Integration of intrinsic muscle properties, feed-forward and feedback signals for generating and stabilizing hopping.

It was hypothesized that a tight integration of feed-forward and feedback-driven muscle activation with the characteristic intrinsic muscle properties is a key feature of locomotion in challenging environments. In this simulation study it was investigated whether a combination of feed-forward and feedback signals improves hopping stability compared with those simulations with one individual type of activation. In a reduced one-dimensional hopping model with a Hill-type muscle (one contractile element, neither serial nor parallel elastic elements), the level of detail of the muscle's force-length-velocity relation and the type of activation generation (feed-forward, feedback and combination of both) were varied to test their influence on periodic hopping. The stability of the hopping patterns was evaluated by return map analysis. It was found that the combination of feed-forward and proprioceptive feedback improved hopping stability. Furthermore, the nonlinear Hill-type representation of intrinsic muscle properties led to a faster reduction of perturbations than a linear approximation, independent of the type of activation. The results emphasize the ability of organisms to exploit the stabilizing properties of intrinsic muscle characteristics.

Effects of frequency-shifted auditory feedback on fundamental frequency of long stressed and unstressed syllables.

Twenty-four normally speaking subjects had to utter the test word /tatatas/ with different stress patterns repeatedly. Auditory feedback was provided by head-phones and was shifted downwards in frequency during randomly selected trials while the subjects were speaking the complete test word. If the first syllable was long stressed, fundamental frequency of the vowel significantly increased by 2 Hz (corresponding to 25.5 cents) under frequency-shifted auditory feedback of .5 octave downwards, whereas under a shift of one semitone downwards a trend of an increase could be observed. If the first syllable was unstressed, fundamental frequency remained unaffected. Regarding the second syllable, significant increases or a trend for an increase of fundamental frequency was found in both shifting conditions. Results indicate a negative feedback mechanism that controls the fundamental frequency via auditory feedback in speech production. However, within a syllable a response could be found only if the syllable duration was long enough. Compensation for frequency-shifted auditory feedback still is quite imperfect. It is concluded that control of fundamental frequency is rather important on a suprasegmental level.

Predictive control of muscle responses to arm perturbations in cerebellar patients.

OBJECTIVES: To examine changes in predictive control of early antagonist responses to limb perturbations in patients with defined lesions of the cerebellum. METHODS: Eight cerebellar patients and eight sex and age matched control subjects participated. Subjects held a handle that was rotated around the elbow joint. They were instructed to hold the forearm at 90 degrees flexion against a mechanical perturbation. Extensor torque (5 Nm) was applied for 140 ms (pulse), or for 1400 ms (step) through an external motor. Motor responses were tested under two different conditions of anticipatory information. In the expected condition, subjects anticipated and received a pulse. Under the unexpected condition, subjects expected steps, but received unexpected pulses. Biceps and triceps EMG as well as angular kinematics were compared between expected and unexpected pulse perturbations to quantify possible effects of prediction. RESULTS: In all healthy subjects, the degree of overshoot in the return flexion movement was significantly less in expected pulse perturbations compared with unexpected trials. The degree of amplitude reduction was significantly smaller in the patient group than in the control group (22.8% v 40.0%). During the expected trials, latency of peak triceps activity was on average 20% shorter in the control group, but 4% larger in the cerebellar patients. CONCLUSIONS: In the expected condition, controls achieved a significant reduction in angular amplitude by generating triceps activity earlier, whereas the ability to use prediction for adjusting early antagonist responses after limb perturbation was impaired in cerebellar patients.

Identification of time-varying stiffness, damping, and equilibrium position in human forearm movements.

Knowledge of how stiffness, damping, and the equilibrium position of specific limbs change during voluntary motion is important for understanding basic strategies of neuromotor control. Presented here is an algorithm for identifying time-dependent changes in joint stiffness, damping, and equilibrium position of the human forearm. The procedure requires data from only a single trial. The method relies neither on an analysis of the resonant frequency of the arm nor on the presence of an external bias force. Its validity was tested with a simulated forward model of the human forearm. Using the parameter estimations as forward model input, the angular kinematics (model output) were reconstructed and compared to the empirically measured data. Identification of mechanical impedance is based on a least-squares solution of the model equation. As a regularization technique and to improve the temporal resolution of the identification process, a moving temporal window with a variable width was imposed. The method's performance was tested by (a) identifying a priori known hypothetical time-series of stiffness, damping, and equilibrium position, and (b) determining impedance parameters from recorded single-joint forearm movements during a hold and a goal-directed movement task. The method reliably reconstructed the original angular kinematics of the artificial and human data with an average positional error of less than 0.05 rad for movement amplitudes of up to 0.9 rad, and did not yield hypermetric trajectories like previous procedures not accounting for damping.

