Simple and associative recognition memory in the hippocampal region

Abstract

Although it is well established that the hippocampal region is involved in the formation of declarative memory, the exact nature of its involvement is unclear. One view is that the hippocampal region is involved only in tasks that require the formation or use of associations. According to this view, the hippocampal region is not involved in traditional tests of recognition memory. An alternative view is that the hippocampal region combines and extends the processing carried out by structures in the parahippocampal gyrus and that it is involved in all forms of declarative memory, including recognition memory. Using event-related functional magnetic resonance imaging (fMRI), we observed hippocampal activity during both traditional and associative recognition memory tasks. Critically, the hippocampal region was no more active in the associative recognition task than in the traditional recognition task.

Figure 1

Areas of significant fMRI signal change related to recollective success (Hits vs. Correct Rejections) are shown in sagittal sections through the hippocampus. The green lines in (a) indicate the location of available data which are shown as colored overlays on the averaged structural images (transformed to the atlas of Talairach and Tournoux 1988). (a and b) fMRI data from the right hippocampal region (26 right) during the first (a) and second (b) administration of the recognition memory test when the participants saw objects at both study and test (Object-Object task). (c) fMRI data from the left hippocampal region (21 left) during the first administration of the recognition memory test when the same participants saw words at both study and test (Word-Word task). (d) fMRI data from the right hippocampal region (26 right) during the first administration of the recognition memory test when the same participants saw objects at study and words (names of objects) at test (Object-Word task). Activation of the hippocampal region was observed in all three recognition memory tasks.

Figure 2

Areas of significant fMRI signal change related to within-subject contrasting of activity associated with trials in which there were Correct Rejections (correct “no” responses to unstudied items) between the first (Test 1) and the second (Test 2) administration of the recognition memory task in the Object-Object condition. The images are shown as a colored overlay on a sagittal section through the right hippocampal region (26 right). Greater activity in the right hippocampal region associated with Correct Rejections was observed in Test 1 relative to Test 2. In Test 1, the Correct Rejections were unfamiliar and likely resulted in hippocampal activity associated with memory encoding. In Test 2, these stimuli were now more familiar and less subject to an “encoding effect.”

Figure 3

Areas showing significantly different activity associated with (a) Hits (correct “yes” responses) and (b) Correct Rejections (correct “no” responses) when contrasting the Object-Object and Object-Word conditions. The images are shown as colored overlays on sagittal sections through the right hippocampal region (26 right). Data from the first administration of the recognition memory test (Test 1) are shown. (a) Successfully recognizing an object in a traditional recognition memory task (Object-Object) and successfully recognizing an object in an associative recognition memory task (Object-Word) yielded no difference in hippocampal activation. (b) Greater activity associated with Correct Rejections was observed in the Object-Object task than in the Object-Word task. During the recognition memory test itself, the automatic encoding of unfamiliar stimuli (the foils in the Object-Object task) elicited increased activity that reduced activity as measured by a Hit versus Correct Rejection contrast.