Computer assisted analysis of relations between single-unit activity and spontaneous EEG

Concurrent Monitoring of Cerebral Electrophysiology and Metabolism after Traumatic Brain Injury: An Experimental and Clinical Study

Multiparameter cerebral monitoring has been widely applied in traumatic brain injury to study posttraumatic pathophysiology and to manage head-injured patients (e.g., combining O(2) and pH sensors with cerebral microdialysis). Because a comprehensive approach towards understanding injury processes will also require functional measures, we have added electrophysiology to these monitoring modalities by attaching a recording electrode to the microdialysis probe. These dual-function (microdialysis/electrophysiology) probes were placed in rats following experimental fluid percussion brain injuries, and in a series of severely head-injured human patients. Electrical activity (cell firing, EEG) was monitored concurrently with microdialysis sampling of extracellular glutamate, glucose and lactate. Electrophysiological parameters (firing rate, serial correlation, field potential occurrences) were analyzed offline and compared to dialysate concentrations. In rats, these probes demonstrated an injury-induced suppression of neuronal firing (from a control level of 2.87 to 0.41 spikes/sec postinjury), which was associated with increases in extracellular glutamate and lactate, and decreases in glucose levels. When placed in human patients, the probes detected sparse and slowly firing cells (mean = 0.21 spike/sec), with most units (70%) exhibiting a lack of serial correlation in the spike train. In some patients, spontaneous field potentials were observed, suggesting synchronously firing neuronal populations. In both the experimental and clinical application, the addition of the recording electrode did not appreciably affect the performance of the microdialysis probe. The results suggest that this technique provides a functional monitoring capability which cannot be obtained when electrophysiology is measured with surface or epidural EEG alone.

Spontaneous neuronal firing patterns in the occipital cortex of developing rats

Spontaneous action potentials have been recorded in the cerebral cortex of developing rats under chloral hydrate anaesthesia. These "spike-trains" were subjected to a multivariate analysis of parameters describing different aspects of temporal patterning within each train. Three parameters which reflected clustering of action potentials in, respectively, the range of: (i) milliseconds (the modal interval, derived from the interval histograms); (ii) seconds (the mean "burst period", i.e. the time elapsing between the onset of successive clusters of relatively short intervals); and (iii) minutes (the coefficient of variation for the number of spikes in consecutive 2 min time bins) showed distinctive developmental patterns.

The electroencephalogram and mental activation

The mental activation (MA) is a simple test consisting of two questions and two commands given during the waking record. The responses are documented by + or - sign notations made by the technologist. In this manner, data were rapidly obtained from 1280 patients; there were 598 patients with EEG records within normal limits and 682 with various degrees of EEG abnormality. The MA serves the following purposes: a. to study the repercussions of the test on the EEG activity, b. to ascertain the optimal level of vigilance, c. to attempt a correlation of MA responses and the degree of EEG abnormality and d. to obtain brief information on the patient's mental state. The test is administered during conventional EEG recordings. It was found that alpha blocking or attenuation was the exception rather than the rule during the test. Diffuse and focal slowing also remained unchanged in the vast majority of the cases, whereas focal intermittent rhythmical delta activity (FIRDA) was blocked or reduced in most patients exhibiting this pattern. Epileptic conditions were also studied and it was found that, in a single observation, even the most difficult part of the test (item D: mental arithmetic) could be carried out during ictal-subclinical regional EEG activity.

Relationship between single-unit activity and the electroencephalogram in a neocortical, cobalt-induced epileptogenic focus

Although the importance of neuronal synchrony in epilepsy has not been disputed, few attempts have been made to examine quantitatively the relationship between this parameter and seizure occurrence. The specific objective of the present investigation was to determine how the amount and type of synchrony between EEG and single-unit activity in an experimental model of focal epilepsy are related to the occurrence of seizures. This was accomplished by examining EEG/single unit relationships in two types of cobalt-induced epileptogenic focus: (1) foci that initiated seizures, and (2) foci that exhibited only interictal spike activity. These relationships were examined during slow-wave sleep, a time when synchronous neuronal activity is thought to be augmented. In control rats and rats that had seizures, the majority of units exhibited a non-random relationship between unit discharge and the EEG. In cobalt-treated rats that were not observed to have seizures, however, the percentage of units exhibiting EEG/single unit relationships was significantly less than that in either controls or rats that had seizures. This observation, paired with observations of the details of the EEG/single unit relationships, led to the hypothesis that cobalt treatment produces a shift from an inhibition dominated synchrony (observed in controls) to an excitation dominated synchrony (observed in rats that had seizures). Intermediate between these two types of synchrony is a less synchronized state (observed in seizure-free, cobalt-treated rats), which probably results from a loss of inhibition dominated synchrony without a concomitant increase in excitation dominated synchrony.

Transition from spindles to generalized spike and wave discharges in the cat: simultaneous single-cell recordings in cortex and thalamus

The relationships between the activity of the cortex and that of a "specific" (n. lateralis posterior, LP) and an intralaminar thalamic nucleus (n. centralis medialis, NCM) were studied in the cat during the transition from spontaneous spindles to generalized spike and wave (SW) discharge following i.m. penicillin injection. The EEG and extracellular single-unit activity were recorded in cortex and thalamus during the spindle stage and at different intervals after penicillin until well developed SW discharges were present. Computer-generated EEG averages and histograms of single-unit activity were triggered by either peaks of cortical or thalamic EEG transients or by cortical or thalamic action potentials. In agreement with previous observations, cortical neurons increasingly fired during the spindle wave as it was transformed into the "spike" of the SW complex, while a period of neuronal silence gradually developed as the "wave" of the SW complex emerged. Similar changes developed in the thalamus, particularly in LP, either concurrently with or more often after the onset of the changes in the cortex. Most neurons in NCM, continued to fire randomly even after well developed SWs and rhythmic neuronal discharges had developed in cortex and LP. Only 4/11 NCM neurons did ultimately exhibit a rhythmic firing pattern similar to that seen in the cortex and LP. The correlation between cortical and thalamic unit activity was low during spindles, but gradually increased during the development of SW discharges. These data confirm that the cortex is the leading element in the transition from spindles to SWs. Increasingly, in the course of this transition, cortical and thalamic neuronal firing becomes more intimately phase-locked. This mutual interrelationship appears to be more pronounced between cortex and "specific" than intralaminar thalamic nuclei.

Simultaneous recording of cortical and thalamic EEG and single neuron activity in the cat association system during spindles

Simultaneous recording of the EEG and single unit activity in cortex and thalamus shows that during spindles the cortical and thalamic EEGs within a given thalamocortical sector are highly correlated, but that the firing of only a fraction of cortical and thalamic neurons shows a close mutual correlation or exhibits a close relationship to the cortical and thalamic EEGs. Thus, only a fraction of thalamic and cortical neurons is involved in generating spindles. The synchronized activity of these neurons, even if some of it is subliminal for action potential discharge, suffices to produce a recordable EEG signal.