I've reproduced this merged figure below, because I intend to use it in my lectures and I missed a figure like this one so far. It shows the transition towards a modern model of synaptic plasticity, with the topmost panel showing the 'invertebrate' presynaptic plasticity and then the 'vertebrate' postsynaptic plasticity as separate as they wre [sic] conceived up until about 1995. The final, lowermost panel shows the combined picture as both in invertebrates and in vertebrates there is sufficient evidence today pointing to both pre- and postsynaptic plasticity in all brains studied in enough detail so far:
This image represents a staggeringly good chronology of the theory of synaptic plasticity as well. To state the obvious, the thing that is terrific about this image is the way it recognizes the leading characteristics of the invertebrate and vertebrate models and then knits them together. It would be really fascinating to work out the actual chronologically of these studies, naming the individuals (many, many people), techniques (many, many of them), and laboratories (many!) who made modest contributions to this story. Then, if we could set the whole story in the context of post-war 'Big Science' we would begin to get somewhere. The image underscores how important it is to think historically about the role that invertebrates in particular have played in defining human self conception. The sea slug, far more than the monkey, Soapy Sam!
The image is by David L. Glanzman (2010) and published in Current Biology (here).