The transition from scotopic to photopic vision in 3-month-old infants and adults: an evaluation of the rod dominance hypothesis

The scotopic to photopic transition was tested in adults and 12-week-old infants using a large field motion nulling technique at a series of luminances between −3.57 and 2.70 log cd m −2. The stimuli were composed of 0.25 cyc deg −1, 5.6 Hz blue/black and yellow/black sinusoidal grating components, superimposed and moving in opposite directions. The contrasts of the two components were traded off to determine motion nulls at each luminance level. An eye movement based response measure was used for infant subjects, whereas self-report was used in adults. In both age groups, the motion null values approached a scotopic asymptote consistent with V′( λ) at the lowest luminance levels, and a photopic asymptote consistent with V 10( λ) at the highest luminance levels. The scotopic to photopic transition was gradual and occurred over about 3 log units between about −2 and 1 log cd m −2 in both groups. The null values for infants and adults were highly similar at each luminance level, and the shapes of the transition curves were virtually identical at the two ages. These data suggest that at each different luminance level, the balance between rod-initiated and cone-initiated signals in the extrafoveal luminance channel is similar or identical in 12-week-old infants and adults.