Functional correlations between neighboring neurons in the primate globus pallidus are weak or nonexistent

The anatomical structure of the basal ganglia displays topographical organization and massive funneling of neuronal projections toward the globus pallidus as well as an axonal collateral system within this nucleus. This structure suggests the formation of correlations between the spiking activities of pallidal cells. Nevertheless, previous studies of remote neurons in the pallidum have reported uncorrelated spiking activity. These correlation results may be challenged, because remote pallidal neurons may be located in different pallidal territories. To further test the independence of pallidal activity, we studied the spiking activity of neighboring pairs recorded by the same electrodes. A narrow peak dominated the correlations of all pairs of neurons recorded on the same electrode. This type of peak is classically interpreted as a sign of strong common input. However, recent mathematical analysis shows that such peaks may derive from a technical inability to detect overlapping spikes by spike-sorting techniques. A long-term shallow trough in the correlation of neighboring neurons may also result from the same effect, which we have termed the "shadowing effect." A comparison of the expected shadowing effect with the actual correlations suggests that no real correlations exist between 93.9% of neighboring pallidal pairs. The remaining 6.1% of the pairs display symmetric long-term positive correlations centered on time 0. Thus, functional interactions between neighboring pallidal neurons do not display any significant differences from the interactions between physically remote neurons in this brain area. Moreover, the combination of anatomical data and current physiological results suggests an active decorrelating process performed in the basal ganglia.

Age-related accelerated tapping response in healthy population

Different types of rapid tapping responses were described in the finger-tapping test. The "Hastening phenomenon" was described as an abnormal motor response in patients with Parkinson's disease. Accelerated tapping has been shown in a healthy elderly sample. It is not clear whether accelerated tapping relates to the hastening phenomenon or characterizes normal aging. We hypothesized that this sample of 21 healthy elderly people showed increased accelerated tapping but not hastening phenomenon. To assess this hypothesis, 20 healthy young and 21 elderly subjects performed a tapping test, requiring responses from 1 to 6 Hz. The healthy elderly sample showed increased accelerated tapping but not increased "hastening phenomenon." We conclude that Accelerated tapping may represent age-related motor processes unlike the hastening phenomenon characterizing Parkinson's disease.

Detection of onset of neuronal activity by allowing for heterogeneity in the change points

We consider situations in which there is a change point in the activity of a cell, that is, some time after an external event the firing rate of the cell changes. The change can occur after a random delay. The distribution of the time to change is considered unknown. Formally we deal with n random point processes, each of these is an inhomogeneous Poisson process, with one intensity until a random time, and a different intensity thereafter. Thus, the change point is not explicitly observed. We present both a simple estimator and the non-parametric maximum likelihood estimator (NPMLE) of the change point distribution, both having the same rate of convergence. This rate is proved to be the best possible. The extension of the basic model to multiple processes per trial with different intensities and joint multiple change points is demonstrated using both simulated and neural data. We show that for realistic spike train data, trial by trial estimation of a change point may be misleading, while the distribution of the change point distribution can be well estimated.

Single-unit activity related to bimanual arm movements in the primary and supplementary motor cortices

Single units were recorded from the primary motor (MI) and supplementary motor (SMA) areas of Rhesus monkeys performing one-arm (unimanual) and two-arm (bimanual) proximal reaching tasks. During execution of the bimanual movements, the task related activity of about one-half the neurons in each area (MI: 129/232, SMA: 107/206) differed from the activity during similar displacements of one arm while the other was stationary. The bulk of this "bimanual-related" activity could not be explained by any linear combination of activities during unimanual reaching or by differences in kinematics or recorded EMG activity. The bimanual-related activity was relatively insensitive to trial-to-trial variations in muscular activity or arm kinematics. For example, trials where bimanual arm movements differed the most from their unimanual controls did not correspond to the ones where the largest bimanual neural effects were observed. Cortical localization established by using a mixture of surface landmarks, electromyographic recordings, microstimulation, and sensory testing suggests that the recorded neurons were not limited to areas specifically involved with postural muscles. By rejecting this range of alternative explanations, we conclude that neural activity in MI as well as SMA can reflect specialized cortical processing associated with bimanual movements.

