Response properties of the perioral reflex in Parkinson's disease

A neurocomputational view of the effects of Parkinson's disease on speech production

The purpose of this article is to review the scientific literature concerning speech in Parkinson's disease (PD) with reference to the DIVA/GODIVA neurocomputational modeling framework. Within this theoretical view, the basal ganglia (BG) contribute to several different aspects of speech motor learning and execution. First, the BG are posited to play a role in the initiation and scaling of speech movements. Within the DIVA/GODIVA framework, initiation and scaling are carried out by initiation map nodes in the supplementary motor area acting in concert with the BG. Reduced support of the initiation map from the BG in PD would result in reduced movement intensity as well as susceptibility to early termination of movement. A second proposed role concerns the learning of common speech sequences, such as phoneme sequences comprising words; this view receives support from the animal literature as well as studies identifying speech sequence learning deficits in PD. Third, the BG may play a role in the temporary buffering and sequencing of longer speech utterances such as phrases during conversational speech. Although the literature does not support a critical role for the BG in representing sequence order (since incorrectly ordered speech is not characteristic of PD), the BG are posited to contribute to the scaling of individual movements in the sequence, including increasing movement intensity for emphatic stress on key words. Therapeutic interventions for PD have inconsistent effects on speech. In contrast to dopaminergic treatments, which typically either leave speech unchanged or lead to minor improvements, deep brain stimulation (DBS) can degrade speech in some cases and improve it in others. However, cases of degradation may be due to unintended stimulation of efferent motor projections to the speech articulators. Findings of spared speech after bilateral pallidotomy appear to indicate that any role played by the BG in adult speech must be supplementary rather than mandatory, with the sequential order of well-learned sequences apparently represented elsewhere (e.g., in cortico-cortical projections).

The role of opponent basal ganglia outputs in behavior

This review is an attempt to explain the role of basal ganglia (BG) outputs in generating movements. Recent work showed that opponent outputs from the BG represent instantaneous body position coordinates during behavior. On the other hand, projection neurons in the striatum, the major input nucleus, as well as dopaminergic neurons that form the nigrostriatal pathway, can represent movement velocity. To explain these findings, a new model is proposed, in which the BG implement the level of transition control in an extended control hierarchy. BG outputs represent descending reference signals that command diverse lower-level position controllers. This model not only explains major neurological symptoms but also makes quantitative and testable predictions.

Multiple factors are involved in the dysarthria associated with Parkinson's disease: a review with implications for clinical practice and research

PURPOSE

Motor speech abnormalities are highly common and debilitating in individuals with idiopathic Parkinson's disease (IPD). These abnormalities, collectively termed hypokinetic dysarthria (HKD), have been traditionally attributed to hypokinesia and bradykinesia secondary to muscle rigidity and dopamine deficits. However, the role of rigidity and dopamine in the development of HKD is far from clear. The purpose of the present study was to offer an alternative view of the factors underlying HKD.

METHOD

The authors conducted an extensive, but not exhaustive, review of the literature to examine the evidence for the traditional view versus the alternative view.

RESULTS

The review suggests that HKD is a highly complex and variable phenomenon including multiple factors, such as scaling and maintaining movement amplitude and effort; preplanning and initiation of movements; internal cueing; sensory and temporal processing; automaticity; emotive vocalization; and attention to action (vocal vigilance). Although not part of the dysarthria, nonmotor factors, such as depression, aging, and cognitive-linguistic abnormalities, are likely to contribute to the overall speech symptomatology associated with IPD.

CONCLUSION

These findings have important implications for clinical practice and research.

