Dissociating the Neural Correlates of Intra-Item and Inter-Item Working-Memory Binding

Contains fulltext : 90165.pdf (publisher's version ) (Open Access)Background - Integration of information streams into a unitary representation is an important task of our cognitive system. Within working memory, the medial temporal lobe (MTL) has been conceptually linked to the maintenance of bound representations. In a previous fMRI study, we have shown that the MTL is indeed more active during working-memory maintenance of spatial associations as compared to non-spatial associations or single items. There are two explanations for this result, the mere presence of the spatial component activates the MTL, or the MTL is recruited to bind associations between neurally non-overlapping representations. Methodology/Principal Findings - The current fMRI study investigates this issue further by directly comparing intrinsic intra-item binding (object/colour), extrinsic intra-item binding (object/location), and inter-item binding (object/object). The three binding conditions resulted in differential activation of brain regions. Specifically, we show that the MTL is important for establishing extrinsic intra-item associations and inter-item associations, in line with the notion that binding of information processed in different brain regions depends on the MTL. Conclusions/Significance - Our findings indicate that different forms of working-memory binding rely on specific neural structures. In addition, these results extend previous reports indicating that the MTL is implicated in working-memory maintenance, challenging the classic distinction between short-term and long-term memory systems.8 p

Taxonomy of types of relational memory, distinguishing intra-item binding (objects and their features that can be either intrinsic or extrinsic) and inter-item binding (two unrelated items that have highly similar characteristics–within-domain–or have less overlap with respect to stimulus characteristics–between-domain).

<p>Taxonomy of types of relational memory, distinguishing intra-item binding (objects and their features that can be either intrinsic or extrinsic) and inter-item binding (two unrelated items that have highly similar characteristics–within-domain–or have less overlap with respect to stimulus characteristics–between-domain).</p

General activation related to the delay period collapsed over all trials against a low level visual fixation baseline.

<p>All reported activations are whole brain corrected for multiple comparisons (FDR<0.05).</p

Differential activity related to the contrast inter-item binding (object/object vs. object/colour + object/location).

<p>All activations are corrected for multiple comparisons (FDR<0.05).</p

Differential activity related to the contrast extrinsic intra-item binding (object/location vs. object/colour + object/object).

<p>*Corrected for multiple comparisons at the cluster level.</p><p>**p<.001 uncorrected.</p><p>All others FDR<0.05 corrected for multiple comparisons.</p

Dissecting medial temporal lobe contributions to item and associative memory formation

A fundamental and intensively discussed question is whether medial temporal lobe (MTL) processes that lead to non-associative item memories differ in their anatomical substrate from processes underlying associative memory formation. Using event-related functional magnetic resonance imaging, we implemented a novel design to dissociate brain activity related to item and associative memory formation not only by subsequent memory performance and anatomy but also in time, because the two constituents of each pair to be memorized were presented sequentially with an intra-pair delay of several seconds. Furthermore, the design enabled us to reduce potential differences in memory strength between item and associative memory by increasing task difficulty in the item recognition memory test. Confidence ratings for correct item recognition for both constituents did not differ between trials in which only item memory was correct and trials in which item and associative memory were correct. Specific subsequent memory analyses for item and associative memory formation revealed brain activity that appears selectively related to item memory formation in the posterior inferior temporal, posterior parahippocampal, and perirhinal cortices. In contrast, hippocampal and inferior prefrontal activity predicted successful retrieval of newly formed inter-item associations. Our findings therefore suggest that different MTL subregions indeed play distinct roles in the formation of item memory and inter-item associative memory as expected by several dual process models of the MTL memory system