A familiarity signal in human anterior medial temporal cortex?

The medial temporal lobe (MTL) comprises the hippocampal complex and amygdala, along with distinct cortical regions, including the parahippocampal, entorhinal, and perirhinal cortices. It has been suggested that different components of the MTL support dissociable memory functions (see, e.g., Eichenbaum et al., 1994). Of particular relevance to the present report is evidence from lesion studies in nonhuman primates suggesting that the perirhinal region plays a key role in visual recognition memory (Meunier et al., 1993; Bachevalier et al., 2002). Consistent with this suggestion, electrophysiological studies have identified neurons in perirhinal and nearby cortical areas of the monkey in which object-selective responses decrease after previous exposure to the object (Brown and Xiang, 1998). These repetition-related decreases can be found over intervals of 24 h, as might be expected of a neural signal contributing to a form of long-term memory. Together, lesion and single neuron evidence has led to the proposal (Brown and Aggleton, 2001) that perirhinal cortex contributes to recognition memory through the assessment of relative familiarity, and that neuronal response decrements provide one basis for such assessments. In the present study, we report that experimentally familiar items elicit smaller hemodynamic responses in human anterior MTL, consistent with the findings from nonhuman primates. Evidence for a role of perirhinal cortex in recognition memory comes almost exclusively from work with experimental animals. There are few data to suggest that the region has an equivalent role in humans (but see Buffalo et al., 1998). We report findings from four memory studies recently conducted in our laboratory using event-related functional magnetic resonance imaging (fMRI). In each case, we found evidence of a reduction in the anterior MTL response to experimentally familiar (“Old”) items relative to experimentally novel (“New”) items—a “New-Old” effect. These studies were conducted independent of one another and are described in detail in separate publications. With the exception of one study (Rugg et al., 2003), however, the data reported in the present study have not been described elsewhere. The four experiments, A–D, shared many methodological features, including scanner hardware, data acquisition parameters, and data analysis methods. They differed along a number of psychological dimensions (see Detailed Methods). In three of the studies, participants explicitly discriminated Old from New items; in the remaining study, the repetition of items was task-irrelevant (experiment D). Two studies used words (experiments A and B), one used pictures (experiment C), and one used faces (experiment D). In two of the studies (experiments B and C), the task required retrieval of contextual information associated with the prior presentation of Old items. The pattern of responses to Old and New items was the same in all cases: Old items elicited a smaller response in anterior MTL than did New items (Fig. 1). The effect Grant sponsor: Wellcome Trust; Grant sponsor: MRC; Grant sponsor: DGAPA, National Autonomous University of Mexico; Grant number: IN303798. *Correspondence to: M.D. Rugg, Insititute of Cognitive Neuroscience, University College London, 17 Queen Square, London WC1N 3AR, UK. E-mail: m.rugg@ucl.ac. uk Accepted for publication 22 August 2002 DOI 10.1002/hipo.10117 HIPPOCAMPUS 13:259–262 (2003)