Sensorimotor adaptation of speech in Parkinson's disease

The basal ganglia are involved in establishing motor plans for a wide range of behaviors. Parkinson's disease (PD) is a manifestation of basal ganglia dysfunction associated with a deficit in sensorimotor integration and difficulty in acquiring new motor sequences, thereby affecting motor learning. Previous studies of sensorimotor integration and sensorimotor adaptation in PD have focused on limb movements using visual and force-field alterations. Here, we report the results from a sensorimotor adaptation experiment investigating the ability of PD patients to make speech motor adjustments to a constant and predictable auditory feedback manipulation. Participants produced speech while their auditory feedback was altered and maintained in a manner consistent with a change in tongue position. The degree of adaptation was associated with the severity of motor symptoms. The patients with PD exhibited adaptation to the induced sensory error; however, the degree of adaptation was reduced compared with healthy, age-matched control participants. The reduced capacity to adapt to a change in auditory feedback is consistent with reduced gain in the sensorimotor system for speech and with previous studies demonstrating limitations in the adaptation of limb movements after changes in visual feedback among patients with PD.

Vocal responses to perturbations in voice auditory feedback in individuals with Parkinson's disease

BACKGROUND

One of the most common symptoms of speech deficits in individuals with Parkinson's disease (PD) is significantly reduced vocal loudness and pitch range. The present study investigated whether abnormal vocalizations in individuals with PD are related to sensory processing of voice auditory feedback. Perturbations in loudness or pitch of voice auditory feedback are known to elicit short latency, compensatory responses in voice amplitude or fundamental frequency.

METHODOLOGY/PRINCIPAL FINDINGS

Twelve individuals with Parkinson's disease and 13 age- and sex-matched healthy control subjects sustained a vowel sound (/α/) and received unexpected, brief (200 ms) perturbations in voice loudness (±3 or 6 dB) or pitch (±100 cents) auditory feedback. Results showed that, while all subjects produced compensatory responses in their voice amplitude or fundamental frequency, individuals with PD exhibited larger response magnitudes than the control subjects. Furthermore, for loudness-shifted feedback, upward stimuli resulted in shorter response latencies than downward stimuli in the control subjects but not in individuals with PD.

CONCLUSIONS/SIGNIFICANCE

The larger response magnitudes in individuals with PD compared with the control subjects suggest that processing of voice auditory feedback is abnormal in PD. Although the precise mechanisms of the voice feedback processing are unknown, results of this study suggest that abnormal voice control in individuals with PD may be related to dysfunctional mechanisms of error detection or correction in sensory feedback processing.

Speech treatment for Parkinson's disease

Researchers estimate that 89% of people with Parkinson's disease (PD) have speech and voice disorders including disorders of laryngeal, respiratory and articulatory function. Despite the high incidence of speech and voice impairment, studies suggest that only 3-4% of people with PD receive speech treatment. Here, we review the literature on the characteristics and features of speech and voice disorders in people with PD and the types of treatment techniques available (medical, surgical and behavioral), with a focus on behavioral therapies. We provide a summary of the current status of the field of speech treatment in PD and recommendations for implementation of the current efficacy of treatment interventions. Directions for future research, including a speculative viewpoint on how the field will evolve in 5 years time, are discussed.

The Lee Silverman Voice Treatment® for voice, speech and other orofacial disorders in patients with Parkinson’s disease

Parkinson’s disease (PD) impairs voice, speech, swallowing and facial expressions, thus affecting communication, food intake, work, socialization and overall health and quality of life. This article reviews current research on the characteristics and features of these disorders in individuals with PD, and the types of treatment techniques available (medical, surgical or behavioral), with emphasis on the Lee Silverman Voice Treatment (LSVT®) as the treatment of choice to improve voice, speech and other orofacial functions. LSVT addresses major deficits underlying voice and speech in PD, namely, impaired scaling of movement amplitude and poor perception and self-regulation of vocal output. Based upon this, LSVT emphasizes increased amplitude of vocal output through intensive training in increased vocal loudness (with good voice quality), recalibration of vocal loudness perception and self-regulation of vocal effort. The potential contribution of LSVT and analogous behavioral treatment approaches to neural plasticity is also addressed, as are directions for future research.

