The purpose of this experiment was to compare S-cone mediated human psychophysical thresholds with in vivo recordings from S-cone excited macaque retinal ganglion cells [+S - (L + M)] in the presence of luminance-modulated and chromaticity-modulated long-wavelength backgrounds. For psychophysics, increment thresholds were measured for 2 degrees 3 ms test pulses on a 12 degrees 2 Hz sine wave background. Test pulse chromaticity was 460 nm. The background was formed by 561 and 645 nm LEDs, either modulated in phase or in counterphase. The time-averaged retinal illuminance was 2500 td, with chromaticity metameric to 588 nm. For electrophysiology, response amplitude to a 9.76 Hz test sine wave was measured on a 0.61 Hz sine wave background, using a 4.7 degrees field. The test was an S-cone isolating (tritan) stimulus. The background was formed by 553 and 636 nm equiluninant LEDs, either modulated in phase or in counterphase. The time-averaged retinal illuminance and chromaticity were 2000 td and 595 nm (for a human obsewer). The psychophysical data revealed that both chromatic and luminance background modulation resulted in threshold elevations. The electrophysiology revealed that the S - (L + M) ganglion cells were modulated by the luminance-modulated background, but not by the chromatic-modulated background. (C) 2000 John Wiley & Sons, Inc.