Motivation effects in a dichotic listening task as evident from functional magnetic resonance imaging in human subjects.

The present study addresses the effect of motivation on cerebral activity using functional magnetic resonance imaging. Five healthy volunteers performed a dichotic listening task in two sets of three trials during which high or low levels of achievement motivation were introduced. They were told that the first set would be used for calculation of intellectual capacity (high achievement motivation) and the second set for scanner calibration (neutral motivation). In three volunteers, high compared with neutral motivation produced activation in the right prefrontal cortex and the dorsal cingulate. We conclude that motivational effects may lead to significant activations and should be controlled in future cognitive imaging studies. We present preliminary evidence that right prefrontal and dorsal cingulate regions might be involved in motivational processes.

Influence of continuous, isobaric rectal distention on smooth muscle tone and cardiovascular activation in healthy volunteers.

UNLABELLED: Interactions between visceral, intestinal stimulation and autonomic cardiovascular function have received increasing attention over the last years. The aim of the study was to investigate the effect of long-term continuous rectal distention on rectal tone and cardiovascular function at the threshold for first sensation of distention and moderate urge to defecate compared to a resting period without distention. Continuous isobaric rectal distention was performed in 13 healthy volunteers with a ten minute pressure tracking procedure at each subject's predetermined individual threshold for either moderate urge or first sensation, separated by a ten minute resting period. Rectal volume, heart rate, systolic and diastolic blood pressure were determined continuously. RESULTS: Rectal tone decreased significantly over time at the threshold for moderate urge and first sensation. Cardiovascular measures remained stable over the distention procedures, except for a significant increase in systolic blood pressure at the threshold for moderate urge. DISCUSSION: The previously reported rectal accommodation response to rectal distention was confirmed. The increase in systolic blood pressure is most likely mediated through sympathetic efferent pathways.

A computer-controlled, long-term recording system for studying eating, drinking, and defecation behavior in miniature pigs.

The long-term observation of ingestive and excretory behaviors in freely ambulating and non socially isolated pigs is an important tool in the investigation of the physiological determinants of these behaviors. A computer-controlled laboratory setup for the recording of feeding, drinking, and defecation behavior in minipigs was developed, allowing for the observation of two pigs at a time for weeks. Four minipigs (29-52 kg) were conditioned to operate feeders with a precise food release per response and were then fed ad lib. The animals had up to 50% of their daily energy intake during the night. Meal size and time spent eating was highly correlated, meal size and the preprandial intermeal interval was moderately correlated, whereas meal size and the postprandial intermeal interval was not. Feeding facilitated defecation, supporting the assumption that the gastrocolonic response is present in the pig. The system has been shown to be highly reliable and valid, and thus provides an excellent tool for the investigation of the rhythmicity of ingestive and excretory behaviors in minipigs.

Temporal analysis of feeding behavior in miniature pigs.

The feeding behavior of four Munich miniature pigs (29-43 kg) housed in pairs of two, was observed for two consecutive weeks. Pigs were conditioned to operate a computer-controlled recording system equipped with feeders delivering a precise food release per response and were then fed ad libitum. In addition to the descriptive analysis of feeding behavior, Walsh-Fourier Spectral Analysis was utilized to investigate the temporal patterns of food ingestion and the synchronisation of feeding patterns between the animals housed together. The pigs had up to 48% of their daily energy intake during the dark cycle and there was a substantial reduction of food intake from the first to the second week. Furthermore, pigs housed together synchronized feeding behavior from week 1 to week 2. The recording system has been shown to be highly reliable and valid and provides an excellent tool for the investigation of ingestive behaviors in miniature pigs.

Facial EMG responses to auditory stimuli.

Three studies were performed to investigate the effects of auditory stimuli (pure tones and environmental noise) of different intensities on surface EMG activity recorded over five facial muscle regions (M. frontalis lateralis, M. corrugator supercilii, M. orbicularis oculi, M. zygomaticus major, M. depressor anguli oris). The results show that with presentation of tones and noises of high intensity (> 85 dB) strong facial EMG reactions over muscles of the upper face (M. frontalis lateralis, M. corrugator supercilii, M. orbicularis oculi) were evoked. Among environmental noises of different valence but the same intensity, baby's crying evoked EMG reactions over facial muscles in the mouth region, possibly indicating that the subjects demonstrated expressions of dislike during this particular stimulation. It is also discussed whether facial EMG reactions to auditory stimulation of different intensities could be connected to changes in muscle tone of the middle ear muscles. The contraction of these muscles modulates sensitivity to auditory stimulation. Thus, facial EMG activity of the muscles of the upper face could serve as an indicator of sensitivity to external auditory stimuli. However, the evaluation of pleasant and unpleasant emotional reactions in response to auditory stimulation seems to be impossible.

