Testing memory for unseen visual stimuli in patients with extinction and spatial neglect.

Visual extinction after right parietal damage involves a loss of awareness for stimuli in the contralesional field when presented concurrently with ipsilesional stimuli, although contralesional stimuli are still perceived if presented alone. However, extinguished stimuli can still receive some residual on-line processing, without awareness. Here we examined whether such residual processing of extinguished stimuli can produce implicit and/or explicit memory traces lasting many minutes. We tested four patients with right parietal damage and left extinction on two sessions, each including distinct study and subsequent test phases. At study, pictures of objects were shown briefly in the right, left, or both fields. Patients were asked to name them without memory instructions (Session 1) or to make an indoor/outdoor categorization and memorize them (Session 2). They extinguished most left stimuli on bilateral presentation. During the test (up to 48 min later), fragmented pictures of the previously exposed objects (or novel objects) were presented alone in either field. Patients had to identify each object and then judge whether it had previously been exposed. Identification of fragmented pictures was better for previously exposed objects that had been consciously seen and critically also for objects that had been extinguished (as compared with novel objects), with no influence of the depth of processing during study. By contrast, explicit recollection occurred only for stimuli that were consciously seen at study and increased with depth of processing. These results suggest implicit but not explicit memory for extinguished visual stimuli in parietal patients.