Remote effects of hippocampal damage on default network connectivity in the human brain.

In the healthy human brain the hippocampus is known to work in concert with a variety of cortical brain regions. It has recently been linked to the default network of the brain, with the precuneus being its core hub. Here we studied the remote effects of damage to the hippocampus on functional connectivity patterns of the precuneus. From 14 epilepsy patients with selective, unilateral hippocampal sclerosis and 8 healthy control subjects, we acquired functional MRI data during performance of an object-location memory task. We assessed functional connectivity of a functionally defined region in the precuneus, which showed the typical properties of the default network: significant task-related deactivation, which was reduced in patients compared to control subjects. In control subjects, a largely symmetrical pattern of functional coherence to the precuneus emerged, including canonical default network areas such as ventral medial prefrontal cortex, inferior parietal cortex, and the hippocampi. Assessment of group differences within the default network areas revealed reduced connectivity to the precuneus in ipsilesional middle temporal gyrus and hippocampus in left hippocampal sclerosis patients compared to controls. Furthermore, left hippocampal sclerosis patients showed lower connectivity than right hippocampal sclerosis patients in left middle temporal gyrus, ventral medial prefrontal cortex, and left amygdala. We report remote effects of unilateral hippocampal damage on functional connectivity between distant brain regions associated with the default network of the human brain. These preliminary results underline the impact of circumscribed pathology on functionally connected brain regions.