HIPPOCAMPUS 25:682–689 (2015)
COMMENTARY
The Mantle of the Heavens: Reflections on the 2014 Nobel Prize for Medicine or Physiology Richard G. M. Morris*
ABSTRACT: The award of the Nobel Prize in Medicine or Physiology in 2014 for the discovery of place and grid cells was both a personal award to three great scientists and also a mark of the maturity of systems neuroscience as a discipline. This article offers both personal and scientific reflections on these discoveries, detailing both how getting to know all three winners had an impact on my life and the research questions that we shared in common work together. It ends with brief C 2015 Wiley reflections on three important outstanding questions. V Periodicals, Inc.
KEY WORDS: associative memory; grid cells; place cells; spatial learning; synaptic plasticity
winners acknowledged their debt to Edward Tolman and his concept of the “cognitive map” (Tolman, 1948). Both discoveries were soon confirmed by others and later established in other species than the rat including humans. It was fitting that accounts of the Nobel Prize in the media should speak of the discovery of “the GPS system of the brain” (http://www. bbc.com/news/health-29504761), though in doing so they tied the discovery firmly to navigation rather than memory.
PERSONAL REFLECTIONS INTRODUCTION When I first heard the news of the 2014 Nobel Prize by an email from Yadin Dudai, I was delighted. There was quickly a happy buzz around the web for this was a popular choice of winners, and a sense that systems neuroscience was also being recognized. I said to myself, to others and later wrote in print: “ Three great friends, three great scientists”. Given this immediate reaction, and focusing almost exclusively on animal research, I shall present both personal and scientific reflections on the award. These merge one with another for their intersection is the joy of life as a scientist—living and breathing what we do. The discovery of “place cells” in London was a fascinating one, and John O’Keefe had the wisdom to realize he had observed something of potential importance. He followed it up and their study became his life’s work, beautifully described in his Nobel Lecture (http://www.nobelprize.org/ mediaplayer/index.php?id=2413). The later discovery of “grid cells,” by Edvard and May-Britt Moser in Trondheim, added a critical and surprisingly hexagonal metric to the puzzle about how mammals recognize their location in space and navigate through it. At the Nobel Lectures in Stockholm, which I had the enormous privilege of attending, all three
Centre for Cognitive and Neural Systems, Edinburgh Neuroscience, The University of Edinburgh, 1 George Square, Edinburgh, United Kingdom *Correspondence to: Richard G. M. Morris, Centre for Cognitive and Neural Systems, Edinburgh Neuroscience, The University of Edinburgh, 1 George Square, Edinburgh, EH8 9JZ, UK. E-mail: [email protected] Received 6 March 2015; Accepted for publication 17 March 2015. DOI 10.1002/hipo.22455 Published online 19 March 2015 in Wiley Online Library (wileyonlinelibrary.com). C 2015 WILEY PERIODICALS, INC. V
A First Meeting I first met John O’Keefe, with Lynn Nadel, in the spring of 1974. I had just finished a D.Phil. in Experimental Psychology at the University of Sussex and wanted to work on the brain—and it was suggested that I visit them at University College London. It was fascinating talking together and an encounter that was to change the direction of my scientific career. I was impressed by their repeated assertion that space mattered, an assertion that was later to find expression in the famous first three sentences of their 1978 book: “Space plays a role in all our behavior. We live in it, move through it, explore it, and defend it. We find it easy enough to point to bits of it: the room, the mantle of the heavens, the gap between two fingers, and the place left behind when the piano finally gets moved” (O’Keefe and Nadel, 1978). Their perspective on learning was refreshingly different to what I had learned both at Cambridge as an undergraduate and at Sussex as a graduate student. Not that Sussex was backward looking—far from it. It was an immensely stimulating place to study, I shared an office with Anthony Dickinson now at the University of Cambridge, and both of us were exposed to new ideas from our supervisors and others—notably sabbatical visitors, such as Herb Terrace (Columbia) and Bob Rescorla (Pennsylvania). Indeed, it was during our time as graduate students that the famous Rescorla and Wagner book chapter was published and a new
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FIGURE 1. Place cells, quantification, and the watermaze. A) The discovery of place cells was put on a firm quantitative footing in various ways, starting with O’Keefe’s own experiments, but notably by Muller et al (1987). Their multicolored pixel approach was soon imitated by others. Quantification is vital in science and a semantically clear method of visualization is an ideal and accessible method of conveying information (from Muller et al., 1987). B) The conceptual link between a “hot-spot” in a uniform arena
that corresponds to the firing field of a place cell and the location of a hidden platform in a watermaze is straightforward (Morris, 1981). C) The watermaze has been used to test numerous hypotheses, as a screening test for cognitive enhancers and to explore a range of basic science and translational issues including the role of N-methyl-D-aspartate receptors in spatial learning. (Morris et al., 1986a, 1986b). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
