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Seeing trees or seeing forests in simultanagnosia : attentional capture can be local or global Dalrymple, Kirsten Anne


Patients with simultanagnosia often demonstrate ’local capture’, meaning that they identify only the local elements of stimuli that contain a hierarchy of both local and global structures. Recent studies, however, have found that these patients may implicitly process the global form. We examined the general applicability of the concept of local capture, and specifically whether the global level of stimuli can be explicitly reported by patients with simultanagnosia. We tested a patient with simultanagnosia with globally biased stimuli such as hierarchical Arcimboldo faces and small, dense Navon letters. With Arcimboldo faces our patient often reported only the face and not the local elements -- the first demonstration of global rather than local capture. With Navon letters, the patient’s ability to report the global letter varied with stimulus density and inversely with stimulus size, so that local capture was found only with large and sparse stimuli. With both faces and letters, the likelihood of global capture by the patient was related to the ease of global reporting in controls, as indexed by their reaction times. This suggests that the patient’s global perception is influenced by the same factors operating in healthy individuals. We conclude that attentional capture in simultanagnosia can be either global or local. Capture likely occurs because of a pathological restriction and/or rigidity of attention, but the type of capture depends upon the competitive balance between global and local salience.

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