Timing of bimanual movements in human and non-human primates in relation to neuronal activity in primary motor cortex and supplementary motor area

This study investigates the timing of bimanual movements in a combined behavioral and physiological approach. Human subjects and rhesus monkeys performed the same bimanual task. In monkeys, we simultaneously recorded neuronal activity in the two hemispheres of primary motor cortex (MI) or supplementary motor area (SMA), and related it to bimanual coordination in the temporal domain. Both for monkeys and humans, the reaction times of bimanual movements never significantly exceeded the reaction times of the slower arm in unimanual movements. Consistent with this, the longest delay between neural activity onset in SMA and MI and movement initiation was observed in unimanual movements of the slower arm and not in bimanual movements. Both results suggest that the programming of bimanual movements does not require more processing time than unimanual movements. They are also consistent with the view that bimanual movements are programmed in a single process, rather than by combining two separate unimanual movement plans. In both humans and monkeys, movement initiation was highly correlated between the arms. However, once movements began, the temporal correlation between the arms progressively declined. Movement decorrelation was accompanied by a net decorrelation of neuronal population activity in MI and SMA, suggesting a functional connection between neuronal interactions and the level of bimanual coupling and decoupling. The similarity of neuronal activities in MI and SMA in relationship to behavioral timing lends support to the idea that both areas are involved in the temporal coordination of the arms.

Dopamine replacement therapy reverses abnormal synchronization of pallidal neurons in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine primate model of parkinsonism

Previous physiological studies have revealed changes in firing rates and synchronization of pallidal neurons in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) primate model of Parkinson's disease. Several primate and human studies have demonstrated that dopamine replacement therapy (DRT) reverses the changes in the pallidal firing rates; however, the effects of DRT on pallidal synchronization have never been explored. To do so, we recorded the simultaneous activity of pallidal neurons of a vervet monkey before and after induction of severe parkinsonism by systemic MPTP treatment. We subsequently recorded the pallidal activity before and after daily administration of oral DRT. We extended the time scale of our correlation studies to +/-5 sec to allow detection of long-duration synchronized neuronal activity. After MPTP treatment, firing rates decreased in the external segment of the globus pallidus (GP(e)) and increased in the internal segment (GP(i)). A reversal of these rate changes occurred during the "on" periods of DRT. The percentage of correlated pairs increased from 16.7% in the normal state to 46.9% after MPTP treatment and was restored to nearly normal values (25% correlated pairs) under the influence of DRT. These changes in rate and correlation were observed at both the population level and at the level of units recorded continuously before, during, and after the clinical transition from "off" to "on" periods. We conclude that changes in both pallidal discharge rates and synchronization are correlated with the clinical manifestations of parkinsonism and its pharmacological treatment.

Pathophysiology of nonparkinsonian tremors

Patients with nonparkinsonian tremors are the second largest group treated with functional neurosurgery. We summarize the present pathophysiological knowledge of these conditions. Essential tremor (ET) may be due to oscillations within the olivocerebellar circuit. There is experimental evidence from animal models for such a mechanism, and clinical data indicate an abnormal function of the cerebellum in ET. Cerebellar tremor may be closely related to the tremor seen in advanced ET. The malfunction of the cerebellum causes a pathological feed-forward control. Additionally an oscillator within the cerebellum or its input/output pathways may cause cerebellar tremor. Almost nothing is known about the pathophysiology of dystonic tremor. Holmes tremor is based on a nigral and a cerebellar malfunction and presents clinically as the combination of tremor in Parkinson's disease and cerebellar tremor. Neuropathic tremor can be extremely disabling and is thought to be due to an abnormal interaction of the disturbances within the periphery and abnormal cerebellar feedback. Unlike the case of Parkinson's disease, functional neurosurgery of nonparkinsonian tremors is not yet based on a solid pathophysiological background.

Pathophysiology of Parkinson's disease: from clinical neurology to basic neuroscience and back

Parkinson's disease (PD) is characterized by motor and nonmotor (cognitive and limbic) deficits. The motor signs of PD include hypokinetic signs such as akinesia/bradykinesia, rigidity and loss of normal postural reflexes, and hyperkinetic signs such as tremor. Dopamine depletion in the striatum is the hallmark of PD and of its animal models, still the pathophysiology of the parkinsonian symptoms and especially of parkinsonian tremor are under debate. The most extreme hypotheses argue about peripheral versus central nervous system origin, intrinsic cellular oscillator versus network oscillators, and basal ganglia-based pathophysiology versus cerebellar-thalamic based pathophysiology. Recent studies support the view that parkinsonian symptoms are most likely due to abnormal synchronous oscillating neuronal activity within the basal ganglia. Peripheral factors do only play a minor role for the generation, maintenance, and modulation of PD tremor and other signs. The most likely candidates producing these neuronal oscillations are the weakly coupled neural networks of the basal ganglia-thalamo-cortical loops. However, the present evidence supports the view that the basal ganglia loops are influenced by other neuronal structures and systems and that the tuning of these loops by cerebello-thalamic mechanisms and by other modulator neurotransmitter systems entrain the abnormal synchronized oscillations. Neurosurgical procedures, such as lesions or high-frequency stimulation of different parts of the loop, might resume the normal unsynchronized activity of the basal ganglia circuitry, and, therefore, ameliorate the clinical symptoms of Parkinson's disease.

