Figure 3.

Feedback inhibition determines frequency selectivity. Gamma frequency is more selective for pulse frequency when feedback inhibition is stronger. This is because a pulse can only evoke an excitatory spike and end the gamma cycle if it arrives under sufficiently low inhibition. In this figure, a pulse arrives after an inhibitory spike with four different delays. V is the membrane potential of a quadratic integrate and fire E‐cell, and s is the saturation of the E–I synapse. The lower branch of the parabola is the stable resting voltage, and the upper branch is the threshold voltage. (A) When E–I connections are strong, only the latest pulse arrives under low enough inhibition to evoke an excitatory spike and shorten the period of this gamma cycle. (B) In a system with the same natural frequency but weaker E–I connections, the three later pulses can all evoke excitatory spikes.