Visual extinction with double simultaneous stimulation: what is simultaneous?

Patients with unilateral lesions to the parietal lobe leading to extinction were tested with stimuli presented in both hemifields. In addition to simultaneous presentation of two stimuli, trials were given with the contralesional or ipsilesional side having an onset slightly before the other. In the first experiment, subjects were asked to identify the stimuli. In agreement with di Pellegrino et al. [Neuropsychologia 35(1997)1215], all patients showed maximal extinction when the stimuli were physically simultaneous. In the second experiment, the same patients were requested to report whether the left or right item had been presented first. In accordance with Rorden et al. [Neuropsychologia 35(1997)421], all three patients required the contralesional item to have a significant lead in order to in order to be judged as occurring first. Taken together, these findings appear to present a paradoxical picture of extinction: the first task implies that items compete maximally when they are physically simultaneous, while the second study suggests that the physical simultaneity is not phenomenally special to the patients. Attempts to reconcile these findings with popular models of extinction are discussed.