Membrane potential fluctuations determine the precision of spike timing and synchronous activity: A model study

It is much debated on what time scale information is encoded by neuronal sp ike activity. With a phenomenological model that transforms time-dependent membrane potential fluctuations into spike trains, we investigate constrain ts for the timing of spikes and for synchronous activity of neurons with co mmon input. The model of spike generation has a variable threshold that dep ends on the time elapsed since the previous action potential and on the pre ceding membrane potential changes. To ensure that the model operates in a b iologically meaningful range, the model was adjusted to fit the responses o f a fly visual interneuron to motion stimuli. The dependence of spike timin g on the membrane potential dynamics was analyzed. Fast membrane potential fluctuations are needed to trigger spikes with a high temporal precision. S low fluctuations lead to spike activity with a rate about proportional to t he membrane potential. Thus, for a given level of stochastic input, the fre quency range of membrane potential fluctuations induced by a stimulus deter mines whether a neuron can use a rate code or a temporal code. The relation ship between the steepness of membrane potential fluctuations and the timin g of spikes has also implications for synchronous activity in neurons with common input. Fast membrane potential changes must be shared by the neurons to produce synchronous activity.