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

Pass the clams, please: A strategy for object retrieval in the octopus.

          I recently blogged about a line of research on octopus reaching movements, but I left out an important study for time's sake.  I promised to cover it promptly, and so I'm making good on that promise here.  To recap:

          It has been shown that reaching movements by octopuses are controlled by the nervous system of the arms relatively autonomously from the central nervous system; that is, a severed arm can complete these movements on its own, given the proper stimulation of the base of the nerve cord of the arm.  This is all well and good, as the movements being studied were rather simple, involving the extension of an arm.  This reaching movement was found to consist of a propagating wave of muscle activity down the arm, a solution to the control of such a flexible appendage that greatly reduces the amount of neural computation that is needed to make such a movement.)

          The study I'll look at today is Octopuses Use a Human-like Strategy to Control Precise Point-to-Point Arm Movements by Sumbre et al (2006).  In it, the authors study a more complicated movement: retrieval of an object using a single arm.  This is a problem of interest to neuroscientists and roboticists, because understanding how the octopus generates controlled, precise movement using an appendage with so many possible movements might give us some insight into the optimal solution of this problem.  This could help reveal general rules of efficiency in neural programming.

          Beginning (as is advisable) at the beginning, the authors videotaped reaching movements in O. vulgaris that were elicited by touching a piece of food to the animal's outstretched arm.  Their observations revealed that octopuses create joints in their arm during this movement, around which they move stiffened segments of the arm much the same way that animals with a skeleton (notably, humans) do.  A series of stills from one of their videos and a lovely diagram of the arm "joints" of an octopus who is about to eat an unhappy-looking fish are shown below:

          It turns out that most of the distance covered in this movement sequence comes from the arm's rotation about the medial joint (the yellow one), much as most of the effective range of human retrieval movements come from flexion of the elbow.  After some more detailed analysis of the kinetics of this movement (which I'll skip here - but please check out the paper if you're interested) the authors go on to probe the pattern of muscle activity that is responsible for this movement as they did in their earlier studies (for a brief explanation of their methods, see my earlier post; for a longer one, see the paper reference therein.)  They found differences in patterns of muscle activity depending upon where in the arm recordings were being made.  When they looked proximally to the medial joint (eg. in arm segment L1 in the above diagram,) they found that a wave of muscle activity propagated away from the body, as it does in reaching movements.  When they recorded from a portion of the arm distal to the medial joint (eg. in arm segment L2,) they found that muscle activity was propagating in the opposite direction - from the tip of the arm towards the base.

          Using these results, the authors offer an elegant explanation of the neural control of this behavior.  They suggest that the initiation of a retrieval movement involves the initiation of two waves of muscle activity, one starting from the tip of the arm and one from the base.  Where these waves meet, the medial joint is formed.  In this way, the octopus nervous system simplifies the problem of finding an efficient way to retrieve an object, a problem which would be hard to solve on the basis of proprioception or neural control contingent on a direct representation of sensory space due to the flexibility of the octopus's body.

          In case you were wondering (I sure was), an experiment that was reported in the supplementary material (though it seems pretty important to me) revealed that retrieval could not be elicited in denervated arms in the way that arm extension can.  Therefore, it appears that some input from the central nervous system is required to initiate this more complicated movement, although it is still possible (and seems likely) that, once initiated, the movement is driven primarily by peripheral mechanisms local to the arm performing the movement.  The authors found that ablation of the anterior basal lobe left octopuses unable to initiate object retrieval, confirming its suspected function as a motor center in the octopus.

          The authors finally put forth a hypothesis of convergent evolution of reaching movements between octopods and vertebrates:

                    It is especially surprising that of all possible
                    geometrical structures and motor control strategies
                    with which a flexible arm can bring an object to the
                    mouth, the octopus generates a quasi-articulated structure
                    with a striking morphological and kinematic resemblance
                    to the multijoint articulated limbs of vertebrates.
                    Because the hypothetical common ancestor of cephalopods
                    and vertebrates dates back to the beginning of
                    Cambrian era (about 540 million years ago), fetching
                    appears to be a genuine and rare case of evolutionary
                    functional convergence, where two independent attributes
                    (morphology and neural control) coevolved to
                    achieve a common goal.

