D1 and D2 both refer to specific types of dopamine receptors.
I'm sure it has something to do with the fact that the D1 receptors are in regions different from D2 receptors.
I know that adenylate cyclase usually triggers a signal transduction cascade that leads to increased cAMP+calmodulin, resulting in increased gene expression of proteins that help promote long-term potentiation of postsynaptic neurons (presumably lowering the absolute value of the voltage threshold that's needed to trigger another action potential, which increases neuron excitability).
But an increase in dopamine is going to cause more neuron excitability in postsynaptic neurons in some regions (those that have D1 receptors), and less in others (those that have D2 receptors). What then explains the fact that increased expression at all dopamine receptors can help modulate attention span?
I know that D3 and D4 are also involved, but they have to be either excitatory, inhibitory, or neutral.