The impact of neuromodulation on cortical computation is still an area of active debate. Recently, fluctuations in synchronization under neuromodulation in cortical areas have been observed. This is important because synchronization in a network influences the read-out of intrinsic membrane properties, i.e. neural excitability. Synchronization shifts a neuron from transmission mode (strong correlation) to intrinsic mode (weak correlation). Here we show how neuromodulation through its impact on synapses may alter the topology of network connections, and how this leads to the observed neuromodulation-dependent fluctuations in synchronization. In our theory, neuromodulation is vitally important for computation, because it is selective for cortical areas, influences the degree of internal synchronization, and therefore defines the actual state of information-processing in the area.