In the Model, R = B + A₁f₁ + A₂f₂

In the model, r = b + a₁f₁ + a₂f₂, [where r = firing rate, b = background firing rate, a = coefficient of a neuron's discharge rate, f₁ = baseline frequency and f₂ = comparison stimulus frequency], in which brain area does a vary the most?

A) S1 cortex
B) S2 cortex
C) Medial prefrontal cortex
D) Medial premotor cortex
E) Primary motor cortex