The first paper I want to talk about in this post is Evidence for a specific short-term memory in the cuttlefish by Agin, Dickel, Chichery, and Chichery (1998).
Agin et al’s experiment ran as follows: cuttlefish of various ages were shown a prawn or shrimp sealed inside of a glass tube during a training phase. As one might expect, the cuttlefish attacked the glass tube, but learned to suppress their attacking as they learned that the prey was inaccessable. Then, after a 5 minute break, the cuttlefish were tested to see how well they could remember not to attack the prey in the tube. There were 3 groups – the first got 20 minutes of training, the second got 5 minutes of training, and the last was exposed to the empty glass jar for 20 minutes as a control group.
In a nutshell, here are their results: Juvenile cuttlefish (15-30 days old) learned to suppress attack in the 20-minute condition only, while for older cuttlefish, a 5 minute exposure was enough for them to learn the lesson and almost completely suppress attack.
This is all well and good by itself. We would more or less expect that cuttlefish should be able to retain this simple memory, knowing how good cephalopods are at that sort of thing. What makes this story more interesting is this groups follow-up paper.
In Effects of learning on cytochrome oxidase activity in cuttlefish brain by Agin, Chichery, and Chichery (2001), the group subjected cuttlefish to the same learning task and then measured cytochrome oxidase levels in their brains at different time points after learning. Cytochrome oxidase expression is increased in metabolically active cells, putatively indicating (in neurons, at least) the overall activity level of the cell (with rather poor spatial resolution, of course.) What this experiment revealed was that the pattern of cytochrome oxidase staining changed depending on how long it had been since the cuttlefish did the learning task. In other words, it looks rather like different populations of neurons are involved in the early stages of memory than the late stages of memory. A similar effect was found using a marker of acetylcholine synthesis in this study, Central acetylcholine synthesis and catabolism activities in the cuttlefish during aging by Bellanger et al .
This is notable because it suggests that cephalopods, like birds and mammals, may have dissociable short- and long-term memory systems. According to the generally accepted theory of human memory, the short-term and long-term memory systems are comprised of distinct but overlapping neural circuits. It would be very interesting if the same dissociation had developed in cephalopods (in fact, this same group has gathered more behavioral evidence supporting this hypothesis since these studies were published, ie. their study Developmental study of multiple memory stages in the cuttlefish, Sepia officinalis, 2006 .) While it seems that the study of cephalopod memory processes is still in its infancy (compared, at least, to the study of human memory processes) and no strong assertions about this system can be made yet, it’s a tantalizing thought that we might be able to find and understand such a striking example of the convergent evolution of neural systems as this.