          I have no particular problem with this hypothesis.  I'd be interested to see if these types of movements are present in other species of cephalopods, both decapods and other octopod varieties.  I think that the case for convergent evolution in this instance will always be a hard sell, because (unlike, for example, the eye) the anatomy of the body parts executing the movement in question are so different.  In addition, the movements studied in this paper were elicited in very specific conditions using octopuses that were trained to extend their arms and wait for food to be pressed to them, leaving it unclear what other types of movements they might make in retrieving an object as well as the relative importance of the aforementioned quasi-articulated arm movements in the ethogram of the octopus.  Those criticisms aside, I'm willing to accept the idea that articulated movement really does represent a good solution to the problem of controlling movement, because it shows up across animalia so often and in so many forms.  Why wouldn't we expect the octopus to get in on the action?

          Thanks for reading!


Sumbre, G., Fiorito, G., Flash, T., & Hochner, B. (2006). Octopuses Use a Human-like Strategy to Control Precise Point-to-Point Arm Movements Current Biology, 16 (8), 767-772 DOI: 10.1016/j.cub.2006.02.069

Neuromuscular Dynamics of Octopus Arm Movements

I was planning on writing an article about cephalopod statocysts (and I still am; I've just had trouble deciding which pieces of research I want to cover and which I want to leave out) to continue on the theme of cephalopod sensory systems.  I've stumbled upon a line research that I just had to blog about, though, so I'm putting off the statocyst post even further.  The research in question is a series of studies by The Octopus Group at the Hebrew University of Jerusalem on the biomechanics and neural control of reaching movements of octopuses.  I read this research some months ago (before I was blogging,) and I was reminded of it while watching Twister (the resident E. dofleini at the Niagara Falls Aquarium) groping about in his enclosure.  I noticed that, as he moved his arms about, the movements almost always started with a bend near the base of the arm, which traveled out to the tip, becoming sharper and moving faster as it proceeded.  It looked for all the world like the way a wave travels through water (or, more geek-ily, the way one imagines spontaneous activity propagating in a spatially extended nervous system.)  The series of studies I will talk about here shows that this is generally the case, and characterizes the way that this happens with some detail, although we still do not know this system in nearly as fine detail as we know the vertebrate neuro-muscular system.  I'm getting ahead of myself, though.

Why do we care about the details of how octopuses move their arms?  First, it's just plain cool - who, upon looking at an octopus moving, hasn't wondered how it can possibly keep track of all those arms?  Second, the octopus arm provides a unique model nervous system for a few reasons.  It is a muscular hydrostat - that it, having no bones, it is a system of muscles that run perpendicularly to each other that maintain a roughly constant total volume; this property of an octopus arm allows it to function like a very flexible vertebrate limb because the muscles can pull against each other to form temporary, semi-rigid structures that allow the arms to bear weight.  As such, it is a novel motor system (in terms of research, that is,) with most of the well-characterized motor systems we know of (ie. human, primate, reptile, etc.) are composed of skeletal muscles, which pull against bones.  Besides this, the task of coordinating the movement of eight almost infinitely flexible arms is a herculean task in terms of neural processing, and it would be very informative (as well as a triumph of systems neuroscience) to understand how this is done.  It has been thought, since the early days of octopus neuroanatomy, that much of the movement of the octopus's arms (and probably those of other cephalopods) is encoded in the nervous system of the arms rather than in the central nervous system (Graziadei, 1971).  This is evidenced by the fact that there is no straightforward representation of the arms in the brain of the octopus, as there is in humans and most other vertebrates, as far as we know, and so it is unlikely that fine motor control comes from the central nervous system.  Supporting the importance of the distributed nervous system of the arms is its incredible scale: the nervous system of the arms is much larger than the central nervous system of octopus, containing around 2/3 of all of the neurons in the animal.  The octopus arm, then, is a unique example of a highly complex, distributed motor system that stands in contrast to the centrally controlled motor systems we are most familiar with.  As with almost every topic in comparative neuroscience (I'm a big sucker for it), I think that the octopus motor system is important because by understanding it, we will understand more about vertebrate nervous systems; that is, we will (pretending for a moment that we could actually solve both systems) understand which features of them are critically related to the specifics of vertebrate and invertebrate neural functioning, physiology, development, and ecology.  We would come closer to understanding why each system evolved the way it evolved.  Finally, we would exercise our tools of modeling neural computation in a way that would allow us to figure out how generalizable they are.  My final verdict: this is a good thing to study.

