The effects of selective electrical stimulation of the rat cochlea on hippocampal field potentials

The hippocampus is a brain structure well known for its importance to spatial learning and memory. As such it is assumed to use multisensory integration in order to generate mathematical maps of the spatial environment. Auditory inputs to the hippocampus have been described to a limited extent. Although it has been demonstrated that natural auditory stimulation can evoke local field potentials (LFPs) and single neuron responses in the hippocampus, and that noise trauma and tinnitus can cause maladaptive hippocampal plasticity, to the best of our knowledge no one has selectively, electrically stimulated the cochlea and recorded electrophysiological responses in the hippocampus. Here, we used unilateral electrical cochlear stimulation in rats while recording LFPs in the hippocampus bilaterally, with a multi-electrode array. We observed triphasic LFP responses bilaterally from unilateral stimulation, but with larger amplitudes on the contralateral side, especially for Phase 1 and 3 of the LFP, for which the latencies were also longer. The latencies under urethane anaesthesia were consistent with auditory information from the cochlea travelling across approximately 6-7 synapses. These data demonstrate that the cochlea contributes a powerful auditory input to the hippocampus, which is also lateralized, and may have implications for spatial cognition in cochlear implant patients.