Visual perception without awareness in a patient with posterior cortical atrophy: impaired explicit but not implicit processing of global information.

A patient with progressive posterior cortical atrophy (PCA) was examined on several tests of visual cognition. The patient displayed multiple visual cognitive deficits, which included problems identifying degraded stimuli, attending to two or more stimuli simultaneously, recognizing faces, tracing simple visual stimuli, matching simple shapes, and copying objects. The patient was also impaired in identifying visual targets contained at the global level within global-local stimuli (i.e., smaller letters that compose a larger letter). Although the patient denied any conscious awareness of the global form, he nevertheless displayed a normal pattern of global interference when asked to identify local level targets. Thus, the patient processed the global information despite not being consciously aware of such information. These results suggest that global-local processing can take place in the absence of awareness. Possible neurocognitive mechanisms explaining this dissociation are discussed.