So now you're bored.  You want to hear about some research!  Well, I won't disappoint; at least, I hope I won't.  We'll start with Gutfreund et al. (1998), one of the early papers out of this research group, which kicked off this line of research by examining the neuromuscular dynamics of octopus reaching movements.  I should note that (presumably for simplicity,) this group generally only studies reaching movements in a single arm - it is not know exactly how their findings might relate to more complicated movements, including those involving multiple arms.  As a disclaimer I am going to leave out description of a large portion of their study, which I encourage you to read in full, for my own convenience, and only present the results that I think are most relevant to the topic at hand.

This authors in this study used electromyography (a method of measuring the electrical activity of muscles) in O. vulgaris to determine how arm muscles are activated in sequence to produce octopus reaching movements.  Briefly, they put electrodes through two points in a single arm of their (anesthetized) test animals, then allowed the animals to wake up and elicited reaching movements by tempting the octopus with either a crab or a target that was associated with food.  They videotaped the reaching movement, which allowed them to compare the electromyogram to the behavior of the octopus.  Reproduced below is their first figure, showing the gross cross-sectional anatomy of the octopus arm, as well as their electrode placement:

The white arrows indicate the position of the electrode, which is the white line running through the muscle.  The striated outer portions of the arm are the muscle, and the round shape in the middle is the nerve cord of the arm.

They found that reaching arm movements usually start with a sharp bend near the base of the arm, which travels outwards until it reaches the tip, accelerating somewhat throughout the extention and then slowing as the arm reaches its target.  Here's a series of images showing the behavior:

 

The authors found that this type of arm extension occurs virtue of a propagating wave of muscle contraction traveling down the arm, from the base to the tip.  Shown here are examples of the type of data they used to confirm this:

The left panel shows two electromyograms from a single trial, the top one from the electrode nearer to the arm tip, and the bottom one nearer to the base of the arm.  The arrows indicate when the bend in the arm reached each electrode.  As is apparent, neuromuscular activity at the proximal site started earlier than that at the distal site, coinciding approximately with the timing of the movement of the bend in the octopuses arm.  The graph shows the correlation between the lag in the electromyogram record between the two sites and the time it took for the bend in the arm to move between the two sites.  It's clear that the propagation of the wave of electrical activity down the arm is highly correlated with the motion of the arm.  The authors continue on to characterize some of the properties of these arm movements in more detail and propose a mathematical model for the movement of the octopus arm, but I'll leave those results out, here.  I recommend this article for it's methodological clarity - too seldom do authors take such pains to make their method so clear and so thoroughly address their research question.

Moving on, the same reearch group (with a different first author) published a paper in Science describing their experiments with isolated arm preparations (Sumbre et al. 2001).  This is where it gets really interesting to me, because this experiment really gets at the distinction between central and peripheral motor control.  The authors made their preparations by either denervating one arm of an octopus that had already been decerebrated (a procedure somewhat akin to an octopus lobotomy) by severing its connection to the brain, or by severing an arm completely.  They then attached the base of the arm to a surface, and stimulated the nerve cord at the base of the arm.  It was found that, in a large percentage of cases (46%, to be exact,) the movement resembled the reaching movement seen in an intact animal.  The figure below (taken from the paper) shows the reaching movement of a normal animal (on the left) and that elicited by stimulating the nerve cord of a denervated arm in a decerebrate animal:

Importantly (for reasons I'll explain in a second,) it appears that the arm movements were initiated, but not sustained by the stimulation.  We can tell the difference because the "reaching" movement continued through to completion even when it began slightly after the experimenters stopped stimulating the arm.  This shows that the brief stimulation started a motor process that was maintained by the intrinsic neuromuscular system of the arm.  The authors also found that similar movements could be elicited in amputated arms by "tactile stimulation of the skin or suckers."  After a brief analysis of the kinetics of the evoked movements, the authors conclude that they, like those of intact animals, are caused by a propagation of muscle activity down the arm.

