Subcortical evoked activity and motor enhancement in Parkinson's disease

Paradoxical kinesia may no longer be a paradox waiting for 100 years to be unraveled

Parkinson's disease (PD) is a progressive neurodegenerative disorder mainly characterized by bradykinesia and akinesia. Interestingly, these motor disabilities can depend on the patient emotional state. Disabled PD patients remain able to produce normal motor responses in the context of urgent or externally driven situations or even when exposed to appetitive cues such as music. To describe this phenomenon Souques coined the term "paradoxical kinesia" a century ago. Since then, the mechanisms underlying paradoxical kinesia are still unknown due to a paucity of valid animal models that replicate this phenomenon. To overcome this limitation, we established two animal models of paradoxical kinesia. Using these models, we investigated the neural mechanisms of paradoxical kinesia, with the results pointing to the inferior colliculus (IC) as a key structure. Intracollicular electrical deep brain stimulation, glutamatergic and GABAergic mechanisms may be involved in the elaboration of paradoxical kinesia. Since paradoxical kinesia might work by activation of some alternative pathway bypassing basal ganglia, we suggest the IC as a candidate to be part of this pathway.

Moving, fast and slow: behavioural insights into bradykinesia in Parkinson's disease

The debilitating symptoms of Parkinson's disease, including the hallmark slowness of movement, termed bradykinesia, were described more than 100 years ago. Despite significant advances in elucidating the genetic, molecular and neurobiological changes in Parkinson's disease, it remains conceptually unclear exactly why patients with Parkinson's disease move slowly. To address this, we summarize behavioural observations of movement slowness in Parkinson's disease and discuss these findings in a behavioural framework of optimal control. In this framework, agents optimize the time it takes to gather and harvest rewards by adapting their movement vigour according to the reward that is at stake and the effort that needs to be expended. Thus, slow movements can be favourable when the reward is deemed unappealing or the movement very costly. While reduced reward sensitivity, which makes patients less inclined to work for reward, has been reported in Parkinson's disease, this appears to be related mainly to motivational deficits (apathy) rather than bradykinesia. Increased effort sensitivity has been proposed to underlie movement slowness in Parkinson's disease. However, careful behavioural observations of bradykinesia are inconsistent with abnormal computations of effort costs due to accuracy constraints or movement energetic expenditure. These inconsistencies can be resolved when considering that a general disability to switch between stable and dynamic movement states can contribute to an abnormal composite effort cost related to movement in Parkinson's disease. This can account for paradoxical observations such as the abnormally slow relaxation of isometric contractions or difficulties in halting a movement in Parkinson's disease, both of which increase movement energy expenditure. A sound understanding of the abnormal behavioural computations mediating motor impairment in Parkinson's disease will be vital for linking them to their underlying neural dynamics in distributed brain networks and for grounding future experimental studies in well-defined behavioural frameworks.

Influence of alarming auditory cues on viscoelastic stiffness of skeletal muscles in patients with Parkinson's disease

BACKGROUND

Patients with Parkinson's disease can show brief normalization of motor activity in response to intense external stimuli - a phenomenon known as paradoxical kinesis. The purpose of the present study was to examine the effect of alarming auditory signals on the level of viscoelastic stiffness of skeletal muscles as an indicator of parkinsonian rigidity.

METHODS

Myotonometry was used to determine the changes of viscoelastic stiffness of skeletal muscles in ten patients in an advanced stage of Parkinson's disease, treated with deep brain stimulation, and ten healthy controls. The measurements were repeated and compared during the stimulation-on and stimulation-off periods, with and without auditory alarming signals.

FINDINGS

The mean values of stiffness measured in the stimulation-off phase (370.4 N/m) were significantly higher than the values obtained in the stimulation-on phase (339.2 N/m) (q = 6.05; P < 0.01) but also in the stimulation-off with alarming signals phase (349.6 N/m) (q = 4.04; P < 0.05). In the normal controls, exposure to the auditory alarming signals did not change the values of viscoelastic stiffness.

INTERPRETATION

These findings demonstrate that the phenomenon of paradoxical kinesis is associated with the changes of muscular rigidity in parkinsonian patients. Results from the study may help to establish new strategies for addressing motor disabilities in patients with Parkinson's disease.

Role of pedunculopontine nucleus in sleep-wake cycle and cognition in humans: A systematic review of DBS studies

BACKGROUND

Animal studies have demonstrated that the pedunculopontine nucleus (PPN) is involved in the control of posture and gait, and that it is also a key structure in controlling basic non-motor functions such as sleep, attention and arousal. In this systematic review we aimed to evaluate all available studies assessing the role of PPN on cognition, nocturnal sleep and alertness in humans. Finally, we attempted to define a model in which PPN acts as an interface structure between motor control and behavior.

METHODS

A systematic search of the computerized databases MEDLINE and PubMed was conducted to identify papers on PPN and cognitive functions, sleep and alertness. Key search terms included: 'PPN', 'arousal', 'sleep', 'cognition', 'memory', 'language', 'attention', 'alertness', 'PPN-DBS', 'Parkinson's and PPN', 'Parkinson's and PPN-DBS'.

RESULTS

Twelve studies met our inclusion criteria and were included. All of them involved PD patients implanted with unilateral or bilateral PPN-DBS, most patients had concomitant DBS of another anatomical structure (subthalamic nucleus or Zona incerta). There is a lack of consistent evidences confirming the effect of PPN-DBS on specific cognitive functions, alertness or sleep in PD. There is heterogeneity between and within surgical centres of study protocols especially regarding DBS targeting, parameters of stimulation and experimental methods. Moreover, the available studies are limited by the small sample size and the short follow-up time. It has been suggested that low frequency stimulation (25 Hz) has a better effect compared to the high frequency one (60-80 Hz) on alertness, however this needs to be confirmed in further studies.

