A study of successive reaction times in the continuous performance of a homogeneous mental task disclosed pauses or lengthened reaction times which appeared at more or less regular intervals. The author describes these pauses under the term "blocks." The results of three different kinds of graphical analysis of the experimental data lead the author to advance the theory that the blocking phenomenon can be explained in terms of at least two fundamental periodicities in the flow of available (neural) energy, and that these periodicities depend on the amount of work done, i.e. the rate at which the available energy is being consumed rather than upon the amount of time which has elapsed. (PsycINFO Database Record (c) 2006 APA, all rights reserved)