In addition, the results of
Experiments 1 and
2 support the hypothesis that the profound lightness effects reported by Gilchrist (
1979,
1988) and Gilchrist et al. (
1983) in their early edge-substitution experiments were likely due to two very different stimulus dimensions being matched across the two conditions. Recall that when the illumination was not visible (Gilchrist,
1979,
1988; Gilchrist et al.,
1983), observers reported that the illumination edge looked like a reflectance edge between black and white backgrounds. Lightness matches indicated that the targets looked mid-gray, one slightly darker than the other due to the simultaneous brightness/lightness contrast effect. This is the result seen in
Experiment 1 for lightness and brightness matches (
Figures 3 and
5) in the corresponding spotlight “OFF” conditions (filled squares and filled circles) for stimulus A, and for lightness and brightness matches in the reflectance condition of
Experiment 2 (
Figure 7, dark-gray bars)—although, in the present study the difference in the appearance of the two test patches in these stimuli would be described as substantially larger than a “slight” difference. When the illumination was clearly visible, however, Gilchrist (
1979,
1988) and Gilchrist et al. (
1983) reported that subjects matched the lightness of the illuminated target to black and the lightness of the shadowed target to white. In the corresponding condition (stimulus B) from
Experiment 1 (
Figure 5, open squares), the inferred-lightness (but not the brightness) of the illuminated target also decreased significantly, i.e., moved toward black. In
Experiment 2 (
Figure 7, white bars and light-gray bars) both the brightness and the inferred-lightness of the illuminated target decreased, however, the decrease was much larger for inferred-lightness. Note that, unlike in the Gilchrist experiments where the shadowed side of the display would have allowed an inferred-lightness judgment of white as reported, the non-illuminated (left-hand) side of illumination-edge stimulus in
Experiments 1 and
2 did not appear to be in shadow, and therefore the brightness and lightness matches appeared the same for the non-illuminated (left-hand) side (
Figure 3, stimulus B, open circles;
Figure 5, stimulus B, open circles;
Figure 7, white and light-gray bars). Thus, Gilchrist's reports (Gilchrist,
1979,
1988; Gilchrist et al.,
1983) that the lightness (apparent reflectance) of the targets was profoundly different in the two conditions of the edge-substitution experiments, even though the target luminances were identical, seems less profound when one considers that the conditions described are those likely to produce lightness matches that are very similar or identical to brightness matches in the first instance (reflectance condition), and that represent inferred-lightness matches in the second instance (illumination condition). This point has caused a great deal of confusion in the literature as it is commonly interpreted that the lightness difference Gilchrist described was actually a huge brightness effect, i.e., an effect on a sensory or phenomenal level, which it does not appear to be (for example, see Kingdom,
2003).