Early stages of sensorimotor map acquisition: neurochemical signature in primary motor cortex and its relation to functional connectivity.

The earliest stages of sensorimotor learning involve learning the correspondence between movements and sensory results-a sensorimotor map. The present exploratory study investigated the neurochemical underpinnings of map acquisition by monitoring 25 participants as they acquired a new association between movements and sounds. Functional magnetic resonance spectroscopy was used to measure neurochemical concentrations in the left primary motor cortex during learning. Resting-state functional magnetic resonance imaging data were also collected before and after training to assess learning-related changes in functional connectivity. There were monotonic increases in γ-aminobutyric acid (GABA) and decreases in glucose during training, which extended into the subsequent rest period and, importantly, in the case of GABA correlated with the amount of learning: participants who showed greater behavioral learning showed greater GABA increase. The GABA change was furthermore correlated with changes in functional connectivity between the primary motor cortex and a cluster of voxels in the right intraparietal sulcus: greater increases in GABA were associated with greater strengthening of connectivity. Transiently, there were increases in lactate and reductions in aspartate, which returned to baseline at the end of training, but only lactate showed a statistical trend to correlate with the amount of learning. In summary, during the earliest stages of sensorimotor learning, GABA levels are linked on a subject-level basis to both behavioral learning and a strengthening of functional connections that persists beyond the training period. The findings are consistent with the idea that GABA-mediated inhibition is linked to maintenance of newly learned information.NEW & NOTEWORTHY Learning the mapping between movements and their sensory effects is a necessary step in the early stages of sensorimotor learning. There is evidence showing which brain areas are involved in early motor learning, but their role remains uncertain. Here, we show that GABA, a neurotransmitter linked to inhibitory processing, rises during and after learning and is involved in ongoing changes in resting-state networks.

Early stages of sensorimotor map acquisition: learning with free exploration, without active movement or global structure.

One of the puzzles of learning to talk or play a musical instrument is how we learn which movement produces a particular sound: an audiomotor map. The initial stages of map acquisition can be studied by having participants learn arm movements to auditory targets. The key question is what mechanism drives this early learning. Three learning processes from previous literature were tested: map learning may rely on active motor outflow (target), on error correction, and on the correspondence between sensory and motor distances (i.e., that similar movements map to similar sounds). Alternatively, we hypothesized that map learning can proceed without these. Participants made movements that were mapped to sounds in a number of different conditions that each precluded one of the potential learning processes. We tested whether map learning relies on assumptions about topological continuity by exposing participants to a permuted map that did not preserve distances in auditory and motor space. Further groups were tested who passively experienced the targets, kinematic trajectories produced by a robot arm, and auditory feedback as a yoked active participant (hence without active motor outflow). Another group made movements without receiving targets (thus without experiencing errors). In each case we observed substantial learning, therefore none of the three hypothesized processes is required for learning. Instead early map acquisition can occur with free exploration without target error correction, is based on sensory-to-sensory correspondences, and possible even for discontinuous maps. The findings are consistent with the idea that early sensorimotor map formation can involve instance-specific learning.NEW & NOTEWORTHY This study tested learning of novel sensorimotor maps in a variety of unusual circumstances, including learning a mapping that was permuted in such as way that it fragmented the sensorimotor workspace into discontinuous parts, thus not preserving sensory and motor topology. Participants could learn this mapping, and they could learn without motor outflow or targets. These results point to a robust learning mechanism building on individual instances, inspired from machine learning literature.

Tactile shape processing.

Neuroimaging techniques may aid in the identification of areas of the human brain that are involved in tactile shape perception. Bodegård et al. (2001) relate differences in the properties of tactile stimuli to differences in areas of cortical activation to infer tactile processing in the somatosensory network.

Compensation for the effects of head acceleration on jaw movement in speech.

Recent studies have demonstrated the ability of subjects to adjust the control of limb movements to counteract the effects of self-generated loads. The degree to which subjects change control signals to compensate for these loads is a reflection of the extent to which forces affecting movement are represented in motion planning. Here, we have used empirical and modeling studies to examine whether the nervous system compensates for loads acting on the jaw during speech production. As subjects walk, loads to the jaw vary with the direction and magnitude of head acceleration. We investigated the patterns of jaw motion resulting from these loads both in locomotion alone and when locomotion was combined with speech production. In locomotion alone, jaw movements were shown to vary systematically in direction and magnitude in relation to the acceleration of the head. In contrast, when locomotion was combined with speech, variation in jaw position during both consonant and vowel production was substantially reduced. Overall, we have demonstrated that the magnitude of load associated with head acceleration during locomotion is sufficient to produce a systematic change in the position of the jaw. The absence of variation in jaw position during locomotion with speech is thus consistent with the idea that in speech, the control of jaw motion is adjusted in a predictive manner to offset the effects of head acceleration.

