Interference between nonspatial stimulus features in the Simon effect.

We investigated whether learning effects influencing the Simon effect, such as those found when learning involves the spatial stimulus dimension, might be also found when learning tasks involve other nonspatial stimulus features, such as color, shape, and orientation. Experiment 1 focused on mutual influences between Simon tasks based on color and shape. The Simon task on color is affected by a previously performed Simon task on shape more than the latter is affected by a previously performed Simon task on color. We hypothesized that this difference depends on the difficulty of the tasks. Because orientation discrimination seems more difficult than both color and shape discrimination (Experiment 2), we predicted that a Simon task on orientation would be influenced by a Simon task on color or shape more than tasks on color or shape would be influenced by a task on orientation. Experiments 3 and 4 confirmed our hypothesis.