We’ve had plenty of discussion of the Stroop Effect on Cognitive Daily, and a cool effect it is — but it’s not the only effect with a catchy name. How about the *spatial numerical association of response codes effect*? Not catchy enough for you? The scholars researching the effect have taken to calling it the SNARC Effect. Now if that isn’t an effect named to win a blogger’s heart, I don’t know what is.

First identified by Dehaene, Bossini, and Giraux in 1993, the SNARC Effect suggests that people represent numbers in the form of an imaginary number line. When asked to indicate if a digit is odd or even, people respond faster to large numbers with their right hand, and faster to small numbers with their left hand. Dehaene et al. reason that this must be because respondents are imagining the numbers on a number line, where smaller numbers are always to the left.

More recent research has confirmed the SNARC Effect, so now study is focused on details of the effect. Samuel Shaki and William Petrusic wondered if we respond to negative numbers the same way.

Negative numbers are obviously very different than positive numbers. Most importantly, they don’t have real physical correlates — though I can have 3 golf balls, I can’t have –3 golf balls (I can assure you: with my unique talent for hitting golf balls into water hazards, if it was possible, I’d have done it).

Shaki and Petrusic devised a simple test to see if response times for negative numbers were also bound by the SNARC Effect. They showed viewers a computer indicating either “larger” or “smaller.” Then two numbers between –9 and +9 were displayed. The two numbers were always just one apart (e.g. 1,2 or –3,–2). The task was to press the button corresponding to the digit which matched the instructions (so, if “larger” flashed, followed by “1,2″, the correct response would be to push the button on the right, with the right hand). Participants understood that “larger” always meant “greater” in the mathematical sense — thus –3 is larger than –4. Here are the results:

These results support the SNARC effect: reaction to smaller numbers is faster with the left hand. However, when the experiment was run in a slightly different format, the results changed. If the experimenters grouped negative numbers together, so participants only had to compare two negative numbers, a kind of reverse SNARC effect was observed:

With negative numbers, the right hand responded faster to smaller numbers, while the traditional SNARC Effect still applied with positive numbers. Shaki and Petrusic argue this result shows that we process numbers differently depending on context. If we’re only considering negative numbers, we skip a step and represent the negative numbers increasing in absolute value from left to right. We only use the traditional “number line” representation when considering both positive and negative numbers.

Shaki, S., & Petrusic, W.M. (2005). On the mental representation of negative numbers: Context-dependent SNARC effects with comparative judgments.

Psychonomic Bulletin & Review, 12(5),931-937.