Topography and dynamics of associative long-term memory retrieval in humans.

The present study investigated the neurophysiological processes underlying associative long-term memory retrieval of objects and spatial positions by means of a modified fan paradigm with cued recall and two neuroimaging methods (electroencephalogram [EEG] and functional magnetic resonance imaging). In an acquisition phase, either one stimulus or two stimuli became associated with a noun. During retrieval, probe stimuli comprising noun pairs were presented, and participants had to recall the respective associations and decided whether the nouns are linked to each other via a commonly associated stimulus. With this design, the quality and quantity of recalled associations was systematically varied, whereas the triggering stimuli and response requirements were held constant in all experimental conditions. Recall time proved to be directly related to the number of associations fanning out from a retrieval cue. Correspondingly, the hemodynamic response (blood oxygen level-dependent [BOLD] signal) and the amplitude of slow negative EEG potentials increased monotonically with the number of associations in both left anterior and bilateral posterior cortical areas. These effects were consistently observed with content-specific topographies for the two distinct materials. Furthermore, the multimethod approach revealed a close temporal link between response times and event-related slow potential changes on the one side and a close topographical and amplitude correspondence between slow potentials and BOLD signal changes on the other. The integrated results suggest that the neuronal dynamics of associative memory retrieval are equivalent for different types of associations, but that the structural basis is clearly content-specific.