Serotonin in the octopus learning system.

          (Note: I apologize if this post seems jargon-ey.  I've tried to explain or reference any hard to get terms, but I do assume that readers know the very basics of neural functioning.  If you need a primer on this, check out wikipedia's page on neurons or this great tutorial.  Feel free to post in the comments if there's anything you want explained more thoroughly, and I'll give it a crack.)

          The Octopus research group in Jerusalem is back with a paper in the August issue of Neuroscience about the function of serotonin in the octopus vertical lobe, Serotonin is a facilitatory neuromodulator of synaptic transmission and “reinforces” long-term potentiation induction in the vertical lobe of Octopus vulgaris.  I'm very excited to blog about this paper - it's the very first time in my short blogging career that I've gotten to cover a study as it was coming out!  You can read my other posts about their work here and here (that second one has a basic description of the technique of stimulation-induced LTP, which I'll be very brief with here.)

          Basically, LTP (long-term potentiation) is one of the mechanisms by which neurons are thought to adjust how they connect to each other during the process of learning - specifically, they become stronger (or potentiated,) meaning that signals are carried across the synapse more effectively.  The authors of this paper use a technique by which they induce LTP in synapses in the octopus vertical lobe (a structure thought to be involved in learning and memory) and study the effects of serotonin (also called 5-HT, which is short for 5-hydroxytryptamine, the terminology I'll be using from now on) on the properties of the induced LTP.  Presumably, this can tell us something about the function of 5-HT in the normal functioning of the vertical lobe, although this point is very debatable.

          Why look at 5-HT?  Well, for starters, it's one of the big neurotransmitters these days (along with such illustrious nearly-lay-term chemicals as dopamine, norepinephrine, GABA and glutamate.)  You hardly need to have a specific reason to study it these days because it's involved in pretty much every process that contemporary neurobiology cares about: consumptive behavior, mood and depression, social cognition, the action of addictive drugs.  More than that, though, it's conserved across all bilaterians, the group of bilaterally symmetrical animals including people, the rest of the vertebrates, the insects, and, among many others, the molluscs!  If there is any neurotransmitter that is interesting to study comparatively, it's 5-HT, as it's been shown to be involved in learning in animals as distantly related to each other as sea slugs, rats, humans, and (now) cephalopods.  If we learn how 5-HT does its job in a wide variety of animals, it will help us understand how neurotransmitters function within nervous systems in general.  This is, we will hopefully agree, a Good Thing. 

          The authors begin with the hypothesis that, as has been shown in Aplysia (a beautiful little sea slug who is relatively widely studied in neuroscience,) 5-HT probably has a role in the modulation of LTP rather than inducing it directly, making it a putative neuromodulator.  It is not hard to imagine how this might be a good thing to have in a memory system.  Let's pretend that our animals has just been injured, or that it has just found a great big source of food.  All of these events call for a general upregulation in the formation of memories, since remembering what happened around these events will help the animal repeat or avoid them in the future, depending on whether they were good or bad.  If a chemical can increase the amount of LTP (a process thought to be involved in learning,) it would make sense that it might be selectively secreted or expressed during times when the animal's memory system needs to pay attention to what's going on, and not when there is nothing of consequence happening.  This is an extremely limited view of the role of neuromodulators in learning, but it illustrates the principal as well as I know how to.  In short, neuromodulators, while not responsible for neurotransmission and plasticity themselves, have some effect on it.  This sort of effect is one of the things that allows the great flexibility of neural systems, one of their key features.

          In the first part of their study, the authors stained slices of the octopus vertical lobe for 5-HT, and then described what they say - this is good old fashioned neuroscience.  They found that 5-HT shows up in fibers from the medial superior frontal lobe (MSF) that innervate large areas of the vertical lobe.  The MSF is thought to be one of the main sources of input of sensory information to the vertical lobe, and this tract of fibers (known as the MSF-VL tract) is thought to be involved in the formation of sensory memories in the octopus, as per J. Z. Young's early lesion experiments in the octopus.  The authors note that this wide spread of 5-HT is typical of neuromodulators, supporting the idea that MSF neurons use 5-HT to modulate LTP in the vertical lobe.

