Modulation of spatial bias in the dual task paradigm: evidence from patients with unilateral parietal lesions and controls.

Lateral attentional bias is common after unilateral brain damage. It has sometimes been proposed that lateral bias is increased by concurrent cognitive demands, perhaps because of lost top-down compensation. However, an important limitation of previous studies is the sole use of right hemisphere patients. Here we employed a dual task paradigm to measure spatial bias on a visual task while manipulating demands of a concurrent auditory task. Bias was examined in patients with left or right parietal lesions and controls. In Experiment 1 the addition of either a non-spatial or spatial auditory task led to a rightward shift in visual bias. This same rightward shift occurred in controls, left parietals and right parietals. Experiment 2 examined whether the participant's response hand affected their bias. In addition, it attempted to distinguish between the hypothesis that modulatory effects are strongly dependent on lateralization of the concurrent task, and the hypothesis that dual tasks cause a general rightward shift. Response hand was found to have no effect on spatial bias. In addition, bias did not differ between left hemisphere (verbal) and right hemisphere (pitch) concurrent tasks, though the trend was for a smaller rightward shift with the verbal task. Our results show that dual tasks do not exacerbate patients' underlying deficits; instead they cause a global shift in attention to the right. This shift may resemble general rightward shifts that have previously been linked to reduced arousal.