Increasing stimulus similarity drives nonmonotonic representational change in hippocampus

Studies of hippocampal learning have obtained seemingly contradictory
results, with manipulations that increase coactivation of memories
sometimes leading to differentiation of these memories, but sometimes not.
These results could potentially be reconciled using the nonmonotonic
plasticity hypothesis, which posits that representational change (memories
moving apart or together) is a U-shaped function of the coactivation of
these memories during learning. Testing this hypothesis requires
manipulating coactivation over a wide enough range to reveal the full
U-shape. To accomplish this, we used a novel neural network image
synthesis procedure to create pairs of stimuli that varied parametrically
in their similarity in high-level visual regions that provide input to the
hippocampus. Sequences of these pairs were shown to human participants
during high-resolution fMRI. As predicted, learning changed the
representations of paired images in the dentate gyrus as a U-shaped
function of image similarity, with neural differentiation occurring only
for moderately similar images.