Color. Our phenomenon is at least superficially similar to the color mixture along a trajectory of a moving object (Nishida et al.,
2007). Although their results did not conflict with the idea that a mechanism contributing to object or global apparent motion perception affects the apparent brightness or color, they did not clarify whether a global motion (or object) mechanism or a lower level mechanism with a small spatiotemporal window was responsible for the color mixture. However, since our stimulus has a larger spatial gap (1.7–3.5 deg) between wedges, our results suggest that the perceptual brightness difference compression is mediated by a mechanism with a larger receptive field than those of V1/V2 neurons (mean and
SD of their size for V1 is about 0.5 deg at 2–7 deg of eccentricity and that for V2 is 1 deg with
SD of 0.32 deg at 1–6 deg of eccentricity; Ito & Gilbert,
1999; Ito & Komatsu,
2004; Kapadia, Westheimer, & Gilbert,
2000). Many physiological and brain imaging studies have demonstrated that whereas MT/hMT activity (Goebel, Khorram-Sefat, Muckli, Hacker, & Singer,
1998; Mikami, Newsome, & Wurtz,
1986) or V1 activity that is fed back from hMT (Muckli, Kohler, Kriegeskorte, & Singer,
2005) processes long-range apparent motion, even lateral interactions between V1 neurons are too small to explain long-range apparent motion (Muckli et al.,
2005). It should be noted that although the color mixture of Nishida et al. and our luminance compression look like similar phenomena, they have some different features. For example, the color mixture was much weakened when the bar was widened, while our brightness compression was observed using wedges that were much wider than their bars (4–12 min). The optimal inter-flash intervals were also different between the two phenomena (about 110 ms in our experiments, and 6.25–75 ms in Nishida et al.), though Nishida et al. showed that the optimal inter-flash interval varied with the bar width. Of course, the strengths of the two phenomena cannot be simply compared because the tasks and evaluation methods were different between the two studies. More experiments will be necessary to clarify the relationship between these two phenomena.