Did you ever get an answer to your question? I know this may be too late now, but I was noseying around and spotted it, so I'll take a stab nonetheless. I could be wrong, but I suspect what you're seeing here may be an extracellular representation of an action potential (and/or it could be a filtering artefact, see below). From the video on the spikerbox website, it looks like you place the electrodes (nails?) into the animal and hope to get close to axons and see spiking activity. Correct me if I'm wrong.
From a brief reading of the literature it would appear, under optimal conditions, the waveform of the extracellular action potential (EAP) is dependent on the cell type and location of the electrode along the neuron (this paper illustrates this nicely). As you move away from the soma and down the axon/dendrites, you tend to see this more prominent initial spike, as you see in your recordings. This is due to the ionic conductances and timings of channel openings. So that's one possible explanation...
Here's the other: the signal is filtered in some way. I notice in your figure your signal is very smooth without any noise and your EAP very long (assuming your x axis is seconds), which suggests to me your acquisition rate is quite low and/or being filtered. I've attached an image of an EAP I acquired (from a cell soma) a while back (with an acquisition rate of 10Khz, i.e. I 'sampled' the signal 10000 times per second to get the trace). They're all the same EAP on the same scale with different types of filters applied. The original signal is the one on the left; note all that baseline noise. The middle one is a low pass filter of 2KHz (gets rid of signals higher than that). The right hand one is a 750 - 1000 Hz band pass filter. This type of filter does funky things to signals if used incorrectly (such as in this example!). It basically only allows signals within a given region and gets rid of signals outside of it. What you end up with is the signal on the right, which looks suspiciously like yours!
So I'd hazard a guess and say it's a problem due to how the system collects and filters data. BUT, there could be a biological basis for it too.
However, there is one thing that does confuse me, and that is the polarity of your waveform. I would normally expect the prominent point of your EAP to be pointing downward. I have two suggestions as to what may be the cause of this:
- You're actually measuring currents here; the units for your Y-axis would help answer that one.
- Some grounding issue.