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BRAIN RHYTHMS—ADRIAN 457 
parts of the cortex which are for the moment unemployed. It is not 
the basic rhythm of unstimulated nerve cells, and there must be some 
kind of competition between the message from the eyes and from the 
source of the a rhythm to decide which shall control the cortical areas. 
To examine this competition in more detail, we must have some way 
of recording the sensory activity of the brain as well as the a activity. 
All the messages which reach the cortex will produce their own elec- 
trical accompaniment, and this can be recorded well enough if elec- 
trodes can be placed on the surface of the brain. But if we can get no 
nearer than the scalp, the potential changes generated in any group 
of nerve cells will usually be obscured by those of other groups nearby, 
and the record will then show us nothing. Fortunately this difficulty 
can be overcome, in part at least, by making all the cells work in unison. 
This can be done, as far as vision is concerned, by making the field more 
or less uniform and lighting it with a flickering light. The nerve cells 
are then forced to work in unison at the frequency of the flicker, and 
we can record their electrical activity through the skull up to fre- 
quencies of about 30 a second. This gives us a method of tracing the 
visual messages in the brain, for by means of the flicker rhythm they 
can be made easy to recognize. 
Provided that the flickering area is in the center of the visual field, 
it need not subtend more than a few degrees at the eye to give a 
potential oscillation at the same frequency in the occipital region, The 
waves are more or less where one would expect them to be, in the right 
occipital region if the left half of the field flickers, and vice versa. But 
the flicker waves are not confined to the visual receiving area: they are 
found also in the neighboring areas, those from which the « waves 
come when the eyes are closed. The flicker area is not so large as the 
« area, but on occasion as much as a quarter of the brain surface seems 
to pulsate with the flicker rhythm. The retinal messages must then 
have spread widely and impressed their rhythm on some of the asso- 
ciation areas of the cortex. 
Now if the eyes are open, the flicker will keep the visual attention 
engaged and the « rhythm at bay. But if the eyes are closed and the 
flickering light is thrown on the closed lids, the subject will be con- 
scious of the flicker but the conditions will favor the « rhythm, since 
closure of the eyes is usually coupled with the withdrawal of attention 
from vision. In these conditions the two rhythms can be seen to com- 
pete for the cortex, and sometimes to cooperate if their frequencies 
allow of it. The flicker rate will sometimes appear in patches with 
the a rate in between, and if a rapid flicker is turned on suddenly the 
area of the brain giving the flicker rhythm may be large at first and 
may then shrink rapidly, giving place to the « waves but persisting 
longer in regions nearer the visual area. If the flicker is made equal 
