An optogenetic system and method for preventing or halting seizures. The system and method use a sensor for monitoring the activity of a neural network containing a group of target neurons, and generating an input signal indicative of said activity, the target neurons being excitatory neurons. Excitatory stimulation is delivered in the form of an optical signal by an optical stimulator to the target neurons, the optical signal being determined based on the input signal, to reduce the overall activity of the target neurons.