Effects of gravitational load on jaw movements in speech.

External loads arising as a result of the orientation of body segments relative to gravity can affect the achievement of movement goals. The degree to which subjects adjust control signals to compensate for these loads is a reflection of the extent to which forces affecting motion are represented neurally. In the present study we assessed whether subjects, when speaking, compensate for loads caused by the orientation of the head relative to gravity. We used a mathematical model of the jaw to predict the effects of control signals that are not adjusted for changes to head orientation. The simulations predicted a systematic change in sagittal plane jaw orientation and horizontal position resulting from changes to the orientation of the head. We conducted an empirical study in which subjects were tested under the same conditions. With one exception, empirical results were consistent with the simulations. In both simulation and empirical studies, the jaw was rotated closer to occlusion and translated in an anterior direction when the head was in the prone orientation. When the head was in the supine orientation, the jaw was rotated away from occlusion. The findings suggest that the nervous system does not completely compensate for changes in head orientation relative to gravity. A second study was conducted to assess possible changes in acoustical patterns attributable to changes in head orientation. The frequencies of the first (F1) and second (F2) formants associated with the steady-state portion of vowels were measured. As in the kinematic study, systematic differences in the values of F1 and F2 were observed with changes in head orientation. Thus the acoustical analysis further supports the conclusion that control signals are not completely adjusted to offset forces arising because of changes in orientation.

Characteristics of velocity profiles of speech movements.

The control of individual speech gestures was investigated by examining laryngeal and tongue movements during vowel and consonant production. A number of linguistic manipulations known to alter the durational characteristics of speech (i.e., speech rate, lexical stress, and phonemic identity) were tested. In all cases a consistent pattern was observed in the kinematics of the laryngeal and tongue gestures. The ratio of maximum instantaneous velocity to movement amplitude, a kinematic index of mass-normalized stiffness, was found to increase systematically as movement duration decreased. Specifically, the ratio of maximum velocity to movement amplitude varied as a function of a parameter, C, times the reciprocal of movement duration. The conformity of the data to this relation indicates that durational change is accomplished by scalar adjustment of a base velocity form. These findings are consistent with the idea that kinematic change is produced by the specification of articulator stiffness.

Similarities in the control of the speech articulators and the limbs: kinematics of tongue dorsum movement in speech.

The kinematics of tongue dorsum movements in speech were studied with pulsed ultrasound to assess similarities in the voluntary control of the speech articulators and the limbs. The stimuli were consonant--vowel syllables in which speech rate and stress were varied. The kinematic patterns for tongue dorsum movements were comparable to those observed in the rapid movement of the arms and hands. The maximum velocity of tongue dorsum raising and lowering was correlated with the extent of the gesture. The slope of the relationship differed for stressed and unstressed vowels but was unaffected by differences in speech rate. At each stress level the correlation between displacement and peak velocity was accompanied by a relatively constant interval from the initiation of the movement to the point of maximum velocity. The data are discussed with reference to systems that can be described with second-order differential equations. The increase in the slope of the displacement/peak-velocity relationship for unstressed versus stressed vowels is suggestive of a tonic increase in articulator stiffness. Variations in displacement are attributed to the level of phasic activity in the muscles producing the gesture.

Human jaw movement in mastication and speech.

The study of jaw movement in humans is a primary source of information about the relationship between voluntary movement and more primitive motor functions. This study focused on the geometric form of the velocity function, as measured by linear voltage displacement transducer. Movement amplitudes, maximum velocities and durations were greater in mastication than in speech. Nevertheless, there were detailed similarities in the shape of the normalized velocity functions. In jaw-closing movements, the normalized functions were similar in form over differences in rate, movement amplitude (speech movements) and the compliance of the bolus (mastication). In opening movements, the functions for mastication and speech were again similar over differences in amplitude and compliance. However, they differed in shape for fast and slow movements. Normalized acceleration and deceleration durations were approximately equal in rapid movements, whereas, for slower movements, deceleration took substantially longer than acceleration.

A system for three-dimensional visualization of human jaw motion in speech.

