Sensorimotor characteristics of speech motor sequences

Summary The present experiment focused on the characteristics of sequential speech movements. Subjects generated two successive lip and jaw closing movements associated with the two p's in sapapple. By selectively manipulating the lower lip perturbation it was possible to discern the role of somatic sensory interactions with the presumed sequential movement programming. Lower lip perturbation duration was manipulated to yield two different load conditions. In the Load On (LN) condition, the perturbation remained on for both closing movements. In the Load On/Off (LNF) condition, the perturbation was removed at variable times prior to the second closing movement. Analyses focused on comparing the EMG and resulting kinematic changes for the second p closure across the two load conditions relative to the normal control (no load) condition. The second p closure was differentially affected by the load conditions resulting in changes in the upper and lower lip compensations. Upper lip changes reflected consistent load duration differences; however, the magnitude of the lower lip EMG and kinematic adjustments did not mirror those of the upper lip. In contrast to the differential upper lip/ lower lip changes observed for the magnitude adjustments, timing adjustments were similar for both upper lip and lower lip suggesting a separation between the specification of magnitude and timing of speech movements. Differential load effects were also observed for the timing of the second closing movements. For the LN condition, the onset of muscle activity and subsequent movement occurred earlier (re: control); for the LNF condition, load removal delayed the onset of muscle activity and the subsequent movement (re: control). Further, the opening movement preceding the second closing movement was modified for both load conditions suggesting that all movements in the sequence, not just closing movements, can be modified. The present results suggest that the programming of speech movement sequences is a dynamic process involving scaling and timing of motor commands relying on various degrees of sensory interaction. The apparent separation in the magnitude and timing specification of the movement sequences suggests the parallel influences of different neural systems. The consequence of this control scheme is that specification of movement parameters for sequential motor acts is a flexible real-time sensorimotor process interacting with less-flexible well-established central motor relations. Further, motor programs for speech may reflect certain generalized movement actions (e.g., oral opening, oral closing) rather than individual words, syllables, or other linguistic categories programmed on a movement-to-movement basis.     

Central patterning of speech movements

Summary Previous speech kinematic studies have demonstrated systematic timing relations among the upper lip, lower lip, and jaw suggesting the operation of a central pattern generator (CPG). The present study evaluated the consistency of these timing relations following unanticipated perturbation of the lower lip. Using this approach, it was also possible to evaluate the influence of sensory information on the timing of motor output and subsequent coordination of the multiple speech movements. Perturbations were applied to the lower lip during the closing movement associated with the first p in sapapple. Muscle activity and movements of the upper lip, lower lip, and jaw were obtained. Changes in movement displacement, velocity and duration, the timing and sequencing of peak velocities, EMG area, and EMG rise time were analyzed for the control and load conditions. Similar to previous perturbation results, significant magnitude compensations from the muscles and movements of the upper lip, lower lip, and jaw were observed. In contrast, movement durations and the sequencing of peak velocities were relatively unaffected by the lower lip load. The timing of peak EMG amplitude and consequently the timing of peak closing velocity for all structures (UL, LL, and J) occurred earlier relative to the preceding opening movement. These results are consistent with the interaction of phasic sensory input with centrally-driven commands resulting in a phase-advanced motor output. Further, as the timing of one structure is modified so were all the functionally-linked components thereby maintaining the necessary coordination. As in other rhythmic motor behaviors such as locomotion and chewing, there appears to be a centrally patterned framework for speech movement coordination.     

