Second Order Conditioning in the Sub-cortical Nuclei of the Limbic System.

Three factor Isotropic sequence order (ISO3) learning is a form of differential Hebbian learning where a third factor switches on learning at relevant moments for example, after reward retreival. This switch enables learning only at specific moments and, thus, stablises the corresponding weights. The concept of using a third factor as a gating signal for learning at relevant moments has been extended in this paper to perform second order conditioning (SOC). We present a biological model of the sub-cortical nuclei of the limbic system that is capable of performing SOC in a food seeking task. The 3rd-factor is modelled by dopaminergic neurons of the VTA which are activated via a direct excitatory glutamatergic pathway, and an indirect dis-inhibitory GABAergic pathway. The latter generates an amplification in the number of tonically active DA neurons. This produces an increase in DA outside the event of a primary reward and enables SOC to be accomplished. [ABSTRACT FROM AUTHOR]