Perceptual similarity affects the learning curve (but not necessarily learning).

What role does item similarity play in motor skill acquisition? To examine this question, we used a modified version of the chord learning task (Seibel, 1963) that entails producing simultaneous finger key presses, similar to playing a chord on a piano. In Experiment 1, difficulty, as indexed by response time (RT) to a particular chord on the first session, was held constant, and chords that were similar to other chords had longer RTs after practice than dissimilar chords. In Experiment 2, we used chords that produced different initial RTs to show that similarity affected asymptotic RT rather than the size of RT decrement achieved with practice. In Experiment 3, we eliminated differences in perceptual similarity by using Chinese characters for stimuli while retaining differences in motoric similarity, which resulted in nearly identical asymptotes for similar and dissimilar chords. Thus, the density effect observed in Experiments 1 and 2 appears to stem from competition triggered by similar stimuli. Because performance differences were immediately re-established when stimulus similarity was introduced in Experiment 3 during transfer sessions, competition appears to emerge among learned, central representations that can be coactivated by multiple stimuli.