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, but little is known about the cognitive strategies involved. In the Landmark test, one could explicitly compare the lengths of the left and right line segments; alternatively, one could compute the centre of mass of the display. We here investigate with functional MRI if these cognitive strategies modulate the neural mechanisms underlying judgements whether pre-transected horizontal lines are correctly bisected (the Landmark test) in normal volunteers. Functional neuroimaging (fMRI) was carried out in 12 healthy volunteers who judged: (a) whether the line segments on either side of the transection mark were of equal length, and (b) whether the transection mark was in the centre of the line. Line centre judgements were made significantly faster than line length comparisons. Increased neural activity common to both strategies was observed in inferior parietal lobes bilaterally and right temporooccipital cortex. Further activations, most likely reflecting general task demands like response selection and motor control, were found in the precentral gyrus bilaterally, supplementary motor area bilaterally, right anterior cingulate, right dorsolateral prefrontal cortex, cerebellar vermis, and right thalamus and right putamen. Explicit length comparisons (relative to line centre judgements) differentially activated left superior posterior parietal cortex, with a tendency toward activation of the equivalent area on the right, while the reverse comparison revealed differential activation in the lingual gyrus bilaterally and anterior cingulate cortex. The activations observed in inferior parietal cortex during task performance using either strategy are consistent with the results of lesion studies. The differential activation of superior posterior parietal cortex following length instructions suggests that explicit comparisons of spatial extent were implicated. The differential activation of bilateral occipital cortex following centre judgements suggests that the centre of a line is extracted at an early stage of visual processing.