# Almeida-Pineda recurrent backpropagation

**Almeida-Pineda recurrent backpropagation** is an error-driven learning technique developed in 1987 by Luis B. Almeida^{[1]} and Fernando J. Pineda.^{[2]}^{[3]} It is a *supervised* learning technique, meaning that the desired outputs are known beforehand, and the task of the network is to learn to generate the desired outputs from the inputs.

As opposed to a feedforward network, a recurrent network is allowed to have connections from any neuron to any neuron in any direction.

## Contents

## Model

Given a set of k-dimensional inputs represented as a column vector:

and a nonlinear neuron with (initially random, uniformly distributed between -1 and 1) synaptic weights from the inputs:

then the output <math>y</math> of the neuron is defined as follows:

y &= \varphi \left ( n \right )

\end{align}</math>where <math>\varphi \left ( \cdot \right )</math> is a sigmoidal function such as that used in ordinary feedforward backpropagation (we will use the logistic function from that page), and <math>n</math> is the net input of the neuron, calculated as follows. Assuming <math>N</math> neurons where <math>k</math> of the neurons are simple inputs to the network, with the weight of the connection from neuron <math>i</math> to neuron <math>j</math> being <math>w_{ij}</math>, the net <math>n_j</math> of neuron <math>j</math> (where <math>j</math> is not an input neuron) is computed using a discrete time approximation to the following equation, iteratively applied to all neurons until the nets settle to some equilibrium state. Initially set <math>n_j</math> to 0 for all non-input neurons.

y_i & \text{ if } i \text{ is not an input } \\ x_i & \text{ if } i \text{ is an input }

\end{cases}</math>Note that if the weights between pairs of neurons are symmetric, that is, <math>w_{ij} = w_{ji}</math>, then the network is guaranteed to settle to an equilibrium state.^{[4]} If symmetry is not held, the network will often settle.^{[5]} Of course, if <math>i</math> is an input, then <math>w_{ji}</math> does not exist.

Once the nets of the neurons are determined, an error phase is run to determine error terms for all neurons *solely for the purpose of weight modification*. As above, these weight modification error terms are computed using a discrete time approximation to the following equation, iteratively applied to all neurons until the error terms settle to some equilibrium state. Initially set <math>e_j = 0</math> for all neurons.

\frac{\mathrm{d} e_j}{\mathrm{d} t} &= -e_j + \frac{\mathrm{d} \varphi \left ( n_j \right ) }{\mathrm{d} n_j} \sum_{i=1}^N w_{ij} e_i + J_j\\ &= -e_j + \varphi \left ( n_j \right ) \left ( 1 - \varphi \left ( n_j \right ) \right ) \sum_{i=1}^N w_{ij} e_i + J_j\\ &= -e_j + y_j \left ( 1 - y_j \right ) \sum_{i=1}^N w_{ij} e_i + J_j

\end{align}</math>where <math>J_j</math> is an error term for neurons which are outputs and have targets <math>t_j</math>:

The weights are then updated according to the following equation:

where <math>\eta</math> is some small learning rate.

## Derivation

The error terms <math>e_j</math> are considered estimates of <math>-\mathrm{d} E / \mathrm{d} n_j</math> during the derivation of the equations for feedforward backpropagation:

\frac{\partial E }{\partial w_{ij}} &= \frac{\mathrm{d} E }{\mathrm{d} n_j} \frac{\partial n_j}{\partial w_{ij}} \\ &= - e_j y_i \\ \Delta w_{ij} &= - \eta \frac{\partial E}{\partial w_{ij}} \\ &= \eta e_j y_i

\end{align}</math>## Objections

While mathematically sound, the Almeida-Pineda model is biologically implausible, like feedforward backpropagation, because the model requires that neurons communicate error terms backwards through connections for weight updates.

## References

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