Figure 1.

The DA neuron population is comprised of neurons that are firing spontaneously, and those that are inhibited from firing due to the influence of the ventral pallidum. The ventral pallidum, in turn, is controlled by a hippocampal ventral subiculum-nucleus accumbens pathway. The state of activity of the subiculum is dependent on the context; with benign contexts the drive is low, but is augmented when the environment warrants a higher level of attention. Whenever the ventral subiculum is activated, it in turn activates the nucleus accumbens, inhibits the ventral pallidum, and removes the inhibitory influence from the DA neurons. Therefore with increased subicular drive, there is an increase in the proportion of DA neurons firing spontaneously. When a phasic, behaviorally relevant event takes place, the pedunculopontine tegmentum provides a rapid, glutamatergic input to the DA neuron population; this is the behaviorally salient signal. However, only DA neurons that are firing spontaneously can respond to the phasic input with burst firing. Therefore, by controlling the number of DA neurons that are firing spontaneously, and therefore capable of being phasically activated, the ventral subiculum controls the gain of the DA signal.