A high performance visual stimulation apparatus, an AV Tachistoscope was used to measure visual recognition ability and the results were applied to assess activation effects by mastication. The equipment can display diagrams and letters on a television screen in a minimum one millisecond display time. When the display time is short, the shown contents can not be recognized but by extending the display time, the shown objects become recognized and the recognition-time threshold can be measured. With this threshold value the cerebral activation effects by psychological and pharmacological means can be assessed. Gum chewing was selected as the activation procedure. The recognition accuracy to visually shown letters was tested in 10 normal persons, and it was significantly improved in the gum-chewing condition(90.8±13.0(%))compared with the control condition(71.5±28.3(%))(p<0.05 by paired t-test). The thresholds of the visual recognition time were 1.9±0.7 and 2.4±1.0 ms for the gum chewing and control conditions, respectively.