With the development of precise three-dimensional motion measurement systems and powerful computers for three-dimensional graphical visualization, it is possible to record and fully reconstruct human jaw motion. In this paper, we describe a visualization system for displaying three-dimensional jaw movements in speech. The system is designed to take as input jaw motion data obtained from one or multi-dimensional recording systems. In the present application, kinematic records of jaw motion were recorded using an optoelectronic measurement system (Optotrak). The corresponding speech signal was recorded using an analog input channel. The three orientation angles and three positions that describe the motion of the jaw as a rigid skeletal structure were derived from the empirical measurements. These six kinematic variables, which in mechanical terms account fully for jaw motion kinematics, act as inputs that drive a real-time three-dimensional animation of a skeletal jaw and upper skull. The visualization software enables the user to view jaw motion from any orientation and to change the viewpoint during the course of an utterance. Selected portions of an utterance may be replayed and the speed of the visual display may be varied. The user may also display, along with the audio track, individual kinematic degrees of freedom or several degrees of freedom in combination. The system is presently being used as an educational tool and for research into audio-visual speech recognition. Interested researchers may obtain the software and source code free of charge from the authors.

An examination of the degrees of freedom of human jaw motion in speech and mastication.

The kinematics of human jaw movements were assessed in terms of the three orientation angles and three positions that characterize the motion of the jaw as a rigid body. The analysis focused on the identification of the jaw's independent movement dimensions, and was based on an examination of jaw motion paths that were plotted in various combinations of linear and angular coordinate frames. Overall, both behaviors were characterized by independent motion in four degrees of freedom. In general, when jaw movements were plotted to show orientation in the sagittal plane as a function of horizontal position, relatively straight paths were observed. In speech, the slopes and intercepts of these paths varied depending on the phonetic material. The vertical position of the jaw was observed to shift up or down so as to displace the overall form of the sagittal plane motion path of the jaw. Yaw movements were small but independent of pitch, and vertical and horizontal position. In mastication, the slope and intercept of the relationship between pitch and horizontal position were affected by the type of food and its size. However, the range of variation was less than that observed in speech. When vertical jaw position was plotted as a function of horizontal position, the basic form of the path of the jaw was maintained but could be shifted vertically. In general, larger bolus diameters were associated with lower jaw positions throughout the movement. The timing of pitch and yaw motion differed. The most common pattern involved changes in pitch angle during jaw opening followed by a phase predominated by lateral motion (yaw). Thus, in both behaviors there was evidence of independent motion in pitch, yaw, horizontal position, and vertical position. This is consistent with the idea that motions in these degrees of freedom are independently controlled.

Compensation for loads during arm movements using equilibrium-point control.

A significant problem in motor control is how information about movement error is used to modify control signals to achieve desired performance. A potential source of movement error and one that is readily controllable experimentally relates to limb dynamics and associated movement-dependent loads. In this paper, we have used a position control model to examine changes to control signals for arm movements in the context of movement-dependent loads. In the model, based on the equilibrium-point hypothesis, equilibrium shifts are adjusted directly in proportion to the positional error between desired and actual movements. The model is used to simulate multi-joint movements in the presence of both "internal" loads due to joint interaction torques, and externally applied loads resulting from velocity-dependent force fields. In both cases it is shown that the model can achieve close correspondence to empirical data using a simple linear adaptation procedure. An important feature of the model is that it achieves compensation for loads during movement without the need for either coordinate transformations between positional error and associated corrective forces, or inverse dynamics calculations.

Independent coactivation of shoulder and elbow muscles.

The aim of this study was to examine the possibility of independent muscle coactivation at the shoulder and elbow. Subjects performed rapid point-to-point movements in a horizontal plane from different initial limb configurations to a single target. EMG activity was measured from flexor and extensor muscles acting at the shoulder (pectoralis clavicular head and posterior deltoid) and elbow (biceps long head and triceps lateral head) and flexor and extensor muscles acting at both joints (biceps short head and triceps long head). Muscle coactivation was assessed by measuring tonic levels of electromyographic (EMG) activity after limb position stabilized following the end of the movements. It was observed that tonic EMG levels following movements to the same target varied as a function of the amplitude of shoulder and elbow motion. Moreover, for the movements tested here, the coactivation of shoulder and elbow muscles was found to be independent - tonic EMG activity of shoulder muscles increased in proportion to shoulder movement, but was unrelated to elbow motion, whereas elbow and double-joint muscle coactivation varied with the amplitude of elbow movement and were not correlated with shoulder motion. In addition, tonic EMG levels were higher for movements in which the shoulder and elbow rotated in the same direction than for those in which the joints rotated in opposite directions. In this respect, muscle coactivation may reflect a simple strategy to compensate for forces introduced by multijoint limb dynamics.

Control of rate and duration of speech movements.

