The effect of proportion manipulation on the size-congruency and distance effects in the numerical Stroop task

In the numerical Stroop task, participants are presented with two digits that differ in their numerical and physical size and are requested to respond to which digit is numerically larger. Commonly, slower responses are observed when the numerical distance between the digits is small (the distance effect) and when the numerical and physical size are incongruent (the size-congruency effect). The current study will use proportion manipulation, which consists of two experimental lists with high versus low frequency of trials belonging to different conditions, as a tool to reduce these effects. Specifically, it will be used to examine how these two interference effects depend on each other, and how a reduction of one effect will affect the other. In Experiment 1, the size-congruency proportions were manipulated; in Experiment 2, the distance proportions were manipulated. The results show that manipulating size-congruency proportions modulates the size-congruency effect but not the distance effect, while manipulating the distance proportions modulates the distance effect but not the size-congruency effect. These results demonstrate for the first time that the distance effect can be modulated by the distance proportions. Furthermore, these results indicate that proportion manipulation is specific and only modulates the variable being manipulated. Together, these results shed new light on the specificity of proportion manipulation in the context of numerical information processing. These results are further discussed in the context of various numerical models that suggest a different relationship between these effects and demonstrate how proportion manipulation can aid to investigate numerical processes.