Distinguishing lateral interaction from uncertainty reduction in collinear flanker effect on contrast discrimination

To clarify the role of lateral interaction and uncertainty reduction in collinear flanker facilitation of contrast discrimination, we investigated the effect of flankers and spatial cueing on the contrast discrimination threshold. The task of the observers was to detect a 1.3 cyc/deg vertical Gabor target superimposed on one of the Gabor pedestals presented at 3 degrees to the left and right of the fixation point. We measured the target threshold vs. pedestal contrast (TvC) functions in four (2 x 2) conditions: With or without the presence of collinear flankers and with or without a spatial cue that indicated the location of the target. The presence of the flankers lowered the target threshold at low pedestal contrasts, but increased the threshold at high contrasts. The presence of spatial cues lowered the target threshold at all pedestal contrasts regardless of the presence of flankers. The flanker effect was similar in cued and non-cued conditions, suggesting that the effect was similar regardless of cues. The TvC functions are well fit by a divisive inhibition model in which the presence of collinear flankers increases both the excitatory and inhibitory terms of a divisive inhibition response function, while cueing increases only the excitatory term.