Fast finger extensions are slower in stutterers than in nonstutterers.

10 adult stutterers and 10 nonstutterers were required to extend as fast as possible their right and left index fingers under isometric conditions in response to an auditory signal. Force developed during finger extension was measured continuously during each trial. From these force records reaction time, contraction time, and peak force were measured. For stutterers, contraction times were longer (about 36 msec.) than for nonstutterers. Possible reasons for this result are discussed.

Mechanical perturbation of jaw movements during speech: effects on articulation and phonation.

12 subjects uttered the testword/papapas/repeatedly with three different speech rates and two stress patterns. On 17% randomly chosen trials, a mechanical load was applied unpredictably to the jaw in the direction of the opening movement. Load onset was triggered by the start of the first phonation. Analysis showed that the opening and closing displacements of the jaw movement in the first syllable were not influenced significantly by the perturbation. The load application prolonged the duration of the jaw movement in unstressed syllables but not in stressed syllables. Further, the mechanical perturbation of the jaw led to increased duration of phonation in unstressed syllables only, the effect for duration of phonation being greater at higher speech rates. These results demonstrate a coupling between articulation and phonation.

Monitoring radio programs and time of day affect simulated car-driving performance.

Whether radio monitoring, task difficulty, and time of day affect driving performance was examined in a computer-controlled, simulated driving task. Driving performance was defined as the absolute deviation of the car position from road midline as displayed on the computer monitor. We found for 12 men and 8 women (i) an expected, marked deterioration of performance with increasing task difficulty, (ii) a deterioration of performance in the morning, and (iii) a deterioration of performance when subjects concurrently listened to radio programs except for the easy task conducted in the morning. The latter is supposed to be caused by a reactive increase of effort, which is induced by the concurrent radio monitoring and stimulated through a perceived impairment of attention.

Nutrient ingestion increases rectal sensitivity in humans.

To determine the effect of nutrient ingestion on rectal perception thresholds (first sensation, consistent urge to defecate, pain), rectosigmoid balloon distentions were performed with a computerized automated pump in eight healthy volunteers (four males, four females, 31.6 +/- 6.02 years). Two measurements of rectal balloon distention were performed on 2 separate days. Day 1 served as a control condition with no meal. On day 2 the subjects received a 600 kcal liquid meal. On the control day, determination of perception thresholds was performed two times with a 10-min break between measurements. On the experimental day, threshold determinations were made before and after the ingestion of the liquid standard meal. The order of the experimental days was counterbalanced. Distention volumes at the urge to defecate and the pain threshold (maximum tolerable volume) were significantly reduced following the meal. The average change from pre- to postprandial measurements of the threshold for urge to defecate was -20.55 +/- 4.22% and for the maximum tolerable volume it was -16.09 +/- 4.4%. These results extend data previously reported from animal studies using similar experimental methods.

The competitive NMDA antagonist CPP protects substantia nigra neurons from MPTP-induced degeneration in primates.

Degeneration of nigrostriatal dopaminergic neurons is the primary histopathological feature of Parkinson's disease. The neurotoxin MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) induces a neurological syndrome in man and non-human primates very similar to idiopathic Parkinson's disease by selectively destroying dopaminergic nigrostriatal neurons. This gives rise to the hypothesis that Parkinson's disease may be caused by endogenous or environmental toxins. Endogenous excitatory amino acids (EAAs) such as L-glutamate could be involved in neurodegenerative disorders including Parkinson's disease. We report in this study that the competitive NMDA antagonist CPP (3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid) protects nigral tyrosine hydroxylase (TH) positive neurons from degeneration induced by systemic treatment with MPTP in common marmosets. This indicates that EAAs are involved in the pathophysiological cascade of MPTP-induced neuronal cell death and that EAA antagonists may offer a neuroprotective therapy for Parkinson's disease.

A neural-network model enabling sensorimotor learning: application to the control of arm movements and some implications for speech-motor control and stuttering.

