The silent substitution stimuli were a subset of those used in a prior report,
41 and full details of their generation may be found there. Briefly, we used the method of silent substitution together with a digital light synthesis engine (OneLight Spectra, Vancouver, BC, Canada) to stimulate targeted photoreceptors. The device produces stimulus spectra as mixtures of 56 independent primaries (∼16 nm full width at half maximum [FWHM]) under digital control, and can modulate between these spectra at 256 Hz. Details regarding the device, stimulus generation, and estimates of precision have been previously reported.
35,46,47 Our estimates of photoreceptor spectral sensitivities were as previously described,
47 with those for the cones based on the field size and age-dependent International Commission on Illumination (CIE) physiological cone fundamentals.
48 The estimates account for subject age, pupil size (which was fixed at 6-mm diameter through the use of an artificial pupil), and our field size of 27.5 degrees. Although the standard specifies fundamentals only for field sizes up to 10 degrees, we obtained the 27.5-degree estimates by extrapolating the formula from the standard routines in the open-source Psychophysics Toolbox.
49–51 Separate background and modulation spectra were identified to provide nominal 400% Weber contrast on melanopsin while silencing the cones for the melanopsin-directed background/modulation pair (Mel), and 400% contrast on each of the L-, M-, and S-cone classes while silencing melanopsin for the luminance-directed modulation/background pair (LMS) (
Fig. 1c). The xy chromaticities of the background spectra for the Mel and LMS stimuli were similar (Mel: ∼0.56, ∼0.40; LMS: ∼0.58, ∼0.38).
48 The background for Mel and LMS pulses were nominally rod-saturating (∼110 photopic cd/m
2 or 3.10 log scotopic trolands for Mel and ∼40 photopic cd/m
2 or 2.99 log scotopic trolands for LMS for sessions 1 and 2; ∼270 photopic cd/m
2 or 3.59 log scotopic trolands and ∼90 photopic cd/m
2 or 3.46 log scotopic trolands for session 3). The xy chromaticities and photopic luminances reported above were calculated using the proposed XYZ functions associated with the CIE 2006 10-degree cone fundamentals (
http://www.cvrl.org).
48 The modulations did not explicitly silence rods or penumbral cones.
47