Effect of duration of pitch-shifted feedback on vocal responses in patients with Parkinson's disease

Study of the pitch-shift reflex is useful for the investigation of how auditory feedback is used in the control of voice fundamental frequency. The present study was an attempt to learn if the basal ganglia are involved in central mechanisms of the pitch-shift reflex by comparing measures of the reflex in a group of Parkinson's disease patients with those measures in a group of control participants. The effect of varying duration of the pitch-shift stimulus (PSS) on the voice fundamental frequency (F0) response in 10 Parkinson's disease (PD) patients and 10 age-matched unaffected participants was investigated. Participants were instructed to vocalize into a microphone while their voice was fed back to them over headphones. This feedback of the vocal signal was shifed in pitch either up or down, with the duration of this shift systematically manipulated at 100 ms, 500 ms, and 1000 ms. Although the participants were on medication, making interpretation of the results problematic with regard to basal ganglia function, it was reasoned that positive effects could nevertheless suggest basal ganglia involvement in this reflex and motivate further research. Results indicated that both groups responded to increased stimulus duration of the pitch-shift stimulus with increases in reflex peak time, magnitude, and end times. However, PD patients had significantly longer peak times and end times than control participants for stimulus durations of 100 ms. These results suggest that basal ganglia dysfunction may affect mechanisms relating to the execution and termination of the pitch-shift reflex for brief stimulus durations. The results also support hypotheses of impaired sensory integration of auditory feedback in PD patients.

Early behavioral changes after nigro-striatal system damage can serve as predictors of striatal dopamine depletion

1. Rats which had received a unilateral injection of 6-OHDA into the substantia nigra were assigned to four lesion groups according to the degree of DA depletion in the neostriatum. In these animals, behavioral changes in the open-field were investigated during the first postoperative week. Overall, this analysis showed that the animals could adequately be characterized by behavior on day 1 and day 7 after lesion. 2. On the first day after lesion, the groups with the severest DA depletions (> 80% and 55-80%) showed an ipsilateral asymmetry in turning. After one week, these groups showed a tendency to recover from this deficit; however, the group with the most strongest lesions (> 80%) was still asymmetric. 3. In scanning behavior, in contrast to turning, all the lesion groups displayed an initial ipsilateral asymmetry. On day 7 after lesion, only the group with DA depletions of > 80% still had an ipsilateral asymmetry. Locomotor activity and rearing were initially reduced after lesion, and showed a tendency to recover, especially in the group with the most severe DA depletions (> 80%). There were no differences between groups neither on day 1 nor on day 7 by grooming, but this behavior increased in all the lesion groups with time. 4. The correlational analyses yielded a positive relationship between the asymmetry in turning and neostriatal DA depletion. Locomotor activity and rearing on day 1 were both negatively correlated with DA depletion. The present results show that a number of behavioral parameters obtained in the open-field are affected by unilateral lesions of the nigro-striatal DA system. The degree of deficit, its time course and relation to lesion size differs among the various behavioral measures. Some of these early behavioral changes after unilateral nigrostriatal lesion are related to DA depletion and should therefore be useful to predict lesion size.Together, these data suggest that the study of such behavioral changes can provide an important tool, to investigate the compensatory mechanisms underlying striatal DA depletion and to understand preclinical states of the Parkinson's disease.

An explanation for reflex blink hyperexcitability in Parkinson's disease. I. Superior colliculus

