Imbalance of incidental encoding across tasks: an explanation for non-memory-related hippocampal activations?

Structural neural correlates of impaired mobility and subsequent decline in executive functions: a 12-month prospective study

Impaired mobility, such as falls, may be an early biomarker of subsequent cognitive decline and is associated with subclinical alterations in both brain structure and function. In this 12-month prospective study, we examined whether there are volumetric differences in gray matter and subcortical regions, as well as cerebral white matter, between older fallers and non-fallers. In addition, we assessed whether these baseline volumetric differences are associated with changes in cognitive function over 12months. A total of 66 community-dwelling older adults were recruited and categorized by their falls status. Magnetic resonance imaging occurred at baseline and participants' physical and cognitive performances were assessed at baseline and 12-months. At baseline, fallers showed significantly lower volumes in gray matter, subcortical regions, and cerebral white matter compared with non-fallers. Notably, fallers had significantly lower left lateral orbitofrontal white matter volume. Moreover, lower left lateral orbitofrontal white matter volume at baseline was associated with greater decline in set-shifting performance over 12months. Our data suggest that falls may indicate subclinical alterations in regional brain volume that are associated with subsequent decline in executive functions.

Mean signal and response time influences on multivoxel signals of contextual retrieval in the medial temporal lobe

INTRODUCTION

The medial temporal lobe supports integrating the "what," "where," and "when" of an experience into a unified memory. However, it remains unclear how representations of these contextual features are neurally encoded and distributed across medial temporal lobe subregions.

METHODS

This study conducted functional magnetic resonance imaging of the medial temporal lobe, while participants retrieved pair, spatial, and temporal source memories. Multivoxel classifiers were trained to distinguish between retrieval conditions before and after correction for mean signal and response times, to more thoroughly characterize the multivoxel signal associated with memory context.

RESULTS

Activity in perirhinal and parahippocampal cortex dissociated between memory for associated items and memory for their spatiotemporal context, and hippocampal activity was linked to memory for spatial context. However, perirhinal and hippocampal classifiers were, respectively, driven by effects of mean signal amplitude and task difficulty, whereas the parahippocampal classifier survived correction for these effects.

CONCLUSION

These findings demonstrate dissociable coding mechanisms for episodic memory context across the medial temporal lobe, and further highlight a critical distinction between multivoxel representations driven by spatially distributed activity patterns, and those driven by the regional signal.

Mechanisms for widespread hippocampal involvement in cognition

The quintessential memory system in the human brain--the hippocampus and surrounding medial temporal lobe--is often treated as a module for the formation of conscious, or declarative, memories. However, growing evidence suggests that the hippocampus plays a broader role in memory and cognition and that theories organizing memory into strictly dedicated systems may need to be updated. We first consider the historical evidence for the specialized role of the hippocampus in declarative memory. Then, we describe the serendipitous encounter that motivated the special section in this issue, based on parallel research from our labs that suggested a more pervasive contribution of the hippocampus to cognition beyond declarative memory. Finally, we develop a theoretical framework that describes 2 general mechanisms for how the hippocampus interacts with other brain systems and cognitive processes: the memory modulation hypothesis, in which mnemonic representations in the hippocampus modulate the operation of other systems, and the adaptive function hypothesis, in which specialized computations in the hippocampus are recruited as a component of both mnemonic and nonmnemonic functions. This framework is consistent with an emerging view that the most fertile ground for discovery in cognitive psychology and neuroscience lies at the interface between parts of the mind and brain that have traditionally been studied in isolation.