Contributions of human hippocampal subfields to spatial and temporal pattern separation

Abstract

We sought to explore the roles of the hippocampal subregions and adjacent medial temporal lobe regions in pattern separation and any differential contributions based on sequential or spatial information. Young adults performed an incidental-encoding task on a sequence of four objects presented on the screen in one of eight locations while we collected high-resolution functional MRI brain scans. We employed five trials of interest: first presentations, exact repetitions, lures in which the same objects were repeated in different locations (spatial lures), lures in which the same objects were presented in a different sequential order (sequential lures), and lures in which both the spatial location and sequence were changed (both lures). We found no evidence for spatial or sequential specialization in the hippocampal subfields, consistent with the hypothesis that the dentate gyrus acts as a universal pattern separator. Likewise, we did not observe specialization for the perirhinal or parahippocampal cortices for spatial or sequential information, though both regions show evidence for associative processing in this task.

Keywords: CA1; CA3; hippocampus; medial temporal lobe; sequence learning.

Figure 1

Alternative predictions for the DG and CA1 hippocampal subfields for the sequential, spatial, and both lures.

Figure 2

Schematic of the experimental paradigm. The numbers 1–4 indicate the ordinal position of the stimulus presentation. Only one stimulus was presented on the screen at a time, and actual stimuli were presented in color.

Figure 3

Behavioral test results indicate comparable memory performance for the spatial and temporal lures.

Figure 4

Parameter estimates in arbitrary units for R/L DG and CA1 extracted for each of the four conditions. A similar pattern was observed for each ROI: greater activity for repeats than lures, with no differences between sequential, spatial, or both lure conditions.

Figure 5

Functional coupling between left DG/CA3, right DG/CA3, left CA1, right CA1 and other MTL regions as a function of spatial-sequence contrast. No evidence for spatial or sequence specialization for any of the subfields.

Figure 6

Parameter estimates extracted for the right and left PRC and PHC for each of the four conditions. Though each condition is different from a first presentation, there is no evidence for sequential or spatial specialization for either ROI.