          In the second part of the study, the authors use a technique where they induce LTP in live slices of octopus brain (cool, right?) by repeatedly stimulating the axons running from the MSF to the vertical lobe.  They measure the "strength" of neurotransmission as fPSP's, or synaptic field potential, which is roughly an indicator of how much electrical activity is generated by activity in many synapses within a small area of the tissue.  I'll only summarize one of their several experiments here, because it is the one that really illustrates the neuromodulatory effect.

          This figure shows the results of an experiment using induced LTP in octopus brain slices.  The experimenters stimulated the brain slices along the MSF-VL tract and recorded the resultant electrical activity in the VL.  Let's start with the first graph.  The y-axis shows the amount of activity recorded in the vertical lobe after a very small electrical stimulation (this is what each data point is.)  The x-axis shows the time from the beginning of the experiment.  At about 30 minutes, MSF-VL neurons were stimulated with a "triplet", which consisted of three pulses in quick succession.  As we can see in the control preparation (the blue line,) this w pas not enough to induce LTP, which would be evident as an increase in the field potential.  In a preparation treated with 5-HT, however, this stimulation was enough to elicit some LTP, which is apparent as a stable elevation of the recorded field potential at times 50 and 60 minutes.  After 60 minutes, each preparation was subject to high-frequency stimulation, which caused maximal LTP in both cases.   The bar graph next to it (B) shows the results of multiple experiments, showing that before high-frequency stimulation, the treatment with 5-HT caused an increase in the LTP resulting from the triple-pulse, indicating that the presence of 5-HT made MSF-VL synapses prone to undergo LTP.  The second line graph (C) shows the results of a set of similar experiments, except that the stimulation was done once per minute.  As is apparent, treatment with 5-HT (shown by the red bar) increased the rate of LTP; however, as indicated in the adjacent bar graph (D), it did not increase the maximum amplitude of LTP.

          It's important to remember that in the active nervous system, it's unlikely that synapses are ever stably at a maximal strength.  That increase in the rate of induction of LTP, modest though it may seem in this experiment, could be crucial in affecting the functioning of a memory system in a behaving animal.  In the "real world", the stimuli involved in learning are often only present for a short time, and the state of any particular synapse in the nervous system is determined by an incredibly complex set of chemical factors.  Neuromodulatory activity (like that argued for in this paper) provides a sensitive mechanism by which the functioning of a neural system could be finely coordinated, allowing the integration of a variety of information into one system that can make a timely decision about whether an action was good enough to repeat or bad enough to avoid in the future.

          For convenience's sake, I skipped a variety of other interesting experiments that the authors did, and I encourage you to get the paper yourself and read it, if you can.  I very much like this type of research, and I like the challenge that blogging about it presents.  Anyways, I hope you've enjoyed this as much as I have!

          Thanks for reading!


Shomrat T, Feinstein N, Klein M, & Hochner B (2010). Serotonin is a facilitatory neuromodulator of synaptic transmission and "reinforces" long-term potentiation induction in the vertical lobe of Octopus vulgaris. Neuroscience, 169 (1), 52-64 PMID: 20433903

Short and long-term memory in cephalopods

          I've heard the assertion that octopuses have short- and long-term memories several times in the past few days, mostly in discussions of the ethics of eating octopuses prompted by ethical questions raised about Paul, the famous German octopod.  It's interesting to me what these people don't say - that they think that having a multiphasic memory process makes octopuses worth not eating (because, well, people have multiphasic memories, and you wouldn't eat them, would you?!?  Sicko.)  While I don't think that memory capacity of an animal is associated in an uncomplicated way with its ability to suffer or its moral status, it seems to me like a nonetheless interesting question.  I'm almost sure that most of the people who use (read: copy and paste) this bit of information to support their beliefs have very little idea of what sort of research is behind it.  Let's face it: developing a working knowledge of behavioral research on cephalopods is something that just isn't on most of the public's mind.  In fact, until I began writing this blog, I had very little knowledge of the subject.  I plan to set the record straight, so that internet users need never make an unfounded or unqualified statement about memory processes in cephalopods again (a lofty goal, huh?)

