Rod modulation of an annular surround can produce brightness contrast in a test field centered at 10° from the fovea. In our research, stimuli originated from a colorimeter that provided 4 primaries in both the circular test and the annular surround fields, and allowed independent modulation of the rods and each of the short (S)-, middle (M)-, and long (L)-wavelength-sensitive cone types. The chromaticity was set so fields had the same appearance as the equal energy spectrum. At 1 photopic troland (td), rod-induced modulation in the test field could be cancelled by either a rod- or a cone-nulling modulation added to the test field. The best cone nulling of rod induction showed residual flicker. Nulling was more effective, though still imperfect, with a cone-nulling stimulus of higher S-cone modulation contrast. Rod induction with square-wave, on-pulse, and off-pulse temporal profiles was closely similar. At higher light levels, 10 and 100 td, rod contrast could not be nulled by rod or cone modulation. The failure to achieve nulls may have been caused by either or both of the following hypotheses: (1) there is a mismatch between the rod and cone temporal waveforms; (2) there is strong rod input to the magnocellular pathway, but negligible rod input to the parvocellular pathway, as shown by single-unit electrophysiological data.

*W′*(

*λ*) represents the CIE

*V′*(

*λ*) expressed at the retinal level and

*F*(

_{s}*λ*) represents transmittance of the prereceptoral filter associated with the standard observer for scotopic photometry. The physiological basis of the

*W′*(

*λ*) distribution is the rhodopsin-absorption spectrum. We assume

*W′*(

*λ*) is invariant across observers because no rhodopsin polymorphisms have been reported for human observers with normal visual function (Sung et al, 1991). Prereceptoral filter transmittances

*F*(

*λ*) are known to vary among observers. For a scotopic luminance match between 2 narrow band lights, reference light

*P*and a test light

_{ref}*P*, the standard observer match is given by where

_{test}*a*is calculated based on

_{s}*V′*(

*λ*). For a scotopic match made by an individual observer where

*F*(

_{o}*λ*) is the prereceptoral filter transmittance for the individual observer, and

*a*is set by the individual observer. Ratio

_{o}*a*/

_{s}*a*represents the ratio of the prereceptoral filtering for the standard observer and an individual observer. Scotopic-luminance matching can be used to assess the prereceptoral difference between an individual observer and the standard observer for wavelengths where the scotopic sensitivity is higher than the photopic sensitivity. In particular, this method is appropriate for primaries 459, 516, and 561 nm.

_{o}*b*,

_{1}*b*, and

_{2}*b*are the tristimulus values calculated from CIE 10° color-matching functions. For an individual observer, the values of the 459-, 516-, and 561-nm primaries are first corrected for the individual prereceptoral filtering differences measured by the scotopic luminance matches, and then they are fixed at the matching values

_{3}*b*,

_{1}*b*, and 1.0. If the observer can make a color match by adjusting only the radiance of the 664-nm primary (

_{2}*b*), his or her receptoral spectral sensitivities at the 4 primary wavelengths can be approximated by linear transforms of the standard observer data. The difference between the 664-nm setting for the individual observer and the standard observer corrects for the individual prereceptoral transmittance at 664 nm.

_{3}*A*, can be calculated as: contrast template, or amplitude template, where

*L*is the time-average illuminance,

*C*is the illuminance contrast of a pair of temporally alternated standard and test lights,

*L*is a normalizing constant, and

_{0}*M*is the optimal modulation threshold for human observer determined by the Weber fraction. The contrast template is appropriate if the time-average illuminance is constant, or if the temporal sensitivity is independent of illuminance level (Weber’s law). The amplitude template is appropriate if the time-average illuminance is varied, or if the temporal sensitivity is dependent on illuminance level. The assumption that temporal sensitivity is independent of illuminance level is true at lower temporal frequencies, but not at higher temporal frequencies where the threshold is dependent on modulation amplitude (Kelly, 1961). Pokorny et al (1989) used the amplitude template to fit data collected at a frequency of 15 Hz. In our experiment, the average illuminance of the standard primary 561 nm was kept constant, but the average illuminance of the test light was varied. The frequency of the square-wave temporal modulation was 6 Hz. The results of fitting were compared with both templates.

_{t}Inducing | Nulling | Retinal Illuminance (td) | Temporal Profile | Fitting Equations | |
---|---|---|---|---|---|

Observer H.S. | Observer S.S. | ||||

Rod | Rod | 1 | Square | y = −0.04+0.95x | y = −0.08+1.19x |

On-pulse | y = −0.05+1.04x | y = −0.07+1.24x | |||

Off-pulse | y = −0.05+0.91x | y = −0.04+0.99x | |||

S+M+L | 1 | Square | y = −0.01+0.445x | y = −0.31+2.82x−3.94x^{2} | |

On-pulse | y = −0.008+0.36x | y = −0.05+0.95x−1.55x^{2} | |||

Off-pulse | y = −0.001+0.37x | y = −0.04+0.88x−1.53x^{2} | |||

Rod | 0.1 | Square | y = −0.05+0.89x | y = −0.12+1.56x | |

On-pulse | y = −0.05+0.98x | y = −0.03+1.14x | |||

Off-pulse | y = −0.05+0.93x | y = −0.08+1.22x | |||

kS+M+L | 1 | Square | y = −0.12+1.41x | y = −0.20+1.66x | |

S+M+L | Rod | 1 | Square | y = −0.031+1.25x |