The unit value or state, which is a part of the unit structure holds the current activation. The activation is a function of the unit inputs, defined by the linked list of input links and the specific activation function. Applied to this activation is the transfer function that determines the unit output. A field called current error is used for some network learning strategies, like back-propagation. The current activation is found as part of the unit structure. However, a pointer to a history is found here too. This allows the possibility to record a history of activations.