Differential contributions of synaptic and intrinsic inhibitory currents to parsing via flexible phase-locking in neural oscillators

Current hypotheses suggest speech parsing is executed by a hierarchy of oscillators in auditory cortex, with auditory cortical theta (3-7 Hz) rhythms playing a key role by phase-locking to syllable boundaries. These oscillators must be "flexible" to reliably synchronize to quasi-rhythmic inputs, whose variable frequency can dip below cortical theta frequency (down to 1 Hz). Using biophysical computational models, we found the flexibility of phase-locking to depend on the types of hyperpolarizing currents that pace neural oscillators. Simulated cortical theta oscillators flexibly phase-locked to slow inputs when these inputs caused both (i) spiking and (ii) the subsequent buildup of outward current sufficient to delay further spiking until the next input. The greatest flexibility in phase-locking arose from a synergistic interaction between intrinsic currents that was not replicated by synaptic currents at similar timescales. Our results suggest synaptic and intrinsic inhibition contribute to regular and flexible phase-locking in neural oscillators, respectively.