An investigation of line and dot forms of the muller-lyer and poggendorff illusions

Both the Müller-Lyer and Poggendorff illusions were found to be reduced by about 70% when all-line figures were replaced by all-dot figures. These data are consistent with some other recent reports and they suggest that Coren's (I970) original finding of a 30% reduction represents an underestimation so that, in his terms, cognitive factors do not account for the larger proportion of these illusions. An additional experiment on mixed line and dot Poggendorff figures provided evidence for a similarity effect, so that larger effects occurred with all-line or all-dot figures than with mixed line-dot configurations. Also, mean effects with line-parallels exceeded those with dot-parallels. However, the interaction between these main effects was not significant. Thus, both figural structure (lines vs. dots) and figural homogeneity (all lines, all dots or mixed) may be independent determinants of the Poggendorff effect.