Subthalamic Neurons Encode Both Single- and Multi-Limb Movements in Parkinson's Disease Patients

Semantic encoding during language comprehension at single-cell resolution

From sequences of speech sounds1,2 or letters3, humans can extract rich and nuanced meaning through language. This capacity is essential for human communication. Yet, despite a growing understanding of the brain areas that support linguistic and semantic processing4-12, the derivation of linguistic meaning in neural tissue at the cellular level and over the timescale of action potentials remains largely unknown. Here we recorded from single cells in the left language-dominant prefrontal cortex as participants listened to semantically diverse sentences and naturalistic stories. By tracking their activities during natural speech processing, we discover a fine-scale cortical representation of semantic information by individual neurons. These neurons responded selectively to specific word meanings and reliably distinguished words from nonwords. Moreover, rather than responding to the words as fixed memory representations, their activities were highly dynamic, reflecting the words' meanings based on their specific sentence contexts and independent of their phonetic form. Collectively, we show how these cell ensembles accurately predicted the broad semantic categories of the words as they were heard in real time during speech and how they tracked the sentences in which they appeared. We also show how they encoded the hierarchical structure of these meaning representations and how these representations mapped onto the cell population. Together, these findings reveal a finely detailed cortical organization of semantic representations at the neuron scale in humans and begin to illuminate the cellular-level processing of meaning during language comprehension.

Subthalamic neurons interact with nigral dopaminergic neurons to regulate movement in mice

AIM

This study aims to address the role of the interaction between subthalamic (STN) neurons and substantia nigra pars compacta (SNc) dopaminergic (DA) neurons in movement control.

METHODS

Fiber photometry and optogenetic/chemogenetic techniques were utilized to monitor and manipulate neuronal activity, respectively. Locomotion in mice was recorded in an open field arena and on a head-fixed apparatus. A hemiparkinsonian mouse model was established by unilateral injection of 6-OHDA in the medial forebrain bundle. Whole-cell patch-clamp techniques were applied to record electrophysiological signals in STN neurons and SNc DA neurons. c-Fos-immunostaining was used to label activated neurons. A rabies virus-based retrograde tracing system was used to visualize STN neurons projecting to SNc DA neurons.

RESULTS

The activity of STN neurons was enhanced upon locomotion in an open field arena and on a head-fixed apparatus, and the enhancement was significantly attenuated in parkinsonian mice. Optogenetic stimulation of STN neurons enhanced locomotion, increased activity of SNc DA neurons, meanwhile, reduced latency to movement initiation. Combining optogenetics with patch-clamp recordings, we confirmed that STN neurons innervated SNc DA neurons through glutamatergic monosynaptic connections. Moreover, STN neurons projecting to SNc DA neurons were evenly distributed in the STN. Either 6-OHDA-lesion or chemogenetic inhibition of SNc DA neurons attenuated the enhancement of locomotion by STN stimulation.

CONCLUSION

SNc DA neurons not only affect the response of STN neurons to movement, but also contribute to the enhancement of movement by STN stimulation. This study demonstrates the role of STN-SNc interaction in movement control.

Human subthalamic nucleus neurons differentially encode speech and limb movement

Deep brain stimulation (DBS) of the subthalamic nucleus (STN), which consistently improves limb motor functions, shows mixed effects on speech functions in Parkinson's disease (PD). One possible explanation for this discrepancy is that STN neurons may differentially encode speech and limb movement. However, this hypothesis has not yet been tested. We examined how STN is modulated by limb movement and speech by recording 69 single- and multi-unit neuronal clusters in 12 intraoperative PD patients. Our findings indicated: (1) diverse patterns of modulation in neuronal firing rates in STN for speech and limb movement; (2) a higher number of STN neurons were modulated by speech vs. limb movement; (3) an overall increase in neuronal firing rates for speech vs. limb movement; and (4) participants with longer disease duration had higher firing rates. These data provide new insights into the role of STN neurons in speech and limb movement.