          If you don't know octopus neuroanatomy very well (and who does?) you might want to check out the figures in this post.  I'll be talking about the vertical and superior frontal lobes of the octopus brain, and I know it sometimes helps to be able to visualize things like that when you're reading about them.  Just so that it's clear: the term "biphasic memory" means that the memory system in question has two discrete parts or processes (ie. short-term and long-term memory.)  A monophasic memory would have only one process, so that memories would last for a certain amount of time and then fade similarly in all circumstances.  A multiphasic memory system (which could be biphasic, triphasic, or more) is a general term to describe memory systems that are clearly more than monophasic, but are not completely characterized yet - and no memory system is.  Now, on to the research!

          J. Z. Young, that demigod of cephalopod neurobehavioral research, published one of the few papers I could find on this topic back in 1970, following up on his earlier work on the subject.  In it, he investigated the development of short and long term memory in O. vulgaris (I assume - he doesn't actually mention what species he uses in this paper, but he almost always used O. vulgaris) as well as the role of two brain areas in memory, the median superior frontal lobe (MSF) and the vertical lobe (VL).  To do so, he performed surgeries to remove one of these two areas of octopuses' brains and put them through a learning task.  In this task, octopuses were trained to either attack a rectangle (rewarded with a piece of fish) or withhold attacking a crab (which was punished with electric shock.)

          It turned out that octopuses whose vertical lobes had been removed were greatly impaired in learning to attack the rectangle.  Young explains this by claiming that the vertical lobe is involved in short-term memory, and that the acquisition of stable behavior day-to-day was impaired because the animals without vertical lobes could not remember events long enough for the training to be effective.  The animals without median superior frontal lobes, however, learned the task just fine, but were impaired in their long-term retention of it., suggesting that the MSF lobe might have some role in retaining learned information.  Interestingly, Young also found (in other experiments) that removing the vertical lobe after a task was learned resulted in a greater retention of the task.  These results suggest that the vertical lobe plays a role in the updating of memory stores, but is not absolutely essential for the recall of memories.

          His results from the attack-withholding task were less clear, but they suggest that animals with lesions, especially those with vertical lobe lesions, were less consistent than intact animals in learning not to attack a crab after being shocked each time they attacked it.

          Basically, Young argues (on the basis of this and some of his other experiments) that octopuses have a memory system that can be disrupted in more than one way; that is, it is possible to dissociate memory acquisition from long term retention, just like in vertebrates.  For the most part, more current research has agreed with his position, as we'll see in this next paper.

          Moving forward (past a lot of great research that I'll skip over for the sake of brevity) to 2008, Shomrat et al. used electrophysiological methods to test this hypothesis.  Before we get into their methods, let's look a bit more closely at the system that we are talking about (this figure is from Shomrat et al. (2008)):

          On the left is a sagittal slice of the supraoesophageal (over-the-oesophagus) mass of the octopus brain.  On the right is a diagram of the memory system in question.  Sensory information flows into the MSF from the arms and eyes before being sent along to the VL.  The VL neurons in turn send out information encoding attack.  It's been established that long-term potentiation (LTP) can occur in this area of the octopus brain, and this is a likely mechanism for the formation of memories in octopus (I blogged about this here - check it out if you need a little more background.)

          The authors' procedure went as so: O. vulgaris who had already been trained to attack a white ball either had their MSF tract cut (at the dashed line in each image,) severing the sensory input to the vertical lobe, or this tract was stimulated, causing LTP at the synapses indicated in the figure.  Shortly after the procedure, the animals were trained to avoid a red ball through electric shock.  It was found that animals with severed MSF tracts were slower than controls to learn to withhold attack, while animals in whom LTP was induced were quicker.  This is all well and good - it confirms what we already thought about the role of the vertical lobe in acquiring memories in the octopus.  The really important result from this paper came when the authors tested the octopuses a day later.  It was found that both MSF tract transection and LTP induction impaired recall after 24 hours.  So even though stimulation of the MSF tract improved short-term memory (presumably by hyper-activating the memory system in the vertical lobe,) it impaired long-term memory.  This suggests that these two processes are not identical; that is, that octopuses have discrete and dissociable short- and long-term memory circuits.  This general finding has been replicated in cuttlefish (see my post on cuttlefish memory) and nautiluses (Crook and Basil, 2008).

