Activity of individual cells in dorsal premotor cortex and parietal area 5. J. Neurophysiol. 78: 2413-2426, 1997. Neuronal activity in primary motor cortex (MI) is altered when monkeys make reaching movements along similar handpaths at shoulder level with two different arm orientations, either in the natural orientation with the elbow positioned below the level of the shoulder and hand or in an abducted orientation with the elbow abducted nearly to shoulder level. The present study examines to what degree two other cortical areas, the dorsal premotor (PMd) and parietal area 5, also show modulation of cell activity related to arm geometry during reaching. The activity of most (89%) of the 207 cells in PMd recorded while monkeys made reaching movements showed a statistically significant change in activity between orientations [analysis of variation (ANOVA), P < 0.01]. A common effect of arm orientation on cell activity was a change in the overall level of discharge either before, during, and/or after movement (67%, ANOVA, task main effect, P < 0.01). Many cells (76%) showed a statistical change in their response to movement direction (ANOVA, task x direction interaction term, P < 0.01), including changes in dynamic range and changes in the preferred direction of cells that were directionally tuned in both arm orientations. Overall, these effects were similar qualitatively but not as strong quantitatively as those observed in MI. A sample of cells was recorded in area 5 of one monkey. Most (95%) of the 79 area 5 cells showed a change in activity when reaching movements were performed using different arm orientations (ANOVA, P < 0.01). As in PMd and MI, many area 5 cells (56, 71%) showed changes in their tonic discharge before, during, and/or after movement, and 70 cells (89%) showed changes in their response to movement direction (ANOVA, task x direction interaction term, P < 0.01). The observed changes in neuronal activity related to posture and movement in MI, PMd and area 5 demonstrate that single-cell activity in these cortical areas is not simply related to the spatial attributes of hand trajectory but is also strongly influenced by attributes of movement related to arm geometry.
Analysis of Variance↗
Mechanics of feline soleus: I. Effect of fascicle length and velocity on force output.
The aim of the present study was to quantify how fascicle length and velocity modify force production in cat soleus. A computerized muscle puller controlled the length and velocity of the whole-muscle. We recorded the force output at the tendon and the length of muscle fascicles using sonomicrometry during whole-muscle isometric and isokinetic contractions. Peak muscle stress was estimated as 31.8 +/- 4.1 N cm-2 (mean and SD) and optimal fascicle length, Lo, was estimated as 3.8 +/- 0.6 cm which corresponds to an optimal sarcomere length of 2.49 +/- 0.08 microns. The isometric force-length data followed closely the expected force-length relationship for cat sarcomeres. The force-velocity relationship was found to be similar in shape between cats, but the per cent increment of force over isometric levels for lengthening contractions was highly variable. Estimates of the kinematics of the fascicles based on whole-muscle length were systematically incorrect; whole-muscle velocity was 21% greater than fascicle velocity. The force-velocity data demonstrated consistent dependencies on fascicle length. At lengths below 0.7 Lo (1.74 microns), the shape of the force-velocity relationship was altered by the inclusion of a passive, repulsive force in the estimate of active isometric force. The shape of the force-velocity relationship changed at lengths greater than 0.7 Lo, but was restricted to lengthening velocities where the increment of force with respect to isometric levels was found to increase with fascicle length. This change in shape in the force-velocity relationship for lengthening contractions reveals a systematic, but previously unknown interdependence between fascicle length and velocity on muscle force production.
Animals↗
Mechanics of feline soleus: II. Design and validation of a mathematical model.
We have developed a mathematical model to describe force production in cat soleus during steady-state activation over a range of fascicle lengths and velocities. The model was based primarily upon a three element design by Zajac but also considered the many different features present in other previously described models. We compared quantitatively the usefulness of these features and putative relationships to account for a set of force and length data from cat soleus wholemuscle described in a companion paper. Among the novel features that proved useful were the inclusion of a short-length passive force resisting compression, a new normalisation constant for connective-tissue lengths to replace the potentially troublesome slack length, and a new length dependent term for lengthening velocities in the force-velocity relationship. Each feature of this model was chosen to provide the most accurate description of the data possible without adding unneeded complexity. Previously described functions were compared with novel functions to determine the best description of the experimental data for each of the elements in the model.
Animals↗