BRAIN RHYTHMS—ADRIAN 455 
nosis, there is the problem of their significance in relation to the 
normal mechanism of the brain and of the mind. Do they tell us 
anything about the neural accompaniment of perception and thought ? 
The statement that one can record the electrical activity of the brain 
through the skull raises the hope that one should be able to detect all 
sorts of brain events connected with consciousness. The hope begins 
to fade when it is realized that the main feature of our records is a 
rhythm from nerve cells which are relatively inactive. But there is 
something of interest to be learned from them. It is true that we 
cannot yet record the detailed activities of different parts, but only 
the gross changes when a whole area goes into action; yet these do give 
us some novel information about the physical accompaniments of 
thought and in particular about the process of attention. 
To begin with, it seemed that the « rhythm was much less interesting. 
It might have been merely a spontaneous beat of the nerve cells in 
parts of the cortex, particularly those concerned with vision—a beat 
developing whenever the cells were not stimulated by messages from 
the eyes. Opening the eyes would break up such a rhythm in the 
receiving area, because the visual pattern there would mean that dif- 
ferent groups of nerve cells would be discharging at different frequen- 
cies. Something of this kind.can certainly happen in the regions 
where a message enters the cerebral cortex, for in records from the 
exposed brain (in anesthetized animals) the arrival of a message in 
the receiving area produces small, rapid waves in place of the slower 
and larger « rhythm. But the a waves are not confined to the 
receiving areas of the brain, and it can be shown that in fact the 
presence or absence of messages from the eyes is not the essential 
condition for the disappearance or return of the « rhythm. 
As far as vision is concerned, what really determines the presence or 
absence of these waves is not whether visual messages are or are not 
coming into the brain, but whether we are or are not attending to 
them—whether we are looking at anything. Man is a visually con- 
trolled animal, and if our eyes are open there is usually something in 
the visual field which catches our attention or some part of it. This is 
not true for all mammals; the rat and the cat seem to rely more on 
other senses and can be quite inattentive to sights. But with us the 
only sure method of shutting out sights from the mind is to close the 
eyes. Normally, therefore, opening the eyes means that we start look- 
ing, or that we become attentive to the visual field. The « waves then 
disappear, and they return when we close our eyes and cease looking. 
But shutting the eyes does not cut out all light from the retina, and the 
a rhythm appears in the brain however much or little light may be 
falling on therclosed lids. Even in a pitch-black room, if we open our 
eyes and start trying to see something, the rhythm goes, although open- 
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