Time Is Precious

摘要

When an activated neuron fires a series of action potentials (spikes), where is the information encoded in the spike train? Conventional wisdom dictates that the exact timing of individual spikes is random and that the sole criterion that could be used to encode information is the mean firing rate during a particular time win- dow (typically set at 20 to 200 ms). With their report on page 559 of this is- sue, lkegaya et al. (1) cast serious doubt on the conventional view. At first glance, the complexity of the cerebral cortex seems to support the conventional view. In a cubic millimeter of cortex, there are on average 20,000 to 100,000 neurons each firing at a rate of a few spikes per second. These spikes travel unattenuated through 4 km of axons and affect other neurons through 800,000,000 synapses (2, 3). A spike arriving at a synapse causes the release of a small amount of neurotransmitter, which evokes a small voltage change in the membrane of the contacted neuron. Most neurons show periods of both elevated and reduced activity (see the figure), prompting the assumption that each synapse is fairly weak. Presumably, the combined activities of all synaptic inputs onto a particular neuron result in a noisy trace that occasionally reaches the firing threshold, resulting in emission of a spike. If many synaptic inputs increase the neuron's firing rate, the chance of reaching the threshold will increase and the neuron will fire spikes at an elevated rate. Yet, the exact timing of the individual spikes remains arbitrary.