The summation index, a concept introduced in a previous paper, assumed a very small value (smaller than that predicted from probability summation) when two superposed test flashes of wavelengths of 630 and 515 mμ, respectively, were presented with a duration of 100 msec. This inhibition phenomenon disappeared, and the probability summation was exhibited when the duration was shortened to 12.5 msec. An explanation of the difference is given in the present paper with the help of the threshold-versus-duration curve. Summation-index curves measured between λ1=630 mμ and λ2=515 mμ as a function of test-stimulus duration support the explanation. Further experiments, where the effect of adapting luminance level upon these curves was investigated, also support the explanation. It is then argued that the inhibition phenomenon should appear for the short duration if the explanation is true and if proper conditions are provided. This phenomenon is indeed observed.
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