The specific paper that I’ll review here is “A Learning and Memory Area in the Octopus Brain Manifests a Vertebrate-Like Long-Term Potentiation” by Hochner et al. It was published in 2003 (7 years ago already!) in the Journal of Neurophysiology (available at this link.) Much as the title suggests, this study showed the presence of long-term potentiation (or LTP) in the octopus vertical lobe.
Let me explain what LTP is, and then the previous paragraph may become a lot more meaningful to some readers. LTP is the mechanism by which synapses (the points of communication between nerve cells) become “stronger”; that is, synapses can transmit information with a varying degree of degradation of the signal, and stronger ones will transmit the information better than weaker ones. First, a picture of a synapse:
The neuron sending the information (the presynaptic neuron) is in yellow, while the neuron receiving the signal (the postsynaptic neuron) is in green. Imagine that the system works like this: an electrical pulse comes flying down the presynaptic axon from the top of the page. When it gets to the end of the axon, it causes (through a variety of rather complicated biochemical mediators) all those synaptic vesicles to dump their contents into the space between the neurons (the synaptic cleft). Their contents are neurotransmitters, which then act on receptors on the postsynaptic neuron. This activity causes electrical currents to be generated in the postsynaptic neuron, and so the electrical signal has bridged the gap and is on its way.
When a synapse is persistently active, it will tend to become stronger (this is known as Hebb’s law – it’s actually only sometimes true, but it’s a good heuristic for now.) This is called long-term potentiation, as the synapse can be said to be potentiated, and this effect will last a while. Now, a lot of things happen during LTP – the synapse may become physically larger or more efficient, and the types of receptors on each side may change. In any case, the overall effect is that the synapse will become better at propagating signals – that is, the same signal in the presynaptic neuron will elicit a larger signal in the postsynaptic neuron.
, the adjustment of the way neurons are “wired” together, which is thought to allow us to do things like learn and remember.) If it’s present in octopus, then it means that there is something about the organization of this type of system that is efficient or effective enough to have evolved largely independently in two very different groups of animals (although we don’t actually know exactly what the last common evolutionary ancestor was between people and octopus, we have a pretty good idea – but that’s for another post. It suffices to say that it mostly likely had a very simple nervous system, meaning that octopus and vertebrate brains evolved mostly independently.)
n, and could account for the memory of octopuses, as we suspect it accounts for much of the memory ability of humans. The rest of the paper is spent elucidating possible mechanisms which could account for the observed LTP, as well as verifying that it is actually LTP and not just an artifact of their procedure – I don’t have the time to go through this at the moment, mostly because it involves a wide array of neurophysiological techniques, which are a workout to explain in and of themselves. (For the curious neurophysiologically-minded readers, I’ll summarize: they find that there are both postsynaptic and presynaptic mechanisms that contribute to LTP in octopuses, as in vertebrates. It is also demonstrated that LTP in the octopus involves a large increase in intracellular calcium concentration, as in vertebrates. Unlike in most vertebrate systems, however, LTP in octopuses is not NMDA-type receptor dependent, although the authors don’t offer an alternative explanation. This is neat, because it suggests that the same sorts of neural systems are likely to evolve with some wiggle room as to the specific mechanisms of their functioning.)