Abstract.The Tibetan Plateau is a storehouse of excess gravitational potential energy accumulated through crustal thickening during India-Asia collision, and the contrast in potential energy between the Plateau and its surroundings strongly influences the modern tectonics of south As•a. The distribution of potential energy anomalies across the region, derived from geopotential models, indicates that the H•malayan front is the optimal location for focused dissipation of excess energy stored in the Plateau. The modem pattern of deformation and erosion in the Himalaya provides an efficient mechanism for such dissipation, and a review of the Neogene geological evolution of southern Tibet and the Himalaya shows that this mechanism has been operational for at least the past 20 million years. This persistence of deformational and erosional style suggests to us that orogens, like other complex systems, can evolve toward "steady state" configurations maintained by the continuous flow of energy. The capacity of orogenic systems to self-organize into temporally persistent structural and erosional patterns suggests that the tectonic history of a mountain range may depend on local energetics as much as it does on far-field plate interactions.

In many networks of oscillatory neurons, synaptic interactions can promote the entrainment of units into phase-coupled groups. The detection of synchrony in experimental data, especially if the data consist of single-trial runs, can be problematic when, for example, phase entrainment is of short duration, buried in noise, or masked by amplitude fluctuations that are uncorrelated among the oscillating units. In the present study, we tackle the problem of detecting neural interactions from pairs of oscillatory signals in a narrow frequency band. To avoid the interference of amplitude fluctuations in the detection of synchrony, we extract a phase variable from the data and utilize statistical indices to measure phase locking. We use three different phase-locking indices based on coherence, entropy, and mutual information between the phase variables. Phase-locking indices are calculated over time using sliding analysis windows. By varying the duration of the analysis windows, we were able to inspect the data at different levels of temporal resolution and statistical reliability. The statistical significance of high index values was evaluated using four different surrogate data methods. We determined phase-locking indices using alternative methods for generating surrogate data and found that results are sensitive to the particular method selected. Surrogate methods that preserve the temporal structure of the individual phase time series decrease substantially the number of false positives when tested on a pair of independent signals.

Physiological evidence indicates that the resting tremor of Parkinson's disease originates in oscillatory neural activity in the forebrain, but it is unknown whether that activity is globally synchronized or consists of parallel, independently oscillating circuits. In the present study, we used dual microelectrodes to record tremor-related neuronal activity from eight sites in the internal segment of the globus pallidus (GPi) from an awake Parkinson's disease patient undergoing stereotaxic pallidotomy. We utilized spectral analysis to evaluate the temporal correlations between multiunit activity at spatially separated sites and between neural and limb electromyographic activity. We observed that some GPi neural pairs oscillated synchronously at the tremor frequency, whereas other neural pairs oscillated independently. Additionally, we found that GPi tremor-related activity at a given site could f luctuate between states of synchronization and independence with respect to upper limb tremor. Consistent with this finding, some paired recording sites within GPi showed periods of transient synchronization. These observations support the hypothesis of independent tremorgenerating circuits whose coupling can f luctuate over time.The motor disorders of Parkinson's disease (PD) and their relief by pallidotomy implicate the basal ganglia in the control of movement. However, neither the nature of the basal ganglia's role in movement control nor the neural mechanisms that underlie this role or the deficits in PD are understood. Most neurophysiological investigations of basal ganglia function and PD symptoms have focused on the mean firing rates of neurons as a descriptive parameter of neural activity. However, this approach limits the assessment of neuronal interactions and their temporal properties. A more thorough understanding of basal ganglia activity and the dynamic symptoms of PD, such as tremor and drug-induced dyskinesia, requires a finer analysis of the temporal structure of neuronal and neuromuscular interactions. This approach has recently been successfully applied to the study of pallidal activity in a primate model of PD (1). Here, we have applied spectral analysis to study the temporal properties of tremor-related activity in the globus pallidus in a single patient undergoing stereotactic pallidotomy.Parkinsonian tremor consists of a rhythmic (4-to 8-Hz) activation of muscles, more prominent in distal musculature, that is manifested under resting conditions and in some cases when maintaining posture. This symptom is one of the cardinal features of PD and occurs in about 75% of the cases (2, 3). ''Tremor-related activity'' is defined as rhythmic neural activity at a frequency in the range of parkinsonian tremor; it does not imply that the neural activity is phase-locked to the tremor. Although tremor-related activity in the basal ganglia and motor thalamus has been reported by many authors, in only a few cases has the temporal relationship between the neural and muscle activity been rigorously examined ...

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