Numerical Stroop effect

In Psychology, related to the standard Stroop effect, the numerical Stroop effect demonstrates the close relationship between numerical values and physical sizes. Digits symbolize numerical values but they also have physical sizes. A digit can be presented as big or small, irrespective of its numerical value. Accordingly, the physical and numerical dimensions may be compatible and create congruent trials (e.g., 5 3) or they may be incompatible and create incongruent trials (e.g., 3 5). Comparing digits in incongruent trials is commonly slower than comparing digits in congruent trials and the difference in reaction time (RT, the time from the onset of the stimulus to the onset of response) is termed the numerical Stroop effect or the size congruity effect. In an experiment, participants carry out a physical or a numerical task in separate blocks of trials. In the numerical task participants respond to the numerical values and ignore the physical sizes and in the physical task participants respond to the physical sizes and ignore the numerical values. Also, it is possible to mix neutral trials with the congruent and incongruent trials. In neutral trials the two digits vary in one dimension only, like the pair 5 3 for the numerical task of trials and 3 5). Comparing digits in incongruent trials is commonly slower than comparing digits in congruent trials and the difference in reaction time (RT, the time from the onset of the stimul 3 for the physical task. Neutral trials enable to measure facilitation (i.e., the difference between neutral and congruent trials) and interference (i.e., the difference between incongruent and neutral trials).

Original experiments

Besner and Coltheart (1979) asked participants to compare numerical values and ignore the physical sizes of the digits (i.e., the numerical task). They reported that the irrelevant physical sizes slowed down responding when physical sizes were incongruent with the numerical values of the digits. Henik and Tzelgov (1982) examined not only the numerical task but also the physical task. The numerical Stroop effect was found in both tasks. Moreover, when the two dimensions were congruent responding was facilitated (relative to neutral) and when the two dimensions were incongruent responding was slowed down (relative to neutral) – the interference component.

Experimental findings

The Stroop effect is asymmetric - color responses are slowed down by irrelevant words but word reading is commonly not affected by the irrelevant colors. Unlike the Stroop effect, the numerical Stroop effect is symmetric – irrelevant physical sizes affect the comparisons of numerical values and irrelevant numerical values affect comparisons of physical sizes. The latter (the effect of irrelevant numerical values) gave rise to the suggestion that numerical values are processed automatically, among other things, because this occurs even when responding to numerical values is much slower than responding to physical sizes. Moreover, processing numerical values depends on familiarity with the numerical symbolic system. Accordingly, young children may show the physical size effect in numerical comparisons but not the effect of numerical values in physical size comparisons.