Spike Timing Dependent Plasticity interpretation A paper I'm looking at titled "Synaptic Modifications in Cultured Hippocampal Neurons: Dependence on Spike Timing, Synaptic Strength, and Postsynaptic Cell Type" (can be found here) showed a relationship between spike timing and LTP/LTD. However, I'm having a bit of difficulty understanding what Figure 7 shows:



Since this photo I've seen published in multiple textbooks and reviews, it seems like a very important figure to understand what is going on. According to what they wrote in the introduction:

"Our results showed that postsynaptic spiking that peaked within a time window of 20 msec after synaptic activation resulted in LTP, whereas spiking within a window of 20 msec before synaptic activation led to LTD"

My understanding is that you have two neurons, a presynaptic neuron and a postsynaptic neuron and then based on their relative firing timings, that tells you whether or not there is an increase or decrease in the synaptic weight provided that the timings are close enough together. What confuses me as a non-biologist is how a postsynaptic spike could occur before the presynaptic spike does? I would think the only way a postsynaptic spike occurs is when the presynaptic neuron fires into the postsynaptic one. And then this time interval... is it from when the timing of the last presynaptic spike to the first spike of the postsynaptic one? Or from the first presynaptic spike since you would need multiple of them to induce a spike from the postsynaptic one? I'd mainly like to ask for more clarity over how to interpret what is happening.
The best explanation for STDP that I've ever seen is Nicky Case's Neurotic Neurons. In this Explorable Explanation, I think you'll find that your misunderstanding lies in the fact that both the pre-synaptic (Neuron A) and the post-synaptic neuron (Neuron B) exist in a network of other neurons. They are not only connected to each other. You are correct in saying that it often takes more than one incoming spike to make a neuron fire. These spikes are usually coming from other neurons (Neuron C, D, E... etc). So it's incorrect to think that "the only way a postsynaptic spike occurs is when the presynaptic neuron fires into the postsynaptic one".

So, to summarize, yes the time interval is the time difference between the last pre-synaptic spike to the first spike of the post-synaptic spike. These neurons exist in a network of neurons. It's probably best to think of STDP as a coincidence detector.