Three models for the spectral shaping of auditory feature selectivity by cortical inhibition. (A) Balanced excitation and inhibition model. (Left) Cotuned frequency tuning curves for excitation (red) and inhibition (blue). CF, characteristic frequency. (Middle) Tuning curves for membrane potential responses resulting from excitation alone (dashed gray curve) and from integrating excitation and inhibition (solid black curve). Note that the tuning curve is scaled down without changes in shape. Red dashed line indicates the level of spike threshold. Green dash line indicates the level of resting membrane potential. Red arrows mark the frequency range for spike response. (Right) Proposed underlying circuit. The recorded cortical excitatory neuron (triangle cell) receives thalamic inputs (excitatory) and inhibition from local inhibition neurons (round cell), which are innervated by the same set of thalamic inputs. Thus, inhibition is disynaptically relayed. “Far” means thalamic input with represented frequency far away from the CF of the recorded neuron. (B) Lateral inhibition model. Note that hyperpolarizing responses (Vm below the resting membrane potential) result in apparent suppressive sidebands. In this case, the inhibitory neurons receive thalamic input with represented frequency far away from the CF of the recorded neuron. (C) Approximately balanced excitation and inhibition model. Note that the inhibitory tuning curve has a more flattened peak than the excitatory tuning curve. The cell is a high-CF cell, so that the excitatory tuning curve is skewed toward the high-frequency side. The relative inhibition is stronger on the left side than the right side of the excitatory tuning curve. The arrow indicates the preferred direction, that is, from high frequency to low frequency (downward FM sweeps). Upward sweeps would activate an earlier strong inhibition, which would suppress later activated strong excitation. Compared with the model in A, the membrane potential tuning is further sharpened. In the circuit, the cortical excitatory neurons connecting to the recorded cell have narrower frequency tuning of spike response compared with the inhibitory neurons connecting to the same cell. As a result, inhibitory inputs are broader than summed excitatory inputs.