How do I interpret this depiction of dopamine neuron signaling?
From the figure caption for that figure:


  Each line of dots shows one trial, the original sequence in each panel being from top to bottom.


The bars at top are a summed histogram. Dots mark the time of spikes (this is called a "raster"). Each line is a different single trial from the same neuron.

(also note: this type of paper is called a 'review' paper, i.e., it isn't original research, although the original research might have been done by the same author. That doesn't mean anything bad, reviews are an excellent place to start understanding a topic, but you should be aware that the figures here are just representative examples or in some cases schematic diagrams. There is a lot of other data behind the conclusions drawn besides these single examples)