Motor Equivalence Research Articles

Lip-jaw and tongue-jaw coordination during rate-controlled syllable repetitions.

The present study investigated the relationship between functionally relevant compound gestures and single-articulator component movements of the jaw and the constrictors lower lip and tongue tip during rate-controlled syllable repetitions. In nine healthy speakers, the effects of speaking rate (3 vs 5 Hz), place of articulation, and vowel type during stop consonant-vowel repetitions (/pa/, /pi/, /ta/, /ti/) on the amplitude and peak velocity of differential jaw and constrictor opening-closing movements were measured by means of electromagnetic articulography. Rather than homogeneously scaled compound gestures, the results suggest distinct control mechanisms for the jaw and the constrictors. In particular, jaw amplitude was closely linked to vowel height during bilabial articulation, whereas the lower lip component amplitude turned out to be predominantly rate sensitive. However, the observed variability across subjects and conditions does not support the assumption that single-articulator gestures directly correspond to basic phonological units. The nonhomogeneous effects of speech rate on articulatory subsystem parameters indicate that single structures are differentially rate sensitive. On average, an increase in speech rate resulted in a more or less proportional increase of the steepness of peak velocity/amplitude scaling for jaw movements, whereas the constrictors were less rate sensitive in this respect. Negative covariation across repetitions between jaw and constrictor amplitudes has been considered an indicator of motor equivalence. Although significant in some cases, such a relationship was not consistently observed across subjects. Considering systematic sources of variability such as vowel height, speech rate, and subjects, jaw-constrictor amplitude correlations showed a nonhomogeneous pattern strongly depending on place of articulation.

Motor control: Mechanisms of motor equivalence in handwriting.

Handwriting is a classic example of how the details of movement can be scale and plane invariant: letter forms reflecting personal style are unchanged, whether one is writing on a piece of paper, on a blackboard or in the sand using the foot. Recent research points to a role for the parietal cortex in such motor equivalence.

Can non-linear muscle dynamics explain the smoothness of handwriting movements?

The paper addresses the issue of the role of non-linear muscle dynamics in determining the smoothness and invariance of handwriting trajectories. In particular, a specific neuromuscular control model is described that has recently been shown to explain the detailed time course of hand stiffness during arm reaching movements. In the paper the model is applied to more complex handwriting trajectories, with the purpose of verifying to which extent the load compensation capabilities of the periphery can subserve motor equivalence. Simulations show the power of the mechanism for movements of “normal” speed. For quicker movements periphery alone is not enough and a central load compensation action is clearly required.

Motor equivalence in the production of /∫/

To explore the idea that speech motor goals are acoustic targets, upper lip protrusion and tongue blade fronting were examined in the sibilant /∫/ for evidence of motor equivalence in eight speakers of American English. Positive correlations across multiple repetitions of /∫/ (motor equivalence) would occur if the upper lip compensated with more protrusion when the tongue blade was further forward and vice versa. This motor equivalence would serve to maintain an adequate front cavity size for good acoustic separation from /s/ and enhance the acoustic stability of /∫/. It was hypothesized that motor equivalence would be found among less prototypical tokens, as elicited in a ‘‘clear+fast’’ speaking condition. Acoustic spectral analyses indicated excellent acoustic separation between the two sibilants, even in the ‘‘clear+fast’’ condition. There were significant positive correlations of the tongue-blade and upper lip movements for 28% of all possible cases, with considerable individual variation. When motor equivalence could be compared across the speaking condition, the tokens produced with the motor equivalent pattern were acoustically less prototypical. No occurrences of negative movement correlations were found. One possible source of individual variation may be the use of saturation (quantal) effects, which could also enhance acoustic stability. [Work supported by NIH.]

Dynamic cortical activity in the human brain reveals motor equivalence.

That animals and humans can accomplish the same goal using different effectors and different goals using the same effectors attests to the remarkable flexibility of the central nervous system. This phenomenon has been termed 'motor equivalence', an example being the writing of a name with a pencil held between the toes or teeth. The idea of motor equivalence has reappeared because single-cell studies in monkeys have shown that parameters of voluntary movement (such as direction) may be specified in the brain, relegating muscle activation to spinal interneuronal systems. Using a novel experimental paradigms and a full-head SQUID (for superconducting quantum interference device) array to record magnetic fields corresponding to ongoing brain activity, we demonstrate: (1), a robust relationship between time-dependent activity in sensorimotor cortex and movement velocity, independent of explicit task requirements; and (2) neural activations that are specific to task demands alone. It appears, therefore, that signatures of motor equivalence in humans may be found in dynamic patterns of cortical activity.

