Impairments of the mental number line for two-digit numbers in neglect

Humans represent numbers along a left-to-right oriented Mental Number Line (MNL). Neglect patients seem to neglect the left part of the MNL, namely the smaller numbers within a given numerical interval. However, until now all studies examining numerical representation have focussed on single-digit numbers or two-digit numbers smaller than 50. In this study, the full range of two-digit numbers was assessed in neglect patients and two control groups. Participants were presented with number triplets (e.g., 10_13_18) and asked whether or not the central number is also the arithmetical middle of the interval. The factors manipulated were decade crossing (e.g., 22_25_28 vs 25_28_31), distance to the arithmetical middle (e.g., 18_19_32 vs 18_24_32), and, most importantly, whether the central number was smaller or larger than the arithmetical middle (e.g., 11_12_19 vs 11_18_19). Neglect patients differed from controls in that they benefited less when the middle number was smaller than the arithmetical middle of the interval. Neglect patients thus seem to have particular problems when accessing the left side of numerical intervals, also when adjusted to two-digit numbers. Such an impaired magnitude representation in neglect seems to have detrimental effects on two-digit number processing as the helpful spatial metric of magnitude cannot be properly activated.