Limitations of attentional orienting. Effects of abrupt visual onsets and offsets on naming two objects in a patient with simultanagnosia.

It has been proposed that the underlying deficit for some simultanagnosics is the inability to bilaterally orient attention in space due to parietal damage. In five experiments, we examine the performance of a patient with simultanagnosia secondary to bilateral occipito-parietal lesions, IC, in naming pairs of line-drawings. With simultaneous presentation and disappearance of objects (Experiment 1), IC typically named a single object. IC's performance dramatically improved when the two drawings alternated every 500 ms (Experiment 2). This improvement was not due to the abrupt onset of the second drawing "capturing attention", as indicated by the results of Experiment 3. Experiments 4 and 5 demonstrated that the crucial factor in improving IC's performance with simultaneous presentation of visual objects was the offset of one of the two stimuli. We propose that IC's impairment in naming two objects is attributable to the inability to "unlock" attention from the first object detected to other objects in the array. Visual offset of the first object disengages attention from the first object, allowing it to be allocated to the second object.