Figure 3.

The basic architecture of the BBG model. Solid standard thickness lines are excitatory projections; and those with rounded terminals are modifiable by learning. Dotted lines indicate an inhibitory pathway. Very thick lines are ascending neuromodulatory (dopamine) projections. The projection from N Acc to PPTN is broken to show that it is indirect (via ventral pallidum). It is a double inhibitory pathway, but is represented as (nett) excitatory for simplicity. The numbers indicate some of the locations at which individual differences were introduced to the model (see text for details). In the model there is a conditionable pathway (via site 4 then ultimately via site 1) by which cortical stimulus representations can activate the midbrain DA cells. There is also a conditionable inhibitory pathway (via site 3 then site 2) by which the cortical stimulus representations can activate DA cells. BBG, Brown, Bullock, and Grossberg; DA, dopamine; LH, lateral hypothalamus; N Acc, nucleus accumbens; PPTN, pendunculopontine tegmental nucleus; VTA, ventral tegmental area; SNpc, substantia nigra pars compacta.