Purpose: Recently, we demonstrated that, contrary to predictions by classical hierarchical vision models, global context affects spatial masking (Barghout and Tyler ARVO 2000). Here we ask whether these effects can be altered by synchrony grouping with a spatially remote stimulus. Methods: We measured threshold elevation as a function of pedestal contrast (TvC) of a vertical 8 c/d micro-Gabor target in five stimulus configurations. The three control configurations controlled for known masking effects: the target alone, the target within a horizontal 2 c/d grating, and the target with the same grating surrounded by a ring whose contrast modulated in synchrony with both 2AFC intervals. The two experimental conditions manipulated global context and perceptual grouping: the target within a grating whose left field was shifted by .5 (producing a line-end induced contour) and the same grating surrounded by a synchronous ring. Results: As predicted by classical masking theories, our control conditions produced no masking or facilitation. Contrary to classical theories, the line-end induced contour produced both low-contrast facilitation and high-contrast masking (Barghout and Tyler ARVO 2000). These changes in threshold due to the induced contour were attentuated when the target was grouped with the surrounding ring by synchrony. Thus, global spatiotemporal context introduced by both induced contours and synchrony grouping can alter local masking effects.