Firing patterns of single units in the prefrontal cortex and neural network models

The occurrences of high-frequency bursts of neural activity and the probability distribution of firing rates of neurons were investigated in the behaving monkey. The activity of 8 to 11 single units was recorded in parallel through six metal microelectrodes from the frontal cortical areas. High-frequency (> 150 Hz) bursts of three or more spikes in succession were extremely rare, occurring at an average rate of 0.068 per second per neuron. The probability of observing a burst in one neuron was not affected by the fact that another adjacent neuron emitted a burst. Thus, if a high-frequency burst represents the 'on' state of a neuron, we failed to demonstrate persistent states in which a group of neurons is turned 'on' together. The firing rates of the neurons were usually low. The probability of a neuron firing a few (2-5) spikes within a narrow (10-100 ms) time window was very low (< 0.003). The probability density of firing rates did not show any sign of inhomogeneity, thus failing to show that the neurons tended to be either in a state of high firing rate or in a state of low firing rate for any periods of time. The low probability of finding a neuron with an elevated firing rate means either that a given cortical area is idle most of the time, or that, in a given process, only a few neurons elevate their firing rates at any one time. The relevance of these findings to neural network models is discussed.