Synaptic Integration in the Cortex Shaped by Network Activity Detlef Heck, Univers ity of Tennessee Health Science Center, Department of Anatomy and Neurobiology, Memphis, TN Neocortical neurons in vivo receive massive synaptic input from the permanently active surrounding network. How this ongoing network activ ity affects the neurons' integrative properties and what function this may imply at the network level remains largely unknown. Most of our knowledge regarding synaptic communication and integration is based on recordings in vitro, where network activ ity is strongly diminished or even absent. Here, we present results from two complementary series of experiments based on intracellular in vivo recordings in anaesthetized rat prefrontal cortex. Specifically , we measured (i) the relationship between the excursions of a neurons membrane potential and the spiking activ ity in the surrounding network; (ii) how summation of several inputs to a single neuron is affected during the different levels of its membrane potential excursions and phases of network activ ity . The combination of these measurements allows to assess how different levels of network activ ity influence synaptic integration. We present direct ev idence that integration of synaptic inputs is linear, independent of the level of network activ ity . However, during periods of high network activ ity , the neurons' response to synaptic input is markedly reduced in both amplitude and duration. This results in a drastic shortening of its window for temporal integration, requiring more precise coordination of presynaptic spike discharges to drive the neuron to spike under conditions of high network activ ity . We conclude that ongoing activ ity , as present in the active brain, emphasizes the need for neuronal cooperation at the network level, and cannot be ignored in the exploration of cortical function.