Lip muscle reflex and intentional response levels in a simple speech task

Summary Sensorimotor integration in human lip muscle was studied by recording muscle activity while subjects produced simple speech utterances in response to mechanical stimulation. On each trial subjects were instructed either to produce the syllable pa or not respond when they detected movement of a small paddle held between the lips. Mechanical stimuli were adequate to elicit reflexes over poststimulus intervals of 15–30 ms (R1) and 30–50 ms (R2). EMG recordings were obtained from upper and lower lip muscles, and EMG levels were calculated for individual trials over several poststimulus time intervals. The independent effects of stimulus magnitude, prestimulus EMG, and reaction time on poststimulus response levels were assessed using multiple regression analysis. R1 and R2 levels were positively correlated with stimulus magnitude, but stimulus magnitude had little modulating effect on intentional lip muscle responses. Both reflex and intentional response levels showed positive associations with prestimulus EMG level. Instructional set had significant modulating effects on reflex responses in 9 of 10 subjects, but the nature of these effects varied among subjects. These various findings are discussed in relation to similar studies on limb motor systems and lip motor control for speech.     

Affect-induced changes in speech production

To determine how sad affect (or brief sad mood) interacts with paralinguistic aspects of speech, we investigated the effect of a happy or sad mood induction on speech production in 49 healthy volunteers. Several speech parameters measuring speech rate, loudness and pitch were examined before and after a standardized mood-induction procedure that involved viewing facial expressions. Speech samples were collected during the self-initiated reading of emotionally "neutral" sentences; there was no explicit demand to produce mood-congruent speech. Results indicated that, after the mood induction, the speech of participants in the sad group was slower, quieter and more monotonous than the speech of participants in the happy group. This speech paradigm provides a model for studying how changes in mood states interact with the motor control of speech.     

Transfer of sensorimotor adaptation between different movement categories

We examined whether task-dependent modulation was evident in a motor learning paradigm. Subjects performed reaching movements before, during, and after exposure to a novel force perturbation while adopting either a spatial goal, "continue towards the target", or an effort goal, "keep your effort profile the same". Before the perturbation, the hand trajectories were moderately straight and accurate regardless of the task. However, during and immediately after the perturbation, the reaches exhibited unambiguous task-dependent differences in both the initial and terminal periods of the reach. With the spatial goal, subjects showed terminal compensations to the force-induced displacements indicative of feedback control. In addition, feedforward control was evident in the smaller path deviations with continued exposure and the initial path aftereffects when the perturbation was removed. In contrast, when adopting an effort goal, subjects showed large and chronically deviated endpoints from the perturbation indicating an absence of feedback compensation. They also showed no feedforward adaptation during repeated exposure or visible aftereffects when the perturbation was removed. Therefore, both feedforward and feedback control mechanisms show task-dependent modulation in a motor learning paradigm.     

Variant and invariant characteristics of speech movements

Summary Upper lip, lower lip, and jaw kinematics during select speech behaviors were studied in an attempt to identify potential invariant characteristics associated with this highly skilled motor behavior. Data indicated that speech motor actions are executed and planned presumably in terms of relatively invariant combined multimovement gestures. In contrast, the individual upper lip, lower lip, and jaw movements and their moment-to-moment coordination were executed in a variable manner, demonstrating substantial motor equivalence. Based on the trial-to-trial variability in the movement amplitudes, absolute positions, and velocities of the upper lip, lower lip, and jaw, it appears that speech motor planning is not formulated in terms of spatial coordinates. Seemingly, object-level planning for speech may be encoded in relation to the acoustic consequences of the movements and ultimately with regard to listener's auditory perceptions. In addition, certain temporal parameters among the three movements (relative times of movement onsets and velocity peaks) were related stereotypically, reflecting invariances characteristic of more automatic motor behaviors such as chewing and locomotion. These data thus appear to provide some additional insights into the hierarchy of multimovement control. At the top of the motor control hierarchy, the overall plan appears to be generated with explicit specification of certain temporal parameters. Subsequently, based upon the plan and within that stereotypic temporal framework, covariable adjustments among the individual movements are implemented. Given the results of previous perturbation studies, it is hypothesized that these covariable velocity and amplitude adjustments reflect the action of sensorimptor mechanisms.     

