Isolusions: Evidence for strong geometric-optical illusions under isoluminance

Abstract

It has long been known that color and luminance are processed together by the same parvo retinal ganglion cells (De Valois & De Valois, 1988). Recently it has been shown that in the primary visual cortex cells respond to oriented chromatic contrast (Johnson et al., Nature Neurosci. 4, 2001), and that the vast majority of color-selective neurons in V1 and V2 are also selective for orientation (Friedman et al., J. Physiol., 548, 2003). On the contrary, a largely segregated processing of unoriented chromatic contrast and oriented luminance contrast has been hypothesized. This idea was bolstered by observations that geometrical-optical illusions vanish under isoluminance (Livingstone & Hubel, J. Neurosci. 7, 1987). Here we examine the strength of various geometric-optical illusions under isoluminance (Delboeuf, Ebbinghaus, Hering, Judd, Mueller-Lyer, Poggendorff, Ponzo, Vertical, Zoellner.) Subjects interactively manipulated computer-generated line drawings to counteract the illusion. In all cases, illusions presented under isoluminance (both for colors drawn from the cardinal L-M or S-(L+M) directions) were as high as the luminance versions (both for high and low contrast). In two control experiments we tested whether subjective deviations from the nominal isoluminance or chromatic aberrations may have caused the observed strong illusions under isoluminance. Again, no significant differences between the illusions under luminance or isoluminance occurred. Our findings show that geometric-optical illusions under isoluminance are as strong as when presented in luminance, supporting the joint processing of oriented color and luminance contrast in early visual areas. Furthermore, the results show that our conscious percept is deceived similarly for both isoluminance and luminance conditions, suggesting that the joint processing for chromatic and luminance defined contours extends well beyond early visual areas.