CONCLUSIONS

PPN-DBS is a promising but yet an experimental procedure. PD represents an encouraging pathological model for future studies aiming to shade light on the role of PPN in cognition, attention and alertness in humans.

What is the therapeutic mechanism of pedunculopontine nucleus stimulation in Parkinson's disease?

Pedunculopontine nucleus (PPN) deep brain stimulation (DBS) is an experimental treatment for Parkinson's disease (PD) which offers a fairly circumscribed benefit for gait freezing and perhaps balance impairment. The benefit on gait freezing is variable and typically incomplete, which may reflect that the clinical application is yet to be optimised or reflect a fundamental limitation of the therapeutic mechanism. Thus, a better understanding of the therapeutic mechanism of PPN DBS may guide the further development of this therapy. The available evidence supports that the PPN is underactive in PD due to a combination of both degeneration and excessive inhibition. Low frequency PPN DBS could enhance PPN network activity, perhaps via disinhibition. A clinical implication is that in some PD patients, the PPN may be too degenerate for PPN DBS to work. Reaction time studies report that PPN DBS mediates a very specific benefit on pre-programmed movement. This seems relevant to the pathophysiology of gait freezing, which can be argued to reflect impaired release of pre-programmed adjustments to locomotion. Thus, the benefit of PPN DBS on gait freezing could be akin to that mediated by external cues. Alpha band activity is a prominent finding in local field potential recordings from PPN electrodes in PD patients. Alpha band activity is implicated in the suppression of task irrelevant processes and thus the effective allocation of attention (processing resources). Attentional deficits are prominent in patients with PD and gait freezing and PPN alpha activity has been observed to drop out prior to gait freezing episodes and to increase with levodopa. This raises the hypothesis that PPN DBS could support or emulate PPN alpha activity and consequently enhance the allocation of attention. Although PPN DBS has not been convincingly shown to increase general alertness or attention, it remains possible that PPN DBS may enhance the allocation of processing resources within the motor system, or "motor attention". For example, this could facilitate the 'switching' of motor state between continuation of pattern generated locomotion towards the intervention of pre-programmed adjustments. However, if the downstream consequence of PPN DBS on movement is limited to a circumscribed unblocking of pre-programmed movement, then this may have a similarly circumscribed degree of benefit for gait. If this is the case, then it may be possible to identify patients who may benefit most from PPN DBS. For example, those in whom pre-programmed deficits are the major contributors to gait freezing.

Reticulospinal Systems for Tuning Motor Commands

The pontomedullary reticular formation (RF) is a key site responsible for integrating descending instructions to execute particular movements. The indiscrete nature of this region has led not only to some inconsistencies in nomenclature, but also to difficulties in understanding its role in the control of movement. In this review article, we first discuss nomenclature of the RF, and then examine the reticulospinal motor command system through evolution. These command neurons have direct monosynaptic connections with spinal interneurons and motoneurons. We next review their roles in postural adjustments, walking and sleep atonia, discussing their roles in movement activation or inhibition. We propose that knowledge of the internal organization of the RF is necessary to understand how the nervous system tunes motor commands, and that this knowledge will underlie strategies for motor functional recovery following neurological injuries or diseases.

REM sleep behaviour disorder: prodromal and mechanistic insights for Parkinson's disease

Rapid eye movement (REM) sleep behaviour disorder (RBD) is characterised by complex motor enactment of dreams and is a potential prodromal marker of Parkinson's disease (PD). Of note, patients with PD observed during RBD episodes exhibit improved motor function, relative to baseline states during wake periods. Here, we review recent epidemiological and mechanistic findings supporting the prodromal value of RBD for PD, incorporating clinical and electrophysiological studies. Explanations for the improved motor function during RBD episodes are evaluated in light of recent publications. In addition, we present preliminary findings describing changes in the activity of the basal ganglia across the sleep-wake cycle that contribute to our understanding of RBD.

The Phenomenology of Parkinson's Disease

The motor symptoms of Parkinson's disease are not limited to the cardinal symptoms of bradykinesia, rigidity, and resting tremor, but also include a variety of interrelated motor phenomena such as deficits in spatiotemporal planning and movement sequencing, scaling and timing of movements, and intermuscular coordination that can be clinically observed. Although many of these phenomena overlap, a review of the full breadth of the motor phenomenon can aid in the diagnosis and monitoring of disease progression.

Comparison of oscillatory activity in subthalamic nucleus in Parkinson's disease and dystonia

OBJECTIVES

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) has been successfully used to treat both Parkinson's disease (PD) and dystonia. Local field potentials (LFPs) recorded from the STN of PD patients demonstrate prominent beta frequency band activity. It is unclear whether such activity occurs in the STN in dystonia, and, if not, whether dystonia has another distinctive neural population activity in the STN.

METHODS

Twelve patients with PD, and eight patients with dystonia underwent DBS electrode implantation targeting the STN. Seven dystonia patients were off medication and one was on aripiprazole and clonazepam. LFPs were recorded from the DBS electrodes in PD in the on/off medication states and in dystonia. Power spectra and temporal dynamics measured by the with Lempel-Ziv complexity of the LFPs were compared among these states.

RESULTS

Normalised power spectra and Lempel-Ziv complexity of subthalamic LFPs differed between dystonia off and PD on/off, and between PD off and on over the low frequency, beta and high gamma bands. Patients with dystonia and off medication had lower beta power but higher low frequency and high gamma power than PD. Spectral power in the low beta frequency (11-20Hz) range was attenuated in medicated PD.

CONCLUSION

The results suggest that dystonia and PD are characterized by different patterns of oscillatory activities even within the same nucleus, and exaggerated beta activity may relate to hypo-dopaminergic status.