Effects of lesions in the mesial frontal cortex on bimanual co-ordination in monkeys

The hypothesis was tested that the mesial frontal cortex, including the supplementary motor area, is engaged in bimanual co-ordination. Three monkeys, trained in a well-co-ordinated bimanual pull-and-grasp task, were subjected to unilateral or bilateral lesions of the mesial frontal cortex. With unilateral lesions, the deficit consisted in a delay in movement initiation of the contralateral arm. With a bilateral lesion, the deficit was more pronounced with marked bilateral delays in movement onset and slowing in reaching. However, in the three monkeys bimanual co-ordination at the moment of goal achievement remained intact with an excellent temporal co-variation of the two limbs. In the two unilateral cases, an adaptive strategy developed after a few sessions, either by catching up during reaching with the limb contralateral to the lesion (monkey M1) or by delaying movement initiation of the limb ipsilateral to the lesion (monkey M2). This outcome is discussed in terms of Lashley's principle of motor equivalence, i.e. invariant goal achievement with variable means. Bilateral lesions led to a transient and near-total impairment in movement self-initiation when all external cues were absent. It is concluded that in monkeys the mesial frontal cortex does not play a crucial role in bimanual co-ordination but rather in movement initiation, especially when sensory cues are absent.

Approximate vocal tract shape invariance without vocal tract shape targets

Under normal conditions, the vocal tract shapes (postures) used by speakers to produce vowels appear to represent only a small subset of the possible shapes that could be used. This seems to suggest that speakers use postural targets (e.g., constriction targets) when planning speech movements. However, postural targets limit the motor equivalent capabilities of a movement controller. An alternative explanation for the approximate postural invariance maintains a high degree of motor equivalence by utilizing only acoustic/perceptual targets and transforming trajectories planned in acoustic/perceptual space into articulator movements through a directional mapping with a ‘‘postural relaxation’’ component. The postural relaxation component biases movements toward more central articulator configurations. Examples of this kind of controller include robotics techniques that utilize a Jacobian pseudoinverse in combination with null space movements formed using an orthogonal projection operator. The current work investigates how such a directional mapping can be learned by a biological motor system during babbling and how it can lead to postural invariance. Simulations of the DIVA model of speech acquisition and production utilizing an adaptive postural relaxation scheme verify the property of approximate postural invariance despite using no postural targets in the planning process. [Work supported by NIH and the Sloan Foundation.]

One-Handed Juggling: A Dynamical Approach to a Rhythmic Movement Task

The skill of rhythmically juggling a ball on a racket was investigated from the viewpoint of nonlinear dynamics. The difference equations that model the dynamical system were analyzed by means of local and nonlocal stability analyses. These analyses showed that the task dynamics offer an economical juggling pattern that is stable even for open-loop actuator motion. For this pattern, two types of predictions were extracted: (a) Stable periodic bouncing is sufficiently characterized by a negative acceleration of the racket at the moment of impact with the ball, and (b) a nonlinear scaling relation maps different juggling trajectories onto one topologically equivalent dynamical system. The relevance of these results for the human control of action was evaluated in an experiment in which subjects (N = 6) performed a comparable task of juggling a ball on a paddle. Task manipulations involved different juggling heights and gravity conditions of the ball. The following predictions were confirmed: (a) For stable rhythmic performance, the paddle's acceleration at impact is negative and fluctuations of the impact acceleration follow predictions from global stability analysis; and (b) for each subject, the realizations of juggling for the different experimental conditions are related by the scaling relation. These results permit one to conclude that humans reliably exploit the stable solutions inherent to the dynamics of the given task and do not overrule these dynamics by other control mechanisms. The dynamical scaling serves as an efficient principle for generating different movement realizations from only a few parameter changes and is discussed as a dynamical formalization of the principle of motor equivalence.

Speech sound acquisition, coarticulation, and rate effects in a neural network model of speech production.

This article describes a neural network model of speech motor skill acquisition and speech production that explains a wide range of data on variability, motor equivalence, coarticulation, and rate effects. Model parameters are learned during a babbling phase. To explain how infants learn language-specific variability limits, speech sound targets take the form of convex regions, rather than points, in orosensory coordinates. Reducing target size for better accuracy during slower speech leads to differential effects for vowels and consonants, as seen in experiments previously used as evidence for separate control processes for the 2 sound types. Anticipatory coarticulation arises when targets are reduced in size on the basis of context; this generalizes the well-known look-ahead model of coarticulation. Computer simulations verify the model's properties.

