Vertical versus horizontal spatial compatibility: Right-left prevalence with bimanual responses

For two-choice tasks in which stimulus and response locations vary along ho rizontal and vertical dimensions, the spatial compatibility effect is often stronger on the horizontal than vertical dimension. Umilta and Nicoletti [ (1990) Spatial stimulus-response compatibility (pp. 89-116). Amsterdam: Nor th-Holland] attributed this right-left prevalence effect to an inability to code vertical location when horizontal codes are present simultaneously. H ommel [(1996) Perception dr Psychophysics, 43, 102-110] suggested instead t hat it reflects a voluntary strategy. This study reports four experiments t hat examine this issue. Experiment I was a conceptual replication of Hommel 's Experiment I, with responses made on a numeric keypad and subjects instr ucted in terms of the vertical or horizontal dimension. The results replica ted Hommel's findings that showed a right-left advantage with horizontal in structions; however, with vertical instructions, we found a benefit of vert ical compatibility alone that he did not. This benefit for vertical compati bility alone was eliminated in Experiment 2 using a varied practice schedul e similar to that used by Hommel. Experiment 3 showed right-left prevalence and a benefit of vertical compatibility alone, even with varied practice a nd vertical instructions, when subjects responded on perpendicularly arrang ed handgrips. These benefits were eliminated in Experiment 4 using Hommel's method of urging subjects to respond only in terms of the instructed dimen sion. With bimanual responses, right-left prevalence is a robust phenomenon that is evident when comparing across vertical and horizontal instructions and, when the right-left distinction is relatively salient, within the ver tical instructions condition alone.