Neurons in the intact brain are bombarded by spontaneous synaptic input that causes increased membrane conductance (i.e. shunting), tonic depolarization, and noisy fluctuations in membrane potential. By comparison, neurons in acute brain slices experience little spontaneous synaptic input and are therefore less leaky, more hyperpolarized, and less noisy. Such differences can compromise the extrapolation of in vitro data to explain neuronal operation in vivo. Here, we replicated three effects of synaptic background activity in acute brain slices, using dynamic clamp to artificially increase membrane conductance, constant current injection to cause tonic depolarization, and time-varying current injection to introduce noise. These manipulations were applied separately and in different combinations in order to resolve their specific influence on neuronal activity. In addition to straightforward effects on passive membrane properties, shunting caused nonlinear effects on spiking. As a result, shunted neurons behaved more like coincidence detectors and less like integrators. Furthermore, shunting caused either divisive or subtractive modulation of firing rate depending on the presence or absence of background noise. These results demonstrate that even simplistic applications of dynamic clamp can reveal interesting phenomena and expand our ability to use in vitro experiments to help understand neuronal operation in vivo.