Effects of seeing and hearing speech on speech production: a response time study

Research demonstrates that listening to and viewing speech excites tongue and lip motor areas involved in speech production. This perceptual-motor relationship was investigated behaviourally by presenting video clips of a speaker producing vowel-consonant-vowel syllables in three conditions: visual-only, audio-only, and audiovisual. Participants identified target letters that were flashed over the mouth during the video, either manually or verbally as quickly as possible. Verbal responses were fastest when the target matched the speech stimuli in all modality conditions, yet optimal facilitation was observed when participants were presented with visual-only stimuli. Critically, no such facilitation occurred when participants were asked to identify the target manually. Our findings support previous research suggesting a close relationship between speech perception and production by demonstrating that viewing speech can ‘prime’ our motor system for subsequent speech production.

Potential interactions among linguistic, autonomic, and motor factors in speech

Though anecdotal reports link certain speech disorders to increases in autonomic arousal, few studies have described the relationship between arousal and speech processes. Additionally, it is unclear how increases in arousal may interact with other cognitive-linguistic processes to affect speech motor control. In this experiment we examine potential interactions between autonomic arousal, linguistic processing, and speech motor coordination in adults and children. Autonomic responses (heart rate, finger pulse volume, tonic skin conductance, and phasic skin conductance) were recorded simultaneously with upper and lower lip movements during speech. The lip aperture variability (LA variability index) across multiple repetitions of sentences that varied in length and syntactic complexity was calculated under low- and high-arousal conditions. High arousal conditions were elicited by performance of the Stroop color word task. Children had significantly higher lip aperture variability index values across all speaking tasks, indicating more variable speech motor coordination. Increases in syntactic complexity and utterance length were associated with increases in speech motor coordination variability in both speaker groups. There was a significant effect of Stroop task, which produced increases in autonomic arousal and increased speech motor variability in both adults and children. These results provide novel evidence that high arousal levels can influence speech motor control in both adults and children.     

Enhancement of spike and wave discharges by GABAmimetic drugs in rats with spontaneous petit-mal-like epilepsy

Certain Wistar rats from our laboratory colony present genetically determined seizures similar to human petit-mal absences. Muscimol, THIP and L-baclofen, agonists of GABA receptors, and gamma-vinyl GABA (GVG), an inhibitor of GABA degradation, enhanced the duration of spontaneous petit-mal-like seizures in a dose-dependent fashion. These findings raise questions as to the role of GABAergic neurotransmission in the occurrence of this type of spontaneous spike and wave discharges.     

Functional and structural aging of the speech sensorimotor neural system: functional magnetic resonance imaging evidence

The ability to perceive and produce speech undergoes important changes in late adulthood. The goal of the present study was to characterize functional and structural age-related differences in the cortical network that support speech perception and production, using magnetic resonance imaging, as well as the relationship between functional and structural age-related changes occurring in this network. We asked young and older adults to observe videos of a speaker producing single words (perception), and to observe and repeat the words produced (production). Results show a widespread bilateral network of brain activation for Perception and Production that was not correlated with age. In addition, several regions did show age-related change (auditory cortex, planum temporale, superior temporal sulcus, premotor cortices, SMA-proper). Examination of the relationship between brain signal and regional and global gray matter volume and cortical thickness revealed a complex set of relationships between structure and function, with some regions showing a relationship between structure and function and some not. The present results provide novel findings about the neurobiology of aging and verbal communication.     

Deficit of sensorimotor integration in normal aging

Sensorimotor performance declines with normal aging. The present study explored age-related changes in sensorimotor integration by conditioning a supra-threshold transcranial magnetic stimulation pulse with a peripheral nerve shock at different interstimulus intervals. Cortical motor threshold of the abductor pollicis brevis muscle, intracortical inhibition and facilitation were measured. We also assessed the influence of median nerve stimulation on motor cortex excitability at intervals which evoked short- and long-latency afferent inhibition (SAI and LAI, respectively) and afferent-induced facilitation (AIF). We observed a marked decrease of the long latency influence of proprioceptive inputs on M1 excitability in the elderly, with the loss of AIF and LAI. The SAI, motor thresholds and intracortical inhibition and facilitation were not age-related. Decreased sensorimotor performance with aging appears to be associated with a decrease in the influence of proprioceptive inputs on motor cortex excitability at longer intervals (probably via higher order cortical areas).     

