Decrease and increase in test-threshold luminance induced by a contiguous annular field

The threshold luminance of the test field (0.6°) presented to the retinal region about 1°09′ below a fixation point was measured under various combinations of luminance and area of contiguous annular field, according to the method of limits. The following results for 2 Ss were obtained. (1) As the luminance of inducing field increased from −2.5 to 0.5 log mL, the test threshold, after once it became lower than the ones measured without inducing field, increased gradually. (2) It seemed to be dependent on the inducing-area where minimum value of threshold appears on the inducing luminance axis.