The magic number four and temporo-parietal damage : neurological impairments in counting targets amongst distractors

Three experiments assessed visual search and target enumeration performance in a patient (DW) with damage to the right temporo-parietal region. DW was extremely accurate in single feature and conjunction visual search tasks. In enumeration tasks in which there were no distracters DW was also extremely accurate, showed a normal subitisation-counting function, and could use global shape as a guide to numerosity. However, enumeration of targets amongst distracters was only accurate for up to three or four items, after which performance decreased rapidly. The results are discussed in relation to recent theories of enumeration