A computerized pulsed-ultrasound system was used to monitor tongue dorsum movements during the production of consonant-vowel sequences in which speech rate, vowel, and consonant were varied. The kinematics of tongue movement were analyzed by measuring the lowering gesture of the tongue to give estimates of movement amplitude, duration, and maximum velocity. All three subjects in the study showed reliable correlations between the amplitude of the tongue dorsum movement and its maximum velocity. Further, the ratio of the maximum velocity to the extent of the gesture, a kinematic indicator of articulator stiffness, was found to vary inversely with the duration of the movement. This relationship held both within individual conditions and across all conditions in the study such that a single function was able to accommodate a large proportion of the variance due to changes in movement duration. As similar findings have been obtained both for abduction and adduction gestures of the vocal folds and for rapid voluntary limb movements, the data suggest that a wide range of changes in the duration of individual movements might all have a similar origin. The control of movement rate and duration through the specification of biomechanical characteristics of speech articulators is discussed.

Computerized measurement of tongue dorsum movements with pulsed-echo ultrasound.

A computerized system for the measurement of tongue dorsum movements with pulsed echo ultrasound is described. The presentation focuses on technical and methodological considerations in the on-line acquisition of vertical tongue movement information, its digital processing and display. Problems associated with transducer placement, peak detection, data averaging, and curve fitting are considered, and validation procedures based on x ray and indicators of measurement reliability are reported. The discussion centers on advantages and disadvantages of the technique and its applications.

Superior lateral pharyngeal wall movements in speech.

Medial movements of the lateral pharyngeal wall at the level of the velopharyngeal port were examined by using a computerized ultrasound system. Subjects produced CVNVC sequences involving all combinations of the vowels /a/ and /u/ and the nasal consonants /n/ and /m/. The effects of both vowels on the CVN and NVC gestures (opening and closing of the velopharyngeal port, respectively) were assessed in terms of movement amplitude, duration, and movement onset time. The amplitude of both opening and closing gestures of the lateral pharyngeal wall was less in the context of the vowel /u/ than the vowel /a/. In addition, the onset of the opening gesture towards the nasal consonant was related to the identity of both the initial and the final vowels. The characteristics of the functional coupling of the velum and lateral pharyngeal wall in speech are discussed.

Control of jaw orientation and position in mastication and speech.

1. The kinematics of sagittal-plane jaw motion were assessed in mastication and speech. The movement paths were described in joint coordinates, in terms of the component rotations and translations. The analysis focused on the relationship between rotation and horizontal translation. Evidence was presented that these can be separately controlled. 2. In speech, jaw movements were studied during consonant-vowel utterances produced at different rates and volumes. In mastication, bolus placement, compliance, and size as well as chewing rate were manipulated. Jaw movements were recorded using the University of Wisconsin X-ray microbeam system. Jaw rotation and translation were calculated on the basis of the motion of X-ray tracking pellets on the jaw. 3. The average magnitudes of jaw rotation and translation were greater in mastication than in speech. In addition, in speech, it was shown that the average rotation magnitude may vary independent of the horizontal translation magnitude. In mastication, the average magnitude of vertical jaw translation was not dependent on the magnitudes of jaw rotation or horizontal jaw translation. 4. The magnitude of rotation and horizontal jaw translation tended to be correlated when examined on a trial by trial basis. Some subjects also showed a correlation between jaw rotation and vertical jaw translation. However, the proportion of variance accounted for was greater for all subjects in the case of rotation and horizontal translation. 5. Joint space paths in both mastication and speech were found to be straight. The pattern was observed at normal and fast rates of speech and mastication and for loud speech as well. Straight line paths were also observed when subjects produced utterances that had both the syllabic structure and the intonation pattern of speech. The findings suggest that control may be organized in terms of an equilibrium jaw orientation and an equilibrium jaw position. 6. Departures from linearity were also observed. These were typically associated with differences during jaw closing in the end time of rotation and translation. Start time differences were not observed in jaw closing and the movement paths were typically linear within this region.

Coordination of mono- and bi-articular muscles in multi-degree of freedom elbow movements.

We investigated the coordination of mono- and bi-articular muscles during movements involving one or more degrees of freedom at the elbow. Subjects performed elbow flexion (or extension) alone, forearm pronation (or supination) alone, and combinations of the two. In bi-articular muscles such as biceps brachii and pronator teres, the amplitude of agonist electromyographic (EMG) activity was dependent on motion in the two degrees of freedom. Agonist burst amplitudes for combined movements were approximately the sum of the agonist burst amplitudes for movements in the individual degrees of freedom. Activity levels in individual degrees of freedom were, in turn, greater than activity levels observed when a muscle acted as agonist in one degree of freedom and antagonist in the other. Other muscles such as triceps, brachialis, and pronator quadratus acted primarily during motion in a single degree of freedom. The relative magnitude and the timing of activity between sets of muscles also changed with motion in a second degree of freedom. These patterns are comparable with those reported previously in isometric studies.

