Spontaneous magnetoencephalographic activity was recorded in awake, healthy human controls and in patients suffering from neurogenic pain, tinnitus, Parkinson's disease, or depression. Compared with controls, patients showed increased low-frequency rhythmicity, in conjunction with a widespread and marked increase of coherence among high-and low-frequency oscillations. These data indicate the presence of a thalamocortical dysrhythmia, which we propose is responsible for all the above mentioned conditions. This coherent activity, the result of a resonant interaction between thalamus and cortex, is due to the generation of lowthreshold calcium spike bursts by thalamic cells. The presence of these bursts is directly related to thalamic cell hyperpolarization, brought about by either excess inhibition or disfacilitation. The emergence of positive clinical symptoms is viewed as resulting from ectopic ␥-band activation, which we refer to as the ''edge effect.'' This effect is observable as increased coherence between low-and high-frequency oscillations, probably resulting from inhibitory asymmetry between high-and low-frequency thalamocortical modules at the cortical level.edge effect ͉ activity ͉ ␥ activity ͉ humans

This article reviews the electroresponsive properties of single neurons in the mammalian central nervous system (CNS). In some of these cells the ionic conductances responsible for their excitability also endow them with autorhythmic electrical oscillatory properties. Chemical or electrical synaptic contacts between these neurons often result in network oscillations. In such networks, autorhythmic neurons may act as true oscillators (as pacemakers) or as resonators (responding preferentially to certain firing frequencies). Oscillations and resonance in the CNS are proposed to have diverse functional roles, such as (i) determining global functional states (for example, sleep-wakefulness or attention), (ii) timing in motor coordination, and (iii) specifying connectivity during development. Also, oscillation, especially in the thalamo-cortical circuits, may be related to certain neurological and psychiatric disorders. This review proposes that the autorhythmic electrical properties of central neurons and their connectivity form the basis for an intrinsic functional coordinate system that provides internal context to sensory input.

SUMMARY1. The electroresponsive properties of guinea-pig thalamic neurones were studied using an in vitro slice preparation.2. A total of 650 cells were recorded intracellularly comprising all regions of the thalamus; of these 229 fulfilled our criterion for recording stability and were used as the data base for this report. The resting membrane potential for thirty-four representative neurones which were analysed in detail was -64 + 5 mV (mean + S.D.), input resistance 42 + 18 MK2, and action potential amplitude 80 + 7 mV.3. Intracellular staining with horseradish peroxidase and Lucifer Yellow revealed that the recorded cells had different morphology. In some their axonal trajectory characterized them as thalamo-cortical relay cells.4. Two main types of neuronal firing were observed. From a membrane potential negative to -60 met, anti-or orthodromic and direct activation generated a single burst of spikes, consisting of a low-threshold spike (l.t.s.) of low amplitude and a set of fast superimposed spikes. Tonic repetitive firing was observed if the neurones were activated from a more positive membrane potential; this was a constant finding in all but two of the cells which fulfilled the stability criteria.5. The l.t.s. response was totally inactivated at membrane potentials positive to -55 mV. As the membrane was hyperpolarized from this level the amplitude of the l.t.s. increased and became fully developed at potentials negative to -70 mV. This increase is due to a de-inactivation of the ionic conductance generating this response. After activation the l.t.s. showed refractoriness for approximately 170 ms. Deinactivation of l.t.s. is a voltage-and time-dependent process; full de-inactivation after a step hyperpolarization to maximal l.t.s. amplitude (-75 to -80 mV) requires 150-180 ms.6. Membrane depolarization positive to -55 mV generated sudden sustained depolarizing 'plateau potentials', capable of supporting repetitive firing (each action potential being followed by a marked after-hyperpolarization, a.h.p.). The a.h.p. and the plateau potential controlled the voltage trajectory during the interspike interval and, with the fast spike, constitute a functional state where the thalamic neurone displayed oscillatory properties.