Humans map numbers upon a mental number line (MNL) on which small integers are placed to the left of larger ones [1-9]. Here, we show that human adults systematically shift the subjective midpoints of number intervals away from the borders separating contiguous tens along the MNL. Number intervals are erroneously bisected further to the right the closer they are to the left starting point of the tens and further to the left the closer they are to the right endpoint of the tens. Similarly, right-brain-damaged patients neglecting the left side of the MNL [10-12] show enhanced pathological rightward deviation in the bisection of number intervals located toward the left starting point of the tens and reduced deviation for intervals located toward the right endpoint of the tens. These data disclose the effects that the recursive grouping of symbolic decimal numerals within the tens has brought on the phylogenetically and ontogenetically foregoing spatial representation of magnitudes [13-16].
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