The main effect of rod activity is to dilute the differential activity of different cone classes, decreasing the chromatic signal, and so potentially decreasing perceived saturation, though it could also have minor effects on hue (Buck, Knight, Fowler, & Hunt,
1998; Stabell & Stabell,
1976). This might have been a factor in the computer experiments, which had lower overall luminance than the natural ones. Secondly, as the number of cones projecting to each midget ganglion cell increases, the proportions of L and M cones contributing to the center and surrounds may equalize (Crook et al.,
2011; Diller et al.,
2004; Lennie, Haake, & Williams,
1991; Mullen, Sakurai, & Chu,
2005; Paulus & Kroger-Paulus,
1983; Solomon et al.,
2005), which would also decrease the chromatic signal and thus might decrease saturation. Although midget ganglion cells may maintain their single-cone center up to about 10°, larger surrounds could potentially have an impact within this region. Potentially acting in the opposite direction, shorter outer segments have less self-screening, so the effective cone sensitivity functions should be slightly narrower for periphery, slightly benefiting the chromatic signal. Importantly however, chromatic signal changes should not be expected to directly produce perceptual saturation differences. There is little indication that perceived saturation actually changes in the region relevant for our study (up to 5°) (Abramov, Gordon, & Chan,
1992). Consistent with this, Experiment 1 revealed no clear pattern for perceived saturation differences between centre and periphery: On our six observers, two reported saturation to be slightly increased, two showed the opposite pattern, and two did not perceive any consistent saturation differences.