Since the pulse duration is relatively short, the timing would need to be exactly right for the currents to add up. This would not be possible considering the mismatch between the 32 neurons. However, I'm not measuring the output current with the oscilloscope, but the voltage difference created by this current over a 20 kOhm resistor. Since the chip's output pin, the board and the oscilloscope's probe all add a certain capacitance to this node, the output voltage will be a low-pass filtered representation of the output current, thereby summing current pulses arriving within a certain delay. The different graphs are obtained by playing mainly with the bias voltages that control the 'membrane leakage', the 'action potential threshold voltage' and the 'refractory period'.
All cells receive the same input.
They generate different output due to the mismatches between the neurons. The membrane leakage current will vary somewhere between 5 and 10 percent from cell to cell for a given setting, and the main influence is coming from the current controlling the refractory period, since this is a small current. The smaller the current, the smaller the relative precision. So for longer refractory periods, you'll get even more variation between the cells. You can find a postscript version of the paper here .