Turning the volume knob on your radio is a gain control mechanism for sound production.6 Gain modulation requires two sources, an amplifier and a modulator (Figure 11.1). When the sources are multiplied (or divided), such interaction generates an output whose magnitude is larger or smaller (in which case the gain is negative) than expected from adding (or subtracting) the sources. Such nonlinear gain modulation is ubiquitous in the brain, with far-reaching consequences, and is achieved by different mechanisms in individual neurons or synapses or at a larger scale in neuronal networks,
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