THE ALPHA rhythm of the electroencephalogram, defined as those wave forms with a frequency of between 8 and 13 per second, depends for its presence on a state of resting wakefulness. When attention is focused on a stimulus, whether this be visual, auditory, tactile, or cognitive, the alpha waves normally disappear or are markedly reduced in amplitude. This phenomenon is generally called alpha blocking, but is also referred to as arousal, activation, and desynchronization. It was first described by Berger in his classic work on the electroencephalogram,² and has been widely studied since then. Although the physiology of alpha blocking is not fully understood, Moruzzi, in a recent review, has stated that this phenomenon "is due to a phasic barrage of ascending reticular impulses, which probably disrupt the slow cortical rhythms by desynchronizing the thalamic pacemaker."¹⁵

The absence of alpha blocking, ie, the