Abstract: The flash light composed of alternative bright and dim phases was used in simple and choice reaction time experiments to investigate the effect of dim-bright ratio (DBR) on signal discrimination. Experimental results showed that the dim phase of the flash light was a noise factor for the discrimination, but with DBR under 0.40 this effect was negligible, while above 0.70 its influence increased abruptly.With small DBR (0.17), simple reaction time to signals within the frequency range of 40 c/min to 420 c/min remained almost unchanged. When DBR reached 0.83, reaction time to both the lower and higher frequencies was remarkably longer than that to the middle frequencies (100—300 c/min). This was due to the fact that at greater DBR the higher frequencies tended to give rise to flicker fusion, whereas at lower frequencies the subjects needed more stimulus repetitions (i.e. bright and dim alternations) to make correct discriminations.

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