Hippocampal Activation is Associated with Binding of Stimulus Features During Memory Encoding

The hippocampus is thought to play a key role in episodic memory. In particular it is thought to be involved in 1) binding together multiple inputs during encoding and 2) recollection as opposed to familiarity based retrieval processes. Eldridge and colleagues [1] showed that the hippocampus is selectively activated when memory retrieval is based on remembering the original learning episode rather than an impression of familiarity. The current experiment tests the hypothesis that the hippocampus is involved in relational binding during memory encoding. Methods Fourteen adolescent subjects performed a modified single trial remember-know experiment. The experiment was comprised of an in-scanner encoding phase, an in-scanner recognition phase, and an out-scanner validation phase. During the in-scanner encoding phase the subjects were shown 60 word pairs (capitalized target word; lowercase paired word) along with their corresponding pictures (color target picture; grayscale paired picture) for 5 seconds, followed by a 10 second ISI. During the in-scanner recognition phase the subjects were shown the 60 target words and 20 foil words and were asked to indicate whether they (1) remembered the word and some of the “features” presented with the word (e.g., the paired word, the color or location of the target picture); (2) knew they saw the word but did not remember any of the “features” presented with the word; (3) thought the word was unstudied. During the out-scanner validation phase the subjects were shown the target words along with the choices of two possible paired words and were asked to indicate which of the two paired words was presented with the target word or whether the target word was unstudied. This presentation was followed by a grayscale picture of the target word and two colors. Subjects were asked to indicate which of the two colors the picture was presented in or whether the picture was unstudied. Results Subjects achieved 78 (±10) percent correct and 10 (±10) percent false positives during the in-scanner recognition phase. The number of encoding trials on which they recognized zero, one, or two stimulus features in the out-scanner validation phase was 4.7 (±2.7), 13.0 (±4.0), and 26.9 (±10), respectively. Increased magnitude of hippocampal activation at encoding was associated with an increased number of stimulus features recognized at retrieval (Figures 1 & 2). Conclusion The principle finding of the study is that the magnitude of hippocampal activation at encoding increases with the number of stimulus features recognized at retrieval. The finding is interpreted to be consistent with the involvement of the hippocampus in relational binding at encoding.