Contextual modulation in contrast perception: the role of flanker contrast.

Collinear modulation corresponds to the increase (facilitation) or decrease (suppression) of contrast sensitivity as measured when a Gabor patch is presented between two collinear high contrast flankers. Recent visual trainings exploited this well-studied contextual modulation phenomenon to improve visual perception in clinical populations. In most cases, training protocols that make use of lateral masking assume as a dogma that the two collinear flankers must have high contrast. However, as shown by Zenger and Sagi (1996) in foveal vision, lateral context contrast has an influence on the target contrast sensitivity. Here, using a 2AFC procedure in a detection task, we investigated to a greater extent the interaction between flanker contrast, target-to-flanker distance, and target sensitivity both in the fovea and in the periphery. Intriguingly, results showed that for the shorter (supposedly inhibitory) target-to-flanker distance (2λ) reducing flankers' contrast also reduced inhibition. Moreover, when the flankers' contrast was close to the threshold, we observed a switch to facilitation. On the contrary, at a larger (facilitatory) target-to-flanker distance (4λ), reducing flanker contrast also reduced collinear facilitation. When the task was performed in the periphery, the expected pattern of results was not found. Our results bring strong experimental support to the predictions of Zenger and Sagi in the fovea but not in the periphery. We conclude that context distance and contrast cannot be neglected among the influential variables affecting collinear modulation. Yet, the effects of lateral context patterns on contrast sensitivity are different in the fovea and the periphery.