Neuronal populations in primary motor cortex encode bimanual arm movements

Previous studies have shown that activity of neuronal populations in the primary motor cortex (MI), processed by the population vector method, faithfully predicts upcoming movements. In our previous studies we found that single neurons responded differently during movements of one arm vs. combined movements of the two arms. It was, therefore, not clear whether the population vector approach could produce reliable movement predictions also for bimanual movements. This study tests this question by comparing the predictive quality of population vectors for unimanual and bimanual arm movements. We designed a bimanual motor task that requires coordinated movements of the two arms, in which each arm may move in eight directions, and recorded single unit activity in the MI of two rhesus (Macaca mulatta) monkeys during the performance of unimanual and bimanual arm movements. We analysed the activity of 212 MI cells from both hemispheres and found that, despite bimanual related activity, the directional tuning and preferred directions of most cells were preserved in unimanual and bimanual movements. We demonstrate that population vectors, constructed from the activity of MI cells, predict accurately the direction of movement both for unimanual and for bimanual movements even when the two arms move simultaneously in different directions.

Bilateral overactivation of the sensorimotor cortex in the unilateral rodent model of Parkinson's disease - a functional magnetic resonance imaging study

Functional magnetic resonance imaging (fMRI) is used to investigate the basal ganglia (BG)-cortex circuit using a rat model of Parkinson's disease (PD). The model involves a unilateral destruction of the right substantia nigra by intranigral injection of the dopaminergic neurotoxin 6-hydroxydopamine. Volume of cortical activity was measured by the blood oxygenation level-dependent contrast method while applying electrical forepaw stimulation. The main findings are the following. (i) Contrary to the predictions of the classic model but in line with recent experimental results (positron emission tomography, fMRI and electrophysiology), an increased cortical activity in the sensorimotor cortex of PD rats compared with sham-operated or normal rats was found. (ii) A diffuse neuronal activity at large cortical areas that were not related directly to the stimulation used, was observed. (iii) No difference was found between the lesion and the nonlesion hemispheres when the left or the right forepaw was stimulated; both cortices show significant overactivation of the sensorimotor cortices in addition to diffuse cortical activation. The last finding could be explained by either corticocortical connections or by bilateral BG-cortex connections. These finding suggest that the mutual influence of the two hemispheres is important in the pathophysiology of the BG-cortex circuit and might be crucial in predicting treatments.

Fragmentation of care for frail older people--an international problem. Experience from three countries: Israel, Canada, and the United States

Cross-national comparisons of healthcare systems can help us to better understand them and to offer possible solutions for problems identified within these jurisdictions. Because multiple discontinuities present in most healthcare systems interfere with the appropriate clinical care of frail older people, we were interested in comparing the situation in three countries with markedly different healthcare systems. At one end of the spectrum we find Canada, with an almost fully socialized system. At the other stands the United States, where market forces are allowed the freest rein in any developed nation. Israel offers an intermediate model with elements held in common with both the U.S. and Canadian systems. Although the problems outlined in this paper can be addressed at the "micro" level, it is through an improvement in the structuring and organization of national systems of care that the appropriate conditions for the care of frail older people can be truly bettered. This international comparison offers insights for policy makers in these three states in particular and other countries in general.

Stepping out of the box: information processing in the neural networks of the basal ganglia

The Albin-DeLong 'box and arrow' model has long been the accepted standard model for the basal ganglia network. However, advances in physiological and anatomical research have enabled a more detailed neural network approach. Recent computational models hold that the basal ganglia use reinforcement signals and local competitive learning rules to reduce the dimensionality of sparse cortical information. These models predict a steady-state situation with diminished efficacy of lateral inhibition and low synchronization. In this framework, Parkinson's disease can be characterized as a persistent state of negative reinforcement, inefficient dimensionality reduction, and abnormally synchronized basal ganglia activity.

