Orientational anisotropy in the human visual system

When a target stimulus in a predesignated location is identified by a keypress response, responses are slightly faster if noise stimuli in adjacent locations are identical to the target than if they are a different stimulus assigned to the same response (a repeated-stimulus superiority effect). An exception to this result has been found in experiments that used randomly intermixed letter and digit stimuli. These experiments showed slower responding for identical noise than for nonidentical, response-compatible noise (a repeated-stimulus inferiority effect). The present study investigated these phenomena in three experiments. Experiments 1 and 2 established that both the superiority and inferiority effects can be obtained in the same experiment. They also provided evidence that the repeated-stimulus inferiority effect is a function of the intermixing of letters and digits and not of the larger target-set size that has been used for mixed lists. Experiment 3 demonstrated that, with unmixed presentation, the repeated-stimulus superiority effect is enhanced by an increase in the number of stimuli assigned to each response. The experiments are consistent with accounts that attribute the repeated-stimulus superiority effect to competition that occurs when different internal recognition responses are activated. Moreover, the experiments suggest that the repeated-stimulus inferiority effect has its basis in processes that occur subsequent to feature extraction.