Variations on a Theme of Mach
Difficulties arise in measuring masking by Mach bands because very low-contrast signals distort the bands. [Henning, Millar, and Hill, J. Opt. Soc. Am. A 17, 1147 (2000)] Adding narrow luminance increments (bright bars) in the dark Mach band widen the dark band, adding decrements (dard bars) narrow the dark band and conversely in the bright bands. Randomizing signal polarity prevents observers from using the distortion of the Mach bands as a cue to the presence of the signal. [Henning, Hoddinott, Wilson-Smith, and Hill, J. Opt. Soc. Am. A 21, 1379 (2004)] Mach bands’ masking of randomly selected bright (incremental) or dark (decremental) bars revealed that detection was worse in both dark and bright Mach bands than on the neighbouring plateaux. Separate analysis of trials containing only one polarity signal revealed 9-c/deg oscillations in performance as a function of location. Oscillations in the two polarities were approximately 180° out-of-phase. Using either very steep ramps to generate the Mach bands or presenting the stimuli t durations shorter than 20 ms, removes the oscillations.