Help needed on alpha1, alpha2, beta1 and beta2 bands

[N020BA at tamvm1.tamu.edu]

   Hello. I need information, especially references, on the following
topics/questions----
 
   (1) What is the basis for dividing the alpha band (10-13 Hz) into the
       alpha1 and alpha2 bands?
   (2) Same as #1, but for beta1 and beta2 bands.
 
   Please reply either to the net (if you think it's a general interest
question) or to N020BA at TAMVM1.TAMU.EDU. Thank you for your help.