Involvement of the medial temporal lobe in priming for new associations

This study was addressed to the question of whether the medial temporal lobe (MTL) plays a critical role in implicit memory for new associations. Priming for new associations was examined in two different tasks in 18 patients with focal lesions all involving the MTL. In Experiment 1, following a study phase for pairs of unrelated words, subjects performed a perceptual identification task on old, recombined, and new pairs of words presented at brief exposure durations. In contrast to control subjects, and despite a normal level of item priming, the patients failed to show superior identification of the old pairs relative to the recombined pairs, the measure of associative priming. In Experiment 2, subjects engaged in speeded naming of the print color for previously studied words presented in the original color or in a different old color, and for unstudied words. Again, in contrast to control subjects and despite a normal level of item facilitation on color naming reaction time (RT), the patients failed to show priming for recently experienced new associations between words and colors. Explicit recognition memory by the patients was abnormal in both experiments. This study records an absence of priming for new associations, in two different tasks in which the nature of the stimuli was considerably different, in a large group of patients with lesions in the MTL. Although some previous research has reported significant associative priming in other tasks for patients with MTL lesions, the present results suggest that this region is critical for forming new associations of the types assessed here.