Neural interactions between motor cortical hemispheres during bimanual and unimanual arm movements

Cortico-cortical connections through the corpus callosum are a major candidate for mediating bimanual coordination. However, aside from the deficits observed after lesioning this connection, little positive evidence indicates its function in bimanual tasks. In order to address this issue, we simultaneously recorded neuronal activity at multiple sites within the arm area of motor cortex in both hemispheres of awake primates performing different bimanual and unimanual movements. By employing an adapted form of the joint peri-stimulus time histogram technique, we discovered rapid movement-related correlation changes between the local field potentials (LFPs) of the two hemispheres that escaped detection by time-averaged cross-correlation methods. The frequency and amplitude of dynamic modifications in correlations between the hemispheres were similar to those within the same hemisphere. As in previous EEG studies, we found that, on average, correlation decreased during movements. However, a subset of recording site pairs did show transiently increased correlations around movement onset (57% of all pairs and conditions in monkey G, 39% in monkey P). In interhemispheric pairs, these increases were consistently related to the mode of coupling between the two arms. Both the correlations between the movements themselves and the interhemispheric LFP correlation increases were strongest during bimanual symmetric movements, and weaker during bimanual asymmetric and unimanual movements. Increased correlations occurred mainly around movement onset, whilst decreases in correlation dominated during movement execution. The task-specific way in which interhemispheric correlations are modulated is compatible with the notion that interactions between the hemispheres contribute to behavioural coupling between the arms.

Normal optic nerve head topography in the early stages of dementia of the Alzheimer type

In view of the existing controversy as to whether or not the optic nerve head (ONH) is altered in Alzheimer disease, we used modern imaging technology to evaluate the ONH structure in individuals with dementia of the Alzheimer type (DAT). Real-time topographical images of the ONH were obtained with a Heidelberg retina tomograph from individuals in the early stages of DAT and age-matched controls. The various ONH parameters examined in this study did not differ significantly between DAT and age-matched subjects. These results suggest that the deficits in visual function that are known to occur in DAT are not related to ONH structural anomalies, at least in the earlier stages of the disease.

Trial to trial variability in either stimulus or action causes apparent correlation and synchrony in neuronal activity

In this report we show that the observed inter-neuronal correlation reflects a superposition of correlations associated with the intrinsic correlation between neurons, and correlations associated with variability in the stimuli presented to, or the actions performed by, the subject. We argue that the effects of either stimulus or action variability on the observed correlation, though generally ignored, can be substantial. Specifically, we demonstrate how observed correlations are effected by trial to trial variability in either stimulus or action. In addition, assuming that all relevant stimuli and actions are known, we outline a method for eliminating their effects on the observed correlation. It is also shown that tuning of correlations to a stimulus or an action might be a direct consequence of variability in that stimulus or action, even in the absence of any modulation of direct inter-neuronal interaction. The effects of stimulus and action variability should therefore be carefully considered when designing and interpreting experiments involving multi-neuronal recordings.

Synchrony of rest tremor in multiple limbs in parkinson's disease: evidence for multiple oscillators

Recent evidence points to involvement of central nervous system oscillators in Parkinson's disease (PD) rest tremor. It remains unknown whether one or multiple oscillators cause tremor in multiple limbs. Based on the prediction that multiple oscillators would cause low coherence even with similar average frequency, we studied 22 PD patients using accelerometers on multiple limbs. Records were digitized and spectral analysis was performed. Peak frequencies in the arms, legs, and chin were similar, indicating that biomechanical factors did not determine the frequency. Coherence between different axes of individual accelerometers and between different segments of the same limb was high. However, coherence between tremor in different limbs was low. There was no consistent pattern across patients of ipsi- vs. contralateral predominance of coherence. These data suggest that tremor in PD is generated by multiple oscillatory circuits, which operate on similar frequencies.

Local field potentials related to bimanual movements in the primary and supplementary motor cortices

We recorded local field potentials (LFP) in primary (MI) and supplementary (SMA) motor areas of rhesus monkey cortex in order to compare movement-evoked potentials (mEP) in bimanual and unimanual movements with single-unit activity recorded concurrently. The mEP was often different during bimanual and unimanual movements (a "bimanual-related" effect), but, unlike the single units, the size of the mEP in both MI and SMA was always greater during bimanual movements than during unimanual movements. This increase primarily reflected an increase in the late positive peak of the mEP, a result that may reflect greater overall cortical activation during bimanual movements. In addition, analysis of the mEP revealed differences between MI and SMA not seen in the single-unit activity. mEP in MI had greater contralateral preference than in SMA. Also, SMA mEP was more correlated to the single-unit activity than in MI. This greater correlation was also more apparent in the late peaks of the mEP than in the early peaks and may reflect a greater influence of recurrent activation in SMA than in MI. Our results further reinforce the idea that unimanual and bimanual movements are represented differently both in MI and in SMA and also show that a complex relationship between spikes of individual neurons and LFP may reflect the different input-output relations of different cortical areas.