Hyperexcitable reflex blinks are a cardinal sign of Parkinson's disease. We investigated the neural circuit through which a loss of dopamine in the substantia nigra pars compacta (SNc) leads to increased reflex blink excitability. Through its inhibitory inputs to the thalamus, the basal ganglia could modulate the brainstem reflex blink circuits via descending cortical projections. Alternatively, with its inhibitory input to the superior colliculus, the basal ganglia could regulate brainstem reflex blink circuits via tecto-reticular projections. Our study demonstrated that the basal ganglia utilizes its GABAergic input to the superior colliculus to modulate reflex blinks. In rats with previous unilateral 6-hydroxydopamine (6-OHDA) lesions of the dopamine neurons of the SNc, we found that microinjections of bicuculline, a GABA antagonist, into the superior colliculus of both alert and anesthetized rats eliminated the reflex blink hyperexcitability associated with dopamine depletion. In normal, alert rats, decreasing the basal ganglia output to the superior colliculus by injecting muscimol, a GABA agonist, into the substantia nigra pars reticulata (SNr) markedly reduced blink amplitude. Finally, brief trains of microstimulation to the superior colliculus reduced blink amplitude. Histological analysis revealed that effective muscimol microinjection and microstimulation sites in the superior colliculus overlapped the nigrotectal projection from the basal ganglia. These data support models of Parkinsonian symtomatology that rely on changes in the inhibitory drive from basal ganglia output structures. Moreover, they support a model of Parkinsonian reflex blink hyper-excitability in which the SNr-SC target projection is critical.

A controlled study of facial mobility treatment in Parkinson's disease

The aim of this study was to evaluate the effects of orofacial physiotherapeutic treatment (OPT) on the facial mobility of Parkinson's disease (PD) sufferers. Sixteen participants with PD were allocated randomly to either the Treatment group or the Control group. A short interview between the physiotherapist and each subject was videotaped, and 10 random frames of the videotape were selected to be used in the facial expression assessment. The quantification of facial expressions was achieved by using an objective microcomputer-based measurement system, based on a mathematical model of the face (FACEM). A facial outline is obtained, as well as 12 facial measures, which represent distances between key facial landmarks. The facial assessment was performed on 3 separate occasions, that is, baseline (pretreatment), posttreatment, and follow-up (4 weeks later). A repeated measures analysis of variance (MANOVA) revealed a significant main effect of Time and a significant interaction effect between Time and Group for the Mouth-Opening Measure, suggesting that after treatment, members of the Treatment group opened their mouths to a greater degree than members of the Control group. Within the Treatment group, significant differences between pretreatment and posttreatment scores (MANOVA) were found for Mouth-Opening Measure and Mid-Top-Lip Measure. Similarly, Mouth-Opening Measure, Mid-Top-Lip Measure, Lower-Lip Thickness Measure, Top Eyelid/Iris Intersect Measure and Lower Eyelid/Iris Intersect Measure were significant across time from baseline to follow-up in the Treatment group only. No significant differences were found on any of the facial measures during the same period for members of the Control group. These findings suggest that OPT can improve facial movement and that this benefit extends in time, beyond the period of OPT itself. Such an increase in facial mobility can be expected to modify the "Parkinsonian facies" and facilitate the display of facial expressions.

Jaw movement dysfunction related to Parkinson's disease and partially modified by levodopa

OBJECTIVES

To test the hypotheses that Parkinson's disease can differentially produce deficits in voluntary and rhythmic jaw movements, which involve different neuronal circuits, and that levodopa treatment improves specific components of the motor deficit.

METHODS

Patients with idiopathic Parkinson's disease and control subjects were tested on a series of jaw motor tasks that included simple voluntary movement, isometric clenching, and natural and paced rhythmic movements. Jaw movements were measured by changes in electromagnetic fields and EMG activity. Patients with Parkinson's disease with fluctuations in motor responses to levodopa were tested while off and on.

RESULTS

During the off state, patients with Parkinson's disease were significantly worse than the control subjects on most tasks. The deficits included a decrease in amplitude and velocity during jaw opening and closing, aberrant patterns and low amplitude of EMG activity during clenching, and low vertical amplitude and prolonged durations of occlusion during rhythmic movements. No decrements were found in the amplitude of voluntary lateral jaw movements or the frequency of rhythmic movements. During the on state, improvements occurred in the patterns and level of EMG activity during clenching and in the vertical amplitude and duration of occlusion during rhythmic movements, although a significant decrement occurred in the lateral excursion of the jaw.

