Spectral efficiency measured by heterochromatic flicker photometry is similar in human infants and adults

Spectral efficiency functions based on heterochromatic flicker photometry (HFP) were measured for three adults and 42 infants using a rapid visually-evoked potential (VEP) method. A 5°-diameter, broadband standard (0.6 cd/m²) was presented in square-wave counterphase (15 Hz) with one of 13 monochromatic lights (420-660 nm; 20 nm steps). The intensity of the monochromatic light was continuously varied while extracting the phase-locked VEP amplitude of the fundamental component. HFP functions measured psychophysically by the method of adjustment were also obtained for the adults. Adult HFP functions from the two methods were found to be essentially the same. Both of these functions were compared to Vos'-modified 2° V(λ) function and to the 10 ° CIEV(λ) function. The mean adult data were slightly better fit to the 2°V(λ) function than to the 10° CIEV(λ) function, although there was an elevation in sensitivity at 420 and 440 nm. Infant HFP functions were similar to Vos' modified V(λ) except for an elevation in efficiency at short wavelengths. The mean infant HFP function agreed better with the 10°CIEV(λ) function than Vos'-modified V(λ) function, but infant sensitivity was elevated by 0.4 log units at 420 nm compared to the 10° CIE observer. The elevation found at short wavelengths for both adults and infants is attributed to individual and age-related variation in the density of the ocular media, and to reduced macular pigment screening resulting from use of a 5° field size. An infant spectral efficiency function based on the HFP data collected in this study was modeled. This function may serve to equate stimuli with respect to luminosity for the average infant.