          Unfortunately, that's just about all that we know at this point: that cephalopods appear to have biphasic memories, meaning that the behavioral evidence of short-term memories can be dissociated from that of long-term memories.  This is hardly (by itself) a basis on which we can imply any sort of consciousness or advanced cognitive capacity, as animal-rights supporters who mention this fact seem to imply.

          In interpreting these results in the context of our knowledge of cephalopods as a whole, we should keep in mind what is meant by short- and long-term memory in humans.  Short-term memory is what happens when newly learned information is bouncing around the cortex somewhere, being continually processed but not permanently encoded somewhere.  These memories will disappear if they are not rehearsed (or otherwise actively retained).  Long-term memory has been (relatively permanently) encoded into neural circuits, so that it can be retrieved after periods when it has not been actively processed in short-term (or working) memory circuits.  These processes have been studied intensely in humans, and can be precisely because we have a complex cognitive system build around them (or on top of or parallel to them, depending on who you ask) that we can access.  As of yet, we don't have the experimental techniques to assess exactly how "human-like" or "vertebrate-like" cephalopod memory systems are, because we can't study them in nearly as much detail as language-based and other cognitive tasks allow us to in humans.  Thus, making any strong conclusions about the nature of cephalopod memory other than that it appears to be multiphasic (with no implied "and-so-cephalopods-are-smart-like-people") is untenable.

          Lastly, I find it frustrating that animal rights activists use our (very primative) knowledge of cephalopod memory systems to try to support their position that eating cephalopods is wrong.  Not only is it an inconclusive (what does memory have to do with suffering and morality?) and nonspecific argument (did anybody think that ungulates, swine and birds don't have complex memory systems?), but it misses some of the big points that the animal rights movement has taught us.  First of all, it implies that cephalopods are somehow special because they are intelligent and human-like.  However, having compassion for animals explicitly demands that we not judge their worth by analogy to our own abilities - this has proved to be an attitude that encourages cruelty to animals simply because we are ignorant of them and their behavioral and cognitive capacities.  If we didn't know about cephalopod memory systems, would they still be worth defending from fishing and consumption as food?  Hopefully, the answer is yes - so why try to use this (admittedly inadequate) argument now that we conveniently have information that appeals to one's emotional predispositions?  I find this to be irresponsible and counter-productive, as it diminshes the credibility of other, more valid arguments against the consumption of cephalopods (or any animal, for that matter) that animal rights activists might use.

          Sorry if this was a bit heavy on editorial material.  Being very concerned about animal welfare myself, I get annoyed when people make the cause look stupid by saying things that are ill-informed, ill-reasoned, or just plain wrong.  Although I wish that people would stop killing cephalopods for food, spinning information to try to get people to agree with a point is dishonest, and at best a very poor strategy for debate, as there's bound to be at least one attentive person on the other side who will point out that you're not being true to the facts - and nobody will listen to you after that.

Thanks for reading!


SHOMRAT, T., ZARRELLA, I., FIORITO, G., & HOCHNER, B. (2008). The Octopus Vertical Lobe Modulates Short-Term Learning Rate and Uses LTP to Acquire Long-Term Memory Current Biology, 18 (5), 337-342 DOI: 10.1016/j.cub.2008.01.056

J. Z. Young (1970). SHORT AND LONG MEMORIES IN OCTOPUS AND THE INFLUENCE OF THE VERTICAL LOBE SYSTEM Journal of Experimental Biology (52), 385-393

Crook, R., & Basil, J. (2008). A biphasic memory curve in the chambered nautilus, Nautilus pompilius L. (Cephalopoda: Nautiloidea) Journal of Experimental Biology, 211 (12), 1992-1998 DOI: 10.1242/jeb.018531

Octopus Sensory Systems: Part 2.5

This will be a quick one - I'll get back to the meat of my series on octopus sensory systems soon, but I wanted to write a post on this article because it struck me as cool (although it has a sort of sensational title.)