The authors' conclusion:

          "The division between the central and
          peripheral levels of the octopus motor control
          system resembles the hierarchical organization
          of motor control systems in other
          invertebrates and vertebrates, even
          though in the octopus it uniquely serves as
          an important component in a goal-directed
          voluntary movement rather than in rhythmical
          or reflexive behaviors."

The peripheral nervous system of the octopus appears to play a much greater role in the programming of movement that does the peripheral nervous system of vertebrates (which can only independently control simple reflexes and some other involuntary movements like peristalsis), even to the extent that it can execute complex movements (like reaching as if to grasp) all by itself.  That propagating wave of muscle activity isn't coordinated by the central nervous system, like coordinated movements are in humans; rather, it's coordinated by the nervous system intrinsic to each arm.  This is convenient for the octopus because it means that it generally does not need to keep track of its arms (that is, it's central nervous system doesn't have to spend a lot of resources monitoring and controlling them) because they largely take care of themselves.  It's a good solution to the problem of having a large number of incredibly flexible appendages.

The exact extent of the arm's abilities to coordinate their own motor activity, as well as activity between arms, remains to be uncovered by more and more detailed experiments on a variety of types of movement, but the general conclusion seems pretty solid to me, and fits nicely with what we know about the neuroanatomy of the octopus.  It would also be interesting to see the results of similar studies in other cephalopods.  I have a sneaking suspicion that one could relate the extent of the peripheral nervous system's "motor autonomy" from the central nervous system to the complexity of arm movement required by a given species' lifestyle.  It would be a neat idea to explore (if I had a laboratory on the Italian coast and a million-dollar grant to study squids.  I can dream, right?)

There's one more article I wanted to cover here, but I don't have time at the moment, and I want to get this up tonight.  It's by the same group, and it applies what the previous studies showed to explain the way that octopuses retract their arms after they have grasped their target.  Hopefully I'll have a shorter post on that before the end of the weekend.

As always, thanks for reading!


Sumbre, G. (2001). Control of Octopus Arm Extension by a Peripheral Motor Program Science, 293 (5536), 1845-1848 DOI: 10.1126/science.1060976


Gutfreund Y, Flash T, Fiorito G, & Hochner B (1998). Patterns of arm muscle activation involved in octopus reaching movements. The Journal of neuroscience : the official journal of the Society for Neuroscience, 18 (15), 5976-87 PMID: 9671683

Graziadei, P.P.C. (1971). The nervous system of the arms. pp. 44-61 in Young, J.Z. The Anatomy of the Nervous System of Octopus vulgaris. Oxford : Clarendon Press.

The Octopus Visual System

Visual neuroscience is one of my favorite sub-fields of neuroscience.  As such, I've decided to write this post about the visual system of Octopus (I will leave out squid and cuttlefish for this post, but theirs are, overall, pretty similar.)

First, let's take a look at the anatomy of the octopus eye, in comparison to a more familiar specimen (this figure is from Ogura et al., 2004):

They look pretty similar, don't they?  Notably, the both have lenses to refract light, a pupil to restrict incoming light, a retinal surface distributed along the back of the eye, and a well-differentiated optic nerve.  There are, however, a few notable differences.  Firstly, octopus eyes and human eyes focus light on the retina in different ways.  Humans bring their eyes into focus by changing the shape of our lenses using the cilliary muscle.  Octopuses, on the other hand, focus by moving the lens closer to or further away form the retina.  This is the same way that a camera focuses, and so people often refer to cephalopod eyes as "camera eyes."  Secondly, the octopus retina is very different from the mammalian retina in terms of its cellular organization.  Let's look at an octopus retina first (From Young, 1962).

