Somatosensory-evoked potentials and slow oscillations in patients with cerebral lesions.

Somatosensory-evoked potentials induced by electrical stimulation of the median nerve were recorded in patients with intracerebral lesions and in normal controls and were studied by means of spectral analysis. The results shown that (1) there is a statistically-significant difference between spectral distributions obtained from normal subjects and those obtained from patients; (2) whereas in normal subjects, specific frequency components of the spectrum are enhanced (are "driven") at specific frequencies of stimulation, in patients with cerebral pathology, such driving is completely or partially abolished; and (3) these differences between normal subjects and patients become more evident at frequencies of stimulation that may vary from one patient to another; for this reason it is suggested that the use of a fairly wide range of frequencies of stimulation may be of great importance when such methods are employed in the evaluation of the functional state of the brain.

Analysis of temporal relations among units and slow waves in rabbit hippocampus.

Arrays of triple microelectrodes were stereotaxically lowered into CA1, CA3 and dentate areas of the dorsal hippocampal formation in anesthetized rabbits. Recordings of action potentials and waves were analyzed on a PDP-11 computer using auto-, cross-, and multiple-correlation programs to determine temporal relations during 90 sec samples of spontaneous activity. It was found that temporal periods of neuronal firing and inhibition were strongly related to the pattern of waves. During periods of high amplitude synchronous waves (theta), the correlation between the activities of different groups of neurons was directly related to the periodicity of the wave. During instances of lower amplitude, desynchronous wave activity, the correlations between spikes recorded from those same cells were less periodic, varying according to the amount of wave synchrony. Variations in wave synchrony due to anatomical location, eserine effects, or spontaneous fluctuations under anesthesia produced corresponding variations in the relations between the activities of different groups of neurons. It is suggested that these relations between neuronal activity and gross waves may be implicated in processes which are at the basis of learning and memory.

Analysis of circulation of neuronal activity in the waking cortex.

It is well known that the rhythmic activity of the cerebral cortex is closely associated with a highly organized circulation of neuronal impulses through the networks in which the activity develops. The present study has attempted to determine by quantitative methods, the consistency of this circulation and of its association with the cortical waves, over relatively long periods. Cortical gross waves and neuronal activity have been recorded by means of arrays of extracellular microelectrodes, in freely moving cats as well as in cats immobilized with flaxedil. Autocorrelations have been performed on trains of rhythmic waves and on associated clusters of action potentials; cross-correlations and multiple correlations have been performed between waves and action potentials and between action potentials generated by different groups of neurons. It has been found that the rhythmicity of the waves, the rhythmicity of the associated clusters of action potentials, the time and phase relations between them, and the circulation of neuronal activity through the networks, remain highly consistent over periods as long as two hours.

Driving of slow oscillations in the human somatosensory system.

Potentials evoked by brief electrical pulses applied to the median nerve, were recorded from the frontal, central and vertex regions of human subjects. Averaging and spectral analysis showed that the responses contained time-locked components ranging in frequency from 0.75 to 4 c/sec and extending over periods as long as 3500 msec after the stimulus. There was no significant difference between evoked potentials induced by stimuli which were accompanied by a twitch in the muscles innervated by the median nerve and evoked potentials induced by stimuli which caused no muscular response. Stimulation by pulses separated by random intervals caused an increase in the amplitude of the late components over a broad frequency spectrum while periodic stimulation caused an increase in amplitude in much narrower ranges. When the frequency of stimulation was progressively increased from 0.25 to 5 pulses/sec it was found that the late oscillations could be "driven" at specific frequencies which caused an increase in the amplitudes of specific frequency components, harmonically related to each other and to the frequency of the stimulus. In a few instances in which a series of 125 individual evoked potentials was studied, it was found that the average computed from the last 50 potentials contained late components of considerably lower amplitudes than the average computed from the first 50 potentials. The administration of barbiturates caused a selective decrease in the amplitude of the late components and a redistribution of energy in the frequency spectrum. The essential characteristics of these late components: their wide distribution over the frontal, central and vertex regions, their occurrence over long periods after the stimulus, the decrease in their amplitude with repeated stimulation, indicating the possibility of "habituation", and their enhancement at specific frequencies of stimulation, suggest that they may be related to the activities of the nonspecific systems of the brain.

Somatosensory evoked responses and slow potential oscillations in human scalp recordings.

Average evoked responses obtained from the frontal, parietal and vertex regions, by electrical stimulation of the median nerve or common peroneal nerve, indicate the presence of time locked components which occur as late as 500 to 3500 msec after the stimulus. Spectral analysis shows that these components are distributed mostly in the 1.5 to 2 and 3.6 to 4 Hz frequently bands, and their presence in the evoked response may be due to the driving, by the stimuli of oscillations of similar frequencies which can be seen in the spontaneous activity recorded from the same region.

Serotonin in the lateral geniculate.

Serotonin was introduced, by means of a fine cannula, into the lateral geniculate body of cats immobilized with Flaxedil and artificially ventilated, while the electrical activity at the point of injection was monitored by means of microelectrodes. Doses of 1.25 to 30 mug dissolved in 0.5 to 2.0 mul of saline produced, in 2-30 min, changes in electrical activity characteristic of synchronization: increase in the rhythmicity and in the amplitude of the spontaneous gross waves and increase in the clustering of the spontaneous neuronal action potentials. At the same time the activity of neurons which produced action potentials of high amplitudes was decreased, the activity of neurons which produced action potentials of low amplitudes was increased. Action potentials of different amplitudes were produced, in this case, by neurons of different types. Thus, in the lateral geniculate as in other thalamic nuclei studied in previous investigations, the synchronization of spontaneous activity seems to require the simultaneous excitation and inhibition of two different types of neurons. The action of serotonin on activity evoked by stimulation with brief flashes of light was limited to the decrease in the amplitude of the average gross response and the inhibition of only one type of neuron. This suggests that, in the lateral geniculate body, serotonin may be implicated in different ways in the different network structures responsible for the development of spontaneous as contrasted with evoked activity.