Figure 2.4.

If we assume the system starts at steady state with a stimulation rate of a₁, which then shifts to a₂ at time t₀, then the area under the activation spike in the time interval [t₀, t₀ + τ] is $s\left(\frac{1}{a_2}-\frac{1}{a_1}\right)(1-e^{-a_2\tau })$.