Effect of the similarity between target and global and local levels in hierarchical stimuli processing.

The hypothesis of global precedence was investigated by manipulating the similarity between target and relevant level, and between target and global and local levels of hierarchical stimuli, both in divided and selective attention conditions. The similarity was measured on the basis of the amount of shared contour between two visual forms. The stimuli were circles, right-facing semicircles, i.e., 1/2 of the circumference missing, and right-facing partial circles with a 1/4 of the circumference missing. The circle was used as a target and, consequently, the similarity between target and semicircle and between it and the partial circle was 50% and 75%, respectively. In the divided attention condition, the results showed a local advantage in the two similarity conditions between target and distractor level (50% and 75%) and a bidirectional and symmetrical interference at both levels of similarity. However, in the 'selective attention condition', a local reaction time advantage was only found when the similarity between target and irrelevant level was of 50%. When attention was directed towards the global level, local interference increased depending on the decrease in the degree of the local level similarity. When the attention was directed towards the local level, there was global interference but it was independent of the degree of the global level similarity. These results show the importance of the physical shape of the local elements used in experiments with hierarchical stimuli, since they may determine the relative speed of global processing, facilitating or impairing it.