In hearing rats, acoustic stimulation of the cochlea paired with electric stimulation
of the vagus nerve (VNS) has been successfully used to restore normal cortical frequency
tuning following noise-induced pathological expansion of receptive fields (Engineer
et al., Nature 470, 2011). Here we explored whether pairing electric stimulation of
the cochlea with VNS (CI/VNS) is effective in reversing deafness-induced degradations
in CI-channel selectivity. Gerbils were bilaterally deafened and implanted with a
CI and an electrode around the ipsilateral (left) vagus nerve. Analogous to prior
pairings of pure tones with VNS (Engineer et al., 2011), single channel CI stimulation
was paired with brief pulses of VNS for several weeks. Acutely deafened gerbils and
deafened gerbils with CI-only stimulation served as controls. Microelectrode mapping
techniques were used to construct spectral cortical maps for multichannel CI stimulation.
In deaf gerbils, paired CI/VNS had no effect on spectral selectivity in the auditory
cortex. To exclude species-specific (rat vs. gerbil) and stimulus-specific (acoustic
vs. electric) differences between the two studies as potential causes underlying the
lack of VNS-induced plasticity, we paired VNS with tones in hearing gerbils. No changes
were observed in the cortical representation of the VNS-paired acoustic signal.
In summary, pairing electric or acoustic stimulation of the auditory nerve with VNS
failed to induce cortical map plasticity. These findings contrast with prior results
obtained in hearing rats. Our results suggest that the potential for paired VNS to
direct AC receptive field plasticity may be species specific. Additional factors that
might have prevented replication of the results by Engineer and colleagues (2011)
will be discussed.