Low-level motor control is defined as adapting an organism to the unique physical properties of its own limbs. The two-jointed arm serves to exemplify that effective low-level motor control demands a neurally medicated inversion of the dynamics, as well as of the kinematics, of a limb system. Reflex-like processing--that is, feedforword of either actual or predicted proprioceptive signals--is thereby assumed to be the principle of the dynamics control. As regards speech-motor control, the overall tool transformation is assumed to transform the force pattern of the articulatory muscles into speech sounds. Like the arm model, the vocal-tract transformation thus defined is also divided into two parts, namely the transformation relating the muscle forces to the mechanospatial states of the vocal tract (which is analogous to the forward dynamics including natural interarticulatory couplings), and the transformation relating the mechanospatial states to the speech sounds. Low-level speech-motor control, then, needs to invert both transformations, each of which can be learned by means of the self-imitation algorithm. Erroneous learning can fail to decouple interarticulatory coupling and therefore lead to abnormal feedback loops through the reflex-like operating neural network, which in turn can cause stuttering if audiophonatoric coupling is involved in learning.

The effect of loperamide on anorectal function in normal healthy men.

Loperamide improves anorectal functioning in patients with diarrhea and incontinence. Loperamide reduces sensitivity of the recto-anal inhibitory reflex and increases internal anal sphincter tone. Additionally, it has an effect on rectal compliance in incontinent patients with diarrhea. We studied the effect of loperamide versus placebo at different distances from the anal verge in 18 healthy male volunteers, using standard anorectal manometry was a double-blind, two-factorial design. We found that the recto-anal inhibitory reflex is most pronounced when stimulated in regions close to the anal canal and that distention stimuli are also perceived best in that region. Both effects are counteracted by loperamide. We found no effect on internal sphincter tone or rectal compliance. These results imply a gradient of sensitivity for rectal perception and the recto-anal inhibitory reflex in healthy volunteers. Loperamide action on both mechanisms suggests a common mediator for both effects.

A neural network model rapidly learning gains and gating of reflexes necessary to adapt to an arm's dynamics.

Effects of dynamic coupling, gravity, inertia and the mechanical impedances of the segments of a multi-jointed arm are shown to be neutralizable through a reflex-like operating three layer static feedforward network. The network requires the proprioceptively mediated actual state variables (here angular velocity and position) of each arm segment. Added neural integrators (and/or differentiators) can make the network exhibit dynamic properties. Then, actual feedback is not necessary and the network can operate in a pure feedforward fashion. Feedforward of an additional load can easily be implemented into the network using "descendent gating", and a negative feedback control loop added to the feedforward control reduces errors due to external noise. A training, which combines a least squared error based simultaneous learning rule (LSQ-rule) with a "self-imitation algorithm" based on direct inverse modeling, enables the network to acquire the whole inverse dynamics, limb parameters included, during one short training movement. The considerations presented also hold for multi-jointed manipulators.

Pattern generating and reflex-like processes controlling aiming movements in the presence of inertia, damping and gravity. A theoretical note.

A model is proposed, in which goal-directed movements of the forearm are controlled by a central pattern generator (CPG) initiated for exactly one period, and by reflex-analogous processes. Movement width is proportional to the amplitude factor of the CPG's output, and to the square of the CPG's period length. The period duration can be freely selected, thus enabling the CPG to accommodate its time scale to the period of others CPG's. Parameters which influence movement accuracy can be adjusted by means of closed control loop, which are discrete with respect to time: The time unit corresponds to the period of the CPG. For instance, momentum adjustment balances the CPG in such a manner that the velocity of the arm becomes zero on termination of the period, while gain adjustment serves to attain a correct movement length in the presence of an inertial load. Friction, stiffness and gravitational force are neutralized by additional reflex-type processes, interpretable as positive feedback loops with adjustable gain factors, using position and velocity signals.

Controlling the dynamics of a two-joined arm by central patterning and reflex-like processing. A two-stage hybrid model.

It is demonstrated, that a two-joined arm performing goal directed movements is controllable (a) by two central pattern generators (CPG) representing sampled data control, each referring to one joint, initiated for exactly one period, and producing the angular movement width about this joint, and (b) by reflex-like processes operating continuously with respect to time. The latter eliminate the effects of dynamic coupling, gravity, inertia, and mechanical impedance on the movement by 'proprioceptive feedforward' of position, velocity and acceleration signals, thus enabling the CPGs to handle the arm segments as if they were independent and free from forces. Higher ordered centers (with respect to the CPGs) therefore only need to control the kinematics, not dynamics, of the arm.