Neuropsychological Evidence

Results from neuropsychological patients with damage to their parietal lobes support the role of spatial attention in solving the binding problem and conjoining the features of objects. In one representative study, an extinction patient was shown two letters in either the contralesional or ipsilesional visual field. One of the letters was a target (F or X) and the other was a distractor (O). The letters were colored, and the patient was instructed to name both the color and the identity of the target letter (i.e., was it F or X and what color was it?). In this task there are two types of errors. The first is a feature error, in which either letter name or color is reported incorrectly. For example, if the patient was presented with a blue F and a red O, reporting a yellow F would be a feature error. The second type of error is a conjunction error, in which a feature of the distractor letter O "migrates" to the target letter, forming an illusory conjunction. For example, if the patient was presented a blue F and a red O and reported a red F, the color of the red O was misconjoined with the target letter F. The extinction patient studied showed many conjunction errors in the contralesional field compared to the ipsilesional field. However, similar numbers of feature errors were made in the contralesional and ipsilesional fields, indicating that feature perception was similar in both fields. Presumably, the damaged parietal-based spatial attention system is unable to permit a correct conjunction of features such as color and shape; the individual features are represented, however, allowing for accurate reports of the features.

Other patients with damage to the parietal lobe attention areas show inabilities to bind features correctly. Patients with bilateral damage to the parietal lobe have Balint's syndrome, which is characterized by an inability to perceive multiple shapes simultaneously (simultanagnosia); Balint's patients can perceive only one object at a time. Recent reports indicated that Balint's patients may show higher than normal rates of illusory conjunctions. If shown a display containing a red X and a blue T and asked to report the name and color of the first letter seen, these patients may often report seeing a blue X and a red T. These misconjunc-tions even occur when the display is present for several seconds.

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