Finding centre: ocular and fMRI investigations of bisection and landmark task performance

The line bisection task is used as a bedside test of spatial neglect patients who typically bisect lines to the right of true centre. To disambiguate the contribution of perceptual from motor biases in bisection, previous research has used the landmark task in which participants determine whether a transection mark is left or right of centre. One recent study using stimuli that reliably leads to leftward perceptual biases in healthy individuals, found that ocular judgements of centre were biased to the right of centre, whereas manual bisections were biased leftwards. Here we used behavioural measures and functional MRI in healthy individuals to investigate ocular and perceptual judgements of centre. Ocular judgements were made by having participants fixate the centre of a horizontal bar that was dark at one end and light at the other (i.e., a 'greyscale' stimulus), whereas perceptual responses were made by having participants indicate whether a transection mark on the greyscales stimuli was to the left or right of centre. Behavioural data indicated a leftward bias in the first, second and longest fixations for bisection. Moreover, greyscale orientation (i.e., dark extremity to the right or to the left), and stimulus position modulated fixations. In contrast, for the landmark task, initial fixations were attracted towards the transection mark, whereas subsequent fixations were closer to veridical centre. Imaging data showed a large bilateral network, including superior parietal and lingual cortex, that was active for bisection. The landmark task activated a predominantly right hemisphere network including superior and inferior parietal cortices. Taken together these results indicate that very different strategies and underlying neural networks are invoked by the bisection and landmark tasks.