Articulatory evidence for acoustic goals for consonants

Evidence has been found for trading relations between tongue-body raising and upper lip protrusion (measured with an EMMA system) for the vowel /u/, reflecting a ‘‘motor equivalence’’ strategy that should help to constrain acoustic variation [Perkell etal., J. Acoust. Soc. Am. 93, 2948–2961 (1993)]. Theoretically, analogous relations in the transformation between the area function and the acoustic transfer function are possible for the consonants /r/ and /∫/, which are also produced with two independently controllable constrictions, formed by the tongue and by the lips. Such relations might occur more among tokens that are least prototypical, i.e., closest to perceptual boundaries. In a preliminary test of these hypotheses, a single speaker has produced the sounds /r/, /∫/, and /u/, embedded in contexts that might induce differences in prototypicality. Motor equivalence was observed for the /u/ in /tut/ (least prototypical, with the highest F2 values) but not in /pub/ or /hud/. For /r/ and /∫/, anterior displacement of the tongue constriction was positively correlated with upper lip protrusion, providing initial support for the hypothesis that movement goals for the consonants also include acoustic components, which are manifested in a tendency to maintain sufficient front cavity length to help assure acoustic-phonetic distinctiveness. [Work supported by NIDCD.]

Development of skilled speech production in children

There has been much recent interest in the manner in which children develop mature speech production capabilities. Investigations of both normal speech acquisition and developmental speech production impairments have contributed to our current knowledge base. A consistent finding is that young children’s speech is highly variable, both in its temporal and spectral attributes, and that this variability diminishes gradually with age. However, the exact manner in which articulatory precision emerges (or fails to emerge in speech disorders) continues to be the subject of debate. For example, some research on anticipatory coarticulation suggests a developmental progression from syllable- to segment-based timing strategies while other research suggests the opposite. Similarly, some studies of compensatory articulation show comparable degrees of motor equivalence in young children and adults, while others show that this aspect of motor control emerges gradually with maturation. This presentation will review key experiments (primarily based on acoustic analyses), consider potential explanations for the controversial data, and discuss methodological alternatives that may assist in the resolution of outstanding questions. Recent evidence indicates that specific motor patterns of different articulators will play a major role.

Current status of the motor program.

Motor program theory has provided physical therapists with one approach to understanding how the brain controls movement. Analogous with computer programs that specify the operations of computer hardware, motor programs are thought to contain commands for muscles that allow movements to occur without the need for continuous peripheral feedback. A review of the physical therapy literature reveals many instances in which motor program theory has been used as a theoretical framework for clinical practice. Yet despite the contribution programming theory has made to the advancement of movement science, the motor program construct is currently under considerable threat. Keele's (1968) original definition no longer seems tenable, given the problems of program storage, motor equivalence, movement flexibility, and context-conditioned variability. The finding that researchers from different disciplines define the motor program in a variety of ways adds difficulty to the task of evaluating the efficacy of the model. A critical appraisal of programming theory and its use in physical therapy suggests that clinicians need to reconsider the usefulness of the motor program model as a basis for movement rehabilitation following brain damage and musculoskeletal disorders.

Skill acquisition, coarticulation, and rate effects in a neural network model of speech production

This work describes a neural network model of speech motor skill acquisition and speech production that explains a wide range of data on contextual variability, motor equivalence, coarticulation, and speaking rate effects. Model parameters are learned during a babbling phase. To explain how infants learn phoneme-specific and language-specific limits on acceptable articulatory variability, the learned speech sound targets take the form of regions, or convexhulls, in orosensory coordinates. This leads to an explanation of coarticulation wherein the target for a speech sound is reduced in size based on context to provide a more efficient sequence of articulator movements. Furthermore, reduction of target size for better accuracy during slower speech (in accordance with Fitt’s law) leads to differential effects for vowels and consonants, as seen in speaking rate experiments that were previously explained by positing separate control processes for the two sound classes. The babbling process also naturally accounts for the formation of coordinativestructures, or groups of articulator movements marshalled together to perform orosensory tasks. Coordinative structures provide motor equivalence, including automatic compensation to perturbations or constraints on the articulators. Computer simulations verify the model’s motor equivalence, coarticulation, and speaking rate properties. [Work partially supported by AFOSR F49620-92-J-0499.]

