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A study of the successive reaction times in the continuous performance of a homogeneous mental task discloses pauses or lengthened reaction times which appear at more or less regular intervals. In such simple tasks as color-naming, form-naming, code substitution, addition, naming opposites, and the like, these pauses occur on the average about three times a minute. In a previous article, (i), the author has described these pauses under the title 'blocks,' and has pointed out that they decrease in frequency and size with practice, increase with fatigue, and vary from individual to individual, being most frequent and longest in slow workers and vice versa. For purposes of quantitative study, blocks were arbitrarily defined as pauses equivalent in length to the time of two or more average responses. This definition is not entirely arbitrary, however, for if a given subject's reaction times are plotted in the form of a frequency polygon, in which the base represents reaction time in eighths of a second, it is found that two distinct modes appear, one of which is due to the central tendency in normal reaction times, while the other shows the central tendency of the blocks. The block mode appears at a point representing approximately double the normal reaction time mode.The relative periodicity of these blocks suggested an interpretation in terms of some hypothetical periodicity in nervous energy or capacity. A number of studies by different authors at widely different times have given evidence of periodic fluctuations in mental efficiency; among which are fluctuations in the threshold for apprehension of minimal stimuli, fluctuations in the focus of attention, fluctuations in performance level, such as those recently described in ' 563

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