The article I'm talking about is Octopuses (Enteroctopus dofleini) Recognize Individual Humans (2010) by Anderson et al. in the Journal of Applied Animal Welfare Science.

The authors used an apparently elegant experimental design to test whether octopuses can tell people from one another across a long period of time  - specifically, this is operationally defined as meaning that they could learn an association between a person's features and a good or bad stimulus.  The experiment was conducted thus:  eight octopuses were captured and habituated to their aquaria.  Then, for 2 weeks, the octopuses had daily interaction with two people, one of whom fed them and one of whom (I'm not joking) poked them with a "bristly stick" (more specifically, "a length of PVC pipe with one end wrapped in Astroturf.")  Then, the octopuses were tested to see if they reacted differently to the two individuals - presumably, if they remember who is who, they should show anticipatory behaviors related to eating or defensive behaviors in response to the appropriate person.

To get a better feel for the task, here are the experimenters, shown in an image taken from the octopus's point of view:

My problem with this experiment is that the term "individual" is usually used in cognitive research to mean some entity who is known to persist despite changes in their appearence in one specific sensory modality.  When we get a haircut, our friends (and, usually, our pet dogs and cats) still recognize us - thus, we are individuals to them.  However, if the visual stimulus of the two keepers didn't change from day to day (and they took pains to make sure that it didn't,) then this seems like little more than a complex visual discrimination task.  It seems, judging from this image, that it would be pretty easy for an octopus to learn an association between, say, a shiny bald head and being jabbed with a stick, regardless of any ability she might have to recognize "individuals" in the cognitive sense.  In any case, we are still a ways away from knowing whether octopuses can recognize individuals, and not just their constant visual features.  With this in mind, let's consider their results.

It turns out that the octopuses learned to move away from the irritator and towards the feeder within two weeks.  In addition, the octopuses showed fewer defensive coloration responses to the feeders than to the irritators, as well as changes in their respiration rate and the orientation of their bodies relative to the people.  In sum, it looks like (in this test, at least) the octopuses succeeded in learning basic traits about the people interacting with them.  I don't think that the title of the paper is fully supported, however - it's hard to make the case that this single study proves that octopuses can identify individuals in any sort of robust way.

This paper is pretty solid (besides its unfounded title,) although it begs a few questions:

1.  How fine of a discrimination can octopuses make?  Would they treat two bald men of similar stature the same?  What if the subjects wear different clothes?  How is this piece of research fundamentally different from Wells' experiments using simple visual cues? These are all important questions if we're actually going to claim that octopuses can identify "individuals" as opposed to simple visual stimuli.

2.  What does this mean functionally to the octopus in the wild?  Is this sort of ability actually used to identify predators and prey items?  Do octopuses remember individuals of any species in the wild?  Unfortunately, there is not much literature on the development of behavior in the octopus, so we can't know how much of octopus behavior is "instinct" and how much of it is based on learning (like that shown in this study.)

3.  How does this generalize to other species of octopus?  This study used Enteroctopus dofleini, the giant pacific octopus, because it is often kept in public aquaria.  However, practically the whole body of research on octopus learning and vision has been done using O. vulgaris and, to a lesser extent, O. cyanea.  We know that cephalopods have a pretty wide diversity of life-styles, so it seems important to me to know how these behaviors occur in different species if findings like this are going to be relevent to the rest of cephalopod research.

If nothing else, this study keeps alive my childish hope that Twister, the resident E. dofleini at the Niagara Falls Aquarium (which I visit almost weekly these days) will someday get to know me, if only in the most basic way.

Anyways, I hope this has been as fun for you as it was for me.  Thanks for reading!


