  Background: The roles of neuromodulation in a neural network, such as in a
cortical microcolumn, are still incompletely understood. Neuromodulation
influences neural processing by presynaptic and postsynaptic regulation of
synaptic efficacy. Neuromodulation also affects ion channels and intrinsic
excitability. Methods: Synaptic efficacy modulation is an effective way to
rapidly alter network density and topology. We alter network topology and
density to measure the effect on spike synchronization. We also operate with
differently parameterized neuron models which alter the neurons intrinsic
excitability, i.e., activation function. Results: We find that (a) fast
synaptic efficacy modulation influences the amount of correlated spiking in a
network. Also, (b) synchronization in a network influences the read-out of
intrinsic properties. Highly synchronous input drives neurons, such that
differences in intrinsic properties disappear, while asynchronous input lets
intrinsic properties determine output behavior. Thus, altering network topology
can alter the balance between intrinsically vs. synaptically driven network
activity. Conclusion: We conclude that neuromodulation may allow a network to
shift between a more synchronized transmission mode and a more asynchronous
intrinsic read-out mode. This has significant implications for our
understanding of the flexibility of cortical computations.
