Search for feature and for relative position: measurement of capacity limitations.

Palmer and his associates (Palmer, Ames & Lindsey (1993). Journal of Experimental Psychology: Human Perception and Performance, 19, 108-130; Palmer (1994). Vision Research, 34, 1703-1721) have confirmed that searches for simple feature targets are not limited by perceptual processing capacity and the effect of set size on performance can be accounted for by integration stage processes only. In this study I used a similar difference threshold method with target and distractor stimuli defined by the relative position of their elements (line drawings of bisected squares) and found clear capacity limitations. Feature search condition, however, with nearly comparable bisected square stimuli did replicate the results of Palmer and associates. This experiment demonstrates that a search for targets defined by relative position in the set of line drawing type of stimuli is fundamentally different from a search for more simple (feature) stimuli and may conform to a strict capacity limited model.