The obtained distributions of number of spikes per second in 30 seconds of baseline recording are shown on top line for cells V1, W1, U1 and O2. The Poisson distribution fitted to each obtained distribution is shown in middle line along with the probability value of the fit as determined by a X² test. In the case of cell U1 for which the Poisson distribution was not an acceptable fit, a normal distribution, as shown, was fitted with the same mean and standard deviation. The Poisson distribution was not an acceptable fit for the baseline distribution of cell O2, but a normal distribution could not be substituted.