Leslie L Baylis | Search PubMed

PubMed2004

Visual neglect can be object-based or scene-based depending on task representation.

Three patients with visual neglect were tested on their ability to detect target letters at ipsilesional and contralesional locations on a monitor, and at different locations within large shapes on the monitor. When patients were asked to detect targets within the entire monitor, they showed neglect for all the contralesional hemifield. In contrast when they were asked to detect targets within a particular object, they showed object-based neglect. In these two conditions the displays, the targets and the response were identical, with the only difference being the space that is represented for the task. These results show that the reference frame of visual neglect may be altered by task-instructions changing how a structured visual scene is represented, with neglect applying to the contralesional side of this represented space.

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PubMed2002

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.

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