Increment threshold for a small (e.g. 1' x 5') line target superimposed on backgrounds of various shapes and sizes was measured to provide a detailed map of the spatial interactions about line targets. This modified "Westheimer paradigm" indicated sensitization in the length direction as well as in the width direction around the line target. The effect of the adaptation field summed over an elongated, end-tapered central region, and showed strong end-zone antagonism beyond the ends of the elongated summation area, as well as flank antagonism to the sides. Secondary disinhibitory and inhibitory areas outside of the antagonistic surround were also demonstrated. When length of the test line was varied, the length of the summation region increased concomitantly, while the length of the end-zones remained fixed. End-zone antagonism was slightly weaker at oblique orientations. These results demonstrate a perceptual analog to neurophysiological end-stopping, and suggest a multilobed y-dimension weighting profile appropriate for models of spatial visual abilities.