Voltage not as expected in LIF Single Neuron Example


I’m just getting going with Nengo and ran the “single_neuron” Jupyter notebook. I’m confused by the membrane voltage plot, so I’m asking for clarification and documentation surrounding the default LIF model. My expectation is that any input value that is below the intercept value will be a negative current and anything above will be a positive current. So, if I change the stim to a constant -0.5:

cos = nengo.Node(lambda t: -.5) # was np.cos(8 * t)

The bottom-right output displays the voltage as 1 - exp(-tau) when I would expect the output to simply be flat, at zero, for the duration of the simulation. I would also expect that if I gave the cell a one-sample input, positive or negative, membrane voltage would rise to a value and then exponentially decay. As it is, from 0.2 sec into the simulation on, if the cell receives an extremely weak positive input, it will spike at that moment.

Any explanation appreciated.