Distinct population code for movement kinematics and changes of ongoing movements in human subthalamic nucleus

The subthalamic nucleus (STN) is theorized to globally suppress movement through connections with downstream basal ganglia structures. Current theories are supported by increased STN activity when subjects withhold an uninitiated action plan, but a critical test of these theories requires studying STN responses when an ongoing action is replaced with an alternative. We perform this test in subjects with Parkinson’s disease using an extended reaching task where the movement trajectory changes mid-action. We show that STN activity decreases during action switches, contrary to prevalent theories. Furthermore, beta oscillations in the STN local field potential, which are associated with movement inhibition, do not show increased power or spiking entrainment during switches. We report an inhomogeneous population neural code in STN, with one sub-population encoding movement kinematics and direction and another encoding unexpected action switches. We suggest an elaborate neural code in STN that contributes to planning actions and changing the plans.

Targeting of the Subthalamic Nucleus in Patients with Parkinson's Disease Undergoing Deep Brain Stimulation Surgery

Precise stereotactic targeting of the dorsolateral motor part of the subthalamic nucleus (STN) is paramount for maximizing clinical effectiveness and preventing side effects of deep brain stimulation (DBS) in patients with advanced Parkinson's disease. With recent developments in magnetic resonance imaging (MRI) techniques, direct targeting of the dorsolateral part of the STN is now feasible, together with visualization of the motor fibers in the nearby internal capsule. However, clinically relevant discrepancies were reported when comparing STN borders on MRI to electrophysiological STN borders during microelectrode recordings (MER). Also, one should take into account the possibility of a 3D inaccuracy of up to 2 mm of the applied stereotactic technique. Pneumocephalus and image fusion errors may further increase implantation inaccuracy. Even when implantation has been successful, suboptimal lead anchoring on the skull may cause lead migration during follow-up. Meticulous pre- and intraoperative imaging is therefore indispensable, and so is postoperative imaging when the effects of DBS deteriorate during follow-up. Thus far, most DBS centers employ MRI targeting, multichannel MER, and awake test stimulation in STN surgery, but randomized trials comparing surgery under local versus general anesthesia and additional studies comparing MER-STN borders to high-field MRI-STN may change this clinical practice. Further developments in imaging protocols and improvements in image fusion processes are needed to optimize placement of DBS leads in the dorsolateral motor part of the STN in Parkinson's disease.

Kinesthetic Cells within the Subthalamic Nucleus and Deep Brain Stimulation for Parkinson Disease

OBJECTIVE

We sought to determine the location of kinesthetic cell clusters within the subthalamic nucleus (STN) on magnetic resonance imaging, adjusted for interindividual anatomic variability by employing the medial STN border as a reference point.

METHODS

We retrospectively localized microelectrode recording-defined kinesthetic cells on 3-Tesla T2-weighted and susceptibility-weighted images in patients who underwent STN deep brain stimulation for Parkinson disease and averaged the stereotactic coordinates. These locations were calculated relative to the nonindividualized midcommissural point (MCP) and, in order to account for interindividual anatomic variability, also calculated relative to the patient-specific intersection of Bejjani line with the medial STN border. Two example patients were selected in order to visualize the discrepancies between the adjusted and nonadjusted theoretic kinesthetic cell clusters on magnetic resonance imaging.

RESULTS

Relative to the MCP, average kinesthetic cell coordinates were 12.3 ± 1.2 mm lateral, 1.7 ± 1.4 mm posterior, and 2.3 ± 1.5 mm inferior. Relative to the medial STN border, mean coordinates were 3.4 ± 1.0 mm lateral, 1.0 ± 1.4 mm anterior, and 1.7 ± 1.5 mm superior on T2-sequences, and on susceptibility-weighted images mean coordinates were 3.2 ± 1.1 mm lateral, 0.8 ± 1.5 mm anterior, and 2.1 ± 1.5 mm superior. The theoretic kinesthetic cell clusters may appear outside the sensorimotor STN when using the MCP, whereas these clusters fall well within the sensorimotor STN when employing the medial STN border as a reference point.

