Two recent studies reported that yes/no recognition can be more impaired by hippocampal lesions than forced-choice recognition when the targets and foils are highly similar. This finding has been taken in support of two fundamental proposals: (1) yes/no recognition tests depend more on recollection than do forced-choice tests; and (2) the hippocampus selectively supports the recollection process. Using the same stimulus materials as in the earlier studies, we tested five memory-impaired patients with circumscribed hippocampal lesions and 15 controls. As in the earlier studies, participants studied 12 pictures of objects and then took either a 12-item forced-choice test with four alternatives or a 60-item yes/no test. Patients were impaired on both tests but did more poorly on the yes/no test. However, a yes/no test based on 12 study items would conventionally involve only 24 test items (i.e., 12 study items and 12 foil items). When we scored only the first 24 test items, the patients performed identically on the yes/no and forced-choice tests. Examination of the data in blocks of 12 trials indicated that the scores of the patients declined as testing continued. We suggest that a yes/no test of 60 items is difficult relative to a 12-item forced-choice test due to the increased study-test delay and due to increased interference, not because of any fundamental difference between the yes/no and forced-choice formats. We conclude that hippocampal lesions impair yes/no and forced-choice recognition to the same extent.

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