Using task-based fMRI to understand attentional mechanisms in healthy controls

Background. Hemispatial neglect is a complex syndrome associated with a reduced capability to orient attentional resources toward the contralateral side of space, that involves a collection of symptoms. We used three functional magnetic resonance imaging (fMRI) tasks (i.e., visual search, VS; line bisection, LB; extinction, EX), performed by 25 healthy subjects, to investigate the behavioral and neural correlates under these symptoms. Procedure. During fMRI, participants were presented with: 1. VS: an array containing 8-12 stimuli such as O-like (target) and Q-like (distractors) shapes projected on both sides 2. LB: a bisected line covering 18° of visual angle 3. EX: a black square (target) of 2x2° that equiprobably appeared on either the left, right or both hemifields Participants pressed one of three response buttons to indicate: left, right and bilateral (i.e., for EX) or central (i.e., for LB). Results fMRI data showed an increased activation in the right angular gyrus comparing bilateral vs. unilateral stimulus in EX, and a bilateral occipital activation comparing high vs. low load stimulus in VS, highlighting that in both cases subjects make an effort performing the task. These results are confirmed behaviourally, in fact subjects are less accurate when they are presented with bilateral (97,8% vs. 99,5% in unilateral) and high load (88,9% vs. 93,1% low load) stimulus. There are no significant activations regarding LB task. Conclusions We found that in healthy conditions the right attentional network is involved in processing simultaneous stimuli and bilateral visual cortex is involved in processing the high load of stimuli. Our findings provide a prior knowledge that can be used to compare with attentional mechanisms in neglect patients performing the same tasks. This will allow us to cluster at least three neglect symptoms, in terms of brain activations raised from compensation mechanisms, due to brain impairments.