Adaptation with negative after-effect.

Psychologists have in the past directed a great deal of effort toward classifying the sensory categories of our world— distinguishing, delimiting, and introspectively analyzing its qualities; but they have spent very little time in noting or studying the functional similarities which cut across these categories. That red, sour and warm are distinct experiences, based on different processes, has been emphasized to the neglect of the fact that they all manifest adaptation and other phenomena in common. Holt and Yerkes (12) prepared in 1903 a laboratory manual of experiments on sensation in which the emphasis was laid upon phenomena common to the various modalities and qualities—latency, after-image, adaptation, contrast, fusion, inhibition, and others—and in general upon the similarities between the senses rather than the differences. But this lead has not been followed in experiments to any significant extent.