Visual search and spatial deficits

Studies of visual search with patients with spatial attentional deficits have shown that the ability to bind basic features properly and thus to search for the conjunction of two spatially contiguous features is compromised. However, the effect of spatial deficits on feature search is more controversial. Here, we explore questions raised by the neuropsychological literature regarding feature processing and demonstrate that features ‘‘pop out’ ’ in the affected visual field, albeit more slowly. The implications for feature processing and selection as well as the relevance for understanding spatial deficits are discussed. The neuropsychological literature has demonstrated that visual search for conjunctions becomes more difficult when visual spatial deficits are present, whereas feature search remains relatively intact. Perhaps the most dramatic example of this difference has been reported in patients with Balint’s syndrome produced by bilateral damage to the dorsal (occipital-parietal) processing stream of the cortex (see Balint, 1909/1995; Freidman-Hill, Robertson, & Treisman, 1995). When symptoms are severe, these patients see only one object at a time (simultanagnosia) and lose the ability to control which object will pop into view next. Another striking symptom is that they are unable to locate even the objects they do see (see Rafal, 2001; Robertson, 2004, for a more complete description of the syndrome). When forced to guess the object’s location, they may protest or perform at chance levels (Friedman-Hill et al., 1995). When asked to point to the object, they are unable to do so correctly, and their errors in spatial reports are unsystematic. These problems are isolated to space external to the body: Locating sensation on the body is typically intact, as is the ability to move bod