458 | ANNUAL REPORT SMITHSONIAN INSTITUTION, 1944 
to or twice the a rate, we may find the two summing up to give very 
large regular waves. Such a combined rhythm usually takes some time 
to build up as the two sets of waves have to be synchronized, but there 
is evidently an interaction between them and a tendency to remain 
synchronized as long as their frequencies are not too far apart. These 
wave effects vary from one subject to another and there are variations 
according to mood, time of day, etc.; in general, the sleepier the subject 
the more the « rhythm will predominate, and the brighter the flicker 
the more persistent will be the flicker rhythm. 
The interaction and interference of these rhythms shows how the 
cortex, or certain parts of it, may be put at the disposal of our atten- 
tion. If we decide to look, or if it is decided for us by something 
“catching our eye,” a change occurs which prevents the « rhythm 
from occupying the regions surrounding the visual receiving area. 
MN Ma aerial 
PSEC: 
FLICKER 
Figure 3.—Hlectroencephalogram from the occipital region, showing the change 
from the a rhythm to the flicker rhythm when the eyes are opened and the sub- 
ject looks at a screen lit by a flickering light. The rate of flicker (17 a second) 
is shown by the photoelectric cell record below. 
If we can turn our whole attention to a watch ticking, the « rhythm 
comes back. What brings this change about? 
The evidence is still rather scanty. It is likely that the whole of 
the cerebral cortex is concerned, in that it is the balance of activity 
in every part which determines where the attention will turn and 
how long it will be held in one field; but the executive act, the direc- 
tion of the attention to the particular field, is probably carried out 
by a relatively small central region in the neighborhood of the thala- 
mus and near the main incoming pathways. It is from there that 
the a rhythm seems to be controlled, and it is at least probable that it 
is the sudden disturbance of this region which causes the loss of 
consciousness after a blow on the head. This central directing 
region must act on information received from the cortex, for it will 
be all the memories and associations stirred up by a stimulus 
which will determine its interest, and these are presumably not 
aroused until the message has reached the cortex. But the central 
region must balance up the conflicting claims of different stimuli and 
