454 | ANNUAL REPORT SMITHSONIAN INSTITUTION, 1944 
we interpret one as sight and another as sound. We know, for exam- 
ple, that if one looks at a bright cross, the initial event in the brain 
will be the activation of a more or less cross-shaped area at the back 
of the occipital lobe, and that if one hears a sound a pattern will be 
reproduced in the temporal lobe corresponding to the areas of vibra- 
tion in the cochlea. All the external events of which we are aware 
are recorded as spatial and temporal patterns of excitation in the sense 
organs. These patterns are reproduced in the brain with a good deal 
of editing, omission of details and heightening of contrasts, and it is 
from them that we reconstruct our external world. 
Now the a rhythm of the electroencephalogram comes from large 
areas in the occipital region and to some extent from the frontal area 
as well. At first sight this seems to leave little room for all these 
diverse patterns of electrical activity. But actually the regions where 
the potential change is at a maximum are not those where the messages 
from the sense organs are received in the brain, but are the neighboring 
“silent” or “association” areas. Also, to make the rhythm appear, the 
AM ewan 
1 SEC. 
ee 
Figure 1.—Normal electroencephalogram showing the a rhythm. The maximum 
potential change is 45 microvolts. 
eyes must be closed and the attention relaxed, so that the brain is rela- 
tively inactive, at least so far as vision is concerned. Thus the regular 
wave sequence is derived from certain parts of the cerebral cortex 
when these have little to do. The cells there are not concerned with 
the incoming signals and so will be free to beat in unison, and if fairly 
large areas are so beating our records from the head will show the « 
waves and will not show the small local irregularities which are prob- 
ably going on all the time so long as we are conscious. 
The cells of the cortex might beat like this when they are left alone 
because this is how they are made; because like heart muscle or cili- 
ated cells they cannot remain alive and inactive; or the beat might be 
imposed on them by rhythmically active cells in some other part of the 
brain which can act as pacemaker to the association areas. Wherever 
the pacemaking region may be, the important fact is that the rhythm 
is much the same from one person to another. For clinical purposes, 
therefore, an electroencephalographic recording can be used as an 
index to show whether the brain is working normally or not, and for 
this reason it has become an important technique of clinical neurology. 
Apart, however, from the value of. these records as a means of diag- 
