The detectability of a Gabor patch is improved by the presence of collinear flanking Gabors, this phenomenon is termed collinear facilitation. In experiment 1, we investigate the effects of adding 2D spatial luminance noise as a means of investigating different transects through the suprathreshold contrast space to see whether facilitation is ubiquitous throughout the contrast domain or whether it is confined to absolute contrast threshold. The results show that adding luminance noise abolishes the facilitation, showing it is confined to absolute threshold. In experiment 2, we assess whether 2nd order stimuli exhibit collinear facilitation and whether 1st order flanks can induce facilitation in 2nd order stimuli and vice versa. Our results suggest that collinear facilitation, albeit weaker, does occur for some 2nd order stimuli but we did not find any 1st/2nd order interactions, suggesting separate 1st/2nd order cortical processing streams, at least at the level at which this phenomenon occurs. Our two main findings, namely the lack of facilitation at suprathreshold contrasts and its presence for 2nd order processing argue against it playing a pivotal role in contour integration which does occur at all contrasts but not for these 2nd order stimuli.
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