Changes in pupil size during the solving of simple multiplication problems can be used as a direct measure of mental activity. The pupil response not only indicates mental activity in itself but shows that mental activity is closely correlated with problem difficulty, and that the size of the pupil increases with the difficulty of the problem. These findings relate to recent Russian research on the pupillary reflex in connection with orienting and brain stimulation.

Increases in the size of the pupil of the eye have been found to accompany the viewing of emotionally toned or interesting visual stimuli. A technique for recording such changes has been developed, and preliminary results with cats and human beings are reported with attention being given to differences between the sexes in response to particular types of material.

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