A model was constructed of the relations between the incoming excitatory postsynaptic potentials and inhibitory postsynaptic potentials to the intracellular membrane potential fluctuations and to the firing rate of a single neuron. From this model, the strength of synapses was assessed in two ways: 1) the ability of several synchronous presynaptic spikes to initiate a postsynaptic spike, termed the synchronous gain of the synapse; and 2) the ability of several asynchronous presynaptic spikes to add a spike to the output spike train, called the asynchronous gain. It was found that for the conditions prevailing in the brain's cortex, the synchronous gain is almost always higher. It appears that the cortical neurons act as coincidence detectors, and that the appropriate code for the higher cortical functions is coincidence and not firing rate.