The impulse response function (IRF) means a response of entire visual system measured by a psychophysical method. As we reported in the FVM 2003, IRFs for S-cone increments in excitation were slower than for luminance modulation, but faster than IRFs for S-cone decrements. In this research, we measured IRFs possibly reflecting interactions between these putative pathways with one bluish pulse and one yellowish pulse. IRFs were derived from thresholds for a series of double-pulses in which the pulses were chromatically modulated at constant luminance (based on 18 Hz HFP) along individually determined tritan lines from an equal-energy white background. Chromatic stimuli were presented as a Gaussian patch (SD = 2.3 degree) in one of four quadrants around a central fixation cross on a CRT screen (a 4AFC method). When S-cone increments and decrements were combined in a pulse pair, there was an interaction that resulted in a threshold reduction. This is consistent with model assumptions that flash interactions between increments and decrements result from the same response to the first flash as obtained under isochromatic conditions, plus a (weaker) response component in the opposite polarity with a time delay. Model analysis indicated that S-cone increment IRFs are faster than S-cone decrement IRF (in terms of the time of peak amplitude and delay for the other polarity phase).