Anderson, R., Mather, J., Monette, M., & Zimsen, S. (2010). Octopuses (Enteroctopus dofleini) Recognize Individual Humans Journal of Applied Animal Welfare Science, 13 (3), 261-272 DOI: 10.1080/10888705.2010.483892

Prawn-in-the-tube (More Cuttlefish Memory)

For several years, a group of researchers in France have been studying the neural correlates of learning in cuttlefish (recently focusing on, among other things, oxytocin-like neuropeptides in the cuttlefish CNS - I'll review this in a later post.)  I reviewed some of their work in an earlier post.  Although this is a fascinating concept, their method has been criticized because they use a single learning task to elicit what they claim are learning-induced neural changes, generally.  Importantly, it is questionable whether their method causes associative learning or habituation.  Associative learning involves the formation of a mental or neural (depending on your conceptual preference) association between some behavior and a consequence of that behavior, such as finding food or feeling pain.  This form of learning has long been thought of as one of the hallmarks of adaptive behavior, and it is certainly central to any claims about cephalopod intelligence - if we could not demonstrate associative learning in cuttlefish, we would have very little ground on which to call them intelligent.  Habituation occurs when we are exposed to some stimulus for long enough that we just stop responding to it.  In the case of habituation, we haven't learned much about the stimulus - simply that it is generally unrelated to any reward or punishment we might get.

So what is this controversial procedure?  The group has given it the obscure name of the prawn-in-the-tube procedure.  It is essentially what is sounds like.  A cuttlefish is presented with a prawn enclosed in a clear plastic tube.  Cuttlefish, being visual predators, will attack the prawn, but their tentacles will hit the tube, and their attack will fail.  Over subsequent presentations, they learn not to attack the tube.  The difficulty is that it is hard to tell whether the cuttlefish are simply habituating to the prawn-in-the-tube stimulus, or whether some sort of sensory feedback from failed attacks is causing them to suppress their attacks - a type of associative learning known as passive avoidance learning.

In this group's research on cuttlefish learning (as well as in an older line of research by J. B. Messenger that used the same procedure) it is vital to know what sort of learning they are inducing in order to interpret their results.  Specifically, they work under the assumption that their procedure induces passive avoidance learning.  This is a pretty big assumption.  As such, they decided to settle this problem with a series of experiments, which they published as The “prawn-in-the-tube” procedure in the cuttlefish: Habituation or passive avoidance learning? (2006) by Agin, Chichery, Dickel, and Chichery. 

This study uses two techniques.  The first is called dishabituation.  In these experiments, a strong competing stimulus is presented alternatively with the "habituated" stimulus.  If this elicits a greater response, the it is likely to be the case that the animal has habituated rather than learned by association.  If the response is still suppressed after the novel stimulus is presented, it must be that the familiar stimulus is repressing behavior, and that passive avoidance learning has taken place.  The logic is that the effects of habituation will decrease if the animal becomes generally aroused by some other stimulus.  Their results show, however, that this is not the case.  Novel stimuli did not dishabituate the cuttlefish to the prawn-in-the-tube assembly.  Strike on against the habituation theory.

The second test that they used involved showing the cuttlefish a piece of bait (a live prawn,) and then removing it from the tank as the cuttlefish attacked, preventing them from ever catching it.  In this test, the cuttlefish never received any sort of tactile feedback when they attacked.  If the prawn-in-the-tube procedure causes habituation, we would expect attacks to decrease in this condition, as there is no reward or punishment to shape the behavior.  If the prawn-in-the-tube procedure works mainly by passive avoidance learning, we would expect that, as there is no negative sensory feedback following unsuccessful strikes, the cuttlefish would not change their response at all during this version of the procedure.  As it turns out, the procedure was almost completely ineffective in inducing any sort of learning in this condition.  The cuttlefish continued to strike regularly at the prawn, and their latency to strike actually decreased.  This experiment clearly does not support the habituation hypothesis.  Strike two!

Where's the third strike?  Oh, yeah, Purdy et al found similar results using a variation of this procedure in their paper Prawn-in-a-Tube Procedure: Habituation or Associative Learning in Cuttlefish? (2006).  Strike three, and the habituation hypothesis is out!

