Level of sustained entorhinal activity at study correlates with subsequent cued-recall performance: a functional magnetic resonance imaging study with high acquisition rate.

Abstract

Functional magnetic resonance imaging (fMRI) with high acquisition rate was performed during the intentional memorizing of words to specify which medial temporal lobe structure is important in determining what words are subsequently remembered in a cued-recall test and to characterize the time course of activation in that structure. Functional images of six healthy young subjects were analyzed by two subject- and voxel-wise statistics: First, to identify brain areas transiently engaged in encoding of words, brain activity during memorizing visually presented words and watching a fixation cross was compared by a Kolmogorov-Smirnov statistic (KS-test). Second, to identify brain areas whose activity correlates with memory encoding success, a Kendall's correlation was calculated between signal intensity at study and performance in a subsequent cued-recall test. Averaged signal intensities were plotted as a function of time to depict the time course of brain activity detected by both statistical tests. The level of slowly modulated, sustained activity in Brodmann area 28 (entorhinal cortex) did not respond transiently as study words appeared, but did correlate positively with subsequent test performance. More left than right activity in Brodmann area 45 (dorso-lateral prefrontal cortex) and bilateral activity in Brodmann area 44 (premotor cortex) exhibited transient hemodynamic responses that did not show any relation to subsequent memory performance. Thus, the study identified a novel pattern of slowly modulated brain activity in human entorhinal cortex that may represent a declarative memory encoding state whose level predicts whether experiences will be remembered or forgotten.