Light enters the eye from the top of this image.  In terms of cellular organization, the limiting membrane (l.m.) at the top of the figure faces towards the front of the eye, with the basal membrane (bas. m.) towards the back of the retina.  The distal segments (dist.) of photoreceptors face the front of the eye, and the proximal segments (pr. s.) sit at the back of the retina, from which they send their projections through the optic nerve to the optic lobes where further processing of visual information takes place.

To compare, let's check out a few diagrams of the mammalian retina (both of these are taken from Wassle and Boycott, 1991 ; the first is their own drawing, and the second is by Ramon y Cajal, one of the early geniuses in neuroscience):

Now, you wouldn't know this from looking at the figures, but the mammalian retina is actually opposite in orientation from the cephalopod retina.  That is, light enters from the bottom of these drawings, and passes through several layers of cells before reaching the photoreceptors at the back of the retina (the photoreceptors in these images are rod-like structures at the top of the images, labeled as "OS" and "IS" (for "outer segments" and "inner segments", respectively) and as "e" and "f" in Cajal's drawing.)

You, being a smart and inquisitive reader, might just now be asking, "what is all that other stuff doing in our retinas?"  It turns out that it processes the information that photoreceptors gather about patterns of light right there in the retina.  By the time retinal ganglion cells (labeled "GLC" and "c" or "d" in the two images) leave the mammalian retina, they are already carrying information about properties of the visual environment, such as where isolated, high contract areas of light are and what the overall level of illumination in different areas of the visual field is.  This information can then be used by visual processing centers in the brain to very efficiently form a picture of the visual world, including identifying objects and movements.

At first glance, it looks like the octopus must miss out on some visual processing power because it does not have this visual processing system intrinsic to the retinal.  It makes up for this, however, in a very interesting way.  Let's look on this drawing of the organization of the octopus optic lobe - the place in the brain where the octopus optic nerve ends up (this image is from Young 1961):

Young makes the argument, in this paper, that the outer layers of the octopus optic lobes (which are nestled right under the eyes and directly innervated by the retina) are likely to have the same function of the complex layers of the mammalian retina, with the deeper layers analagous to mammalian central visual processing areas.  As far as I know, there has been no direct test of this hypothesis.  However, it is an interesting and (at least circumstantially) sensible idea, and it must be the case that octopus forms perceptions of objects and movements are some central location - after all, it does have rather good vision.  The optic lobe, after presumably making sense of what the retina is saying, sends projections all over the superoesophageal mass of the brain.

Why does this matter?  Who gives a bibble?  Well, the evolution of the eye as long been used as a sort of model evolutionary system, that is, we can probably use it to understand evolution, and to determine what principals of genetic selection drive evolution.  It's suitable for this for a few reasons: we have extant animals with a variety of types of eyes, we have extant animals with no eyes, the eye is relatively accessable and easy to study (and so the eyes of mammals and insects has already been extensively studied.)  As a discrete organ, it is also a relatively straightforward task to characterize morphological and cytoarchitectural variation between species.  In addition, the visual system is interesting to study because it is one of the few systems that we can understand and model on a fine-grained computational level.  Thus, the differences between cephalopod and human eyes are important because they can not only tell us things about how our eyes work, but help us figure out the way that evolution works.  Evolution, as we know, is important to understand because it is, quite literally, the reason we are here.

It seems that there is probably an optimal organization to any visual processing system that involves progressive layers of increasingly synthetic processing of incoming visual information (for example, the progressive abstraction of visual input in humans from light to features, and then to objects and scenes.)  This point is strengthened by the fact that insects appear to share this style of processing visual information, albeit with anatomical differences (for more information on insect visual processing, see this paper).

 I had wanted to talk about the vestibular system of cephalopods in this post as well, but it's already lengthy and I need to get going.  I'll do a post on it another time.

Thanks for reading!

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!