You’ve probably heard of the Stroop effect (if not, see here), now let me introduce you to the SNARC.
The Spatial Numerical Association of Response Codes (SNARC) effect is the observation that people are faster to make a judgment about a number if the hand they use to respond is congruous with the size of the number in question – with the left hand being quicker for smaller numbers and the right quicker for larger numbers. It suggests we automatically associate smaller numbers with the left side of space and larger numbers with the right-hand side, and it reinforces the age-old notion that mentally we represent numbers as if they are located along a line.
For example, when instructed to respond to even numbers by pressing a button with their left hand, and to respond to odd numbers by pressing a button with their right hand, people will be quicker responding to ‘2’ compared with ‘98’, whereas they will be quicker responding to ‘97’ compared with ‘3’.
The SNARC effect can also operate in the vertical dimension, with people associating larger numbers with upper space and smaller numbers with lower space.
Now Wim Gevers and colleagues have shown that the vertical and horizontal effects interact. Imagine a lower left-hand key must be pressed for even numbers and an upper-right hand key must be pressed for odd numbers. In this case, the SNARC effect will be particularly large when responding to small, even numbers.
However, the researchers also showed that a given spatial advantage (i.e. left versus right; upper versus lower) is only activated if it is relevant to the response. For example, if people are instructed to press an upper button in response to an odd number, and a lower button to an even number, with no left/right dimension, the usual left-hand advantage for small numbers will disappear or be reduced.
Gevers, W. Lammertyn, J., Notebaert, W., Verguts, T. & Fias, W. (2006). Automatic response activation of implicit spatial information: Evidence from the SNARC effect. Acta Psychologica, 122, 221-233.
For more on the SNARC, see full-text seminal paper here.