The effect of edge blur on grating induction magnitude

In order to assess the contribution of high spatial frequency channels (i.e. local, edge-dependent mechanisms) to the grating induction effect, grating induction magnitude was measured as a function of systematic amounts of blurring of the inducing/test field boundary for four test field heights which spanned a two octave range (0.25-2.0°). Measurements were obtained from two subjects using both the cancelling procedure of McCourt [(1982) Vision Research, 22, 119-134] and a contrast matching paradigm. The two measures yielded similar outcomes: consistent with previous results, both matching and cancelling contrast decreased monotonically with increasing test field height. The effect of blurring the edge was to produce a small (η² = 0.6-5.1%), but significant (P < 0.001), increase in grating induction magnitude. A second experiment utilized the contrast matching paradigm to investigate the effect of edge blur at extreme values (i.e. zero blur and maximum blur) on grating induction magnitude across a four octave range of spatial frequency (0.0625-1.0 c/deg). The results of the matching procedure were again consistent with those obtained previously using the cancelling technique: grating induction magnitude decreased monotonically with increasing spatial frequency. The effect of blurring was to produce a modest (η² = 1.0-2.8%), but significant (P < 0.001), elevation in induction magnitude. These results lead to the conclusion that, unlike some other brightness effects, visual spatial filters selectively sensitive to high spatial frequencies, or to the edges which they sharpen, are not essential for the production of the grating induction effect.