The Spatial Nature Of The Mental Number Line In Neglect Patients

Objective The space orientation of the mental number line is currently a subject of investigation in the numerical cognition literature. We investigated the spatial nature of the mental number line and the neural mechanisms of neglect in hemispatial neglect patients.Methods Our screening tests used the physical line bisection task, Albert cancellation test and tests of simple arithmetic. All participants showed intact numerical and arithmetic skills. Eight right-brain-damaged patients with persistent left neglect, eight aged- and education-level-matched right-brain-damaged patients without spatial neglect,and eight healthy control subjects participated the mental number bisection task. Participants heard two anterograde(for example, 2–4) or reverse(for example, 4–2)numbers with fixed number intervals . The size of the interval could be three (for example, 2–4), five(for example, 1–5), seven or nine.All participants were asked to answer the midpoint number without making calculations.Result Neglect patients made many errors of the midpoint number in the mental number bisection task.The performances were affected by the size of the number interval.In the anterograde presenting test ,the error rate of the subjective midpoint of neglect patients was ( 44.63±12.49 ) % . For longer intervals, right-shifted errors were more common .when the interval sizes of numbers were three,five,seven or nine,the mean deviations of the subjective midpoint were(-4.25±7.87)%,(12.63±19.06)%, (58.38±24.30)%,(111.50±53.54)% respectively.Reverse presentation produced the same performance as anterograde presentation.The error rate of the subjective midpoint of neglect patients wa(s46.00±12.10)%.When the interval sizes of numbers were three,five,seven or nine,the mean deviations were(-18.40±25.35)%,(-1.60±214)%,(56.80±27.04)%,(108.20±55.10)%.The results of analysis of variance were F=1.705, P>0.05 between the performances when numbers were anterograde and reverse presented .The results of correlative analyse were r=0.887, P<0.05 between the performances of the mental number bisection task and the line bisection task.Whatever the numbers were anterograde or reverse presented ,there were left-shifted errors of the subjective midpoint in two neglect patients when the interval size of numbers were three.Control subjects made few errors . When the numbers were anterograde and reverse presented, the error rate of the subjective midpoint of the right-brain-damaged patients without spatial neglect were(3.34±4.32)%,(5.21±5.21)% respectively. the error rate of healthy control subjects was (0.00±0.00)%.The performances of control subjects ware not affected by interval sizes. The magnitude of numbers used in task did not influenced any performances of the groups.Conclusions The performances of neglect patients in the mental number bisection task were affected by the size of the number interval.And the performances of control subjects were not affected by interval size.This demonstrates the systematic subjective midpoint shift observed in our test is produced by neglect. Representation exists disorder in neglect patients.There were left-shifted errors of the subjective midpoint in two neglect patients when the interval size of numbers were three.This is closely similar to the cross-over effect in the physical line bisection task. The performances of neglect patients had relevance in the mental number bisection task and the physical line bisection task,which demonstrates the spatial nature of the mental number line isomorphism to the spatial nature of physical lines. Reverse presentation produced the same performances as anterograde presentation,which confirms the mental number line is orientated in a left-to-right manner.