Figure 2.

(A) Background spectrum (left panel) to which the observer is light-adapted, eliciting a pattern of responses in the photoreceptors (right panel). (B) Increase in emitted light near the melanopsin peak relative to the background spectrum (left panel; dashed line = background spectrum, red line = modulation spectrum) leads to an increase in the excitation of all photoreceptors (middle panel), or equivalently, positive contrast on the photoreceptors (right panel). (C) To balance the excitation of the S cones, a decrease in emitted short-wavelength light (left panel) leads to silencing of the S cones (middle panel), or equivalently, zero contrast on the S cones (right panel). (D) To balance the excitation of the L and M cones, a decrease in emitted medium-wavelength light (left panel) leads to a reduction in L and M cone activity (middle panel) but not yet zero contrast on the L and M cones (right panel); indeed, the contrast seen by the L and M cones is now negative. (E) To silence the excitation of the L and M cones, a decrease in emitted long-wavelength light (left panel) leads to balancing of the L and M cones (middle panel), or equivalently, zero contrast on the L and M cones (right panel). The contrast seen by melanopsin is 50%. (F) The modulation spectrum shown in (E) yields positive contrast relative to the background spectrum but the spectrum can also be “mirrored” around the background spectrum, thereby leading to a negative modulation of melanopsin (and rods).