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Thorsten Hansen, Karl Gegenfurtner; Uncovering multiple higher order chromatic mechanisms in cone contrast space. Journal of Vision 2010;10(7):388. doi: 10.1167/10.7.388.
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Despite good psychophysical and physiological evidence, the number and nature of multiple higher-order chromatic mechanisms is still under debate. This is mainly due to several studies that defined their stimuli in cone contrast space (CCS) and failed to find support for higher order mechanisms. We measured detection thresholds for chromatic directions in cone contrast space using a noise masking paradigm (Hansen & Gegenfurtner (2006), Journal of Vision, 6(3):5, 239–259). Our choice of masking directions (38 and 47 deg) was guided by an analysis of the nonlinear mapping of angles between cone contrast space and a post-receptoral color space (DKL). When the noise contrast was sufficiently high (40% rms cone contrast), we found clear evidence for selective masking, indicating multiple mechanisms tuned to these directions. Why did earlier studies in CCS fail to find evidence for higher order chromatic mechanisms? First, the noise directions in CCS tested in previous studies (90 deg ΔM/M, 135 deg isolum) map to almost identical angles in DKL space (7.1 and 1.6 deg), implying that effectively only one higher order mechanisms (L–M) was stimulated. Second, the masking contrast in these studies was generally very low (<10%), resulting in insufficient power to differentially activate higher order mechanisms. We conclude that CCS as a receptoral color space is not well-suited for the study of higher order mechanisms. Rather, these higher-order mechanisms seem to be evenly distributed in post-receptoral DKL color space. DKL color space is a natural choice to study higher order mechanisms because it reflects the input signals arriving in visual cortex. Higher order mechanisms exist in CCS, but are more difficult to find.
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