Decade breaks in the mental number line? Putting the tens and units back in different bins

Most models of number recognition agree that among other number representat ions there is a central semantic magnitude representation which may be conc eptualized as a logarithmically compressed mental number line. Whether or n ot this number line is decomposed into different representations for tens a nd units is, however, controversial. We investigated this issue in German p articipants in a magnitude comparison (selection) task in which the larger of two visually presented Arabic two-digit numbers had to be selected. Most importantly, we varied unit-decade-compatibility: a number pair was define d as compatible if the decade magnitude comparison and the unit magnitude c omparison of the two numbers would lead to the same response (e.g. 52 and 6 7) and as incompatible if this was not the case (e.g. 47 and 62). While con trolling for overall numerical distance, size and other variables, we consi stently found compatibility effects. A control experiment showed that this compatibility effect was not due to perceptual presentation characteristics . We conclude that the idea of one single number line representation that d oes not additionally assume different magnitude representations for tens an d units is not sufficient to account for the data. Finally, we discuss why decade effects were not found in other experimental settings. (C) 2001 Else vier Science B.V. All rights reserved.