What happens to the staircase Gelb effect when the highest luminance is not white?

In the staircase Gelb effect, five squares cut from achromatic Munsell (aM) papers 2.0, 4.0, 6.0, 8.0, and 9.5 are arranged in a row from the darkest to the lightest and illuminated by a spotlight, often referred to as “Gelb illumination”. The perceptual outcome is a compressed lightness range, from middle grey to white, or super-white. The illusion has been extensively used as a case study for the Anchoring Theory (Gilchrist et al, 1999, Psychological Review, 106, 786-834). According to such theory, the highest luminance (hL) of the configuration is assigned the value of white in the local framework. We studied the role played by such hL in the compression rate of the illusion, by manipulating the hL target – four levels: 9.5, 9.25, 9.0, and pastel yellow (Munsell 5Y 9/4 with luminance between the values for 9.25 and 9.0). Results show that the achromatic hLs are off the aM scale, appearing either luminous or super-white; the brightness of the yellow hL target appears also greater than 9.5. The compression effect drops as the hL is lowered. This “decompression” is statistically significant for targets 2.0 and 4.0 with hL 9.0 and yellow