Arrows indicate the direction of information flow. A typical neuron receives inputs from the sensory periphery via glutamate, which depolarizes the membrane potential (“+”). The glutamate-gated ion channels and synapses that mediate this response are referred to as layer 1. They define the neuron's stimulus (the “excitatory center” of its receptive field). The function of layer 1 is to provide current information about the external world. Those individual inputs that are most successful in depolarizing the neuron, and which are most closely correlated with reward, are selected according to a Hebbian or error-maximizing rule (equation 4). The neuron's other ion channels constitute layer 2. The function of layer 2 is to use prior information to predict membrane voltage, and thereby predict the conductance of layer 1 and glutamate concentration as well. The membrane voltage is determined by the difference between the output of layer 1 and its expected output as determined by layer 2 (equation 1), and it therefore functions as a prediction error. In predicting voltage, layer 2 acts to drive voltage towards a point near the middle of its range where the error is zero. The ion channels of layer 2 are selected to perform this function by an anti-Hebbian or error-minimizing rule (equation 3). Many of these ion channels are inhibitory (“−”) and tend to open when the neuron is depolarized, whereas others are excitatory (“+”) and tend to open when the neuron is hyperpolarized. Some are gated by membrane voltage and provide prior temporal information, whereas others are gated by neurotransmitters and contribute prior spatial information.