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# Node Perturbation

Node Pertubation is an algorithm for learning with a global error signal introduced by Williams 1992 that improves on Weight Pertubation by identifying that the gradient of the global error can be estimated by adding noise into the bias alone. Consider adding noise $\gamma_i^l$:

$\tilde{x}_i^l = f \Big(\sum_j W_{ij}^l \tilde{x}_j^{l-1} + (b_i^l + \gamma_i^l) \Big)$

We can use the same Weight Perturbation approach to estimate $\frac{\partial E}{\partial b_a^c}$, but how can we estimate $\frac{\partial E}{\partial W_{ab}^c}$?

Define the unperturbed, pre-activation value:

$\nu_i^l = \sum_j W_{ij}^l x_j^{l-1} + (b_i^l + \gamma_i^l)$

Per the chain rule,

$\frac{\partial E}{\partial W_{ab}^c} = \frac{\partial E}{\partial \nu_a^c} \frac{\partial \nu_a^c}{\partial W_{ab}^c} = \frac{\partial E}{\partial \nu_a^c} x_b^c$

We can estimate $\frac{\partial E}{\partial \nu_a^c}$ using a similar trick as WP. To first order, the difference between the perturbed and unperturbed error is

$\tilde{E}_{tr} - E_{tr} \approx \sum_{l, i} \frac{\partial E}{\partial \nu_i^l} \gamma_i^l$

Multiply both sides by $\gamma_a^c$ and take the expectation with respect to $\gamma_a^c$:

$\langle (\tilde{E}_{tr} - E_{tr}) \gamma_a^c \rangle_{\gamma_a^c} = \frac{\partial E}{\partial \nu_a^c} \sigma^2$

Thus,

\begin{align*} \frac{\partial E}{\partial W_{ab}^c} &= \frac{\partial E}{\partial \nu_a^c} x_b^c\\ &= \frac{1}{\sigma^2}\langle (\tilde{E}_{tr} - E_{tr}) \gamma_a^c \rangle_{\gamma_a^c} x_b^c \end{align*}

Since $tilde{x} \approx x$ to first order in $\gamma$, NP approximates gradient descent using the following update equations:

\begin{align*} \Delta W_{ij}^l &= - \eta \tilde{E}_{tr} \gamma_i^l x_j^l \\ \Delta b_i^l &= - \eta \tilde{E}_{tr} \gamma_i^l \end{align*}