Figure 1. Biconditional discrimination task logic and network architecture.

A. In an example of this task, two of four possible stimuli (A, B, C and D) are presented simultaneously to a subject [9]. If either both A and B are present or neither is present, the subject should make one response (such as release a lever). If either A or B but not both are present, the subject should make an alternative response (such as hold the lever until the end of the trial). To perform this task successfully, neurons must generate responses to specific stimulus-pairs (e.g. A+B). A response to a single stimulus (e.g. A) is not sufficient to drive the correct response in one pairing without activating the incorrect response for the opposite pairing of that stimulus. B. The network consists of Poisson input groups that randomly project to a random recurrent network of excitatory (red) and inhibitory (cells). Excitatory-to-excitatory connections (arrows) and inhibitory-to-excitatory connections (balls) are probabilistic and plastic. All-to-all inhibitory-to-inhibitory synapses are also present but not plastic. In the relevant simulations, STDP occurs at excitatory-to-excitatory and input-to-excitatory synapses, while LTPi occurs at inhibitory-to-excitatory synapses. Inhibition is feed forward only (i.e. the network does not include recurrent excitatory-to-inhibitory synapses). C. Excitatory cells from the Associative layer project all-to-all, initially with equal synaptic strength to excitatory cells in both the hold and release pools of the decision-making network. The decision-making network consists of two excitatory pools with strong recurrent connections, which compete via cross-inhibition [29]. Strong self-recurrent excitation ensures bistability for each pool, while the cross-inhibition generates winner-take-all (WTA) dynamics such that only one population can be active following the stimulus, resulting in one decision. Whether the motor output (based on the decision of hold versus release) is correct for the corresponding cue, determines the presence of Dopamine (DA) at the input synapses, according to the rules of the task in part A.