The effect of collinear context on the filter mediating the detection of a Gabor stimulus was investigated by using the classification image method. Classification images were estimated for a 1.5 cpd horizontal Gabor target and the same target flanked by two collinear Gabors horizontally 1.7 degrees displaced from the target. The target was masked by a low-contrast white-noise mask. Obtained classification images were fitted by Gabor functions. The results show that collinear flankers increase the length of the classification image profiles along the collinear axis. At the same time, modest facilitory effects were observed in most subjects. The specificity and the amount of context-induced elongation in the classification images makes it hard to be explained by uncertainty reduction alone. In previous studies, collinear facilitation has been reported to abolish due to perceptual learning. We report a possibly related phenomenon: classification image data was re-analyzed in two parts consisting of the early and the late trials. In the latter trials, differences between the classification images in flankers and no-flankers condition are no longer significant.