Coordination of multiple muscles in two degree of freedom elbow movements.

The present study quantifies electromyographic (EMG) magnitude, timing, and duration in one and two degree of freedom elbow movements involving combinations of flexion-extension and pronation-supination. The aim is to understand the organization of commands subserving motion in individual and multiple degrees of freedom. The muscles tested in this study fell into two categories with respect to agonist burst magnitude: those whose burst magnitude varied with motion in a second degree of freedom at the elbow, and those whose burst magnitude depended on motion in one degree of freedom only. In multiarticular muscles contributing to motion in two degrees of freedom at the elbow, we found that the magnitude of the agonist burst was greatest for movements in which a muscle acted as agonist in both degrees of freedom. The burst magnitudes for one degree of freedom movements were, in turn, greater than for movements in which the muscle was agonist in one degree of freedom and antagonist in the other. It was also found that, for movements in which a muscle acted as agonist in two degrees of freedom, the burst magnitude was, in the majority of cases, not different from the sum of the burst magnitudes in the component movements. When differences occurred, the burst magnitude for the combined movement was greater than the sum of the components. Other measures of EMG activity such as burst onset time and duration were not found to vary in a systematic manner with motion in these two degrees of freedom. It was also seen that several muscles which produced motion in one degree of freedom at the elbow, including triceps brachii (long head), triceps brachii (lateral head), and pronator quadratus displayed first agonist bursts whose magnitude did not vary with motion in a second degree of freedom. However, for the monoarticular elbow flexors brachialis and brachioradialis, agonist burst magnitude was affected by pronation or supination. Lastly, it was observed that during elbow movements in which muscles acted as agonist in one degree of freedom and antagonist in the other, the muscle activity often displayed both agonist and antagonist components in the same movement. It was found that, for pronator teres and biceps brachii, the timing of the bursts was such that there was activity in these muscles concurrent with activity in both pure agonists and pure antagonists.(ABSTRACT TRUNCATED AT 400 WORDS)

Origins of the power law relation between movement velocity and curvature: modeling the effects of muscle mechanics and limb dynamics.

1. When subjects trace patterns such as ellipses, the instantaneous velocity of movements is related to the instantaneous curvature of the trajectories according to a power law-movements tend to slow down when curvature is high and speed up when curvature is low. It has been proposed that this relationship is centrally planned. 2. The arm's muscle properties and dynamics can significantly affect kinematics. Even under isometric conditions, muscle mechanical properties can affect the development of muscle forces and torques. Without a model that accounts for these effects, it is difficult to distinguish between kinematic patterns that are attributable to central control and patterns that arise because of dynamics and muscle properties and are not represented in the underlying control signals. 3. In this paper we address the nature of the control signals that underlie movements that obey the power law. We use a numerical simulation of arm movement control based on the lambda version of the equilibrium point hypothesis. We demonstrate that simulated elliptical and circular movements, and elliptical force trajectories generated under isometric conditions, obey the power law even though there was no relation between curvature and speed in the modeled control signals. 4. We suggest that limb dynamics and muscle mechanics-specifically, the springlike properties of muscles-can contribute significantly to the emergence of the power law relationship in kinematics. Thus, without a model that accounts for these effects, care must be taken when making inferences about the nature of neural control.

Lower pharyngeal wall coarticulation in VCV syllables.

Speech movements of the lower pharyngeal wall were recorded in two subjects using pulsed-echo ultrasound. The focus of the study was the pattern of coarticulation of pharyngeal wall movements. Using nonsense utterances as test material, both anticipatory and carryover coarticulatory effects were observed. The identity of the final vowel in VCV sequences affected the kinematic characteristics of the initial VC transition. Both the amplitude and the duration of the movement between the initial vowel and the consonant were greater when the final vowel was /u/ rather than /a/. Similarly, the initial vowel affected the kinematic characteristics of the final CV transition. The amplitude of the movement from the consonant to the final vowel was greater with the initial vowel /u/ as opposed to /a/. The coarticulatory patterns observed in this study are similar to those previously reported for the tongue dorsum and upper pharynx [Parush et al., J. Acoust. Soc. Am. 74, 1115-1125 (1983); Parush and Ostry, J. Acoust. Soc. Am. 80, 749-756 (1986)].