Goal-directed arm movements in absence of visual guidance: evidence for amplitude rather than position control

Summary 1. The control of pointing arm movements in the absence of visual guidance was investigated in unpracticed human subjects. The right arm grasped a lever which restricted the movement of the right index fingertip to a horizontal arc, centered between the axes of eye rotation. A horizontal panel directly above the arm prevented visual feedback of the movement. Visual stimuli were presented in discrete positions just above panel and fingertip. A flag provided visual feedback on fingertip position before each pointing movement (Exp. A and B), or before a movement sequence (Exp. C). 2. When subjects pointed from straight ahead to eccentric stimulus positions (Exp. A), systematic and variable pointing errors were observed; both kinds of errors increased with stimulus eccentricity. When subjects pointed from 30 deg left to stimuli located further right (Exp. B), errors increased with stimulus position to the right. Taken together, these findings suggest that pointing accuracy depends not primarily on stimulus position, but rather on required movement amplitude. 3. When subjects performed sequences of unidirectional movements (Exp. C), systematic and variable errors increased within the sequence. A quantitative analysis revealed that this increase can be best described as an accumulation of successive pointing errors. 4. We conclude that both findings, error increase with amplitude, and accumulation of successive errors, when considered together strongly support the hypothesis that amplitude, rather than final position, is the controlled variable of the investigated movements.     

Control of bimanual rhythmic movements: trading efficiency for robustness depending on the context

This paper investigates how the efficiency and robustness of a skilled rhythmic task compete against each other in the control of a bimanual movement. Human subjects juggled a puck in 2D through impacts with two metallic arms, requiring rhythmic bimanual actuation. The arms kinematics were only constrained by the position, velocity and time of impacts while the rest of the trajectory did not influence the movement of the puck. In order to expose the task robustness, we manipulated the task context in two distinct manners: the task tempo was assigned at four different values (hence manipulating the time available to plan and execute each impact movement individually); and vision was withdrawn during half of the trials (hence reducing the sensory inflows). We show that when the tempo was fast, the actuation was rhythmic (no pause in the trajectory) while at slow tempo, the actuation was discrete (with pause intervals between individual movements). Moreover, the withdrawal of visual information encouraged the rhythmic behavior at the four tested tempi. The discrete versus rhythmic behavior give different answers to the efficiency/robustness trade-off: discrete movements result in energy efficient movements, while rhythmic movements impact the puck with negative acceleration, a property preserving robustness. Moreover, we report that in all conditions the impact velocity of the arms was negatively correlated with the energy of the puck. This correlation tended to stabilize the task and was influenced by vision, revealing again different control strategies. In conclusion, this task involves different modes of control that balance efficiency and robustness, depending on the context.     

Task-specific adaptations of postural sway in sitting infants

When engaging in manual or visual tasks while sitting, infants modify their postural sway based on concurrent task demands. It remains unclear whether these modulations are sensitive to differences in concurrent task demands (holding a toy vs. looking at a toy being held by someone else), and whether the properties of the support surface impact these adaptations. We investigated infants’ ability to modify postural sway when holding a toy or visually attending to a toy someone else was holding while sitting on different support surfaces. Twenty-six independently sitting infants sat on solid and compliant surfaces placed on a force plate while looking at or holding a toy. Measures of postural sway were calculated from the center of pressure data. Visually attending to a toy was associated with less sway and lower sway velocity than when holding a toy. Surprisingly, surface compliance did not affect sway and there were no interaction effects. Whereas sway modulations may facilitate infants’ performance on both manual and visual concurrent tasks, the visual task placed more constraints on the postural system leading to greater adaptations in postural sway. These findings provide insights into how infants are allocating attention and coordinating perceptual-motor information in developing sitting skills.     

