THE application of electro-encephalography to organic brain lesions has mainly been limited to the study of cases with involvement of the convexity of the hemispheres. In these an expanding lesion is usually associated with potential changes at a frequency well below the normal range-i to 3 c/s. It is generally accepted that discharges at this frequency are pathological when their amplitude approaches that of the normal rhythms (2O0,V). When localized by multi-channel recording, even lower potentials indicate cortical abnormality. These low frequency or " delta" rhythms (Walter, 1936, 1937, 1938) are not specifically diagnostic of new growth for they are also seen in traumatic and degenerative conditions, and in idiopathic epilepsy. One of the main difficulties in interpreting abnormal e.e.g. records is that the waveform is rarely simple or repetitive, as in the electro-cardiogram. Many observers have remarked that in cases of new growth, the waveform of the slow potential changes is particularly irregular and complex, whereas in cases of idiophathic epilepsy the discharge tends to be more stereotyped and rhythmic. An irregular and complex waveform is hard for the unaided eye to interpret, and it is also likely to contain many components. The e.e.g. 'is simply a graph of voltage difference against time. The interpretation of such a graph depends upon the capacity of the eye and brain to analyse a sinuous line in a conventional co-ordinate system. Most people understand readily the meaning of a simple curve, and a little practice gives facility in appreciating a simple periodic one, such as a sine curve. When, however, a periodic curve contains several components, appreciation is more difficult and when the periodicity is irregular and the components variable, the efficiency of the direct visual analysis falls off and fatigue quickly sets in. NQw in the case of t'he e.e.g. a simple periodic curve is an extreme rarity, for only occasionally are two con-
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