The greyscales task: a perceptual measure of attentional bias following unilateral hemispheric damage

The two cerebral hemispheres in humans have been suggested to control contralaterally opposed attentional biases. These biases may be revealed by unilateral hemispheric damage, which often causes contralesional spatial neglect, particularly when the right hemisphere (RH) is affected. Subtle attentional biases have also been observed in normal observers in tasks requiring judgements of horizontal spatial extent, brightness, numerosity and size. Here, we examined attentional biases for judging the darker of two left-right mirror-reversed brightness gradients under conditions of free viewing (the greyscales task). We compared performances of patients with damage to the RH (n=78) and left hemisphere (LH; n=20) with those of normal controls (n=20). Controls showed a small but significant leftward bias, implying a subtle asymmetry favouring the RH. In contrast, RH and LH patients showed extreme rightward and leftward biases, respectively, both of which differed significantly from that of controls. For the patient groups, performance on clinical tests of neglect (cancellation and line bisection) did not predict their greyscales scores. Pathological biases were present in patients without clinical neglect or visual field defects, suggesting that the attentional bias measured by the greyscales task can be dissociated from clinical neglect and visual sensory loss. The greyscales task offers an efficient means of quantifying pathological attentional biases in unilateral lesion patients; it is easy to administer and score, and may be particularly useful for clinical trials of recovery and rehabilitation following stroke.