Spatiotemporal stability and patterning of speech movement sequences

In order to examine the stability and patterning of speech movement sequences, movements of the lip were recorded as subjects produced a phrase at normal, fast, and slow rates. Three methods of analysis were employed. First, a new index of spatiotemporal stability was derived by summing the standard deviations computed across amplitude- and time-normalized displacement records. This index indicated that normal and fast rates of speech production result in more stable movement execution compared to slow rates. In the second analysis, the relative time of occurrence of the peak velocity of the three middle opening movements of the utterance was measured. For each of the three peaks, the preservation of relative timing was assessed by applying Genter's (1987) slope test. The results clearly indicate that the relative timing of these events does not remain constant across changes in speech rate. The relative timing of the middle opening gestures shifted, becoming later as utterance duration increased. In a third analysis, pattern recognition techniques were applied to the normalized displacement waveforms. A classification algorithm was highly successful in sorting waveforms into normal, fast, and slow rate conditions. These findings were interpreted to suggest that, within a subject, three distinct patterns or movement templates exist, one for each rate of production. Speech rate appears to be a global parameter, one that affects the entire command sequence for the utterance.     

Facilitatory and interfering effects of neighbourhood density on speech production: Evidence from aphasic errors

In a system where tens of thousands of words are made up of a limited number of phonemes, many words are bound to sound alike. This similarity of the words in the lexicon as characterized by phonological neighbourhood density (PhND) has been shown to affect speed and accuracy of word comprehension and production. Whereas there is a consensus about the interfering nature of neighbourhood effects in comprehension, the language production literature offers a more contradictory picture with mainly facilitatory but also interfering effects reported on word production. Here we report both of these two types of effects in the same study. Multiple regression mixed models analyses were conducted on PhND effects on errors produced in a naming task by a group of 21 participants with aphasia. These participants produced more formal errors (interfering effect) for words in dense phonological neighbourhoods, but produced fewer nonwords and semantic errors (a facilitatory effect) with increasing density. In order to investigate the nature of these opposite effects of PhND, we further analysed a subset of formal errors and nonword errors by distinguishing errors differing on a single phoneme from the target (corresponding to the definition of phonological neighbours) from those differing on two or more phonemes. This analysis confirmed that only formal errors that were phonological neighbours of the target increased in dense neighbourhoods, while all other errors decreased. Based on additional observations favouring a lexical origin of these formal errors (they exceeded the probability of producing a real-word error by chance, were of a higher frequency, and preserved the grammatical category of the targets), we suggest that the interfering effect of PhND is due to competition between lexical neighbours and target words in dense neighbourhoods.     

Fine-tuning of auditory cortex during speech production

The cortex suppresses sensory information when it is the result of a self-produced motor act, including the motor act of speaking. The specificity of the auditory cortical suppression to self-produced speech, a prediction derived from the posited operation of a precise forward model system, has not been established. We examined the auditory N100 component of the event-related brain potential elicited during speech production. While subjects uttered a vowel, they heard real-time feedback of their unaltered voice, their pitch-shifted voice, or an alien voice substituted for their own. The subjects' own unaltered voice feedback elicited a dampened auditory N100 response relative to the N100 elicited by altered or alien auditory feedback. This is consistent with the operation of a precise forward model modulating the auditory cortical response to self-generated speech and allowing immediate distinction of self and externally generated auditory stimuli.     

Observation of static gestures influences speech production

Research investigating ‘mirror neurons’ has demonstrated the presence of an observation–execution matching system in humans. One hypothesized role for this system might be to aid in action understanding by encoding the underlying intentions of the actor. To investigate this hypothesis, we asked participants to observe photographs of an actor making orofacial gestures (implying verbal or non-verbal acts), and to produce syllables that were compatible or incompatible with the gesture they observed. We predicted that if mirror neurons encode the intentions of an actor, then the pictures implying verbal gestures would affect speech production, whereas the non-verbal gestures would not. Our results showed that the observation of compatible verbal gestures facilitated verbal responses, while incompatible verbal gestures caused interference. Although this compatibility effect did not reach statistical significance when the photographs implied a non-verbal act, responses were faster on average when the gesture implied the use of similar articulators as those involved with the production of the target syllable. Altogether, these behavioral findings compliment previous neuroimaging studies indicating that static pictures portraying gestures activate brain regions associated with an observation–execution matching system.     

