Cognitive studies of episodic memory have shown that performance in long-term memory tests is determined by two major principles, one relating to the conditions of acquisition or encoding (1, 2) and the second dealing with the relation between encoding and retrieval operations (3, 4). The first principle reflects the observation that more meaningful analyses of stimuli are associated with higher levels of subsequent retention. Craik and Lockhart (1) suggested that incoming stimuli can be analyzed to different levels, ranging from shallow sensory analyses (form, pitch, color, etc.) to deeper semantic analyses involving meaning and implications. The level at which a stimulus is analyzed depends on factors such as the meaningfulness of the stimulus to the subject, the amount of attention devoted to its analysis, and the subjectÊs purpose and intentions with respect to the stimulus. It was emphasized by Craik and Lockhart that intention to memorize is not an important determinant of subsequent retrieval; rather, it is the type of encoding operation, carried out for whatever purpose, that substantially controls episodic memory performance. In a series of experiments designed to explore this notion, Craik and Tulving (2) demonstrated that correct recognition of verbal stimuli ranged from 15% to over 80%, depending only on the type of encoding that the subject was induced to perform. This dependence of memory performance on different types of encoding operations is termed the „levels of processing effect.‰ It is an extremely robust effect, can be obtained reliably in a single subject, and has been shown by several other investigators (5, 6). Despite the obvious relevance of this finding to an understanding of memory processes, essentially nothing is known about its neural basis.