The shape of the human spectral sensitivity function depends on how it is measured. In the increment threshold (IT) technique, sensitivity is typically measured as the inverse of threshold for detection of increments of monochromatic light presented for relatively long durations on achromatic backgrounds. Spectral sensitivity functions derived from IT techniques have long been used to reveal contribution from opponent color channels. Although IT functions have been studied extensively, little attention has been given to spectral sensitivity functions derived from decrement thresholds (DT), partly due to technical challenges of producing appropriately controlled stimuli. Comparison of IT and DT spectral sensitivities may be of interest because there are known asymmetries in the visual system between on- and off-pathways and between increment and decrement responses within these pathways.? Consequently, spectral sensitivity functions obtained using DT measures may reveal a different complement of contributing mechanisms than those that produce IT functions. Here we report measurements of DT and IT functions using conditions known to reveal strong contribution from opponent pathways.

IT and DT derived spectral sensitivities were essentially identical over much of the visible spectrum. However, decrement sensitivity was slightly greater than increment sensitivity in the shorter wavelengths at modest light levels. This difference was not present at higher light levels, implicating rod pathways as a possible source of the difference rather than asymmetries in opponent mechanisms. In sum, it appears that under conditions shown to reveal strong contribution from opponent mechanisms, decrement spectral sensitivities are either 1) likely determined by a similar complement of spectrally opponent mechanisms as those that define increment spectral sensitivities or 2) that the conditions typically employed for IT determination are insensitive to underlying asymmetries.