Time-dependent influence of sensorimotor set on automatic responses in perturbed stance

 These experiments tested the hypothesis that the ability to change sensorimotor set quickly for au-tomatic responses depends on the time interval between successive surface perturbations. Sensorimotor set refers to the influence of prior experience or context on the state of the sensorimotor system. Sensorimotor set for postural responses was influenced by first giving subjects a block of identical backward translations of the support surface, causing forward sway and automatic gastrocnemius responses. The ability to change set quickly was inferred by measuring the suppression of the stretched antagonist gastrocnemius responses to toes-up rotations causing backward sway, following the translations. Responses were examined under short (10–14 s) and long (19–24 s) inter-trial intervals in young healthy subjects. The results showed that subjects in the long-interval group changed set immediately by suppressing gastrocnemius to 51% of translation responses within the first rotation and continued to suppress them over succeeding rotations. In contrast, subjects in the short-interval group did not change set immediately, but required two or more rotations to suppress gastrocnemius responses. By the last rotation, the short-interval group suppressed gastrocnemius responses to 33%, similar to the long-interval group of 29%. Associated surface plantarflexor torque resulting from these responses showed similar results. When rotation and translation perturbations alternated, however, the short-interval group was not able to suppress gastrocnemius responses to rotations as much as the long-interval group, although they did suppress more than in the first rotation trial after a series of translations. Set for automatic responses appears to linger, from one trial to the next. Specifically, sensorimotor set is more difficult to change when surface perturbations are given in close succession, making it appear as if set has become progressively stronger. A strong set does not mean that responses become larger over consecutive trials. Rather, it is inferred by the extent of difficulty in changing a response when it is appropriate to do so. These results suggest that the ability to change sensorimotor set quickly is sensitive to whether the change is required after a long or a short series of a prior different response, which in turn depends on the time interval between successive trials. Different rate of gastrocnemius suppression to toes-up rotation of the support surface have been reported in previous studies. This may be partially explained by different inter-trial time intervals demonstrated in this study. Received: 7 October 1997 / Accepted: 31 August 1998     

Error accumulation and error correction in sequential pointing movements

Human subjects pointed, without seeing their arm, at visual targets presented in repeated sequences in a frontal plane. Required movement direction could change within the sequence by 0, 45, 90, 135 or 180°. Hand position was recorded contact-free in three dimensions (3D). From the recordings, the pointing errors towards each target were transformed into a Cartesian coordinate system with the x-axis representing the mean direction of all movements towards that target. We then investigated the relationship between successive errors by applying linear regression analysis separately to the three Cartesian error components. For the x-component, we found that successive errors were positively correlated throughout the experiment, which confirms our previous finding that errors in sequential pointing tend to accumulate (Bock and Eckmiller 1986; Bock et al. 1990). Correlation dropped by nearly 50% following a direction change of 90° or more, suggesting that accumulation is reduced but not abolished by large changes in movement direction. The slope of the regression line averaged 0.6, which indicates the existence of a complementary trend towards error correction, contributing about 40% towards motor performance. Changes of movement direction affected slope and correlation in a closely similar way, suggesting that reduced accumulation is paralleled by increased correction. For the y- and z-components, we found that successive errors were positively correlated as well, but were not reduced following even large direction changes. This apparent discrepancy can be resolved by assuming separate neural mechanisms for amplitude and for direction control, differing in their sensitivity to direction changes. We discuss our findings in the context of the two predominant motor control hypotheses, the amplitude and the position control models.