The effects of stimulus symmetry on landmark judgments in left and right visual fields

Line bisection impairment is greater following right compared to left hemisphere damage, suggesting that some of the underlying visuo-spatial mechanisms may be lateralised. One important perceptual component of line bisection is estimating stimulus midpoint. Here, we used a modified version of the perceptual landmark task to examine, in healthy individuals, how the midpoint of a stimulus is apprehended, and if the cerebral hemispheres are equally as efficient in performing such a task. We show that the right, relative to the left, hemisphere is both faster and more accurate at apprehending prebisected lines, but no better at apprehending misbisected lines. We then demonstrate that the right hemisphere advantage stems from a specialisation in detecting stimulus symmetry; by associating prebisected lines with the presence of display symmetry, transect location can be inferred without having to explicitly calculate the midpoint of lines. The findings provide a further reason why right hemisphere damage is so deterimental to perceptual line bisection. In addition, the data indicate that the detection of visual symmetry is preferentially lateralised to the right hemisphere.