CONCLUSIONS

Parkinson's disease affects the central programming of functionally related muscles involved in voluntary and rhythmic jaw movements and levodopa replacement influences only certain aspects of jaw movement, most likely those requiring sensory feedback.

Abnormalities in long latency responses to superior laryngeal nerve stimulation in adductor spasmodic dysphonia

Sensorimotor responses to repeated electrical stimulation of the superior laryngeal nerve were compared in 8 patients with adductor spasmodic dysphonia (ADSD) and 11 normal controls to determine if adductor response disinhibition occurred in ADSD. Pairs of electrical pulses were presented at interstimulus intervals varying from 100 to 5,000 milliseconds (ms). Three responses were measured in thyroarytenoid muscles: ipsilateral R1 responses at 17 ms and ipsilateral and contralateral R2 responses between 60 and 75 ms. Conditioned response characteristics, the percent occurrence and percentage amplitude of initial responses, were measures of response inhibition. As a group, the patients had reduced response inhibition: their conditioned ipsilateral R1 response amplitudes were increased, as was the frequency of their conditioned contralateral muscle responses (p < or = .002) compared to normal. However, the patients' initial responses were normal in latency and frequency characteristics, demonstrating that the brain stem mechanisms for these responses were intact. These results suggest a central disinhibition of laryngeal responses to sensory input in ADSD.

The role of the basal ganglia in nociception and pain

The involvement of the basal ganglia in motor functions has been well studied. Recent neurophysiological, clinical and behavioral experiments indicate that the basal ganglia also process non-noxious and noxious somatosensory information. However, the functional significance of somatosensory information processing within the basal ganglia is not well understood. This review explores the role of the striatum, globus pallidus and substantia nigra in nociceptive sensorimotor integration and suggests several roles of these basal ganglia structures in nociception and pain. Electrophysiological experiments have detailed the non-nociceptive and nociceptive response properties of basal ganglia neurons. Most studies agree that some neurons within the basal ganglia encode stimulus intensity. However, these neurons do not appear to encode stimulus location since the receptive fields of these cells are large. Many basal ganglia neurons responsive to somatosensory stimulation are activated exclusively or differentially by noxious stimulation. Indirect techniques used to measure neuronal activity (i.e., positron emission tomography and 2-deoxyglucose methods) also indicate that the basal ganglia are activated differentially by noxious stimulation. Neuroanatomical experiments suggest several pathways by which nociceptive information may reach the basal ganglia. Neuroanatomical studies have also indicated that the basal ganglia are rich in many different neuroactive chemicals that may be involved in the modulation of nociceptive information. Microinjection of opiates, dopamine and gamma-aminobutyric acid (GABA) into the basal ganglia have varied effects on pain behavior. Administration of these neurochemicals into the basal ganglia affects supraspinal pain behaviors more consistently than spinal reflexive behaviors. The reduction of pain behavior following electrical stimulation of the substantia nigra and caudate nucleus provides additional evidence for a role of the basal ganglia in pain modulation. Some patients with basal ganglia disease (e.g., Parkinson's disease, Huntington's disease) have alterations in pain sensation in addition to motor abnormalities. Frequently, these patients have intermittent pain that is difficult to localize. Collectively, these data suggest that the basal ganglia may be involved in the (1) sensory-discriminative dimension of pain, (2) affective dimension of pain, (3) cognitive dimension of pain, (4) modulation of nociceptive information and (5) sensory gating of nociceptive information to higher motor areas. Further experiments that correlate neuronal discharge activity with stimulus intensity and escape behavior in operantly conditioned animals are necessary to fully understand how the basal ganglia are involved in nociceptive sensorimotor integration.