Actually, these results could presumably be overturned by some more sensitive or definitive test in the future.  For the moment, however, these studies allow the cuttlefish memory research community to investigate the neural bases of memory in the cuttlefish with a reasonable amount of certainty that they are studying associative learning.  They also make a nice general point about the sort of fine-grained analysis that's needed in order to study complex psychological processes like learning and memory, as well as emphasizing the importance of being critical of the assays that one uses to study these things.

Thanks for reading!

 (Sepia apama.  Photo by Nick Hobgood, used under a Creative Commons license.)

LTP in the Octopus Memory System

I’ll get back to octopus behavior in the subsequent posts, but I want to digress into octopus neurobiology for a minute.  We know that octopuses can learn, and our buddy J. Z. Young proposed that their memory system is much like ours – as evidence, he showed that the structure of the octopus vertical lobe (a little chunk of brain tissue that sits right at the top of the octopus brain – see P. Z. Myers’ post on the subject for a quick introduction to the brain of octopus) may have a lot in common with the structure of the mammalian hippocampus (which is a place in the human brain that is critical for memory – it’s shown here.) 

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.

In this study, electrical pulses were sent through the MSF (medial superior frontal) tract – a tract that runs parallel to the brain surface and interacts with vertical lobe neurons.  Simultaneously, recordings were made from neurons in the vertical lobe that could receive signals from the MSF tract.  What the experimenters were testing was whether they could induce LTP in octopus neurons by stimulating them.  This procedure is known to work in vertebrates, and is thought to be responsible for much of vertebrate neural plasticity (that is, 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.)

If you’ve read my previous post or another piece of writing about the squid giant axon, let me use this example to drive home its significance.  The techniques of neural stimulation and recording in this paper, as well as the theories that the authors employ about the structure and function of neurons, all descend directly from work done on the squid giant axon.  It really is a big deal.

So, with the basic experimental design and that little editorial out of the way, let’s hit the meat of the paper:

All of this groups work was done in vertical lobe slices; that is, they anesthetized the octopus by submerging it in a weak ethanol solution, removed a slice of its brain, and kept the brain slice alive in a solution of artificial seawater and antibiotics for a day before experimenting on it.

This figure shows the anatomy of the vertical lobe/MSF tract system.  To make it clear, if you imagine an octopus sitting on the ground, the octopus’s tentacles and mouth would be to the right of this figure, and its mantle would be to the left.

This figure shows the location of recording and stimulation electrodes.  The graphs are tracings of the voltage recorded by the recording electrode.  The authors identify two signals – the large one (TP) is from neurons in the MSF tract, and the small one after it (shown in this figure by arrow heads) is from the vertical lobe neurons that the MSF tract makes synapses with.  They are delayed in time simply because it takes some time for a signal to travel down a neuron.  In this case, the authors measured the size of each signal, measured as the maximum height of the tracing.

This is a summary of the results of this experiment.  After repeated stimulation, most of their test preparations showed a large significant increase in the strength of the synapse, meaning that the same presynaptic signal generated a larger postsynaptic signal.  This is a sort of weird graph, so let me explain it:  the horizontal axis shows the significant of the trial - the ones to the left are significant, whereas that group on the right is not significant (meaning they didn't actually show any change.)  The vertical axis shows how strong the synapse was after LTP-inducing stimulation, proportional to how strong it was before - that is, "2" means that the synapse is twice as strong after stimulation as it was before, "3" means it is 3 times as strong, etc.

In this figure, the top graph represents the size of the recorded signals in postsynaptic neurons of the vertical lobe (that is, field-type postsynaptic potentials, or fPSP.)  The bottom graph represents recordings from the presynaptic MSF neurons.  The arrows show the beginning and the end of LTP-inducing stimulation.  This figure is very informative, as it shows us that the synapse is indeed selective strengthened.  The presynaptic signal (TP – bottom graph) does not increase, but the postsynaptic potential (fPSP – top graph) becomes at least twice as strong as it was prior to stimulation.  To sum it up, the presynaptic signal stays the same, but because the synapses have become better at transmitting the signal, the postsynaptic signal is larger.

