From the earliest studies of the human electroencephalogram, the disappearance ("blocking") of the alpha activity upon presentation of a visual stimulus has been regarded as among the most consistent of the electrical responses of the brain to external stimulation. While the effect of light upon the electroencephalogram has varied considerably in degree from one subject to another, the response in a single subject in the waking state has generally been considered to be a consistent one, i. e., a response which does not habituate. The predictability of this response has led to its being employed as the unconditional stimulus in numerous experiments on the development of conditioned cerebral responses.
While studying the development of conditional cerebral responses in man, it has been observed both by me and by others,1 that repeated presentation of paired sound and light stimuli in the same temporal relationship leads to the attenuation of the