Rhythmic slow discharges in the electroencephalogram.

" RHYTHMS" have been a part of the lingua franca of the electroencephalographist from the beginning, but in fact truly rhythmic disturbances are exceptional, more particularly among the slower discharges, where great irregularity is the rule. However, in connection with epilepsy it was noted at an early date that waves of a period of about onethird of a second did tend to occur in bursts of fair regularity, both of period and of form. Somewhat similar waves have been seen in association with brain tumours (Walter, 1936) and with trauma (Jasper, Kershman, and Elvidge, 1940) and have aroused speculation as to the possible association of such waves with epilepsy. The author's attention was drawn particularly to these "rhythmic delta " discharges by a number of cases of tumours which seemed to present a more or less specific appearance in the EEG, and accordingly it was decided to investigate their occurrence and possible significance. In the epileptic the rhythmic slow discharge may consist of waves which approximate closely to the sinusoidal form or may present the more complex appearance of the " spike-and-wave ". The latter is extremely variable in its manifestations and certainly no dividing line can be drawn between it and the simple sine wave, every degree of spike formation being seen from patient to patient and even, at times, in a single record. The spike-and-wave complex, therefore, cannot be excluded from this discussion, and indeed, because of its well established association, must occupy a high place, although the chief interest is centred on discharges of " organic " rather than epileptic origin. Many of the slow rhythms had been noted to be subject to change when the patients' eyes were opened or closed, and consequently the occurrence of a response to this type of stimulus has also been investigated; in order to obtain a more complete picture of this phenomenon the frequency range considered was up to 8/sec. Many of the disturbances are so outstanding that no difficulty is experienced in placing them, but there is a borderline of ill-definition where it becomes difficult to decide, and objectivity is en-