Temporal coordination in bimanual actions

The issue of bimanual temporal coordination in human subjects is discussed for three selected movement paradigms: (i) simple, symmetric, bimanual finger movements, (ii) bimanual unloading, and (iii) a complex bimanual pull and grasp task. Temporal synchronization was found for all three experiments and was least variable for the first experiment. In the second experiment, synchronization concerned unloading with the index finger of one hand (electromyographic activation of the first dorsal interosseus muscle) and the postural adjustment of the load-bearing index finger of the other hand (electromyographic deactivation of the first dorsal interosseus muscle). In the third experiment, a goal-related temporal invariance was observed, even in the absence of visual guidance. Possible neural mechanisms for the observed temporal coordination of the three types of bimanual movements are discussed, as well as the concepts of goal invariance and motor equivalence.

Across session spatial stability of the tongue-jaw complex in alveolar closure

The movements of the tongue, lips, and jaw were transduced by electromagnetic midsagittal articulography. A single session included 20 repetitions of /pap/, /tat/, and /sas/ imbedded in a carrier phrase at normal, slow, and fast speech rates. Seven talkers completed three sessions at 1-week intervals. Reported here are measures of across session stability of tongue and jaw spatial synergies for alveolar closure at normal rates. Preliminary analyses indicate that idiosyncratic organization of the tongue and jaw for alveolar closure is relatively stable within and across sessions. However, across session stability, in terms of the relative contribution of the tongue and jaw towards closure and motor equivalence covariability of the summed tongue-jaw, the jaw, and the derived independent tongue displacements, is sensitive to variations in normal speech rate. For example, tongue-jaw synergies varied when the difference between session average duration of the tongue and/or jaw gestures approached 30%. [Work supported by NIH DC-00121 to Haskins Laboratories and University of Illinois research grant.]

A Self-Organizing Neural Model of Motor Equivalent Reaching and Tool Use by a Multijoint Arm

Abstract This paper describes a self-organizing neural model for eye-hand coordination. Called the DIRECT model, it embodies a solution of the classical motor equivalence problem. Motor equivalence computations allow humans and other animals to flexibly employ an arm with more degrees of freedom than the space in which it moves to carry out spatially defined tasks under conditions that may require novel joint configurations. During a motor babbling phase, the model endogenously generates movement commands that activate the correlated visual, spatial, and motor information that are used to learn its internal coordinate transformations. After learning occurs, the model is capable of controlling reaching movements of the arm to prescribed spatial targets using many different combinations of joints. When allowed visual feedback, the model can automatically perform, without additional learning, reaches with tools of variable lengths, with clamped joints, with distortions of visual input by a prism, and with unexpected perturbations. These compensatory computations occur within a single accurate reaching movement. No corrective movements are needed. Blind reaches using internal feedback have also been simulated. The model achieves its competence by transforming visual information about target position and end effector position in 3-D space into a body-centered spatial representation of the direction in 3-D space that the end effector must move to contact the target. The spatial direction vector is adaptively transformed into a motor direction vector, which represents the joint rotations that move the end effector in the desired spatial direction from the present arm configuration. Properties of the model are compared with psychophysical data on human reaching movements, neurophysiological data on the tuning curves of neurons in the monkey motor cortex, and alternative models of movement control.

Motor equivalence in the transformation from vocal‐tract configurations to the acoustic transfer function: Adaptation to a bite block

In a preliminary study [Perkell et al., J. Acoust. Soc. Am. 93, 2948 (1993)] subjects demonstrated a trading relation (negative correlations) between tongue‐body raising and lip protrusion in production of the vowel /u/. Data analyses indicated an increased tendency for the trading relation to occur at high values of F2. The tentative hypothesis is that when more than one articulator contributes to producing a particular acoustic cue and a source of variation displaces one of the articulators away from its target, compensating (complementary) adjustments are made in the programmed displacements of the other contributing articulator(s) to help constrain the acoustic variation. These adjustments should be more strongly expressed in tokens that are close to phonemic boundaries, where variation in the displacement of one of the contributing articulators could result in production of the wrong sound. This work has been extended with two new subjects. halfway through the experiment a bite block was placed in the subject’s mouth to introduce an additional source of variation. One subject strictly used coordination (i.e., positive correlation) to produce /u/ and did not alter this strategy with the bite block. Negative correlations were found for the other subject in subsets with relatively high values of F2. [Work supported by NIDCD.]

