456 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1944 
ing the eyes cannot have altered the illumination of the retina. Sooner 
or later, when we have given up the attempt to see, the waves will 
return although the eyes are still open. 
What matters, therefore, is not the excitation of the retina but the 
turning of attention to the visual field or away from it. This can be 
shown even more clearly in another way. In daylight and with the 
eyes open, the attention can rarely be abstracted completely from 
vision except for short periods; something keeps on “catching the eye” 
and coming into the mind, even though we have most of our attention 
fixed on other things, a sound or a smell, for example. The reason why 
the visual field cannot be completely ignored is that the picture of it in 
our brain has patterns and sequences which arouse interest by recalling 
memories or starting some new train of activity. But if we can make 
the visual field convey less meaning, it will cease to be so attractive 
and our attention can leave it more easily. A simple way of securing 
this is to wear spectacles which will throw everything out of focus. 
é' t 
ser ovirantsiition, etter dingy DAN ate ed arnnnl rr 
ATTENTION 
TO VISION HEARING VISION 
1 SEC. 
es 
Figure 2.—The a rhythm appears when the attention is transferred from vision 
to hearing. The visual field has been made unattractive by +10 D spectacles. 
During the middle section of the record the attention is concentrated on the 
tick of a watch. 
When this is done, although the eyes remain open, the a rhythm will 
appear much more readily than when the visual field is in its proper 
focus. With the field blurred, we have only to listen intently to a 
sound and the « waves will begin, to cease again if we transfer our 
attention back to vision. Here, too, there has been no change in the 
illumination of the retina but only the shift of attention. It may be 
noted that it is not only the intellectual interest of the field which holds 
our attention. Any movement in it or any sudden change of illumi- 
nation will do so; and there is a great variation in the ability of differ- 
ent persons to detach the attention from vision, and in the same person 
at different times. 
All this shows that the « rhythm is an activity which appears in the 
cerebral cortex when the attention is not directed to vision, and dis- 
appears when it is. The mental act of looking somehow prevents the 
a waves from developing in certain parts of the brain, parts which are 
likely to be concerned in analyzing the visual pattern. The a rhythm 
is therefore a rhythm of inattention, a positive activity which fills those 
