Hippocampal differentiation without recognition: an fMRI analysis of the contextual cueing task

Abstract

A central role of the hippocampus is to consolidate conscious forms of learning and memory, while performance on implicit tasks appears to depend upon other structures. Recently, considerable debate has emerged about whether hippocampal-dependent tasks necessarily entail task awareness. In the contextual cueing task, repetition facilitation is implicit, but impaired in patients with amnesia. Whether the hippocampus alone or other MTL structures are required is unclear. Event-related functional magnetic resonance imaging revealed hippocampal activity that differentiates novel from repeated arrays. This pattern of results was observed without recognition of the repeating arrays. This finding provides support for the claim that the hippocampus is involved in processes outside the domain of conscious learning and memory.

Figure 1

Typical stimulus array. The task was to locate the rotated “T” from among the rotated L distractors. The position and color of the distractors serves as a context that determines the location of the target.

Figure 2

Reaction time data for repeated versus novel arrays. Means ± SEM computed for each of 20 blocks.

Figure 3

The hippocampus differentiates novel from repeated arrays. (A) Main effect of array type: activity is decreased for repeated items. At left is the Impulse Response Function (IRF; mean percent signal change ± SEM) for the largest of the activations (circled in yellow) in the left posterior hippocampus. The IRF shows a decrease in functional activity for repeated items relative to novel items and baseline. Maps of hemodynamic activity are shown on the right. Coordinates (Talairach and Tournoux 1988) and volumes are provided adjacent to each activation. The top left image provides the index (Y-coordinate) for the positions of three coronal slices for four distinct clusters. (B) The interaction of array type by reaction time: only repeated items show activation that is inversely correlated with reaction time. Faster reactions correspond to greater activity. At the left is an interaction plot (the area under the curve for the IRF) for the larger of the two activations. For graphing purposes, the continuous RT variable was split along its median. The functional activity shows that repeated arrays at faster RTs show increased hemodynamic activity compared with novel arrays and repeated arrays at slower RTs, which do not differ significantly from zero. Maps of hemodynamic activity are shown on the right.