Objective: To investigate the spatial nature of mental number line.
Methods: Number bisection test was conducted on 8 right-brain-damaged patients with persistent left neglect, 8 right-brain-damaged patients without spatial neglect, and 8 healthy control subjects, all aged- and education-level-matched and right-handed.
Results: The error rate in subjective midpoint of the 8 neglect patients was (44.63+/-12.49)% (forward direction) and (46.00+/-12.10)% (reverse direction), both significantly higher than those of the right brain damage control group [(3.34+/-4.32)% and (5.21+/-5.21)% respectively] and healthy control group [(0.00+/-0.00)% and (0.00+/-0.00)% respectively] (all P<0.01). The longer the interval between the numbers, the higher right-shift error rate: corresponding to the interval sizes as 3, 5, 7, and 9, the mean deviation rates of anthrone presentation were (-4.25+/-7.87)%, (12.63+/-19.06)%, (58.38+/-24.30)%, and (111.50+/-53.54)% respectively, and the mean deviation rates of reverse presentation were (-18.40+/-25.35)%, (-1.60+/-21.4)%, (56.80+/-27.04)%, and (108.20+/-55.10)% respectively. Analysis of variance of the anthrone presentation and reverse presentation showed the result: F=1.705 and P=0.262. Correlation analysis of the mental number bisection task and line bisection task showed the result: r=0.887 and P=0.045. The control subjects made very few errors in subjective midpoint and their performance was not affected by interval sizes. The magnitude of numbers used did not influence the performance in any group.
Conclusion: The performance of neglect patients in bisecting mental number line closely mirrors their difficulty in bisecting physical lines. This demonstrates the spatial nature of the mental number line isomorphism to physical lines. The mental number line is orientated in a left-to-right manner. Disorder exists at the characterization level in neglect patients.