[Psychosocial survey of drunken drivers within the KAPUBRA project. A new interview technique suitable for matching the right treatment with the right client]

Three different programs for individuals convicted of drunken driving are being evaluated in a randomized design in collaboration between researchers at the Karolinska Institute and the Prison and Probation Service in Sweden. In the years 1996-1998, 912 clients were interviewed by means of a structured interview, the Addiction Severity Index (ASI), which covers seven problem areas (medical status, alcohol use, employment status, drug use, legal status, family/social and psychiatric status). So far about half of the subjects have been re-investigated two years after leave. Initially, the group being investigated had problems particularly in the areas of criminality and alcohol use. Two years later a positive trend in most of the problem areas could be observed for clients in follow-up.

Neuroretinal function is normal in early dementia of the Alzheimer type

We recently demonstrated that retinal ganglion cell function, optic nerve head parameters and the retinal nerve fiber layer thickness are not altered in early dementia of the Alzheimer type (DAT). Our current objective was to assess whether the function of cells located more distally in the retina is also unaffected by the disease. We evaluated 23 individuals with early to moderate DAT and 23 healthy age-matched subjects, all displaying clinically normal visual function. Scotopic and photopic flash electroretinograms (fERGs) and oscillatory potentials (OPs) were recorded. The amplitude and latency of the retinal potentials did not differ between DAT and control subjects. Our current results showing normal fERGs and OPs in early DAT indicate that the underlying neurons giving rise to these signals are not impaired by the disease process. These data support and extend our recent findings suggesting that visual deficits in DAT do not stem from neuroretinal dysfunction.

Failure in identification of overlapping spikes from multiple neuron activity causes artificial correlations

Recording of multiple neurons from a single electrode is common practice during extra-cellular recordings. Separation and sorting of spikes originating from the different neurons can be performed either on-line or off-line using multiple methods for pattern matching. However, all spike sorting techniques fail either fully or partially in identifying spikes from multiple neurons when they overlap due to occurrence within a short time interval. This failure, that we termed the 'shadowing effect', causes the well-known phenomenon of decreased cross-correlation at zero offset. However, the shadowing effect also causes other artifacts in the auto and cross-correlation of the recorded neurons. These artifacts are significant mainly in brain areas with high firing rate or increased firing synchrony leading to a high probability of spike overlap. Cross correlation of cells recorded from the same electrodes tends to reflect the autocorrelation functions of the two cells, even when there are no functional interactions between the cells. Therefore, the cross-correlation function tends to have a short-term (about the length of the refractory period) peak. A long-term (hundreds of milliseconds to a few seconds) trough in the cross-correlation can be seen in cells with bursting and pausing activities recorded from the same electrode. Even the autocorrelation functions of the recorded neurons feature firing properties of other neurons recorded from the same electrode. Examples of these effects are given from our recordings in the globus pallidus of behaving primates and from the literature. Results of simulations of independent simple model neurons exhibit the same properties as the recorded neurons. The effect is analyzed and can be estimated to enable better evaluation of the underlying firing patterns and the actual synchronization of neighboring neurons recorded by a single electrode.

Reinforcement-driven dimensionality reduction--a model for information processing in the basal ganglia

Although anatomical studies of the basal ganglia show the existence of extensive convergence and lateral inhibitory connections, physiological studies failed to show correlated neural activity or lateral interaction in these nuclei. These seemingly contradictory results could be explained with a model in which the basal ganglia reduce the dimensionality of cortical information using optimal extraction methods. Simulations of this model predict a transient change in the efficacy of the feed-forward and lateral synapses following changes in reinforcement signal, causing an increase in correlated firing rates. This process ultimately restores the steady-state situation with diminished efficacy of lateral inhibition and no correlation of firing. Our experimental results confirm the model's predictions: rate correlations show a drastic decrease between the input stage (cortex) and output stage (pallidum). Moreover, preliminary analysis revealed that pallidal correlations show a transient increase following discrepancies between the animal's predictions and reality. We therefore propose that by using a reinforcement-driven dimensionality reduction process the basal ganglia achieve efficient extraction of cortical salient information that may then be used by the frontal cortex for execution and planning of forthcoming actions.