Task instructions influence the cognitive strategies involved in line bisection judgements: evidence from modulated neural mechanisms revealed by fMRI

Manual line bisection and a perceptual variant thereof (the Landmark test) are widely used to assess visuospatial neglect in neurological patients, bu t little is known about the cognitive strategies involved. In the Landmark test, one could explicitly compare the lengths of the left and right line s egments; alternatively, one could compute the centre of mass of the display . We here investigate with functional MRI if these cognitive strategies mod ulate the neural mechanisms underlying judgements whether pre-transected ho rizontal lines are correctly bisected (the Landmark test) in normal volunte ers. Functional neuroimaging (fMRI) was carried out in 12 healthy volunteer s who judged: (a) whether the line segments on either side of the transecti on mark were of equal length, and (b) whether the transection mark was in t he centre of the line. Line centre judgements were made significantly faste r than line length comparisons. Increased neural activity common to both st rategies was observed in inferior parietal lobes bilaterally and right temp orooccipital cortex. Further activations, most likely reflecting general ta sk demands like response selection and motor control, were found in the pre central gyrus bilaterally, supplementary motor area bilaterally, right ante rior cingulate, right dorsolateral prefrontal cortex, cerebellar vermis, an d right thalamus and right putamen. Explicit length comparisons (relative t o line centre judgements) differentially activated left superior posterior parietal cortex, with a tendency toward activation of the equivalent area o n the right, while the reverse comparison revealed differential activation in the lingual gyrus bilaterally and anterior cingulate cortex. The activat ions observed in inferior parietal cortex during task performance using eit her strategy are consistent with the results of lesion studies. The differe ntial activation of superior posterior parietal cortex following length ins tructions suggests that explicit comparisons of spatial extent were implica ted. The differential activation of bilateral occipital cortex following ce ntre judgements suggests that the centre of a line is extracted at an early stage of visual processing. (C) 2001 Elsevier Science Ltd. All rights rese rved.