Memory for past events can be based on recollection or on assessments of familiarity. These two forms of human memory have been studied extensively by philosophers and psychologists, but their neuroanatomical substrates are largely unknown. Here we examined the brain regions that are involved in these two forms of memory by studying patients with damage to different temporal lobe regions. Our results come from (i) structural covariance modeling of recall and recognition, (ii) introspective reports during recognition and (iii) analysis of receiver operating characteristics. In sum, we found that the regions disrupted in mild hypoxia, such as the hippocampus, are centrally involved in conscious recollection, whereas the surrounding temporal lobe supports familiarity-based memory discrimination.

Previous studies using the process dissociation and the remember-know procedures led to conflicting conclusions regarding the effects of anterograde amnesia on recollection and familiarity. We argue that these apparent contradictions arose because different models were used to interpret the results and because differences in false-alarm rates between groups biased the estimates provided by those models. A reanalysis of those studies with a dual-process signal-detection model that incorporates response bias revealed that amnesia led to a pronounced reduction in recollection and smaller but consistent reduction in familiarity. To test the assumptions of the model and to further assess recognition deficits in amnesics, we examined receiver operating characteristics (ROCs) in amnesics and controls. The ROCs of the controls were curved and asymmetrical, whereas those of the amnesics were curved and symmetrical. The results supported the predictions of the model and indicated that amnesia was associated with deficits in both recollection and familiarity.

Recent positron emission tomography (PET) studies have identified neuronal components of widespread novelty-assessment networks in the brain. Wepropose that the efficacy of encoding on-line information into long-term memory depends on the novelty of the information as determined by these networks, and report a test of this "novelty/encoding" hypothesis. Subjects studied a list of words. Half of the words were "familiar" by virtue of their repeated presentation to the subjects before the study of the critical list; the other half were novel, in that they had not previously been encountered in the experiment. The results conformed to the prediction of the novelty/encoding hypothesis: accuracy of explicit (episodic) recognition was higher for novel than for familiar words.Previous research using the technique ofpositron emission tomography (PET) has indicated that the human brain contains widely distributed novelty-detection networks. Cortical and subcortical regions in the expanded limbic system as well as in the temporal lobes respond more actively to novel stimuli than to otherwise comparable familiar stimuli (Tulving, Markowitsch, Kapur, Habib, & Houle, 1994). These findings with humans are consistent with the results of single-unit recording studies that have revealed the existence of neurons in the homologous regions of monkey brains that respond more vigorously to novel than to familiar or recently seen stimuli (Fahy,Riches, & Brown, 1993; Li, Miller, & Desimone, 1993;Riches, Wilson, & Brown, 1991;Wilson & Rolls, 1993).The discovery ofspecific neuronal networks in the brain that are differentially involved in the processing of novel rather than familiar perceptual inputs makes excellent sense in light of massive behavioral evidence showing that all animals routinely respond differentially to novel versus familiar stimuli. These networks also make good evolutionary sense. Distinguishing between what is novel and what is already known, and distinguishing between degrees and kinds of novelty, is one of the elemental requirements for reacting adaptively to the happenings in one's proximate environment.

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