Behavioral asymmetries and recovery in rats with different degrees of unilateral striatal dopamine depletion

A detailed behavioral analysis during the first postoperative week was performed in rats which had sustained various degrees of unilateral neostriatal dopamine (DA) lesions by administration of the neurotoxin 6-hydroxydopamine into the substantia nigra. These animals were assigned to different groups according to their residual DA levels in the damaged neostriatum (as percentage of the intact side). On the first day after toxin injection into the substantia nigra, turning asymmetries (tight turns) toward the side of the lesion were observed in animals with a mean residual DA level of 32% or less. Out of these, the strongest asymmetries were observed in animals with a mean residual DA of 3%. After one week, the asymmetry in tight turns had totally recovered except in those groups with mean residual DA levels of 17% or less. Partial recovery was found in animals with mean residual DA of 9 and 17%, whereas no indication for recovery was found in animals with the most severe lesions (mean residual DA 3%). Measurement of thigmotactic scanning also revealed an asymmetry for the side of the lesion on the first post-operative day. This asymmetry was observed over a wider range of DA lesion than that observed in turning, namely up to a mean residual DA level of 78%. Furthermore, recovery to symmetry was observed in all lesion-groups except in those with more severe lesions (mean residual DA 17% or less). In contrast to turning, the strongest asymmetries were not displayed by the animals with the most severe lesions. Furthermore, locomotor activity was affected by the lesion, since on the first postoperative day locomotion was reduced in animals with mean residual DA of 39% or less. On day 7, this lesion-dependent deficit had recovered to control levels. Finally, the analysis of net turns allowed the prediction of lesion size in animals with residual DA levels of less than 15%. These results are discussed with respect to mechanisms of recovery, the role of lesion size, and the value of different behavioral measures to predict the degree of DAergic lesion.

Management of orofacial manifestations of Parkinson's disease with splint therapy: a case report

Patients with Parkinson's Disease display a number of orofacial manifestations. These manifestations are secondary to motor and sensory deficits, resulting in bradykinesia, muscle rigidity and tremor, and difficulties with speech, swallowing, proprioception, tactile sensitivity, and hard- and soft-tissue trauma. This case describes the use of a bruxism splint which benefited a PD patient by diminishing the occurrence of orofacial pain secondary to muscle tremor and rigidity while reducing the cumulative damage to the oral structures common with the disease. The impact of therapy on sleep, speech, and swallowing difficulties is also discussed.

Behavioral indices of moderate nigro-striatal 6-hydroxydopamine lesion: a preclinical Parkinson's model

Asymmetries in turning and scanning were investigated in rats with different degrees of neostriatal dopamine depletion after unilateral injections of 6-hydroxydopamine into the substantia nigra. Animals with severe lesions, i.e., residual dopamine levels of < 20%, spontaneously turned ipsiversive and showed more scanning behavior with the side ipsilateral to the lesion. These asymmetries were reversed by the dopamine receptor agonist apomorphine. Animals with less severe dopamine depletion, i.e., residual dopamine levels of 20-65%, did not show an asymmetry in spontaneous turning, but an ipsilateral asymmetry in scanning was still observed, indicating a greater sensitivity of this measure for moderate striatal dopamine depletions. Furthermore, in animals with residual dopamine levels of 45-65%, the dopamine receptor agonist apomorphine did not lead to a behavioral reversal as with severe lesions, but induced ipsilateral scanning and ipsiversive turning. These ipsiversive asymmetries are discussed in relation to asymmetries in self-regulatory mechanisms of the nigro-striatal dopamine system, such as dopamine autoreceptors controlling the release of this transmitter. Dopamine receptor-stimulated behavioral asymmetry in animals with moderate depletions of dopamine is suggested as a preclinical model to study mechanisms affected in the early state of Parkinson's disease.

