Uncovering the functional relationship between temporal lobe amnesia and diencephalic amnesia depends on determining the role of the fornix, the major interlinking fiber tract. In this study relating fornix volume with memory, we made magnetic resonance imaging-based volume estimates of 13 brain structures in 38 individuals with surgically removed colloid cysts. Fornix status was assessed directly by overall volume and indirectly by mammillary body volume (which atrophies after fornix damage). Mammillary body volume significantly correlated with 13 out of 14 tests of episodic memory recall, but correlated poorly with recognition memory. Furthermore, as the volumes of the left fornix and the left mammillary bodies decreased, the difference between recall and recognition scores increased. No other structure was consistently associated with memory. These findings support models of diencephalic memory mechanisms that require hippocampal inputs for recall, but not for key elements of recognition.

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To understand recognition memory, the detection of stimulus repetition, it first is necessary to resolve the debate between 2 fundamentally different models of recognition. Contemporary single-process models assume that recognition memory relies solely on the neural system required for the recall of prior events. Dual-process models assume that recognition comprises 2 independent forms of memory: one supports recall, and the other detects repeated stimuli by signaling their familiarity, the feeling of previous occurrence without the recall of any associated information. These 2 models were contrasted in patients who had undergone surgical removal of a colloid cyst, a condition associated with memory loss when accompanied by fornix and/or mammillary body atrophy. Comparisons were made between 2 groups of 9 patients that differed only with respect to the extent of mammillary body atrophy. Only the more atrophied group was impaired on tests of recall, but both groups showed normal recognition levels on a task that equates recall and recognition performance in normal participants. To explore the nature of this spared recognition, we estimated recall-based recognition and familiarity-based recognition using 3 distinct methods: self-report, receiver operating characteristics, and structural equation modeling. All 3 methods showed impaired recall-based recognition accompanied by intact familiarity in the most atrophied group, as predicted only by dual-process models. When structural equation modeling was applied to all 62 colloid cyst patients, the recall/familiarity dualprocess model best explained the patients' memory pattern. The convergent evidence that mammillary body atrophy impairs recall but spares familiarity-based recognition appears irreconcilable with single-process models.amnesia ͉ colloid cyst ͉ fornix ͉ mammillary body ͉ recognition

Some patients with relatively selective hippocampal damage have shown proportionate recall and recognition deficits. Moreover, familiarity as well as recollection have been found to be impaired in some of these patients. In contrast, other patients with apparently similar damage presented with relatively preserved recognition despite having severely impaired recall, and some of these patients have been shown to have preserved familiarity. We report here the case of an amnesic patient who suffered bilateral hippocampal damage and temporoparietal atrophy after carbon monoxide poisoning. On tests matched for difficulty, his recall performance was more severely impaired than his recognition memory, for verbal as well as for visual materials. Moreover, he performed within the range of healthy matched subjects on nine recognition tests out of ten. In a task using the process dissociation procedure, the patient's familiarity was preserved although his recollection was impaired. These findings indicate that recall and recognition memory can be dissociated in amnesic patients with hippocampal lesions even when temporoparietal cortical atrophy is also present.

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