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1. Extracellular records have been made of spontaneous unitary activity in the cortex of the anaesthetized rat. If the animal is anaesthetized with sufficient urethane almost to abolish reflex contractions of leg muscles when a hind paw is stimulated, two types of spontaneously active unit are found. One type of unit is found in all areas of the cortex which have been studied, and gives high voltage discharges. The other has only been found in restricted areas of cortex and gives low voltage discharges.

2. The activity of the former type of unit has been analysed. In the unstimulated animal, groups of relatively frequent discharges are separated by periods of quiescence. Apart from this, the discharge pattern appears irregular on naked-eye examination. Synchronous with these periods of frequent discharge, the electrocorticogram exhibits rapid oscillations. Peripheral stimulation of the animal can modify the pattern of discharge of this type of unit. Details of these observations are described.

3. The results are described of experiments in which simultaneous records have been taken from pairs of units in the cerebral cortex. The intensity of correlation between the two units has been measured as a function of temporal displacement (lag) between the two records.

4. With fairly deep urethane anaesthesia, the highest correlation between the two unitary discharges occurs at zero lag if the two units are close together or if they are far apart but in the same coronal line, e.g. if the recording electrodes lie in homologous areas of the two cerebral hemispheres. If one unit is in the anterior pole of a hemisphere and the other in the posterior pole, the discharges are usually synchronous. However, it has sometimes been found that the anterior of the pair of units discharges impulses which precede those of the posterior unit by up to 100 msec. The highest intensity of correlation is then appropriately displaced to 100 msec.

5. The correlation fluctuates rhythmically with a period of 1·8-2 sec, slowly damps out and is symmetrical around the point of highest correlation.

6. Increasing the sensory bombardment of the cortex reduces or abolishes the correlation between the activity of pairs of units.

7. Under Nembutal anaesthesia sufficient to abolish reflex contractions of leg muscles when a hind paw is pinched, no spontaneous cortical unitary activity has been found. When anaesthesia is ligher, spontaneous activity does occur and consists of bursts of up to six impulses at a frequency of 300-500/sec. When simultaneous records from two units have been made, no correlation has been found between their activity.