Changes in motoneuron excitability of masseter muscle following exteroceptive stimuli in Parkinson's disease

Changes in motoneuron activity of masseter muscle to exteroceptive stimuli were evaluated in parkinsonian patients. Two different electrophysiological procedures were applied, consisting of exteroceptive suppression of the masseter or excitability curves of the masseteric reflex obtained by using exteroceptive conditioning stimuli. Seven patients not yet treated with dopaminergic or anticholinergic drugs were compared with 10 age-matched normal volunteers. Only the second phase of exteroceptive suppression was examined because correct measurement of the first phase was impossible due to the stimulus artefact. No significant differences were observed in exteroceptive suppression between parkinsonian patients and normals. Early and late inhibitory phases of the excitability curve of the masseteric reflex were obtained in both normals and patients. However, parkinsonians showed less inhibitory change than normals in both early and late phase of the curve. This study confirms that reduced inhibition of the masseteric reflex to exteroceptive stimuli is present in Parkinson's disease. The excitability curve of the masseteric reflex represents a more reliable method than exteroceptive suppression in detecting these abnormalities.

Perioral reflexes in orofacial dyskinesia and spasmodic dysphonia

In order to assess the clinical utility of trigemino-facial reflexes in lower facial muscles, we studied perioral reflexes to mechanical and electrical stimulation in 13 patients with spasmodic dysphonia and orofacial dyskinesia and in 7 healthy subjects. Mechanical stimulation of the upper lip of all patients and electrical stimulation of the infraorbital nerve of patients with orofacial dyskinesia elicited larger perioral reflexes than in controls. In the majority of patients, hyperexcitable perioral reflexes were accompanied by increased gain of the blink reflex. In 4 patients, however, trigemino-facial reflexes were enhanced selectively in either the perioral muscles or orbicularis oculi. Our findings suggest that the quantitative assessment of perioral reflexes may provide information about the excitability of brainstem interneurons in cranial dystonia that is complementary to blink reflex studies.

Levodopa induced ON-OFF motor fluctuations in Parkinson's disease related to rhythmical masticatory jaw movements

The motor disturbance in Parkinson's disease affects all voluntary movement, including innate rhythmical processes such as gait, breathing, and chewing. While there are good descriptions and pathophysiological hypotheses of the changes in gait less is known about the way masticatory movements are affected. By means of a three-dimensional optoelectronic recording technique the differences were investigated in mandibular movement displacement, velocity, and masticatory cycle duration during levodopa induced OFF and ON states. Recordings were made before ordinary morning medication and one hour after medication with 200 mg levodopa and a decarboxylase inhibitor (madopar). There were no differences before and after medication in the opening and closing duration of the masticatory cycle, but a significantly decreased occlusal level phase duration in the ON state was seen. Mandibular velocity and mandibular movement amplitude were significantly greater after medication. Thus the changes in dopamine transmission selectively influenced parts of the masticatory cycle but not the fundamental rhythmical pattern.

Gain and spatial characteristics of human lip-muscle reflexes

Mechanically evoked early and late excitatory reflexes (E1 and E2) and suppression responses (S) were studied in human lip muscle. Acceleration pulses were applied at 30-150 m/s2 independently to the upper and lower lips during lip rounding and lower-lip depression postures, and to both lips during a lip-press posture. E1 responses were prevalent during lip rounding and press gestures and S responses during lip depression. Reflex magnitudes were well correlated with stimulus acceleration for the 3 response components, with E1 responses showing the strongest association. The slopes of linear equations relating reflex and stimulus magnitudes, i.e. reflex gains, for E1 and E2 responses were highly variable across subjects. This variability was partially related to subject gender, females showing larger reflex gains. Two novel findings on the spatial aspects of lip-muscle reflexes are that: (1) S responses in lower-lip muscle are more prevalent to upper-lip versus lower-lip stimulation, and (2) E1 and E2 gains in lower-lip muscle are larger for stimulation of the lower lip compared to stimulation of the upper lip. Further testing suggested that this latter pattern differs with form of stimulation, with a laterally directed sliding stimulus on the upper lip producing predominant effects in both the upper- and lower-lip muscle, and an indenting stimulus producing the largest reflexes in muscle of the stimulated lip.