Trading relations between tongue-body raising and lip rounding in production of the vowel /u/: a pilot "motor equivalence" study.

Articulatory and acoustic data were used to explore the following hypothesis for the vowel /u/: The objective of articulatory movements is an acoustic goal; varying and reciprocal contributions of different articulators may help to constrain acoustic variation in achieving the goal. Previous articulatory studies of similar hypotheses, expressed entirely in articulatory terms, have been confounded by interdependencies of the variables being studied (e.g., lip and mandible displacements). One case in which this problem may be minimized is that of lip rounding and tongue-body raising (formation of a velo-palatal constriction) for the vowel /u/. Lip rounding and tongue-body raising should have similar acoustic effects for /u/, mainly to lower F2. In multiple repetitions, reciprocal contributions of lip rounding and tongue-body raising could help limit F2 variability for /u/; thus this experiment looked for complementary covariation (negative correlations) in measures of these two parameters. An electro-magnetic midsagittal articulometer (EMMA) was used to track movements of midsagittal points on the tongue body, upper and lower lips, and mandible for large numbers of repetitions of utterances containing /u/. (Interpretation of the data was aided by results from area-function-to-formant modeling.) Three of four subjects showed weak negative correlations, tentatively supporting the hypothesis; a fourth showed the opposite pattern: positive correlations of lip rounding and tongue raising. The results are discussed with respect to ideas about motor equivalence, the nature of speech motor programming, and potential improvements to the paradigm.

Three-dimensional kinematic analysis of frog hindlimb movement in reflex wiping.

The three-dimensional kinematics of the hindlimb back-wipe were examined in spinal frogs. The component movements were identified and the relationship between stimulus position and hindlimb configuration was assessed. The planes of motion of the hindlimb were examined throughout the movement. The back-wipe comprises three essential phases: a placing phase (I), in which the foot is drawn over the back of the frog and placed in a position near to the stimulus; a pre-whisk phase (II), in which the endpoint of the foot moves away from the stimulus; and a whisk/extension phase (III), in which the stimulus is removed. The pre-whisk phase contributes to force production for the whisk/extension (III). In the placing phase a systematic relationship was found between limb endpoint position and stimulus position in the rostro-caudal direction. The hip, knee and metatarsal joint angles were related to the position of the endpoint in the rostro-caudal direction. However, different frogs tended to adopt different strategies to remove the stimulus. In one strategy, when the knee angle was strongly related to the rostro-caudal stimulus position, the metatarsal angle was weakly related and vice versa. Other strategies were observed as well. There was no adjustment in limb endpoint position for stimulus placement in the medial-lateral direction. Consistent with this finding, the point on the foot at which stimulus contact occurred changed systematically as a function of medial-lateral stimulus placement. Thus, in order to remove the stimulus in different medial-lateral positions, the frog used a different part of the foot rather than moving the foot in the direction of the stimulus. In two frogs a relationship was observed between the elevation of the femur and the medial-lateral stimulus position. The motion planes of the hindlimb were studied by examining the instantaneous plane of motion of the endpoint and the planes of motion of adjacent limb segments. The motion of the endpoint was found not to be planar in any phase of the wipe. In contrast, planar motion of the femur and tibia was observed for all phases. Systematic changes in the orientation of these planes characterized the different phases. The position of the hindlimb was found to be variable prior to the placing phase. This variability was not related to stimulus position. However, in trials with multiple wipes, once an initial limb configuration was assumed, the limb returned to this configuration before each wipe in the sequence. Evidence for motor equivalence was sought in two ways.(ABSTRACT TRUNCATED AT 400 WORDS)

Neural representations for sensory-motor control, I: Head-centered 3-D target positions from opponent eye commands

This article describes how corollary discharges from outflow eye movement commands can be transformed by two stages of opponent neural processing into a head-centered representation of 3-D target position. This representation implicitly defines a cyclopean coordinate system whose variables approximate the binocular vergence and spherical horizontal and vertical angles with respect to the observer's head. Various psychophysical data concerning binocular distance perception and reaching behavior are clarified by this representation. The representation provides a foundation for learning head-centered and body-centered invariant representations of both foveated and non-foveated 3-D target positions. It also enables a solution to be developed of the classical motor equivalence problem, whereby many different joint configurations of a redundant manipulator can all be used to realize a desired trajectory in 3-D space.