Stimulus symmetry affects the bisection of figures but not lines: evidence from event-related fMRI

Many geometric shapes retain their symmetry when bisected, but appear asymmetrical when misbisected. We have previously shown that this correspondence can guide the accuracy and speed of perceptual bisection (Landmark) judgments. Using event-related fMRI, here we examined whether the behavioural effects of symmetry are also evident at the neural level. The data showed that the presence/absence of symmetry modulates the activity of right anterior cingulate gyrus, an area associated with a variety of higher level attentional functions. A previous visual half-field study also showed that bisected lines are apprehended more quickly and accurately than misbisected lines in right, but not left, visual field. We were able to localise this advantage to right superior temporal gyrus. Significantly, we found no evidence that symmetry played a role in apprehending the midpoint of the line stimuli traditionally used to assess visual neglect. The data clarify the effects of visual symmetry on bisection behaviour, and highlight novel dissociations within the neural systems thought to underline Landmark performance.