Units in the cerebral cortex of the anaesthetized rat and the correlations between their discharges

Abstract

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.

Variations in the time course of the synchronization of intercostal motoneurones in the cat

“…However, their histogram peaks, in particular those obtained from the cortex, showed a very wide range of durations (cf. Holmes & Houchin, 1966) and could not confidently be ascribed to particular inputs.…”

Analysis of the activity of one type of spontaneously discharging unit in the cortex cerebri of the anaesthetized rat

“…As a result of observations on the synchrony of discharges in certain cortical units (Holmes & Houchin, 1966), we were led to suspect that patterning existed in the discharge of each unit. Some mathematical functions have been computed on the discharge of single units and the patterning elucidated in this way forms the subject of the present publication.…”

“…In conclusion, the forecast made in a previous publication (Holmes & Houchin, 1966) APPENDIX The data consist of a sequence of time intervals. All the statistical functions except the serial correlation coefficients are defined as sums of delta functions, with the notation of the previous paper (Holmes & Houchin, 1966).…”

“…Methods were identical with those described by Holmes & Houchin (1966). In the present investigation, seventy-six Sprague-Dawley, Wistar and Hooded Lister rats weighing between 150 and 300 g have been used.…”

“…Holmes & Houchin (1966) reported that, when anaesthesia was lighter and a brisk withdrawal reflex was present, the type 1 cortical units discharged more frequently. This change was due to a reduction in the frequency of occurrence of long intervals with no significant change in the frequency distribution of short intervals.…”