This is good evidence that LTP takes place in the memory system of the octopus brain, 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.)

Why does this study matter?  It implies that this specific type of organization and functioning of a memory system is somehow “special” – that is, it works so much better than an alternative arrangement that it was selected for in (at least) two independent cases.  In terms of studying octopus biology, it also means that the great wealth of information on vertebrate neural systems is likely to be applicable (at least in a modified form) to the study of cephalopod nervous systems.  In terms of studying vertebrate biology, it is possible that studying how this system work in octopus could give us new insights into the function of vertebrate memory systems.  Lastly, the methods used in this paper are just incredibly cool.  C’mon, people – keeping octopus brains alive in a bath!  Imagine how awesome it would be to explain your job to somebody at a dinner party if you were the experimenter.

If you read this and find yourself with any questions, or noticing any errors, please let me know.  I know this was a bit technical, but I think it’s misleading to present science as if it were possible to really grasp it without being at least a bit technical.  I think to really understand the importance of research like this, you have to understand the procedures used, at least basically.  In any case, I hope this post was informative and interesting.

Thanks for reading!

Detour Experiments in Octopus

Today I’ll review the earliest Octopus behavioral research study I could find (that is, except for a few very old papers in French, that I shamefully do not have the skill to read, although I am working on translating a few of them, bit by bit.) This is a study by Paul Schiller published in the Journal of Comparative Psychology in 1949, titled “Delayed Detour Response in the Octopus”. It’s a very early experiment on the ability of octopus to apply detours to a learned task (that is, you teach the animal to go somewhere for a reward, usually food, and then you put a barrier in its way. Depending on the character of the animal’s “intelligence”, it may or may not be able to successfully pass the barrier to get the reward.) If you have access to scholarly databases, you can probably get ahold of it (I got mine for Scirius, and I think Ovid has it as well) – unfortunately, I can’t link to a free .pdf of the article here.


Interestingly, Schiller begins his description of his methods by describing a procedure that does not work with octopus:

                    The conventional technique of using two inverted cans, one covering a baited, the other
                    an unbaited container, both of them previously exposed to the vision of the octopus, was
                    tried on 4 animals with rather discouraging results. Both cans were attacked and lifted indiscriminately
                    or, if not far enough from each other, simultaneously. This happened often
                    even in the preliminary stage when the covering cups were transparent. The tendency to
                    crawl in or lift up the containers was so powerful that the animal did not regard the bait at
                   all unless specifically trained to do so.

This makes a lot of sense – it turns out, as shown in this and later experiments on octopus, that their top performance in response-selection tasks is somewhere around 70-80% correct responses. They are “curious” enough that they will choose to investigate the “wrong” stimulus regularly. This makes sense for a foraging, active predator, who is more successful if they inspect many new areas of their environment than if they are entirely predictable.

Shown in this figure is the apparatus he settled on. The octopus is confined in the starting compartment and allowed to investigate a crab in a beaker through a screen. Then, the entrance door is opened, and the octopus learned to move through the opaque corridor to receive the crab. It was found that, after learning this, Schiller’s octopuses made 75% correct responses – well above chance (which is 50%, in this set-up.) Furthermore, Schiller found that the longer it takes the octopus to get through the corridor, the worst its chance of being correct. He also finds that, using a female whose reward is returning to her nest instead of a crab, that disorientation of her body posture by making her crawl through a small hole destroyed her ability to make the correct choice in the delayed detour task:

              It seems, with this one animal now under the more powerful motivation of her
              nest instead of food, that a delay of at least one minute does not interfere with
              the correct choice. The same amount of delay, however, if it involves disorganization
              of the bodily posture while in locomotion, prevents a successful delayed
              choice. There is no need to assume central representative factors for the delayed
              detour performance which, in the octopus, may be mediated by locomotional
              cues.

Basically, although we can explain detour performance in (for example) rats by showing that they probably have some flexible internal representation of the test space (see Tolman's discussion of cognitive maps for more information,) it appears that this same ability in octopus can be explained by intervening postural and sensory cues, without recourse to more complicated cognitive processes.

Thanks for reading!