CONCLUSIONS

By using the medial STN border as a patient-specific anatomic reference point in STN deep brain stimulation for Parkinson disease, we accounted for interindividual anatomic variability and provided accurate insight in the clustering of kinesthetic cells within the dorsolateral STN.

Degradation of Neuronal Encoding of Speech in the Subthalamic Nucleus in Parkinson's Disease

BACKGROUND

Most of the patients with Parkinson's disease suffer from speech disorders characterized mainly by dysarthria and hypophonia.

OBJECTIVE

To understand the deterioration of speech in the course of Parkinson's disease.

METHODS

We intraoperatively recorded single neuron activity in the subthalamic nucleus of 18 neurosurgical patients with Parkinson's disease undergoing implantation of deep brain stimulator while patients articulated 5 vowel sounds.

RESULTS

Here, we report that single subthalamic neurons encode individual vowel phonemes and employ 1 of 2 encoding schemes: broad or sharp tuning. Broadly tuned units respond to all examined phonemes, each with a different firing rate, whereas sharply tuned ones are specific to 1 to 2 phonemes. We then show that in comparison with patients without speech deficits, the spiking activity in patients with speech disorders was lower during speech production, overt or imagined, but not during perception. However, patients with speech disorders employed a larger percentage of the neurons for the aforementioned tasks. Whereas the lower firing rates affect mainly sharply tuned units, the extra units used a broad tuning encoding scheme.

CONCLUSION

Our findings suggest mechanisms of neuronal degradation due to Parkinsonian speech disorders and their possible compensation. As impairment in sharply tuned units may be compensated by broadly tuned ones, the proposed compensation model appears to be suboptimal, lending support to the persistence of speech disorders in the course of the disease.

Pace of movement: the role of single neurons in the subthalamic nucleus

OBJECTIVE

The ability to modulate the pace of movement is a critical factor in the smooth operation of the motor system. The authors recently described distinct and overlapping representations of movement kinematics in the subthalamic nucleus (STN), but it is still unclear how movement pace is modulated according to the demands of the task at the neuronal level in this area. The goal of this study was to clarify how different movement paces are being controlled by neurons in the STN.

METHODS

The authors performed direct recording of the electrical activity of single neurons in the STN of neurosurgical patients with Parkinson's disease undergoing implantation of a deep brain stimulator under local anesthesia while the patients performed repetitive foot and hand movements intraoperatively at multiple paces.

RESULTS

A change was observed in the neuronal population controlling the movement for each pace. The mechanism for switching between these controlling populations differs for hand and foot movements.

CONCLUSIONS

These findings suggest that disparate schemes are utilized in the STN for neuronal recruitment for motor control of the upper and lower extremities. The results indicate a distributed model of motor control within the STN, where the active neuronal population changes when modifying the task condition and pace.

Electrophysiological differences between upper and lower limb movements in the human subthalamic nucleus

OBJECTIVE

Functional processes in the brain are segregated in both the spatial and spectral domain. Motivated by findings reported at the cortical level in healthy participants we test the hypothesis in the basal ganglia of Parkinson's disease patients that lower frequency beta band activity relates to motor circuits associated with the upper limb and higher beta frequencies with lower limb movements.

METHODS

We recorded local field potentials (LFPs) from the subthalamic nucleus using segmented "directional" DBS leads, during which patients performed repetitive upper and lower limb movements. Movement-related spectral changes in the beta and gamma frequency-ranges and their spatial distributions were compared between limbs.

RESULTS

We found that the beta desynchronization during leg movements is characterised by a strikingly greater involvement of higher beta frequencies (24-31 Hz), regardless of whether this was contralateral or ipsilateral to the limb moved. The spatial distribution of limb-specific movement-related changes was evident at higher gamma frequencies.

CONCLUSION

Limb processing in the basal ganglia is differentially organised in the spectral and spatial domain and can be captured by directional DBS leads.

SIGNIFICANCE

These findings may help to refine the use of the subthalamic LFPs as a control signal for adaptive DBS and neuroprosthetic devices.