Once upon a time, as a young undergraduate, I took a course in neurobiology (which turned out to be rather influential in my life, but that’s another story). The professor, Johnny Palka, took pains at the beginning to explain to his class full of pre-meds and other such riff-raff that the course was going to study how the brain works, and that we were going to be looking at invertebrates almost exclusively—and he had to carefully reassure them that flies and squid actually did have brains, very good brains, and that he almost took it as a personal offense when his students implied that they didn’t. The lesson was that if you wanted to learn how your brain worked, often the most fruitful approach was an indirect one, using comparative studies to work out the commonalities and differences in organization, and try to correlate those with differences and similarities in function.
At about that time, I also discovered the work of the great physiologist, JZ Young, who had done a great deal of influential work on the octopus as a preparation for studying brain and behavior. (Young, by the way, went by the informal name “Jay-Zed”, and there you have another clue to my affectation of using my first and middle initial as if it were a proper name.) It was around then that I was developing that peculiar coleoideal fascination a few of the readers here might have noticed—it was born out of an appreciation of comparative biology and the recognition that cephalopods represented a lineage that independently acquired a large brain and complex behavior from the vertebrates. To understand ourselves, we must embrace the alien.
Young’s attempts to understand mechanisms of learning in memory in the octopus were premature, unfortunately—they have very complex brains, and we made much faster progress using simple invertebrates, like Aplysia, to work out the basics first—but it’s still the subject of ongoing research. I was very pleased to run across a general overview of the octopus brain in The Biological Bulletin.
We know that the octopus is amazingly smart. They are capable of associative and observational learning, they are curious and adaptive, and can invent new solutions to problems. They have a large brain relative to their body size, containing about 500 million cells, and they have condensed the classically distributed invertebrate central nervous system into a dense, discrete brain. One of the problems that stymied Young was that, rather than retaining the very large and accessible identifiable neurons we associate with invertebrates, the cephalopods have paralleled the vertebrates, microminiaturizing neurons to pack more cells into a given space. They’ve also built layered structures into their brains, and thrown the tissue up into folds that increase surface area, much as the vertebrate cortex has.
So what does an octopus brain look like? Superficially, nothing like ours.
Right away, you should notice one major peculiarity: the gut runs through the middle of it, separating the brain into a supra- and sub-esophogeal ganglion. Look at it again, though: there are lobes and nuclei and tracts, and even without knowing anything about function, you can tell that this is a modular brain of considerable complexity. Details are different, but the key thing is that we see specialized subunits emerging—processing centers that carry out dedicated tasks.
This paper focuses on one particular area, the Vertical Lobe, or VL, which is at the top of this image. Here’s a closeup:
What does the VL do? Octopuses are tough and resilient, and as it turns out you can do some fairly invasive surgeries, stitch them up, and they recover just fine. The VL can actually be extirpated, and VLless octopuses, once they’ve got over the surgery, seem perfectly normal in swimming, feeding, and other ordinary behavioral functions. Deficits show up, though, when they are tested on learning and memory tasks: long term memory function is lost, and learning is greatly impaired. The VL and the median superior frontal lobe (MSF) together form a structure that is functionally analogous to the vertebrate hippocampus.
They also exhibit a microstructure of a sort we see over and over again in the vertebrate brain: a matrix of parallel fibers produced by many small cells, a set of input fibers (the MSF tract) crossing them orthogonally, and a small set of large output neurons upon which all this arrayed activity converges. It’s an elegant way to set up a huge number of synaptic connections and sample from a large field of possibilities, independently evolved in the octopus, and therefore may represent an optimal mechanism for generating behavioral flexibility. Again, there are differences in the details; octopus neurons, like many other invertebrate neurons, do not involve the cell body in generating electrical activity (very much unlike our neurons), and there are certainly substantial differences in the molecular biology and pharmacology of the channels involved. The striking features, though, are the convergences, which tell us what properties of a neuronal array might be necessary for learning and memory.
The findings emerging from recent electrophysiological studies in the octopus suggest that a convergent evolutionary process has led to the selection of similar networks and synaptic plasticity in evolutionarily very remote species that evolved to similar behaviors and modes of life. These evolutionary considerations substantiate the importance of these cellular and morphological properties for neural systems that mediate complex forms of learning and memory. In particular, the similarity in the architecture and physiological connectivity of the octopus MSF-VL system to the mammalian hippocampus and the extremely high number of small interneurons in its areas of learning and memory suggest the importance of a large number of units that independently, by en passant innervation, form a high redundancy of connections. As these features are found in both the octopus MSF-VL system and the hippocampus, it would appear that they are needed to create a large capacity for memory associations.
Even something as specific as the neurobiology of learning and memory benefits from the evolutionary approach. By comparing different systems, we can identify the commonalities linked to function, and thereby come closer to finding generalizable rules and principles rather than the usual welter of details.
Hochner B, Shomrat T, Fiorito G (2006) The Octopus: A Model for a Comparative Analysis of the Evolution of Learning and Memory Mechanisms. Biol. Bull. 210: 308-317.