era of thinking about the role of “expectancy” in reinforcement for learning was ushered in (Rescorla and Wagner, 1972). The animal learning framework was, however, one in which associative processes, the fundamental glue that enabled learning to occur, operated in a kind of vacuum. There was little sense then, as I remember it, that learning happened in a context, still less a spatial context. So O’Keefe and Nadel’s challenge to this framework in the form of dissociable “locale” and “taxon” processes was refreshingly new. John showed me his in vivo recording laboratory and was then trying to be responsive to the skepticism of others who were questioning the existence of place cells. This was over 40 years ago, methodology was more limited than now, and he was planning a study with a still camera that, with a long shutter time, would simultaneously capture the movements of the animal (a flashing light of one color on the animal’s head) and of the firing of a place cell (a different color). He conveyed a sense of wanting to be rigorous about what he believed was an important discovery. The late Bob M€uller, with John Kubie and Jim Ranck, was later to take quantitative monitoring of place cells to new levels of sophistication (Fig. 1A; Muller et al., 1987). Not being an electrophysiologist, I wondered what kind of behavioral experiments on spatial learning using lesions or other manipulations could best test the idea of “place” as something not determined by local cues, unlike conventional discrimination learning, and requiring the integrity of the hippocampus. In John’s view, the sense of location was worked out by a dedicated brain system that used information from diverse sensory modalities. Lynn had a circular track in which he could deliver water reward at various places, and he went on to establish the deleterious effects of fornix lesions in this apparatus (O’Keefeet al., 1975), but I was unsure about this approach—as it did not really let the animal navigate. I
was on my way to a Research Fellowship at the University of Durham and, arriving there in 1974, I set up a large circular apparatus with an uniform floor that could be rotated, and with food delivered at various places in it that the animals had to find—but the task did not work very well. Nor was my time in Durham as enjoyable as life had been in Sussex and so I left academic life for a period. I was not long away and, in 1977, found myself fortunate to be appointed to a Lectureship in Physiological Psychology (sic) at the University of St. Andrews.
The Watermaze After I had got my first-year undergraduate lectures underway, the key research issue for me remained that place cells fired where they did irrespective of local cues. They could not be strictly sensory cells, whether unimodal or polymodal, and they had to depend on some kind of spatial memory processing. How to get at this key concept? I also wanted to study the possible relationship between learning and plasticity, rather than just spatial navigation. To achieve these aims, I reasoned that I had to get rid of local cues completely but in a true learning task and one in which I could, in addition to lesions, manipulate neuronal plasticity. The department had no animal facilities and so I was assigned laboratory space in the remarkable but somewhat antiquated Gatty Marine Laboratory located on the West Sands of St Andrews’ north facing and often bleak shoreline—where the opening shots of the film “Chariots of Fire” was soon to be filmed. It was a slightly strange place to work, quite apart from not infrequently having to battle my way down the shore path through the winds of a northerly winter gale that had blown in from Russia. Once indoors, I got to my laboratory past tank Hippocampus
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after tank of sea creatures of various shapes and forms, some of whom might have been the subject of the former Director Graham Horridge’s recently completed studies of invertebrate interneurons (Horridge, 1968). One day, it occurred to me that rats would be able to learn while swimming and that this might help solve the local cue problem. I knew rats could swim, but I wondered if they could learn to escape from water at a specific place onto a platform that was hidden beneath the water surface and so was neither visible nor audible, offered no olfactory cues, and could not be identified using somatosensory cues until the animal had already got to it. This might be the solution to the local cue problem. The first “watermaze” was built from hardboard and fiberglass yacht resin by myself with the help of Chris Barman, an animal technician. We completed it in the workshop over the weekend, these being the days when staff still had access to workshops at weekends and health and safety officers were still over the horizon. To my amazement and delight, the rats learned the task very quickly (Fig. 1B). I ran some essential control conditions and a paper on “place navigation” followed soon (Morris, 1981). The discussion in that paper reflects my “animal learning” background and while I did not pursue it, John Pierce (who was also a graduate student at Sussex) has latterly gone on to conduct imaginative studies exploring the possible associative basis of spatial learning (Pearce, 2009). The observation that spatial learning in the watermaze is severely impaired by hippocampal lesions was made a year later—in collaboration with Paul Garrud, John O’Keefe, and Nick Rawlins (Morris et al., 1982). Paul and I tracked the animals by tracing a path with a felt tip pen onto clear film that we had taped over a video monitor. A year or so later, the British Broadcasting Corporation (BBC) introduced the “BBC Computer” with 128 K of random access memory and an easily learned software language called BBC Basic. I wrote a little program and, using a commercially available tracking device that John O’Keefe had by then started to use from HVS Image C , we were soon able to follow the paths of the swimming rats V directly. By tracking the black head of a hooded rat, we even managed to do without the little “police car” lights attached to the animal. This was a step toward better objectivity for, to my knowledge, studies of spatial learning had hitherto relied on an observer reports. A series of lesion studies followed, in which we also observed deleterious effects of both fornix lesions and retrosplenial lesions encompassing the entorhinal cortex (Schenk and Morris, 1985; Morris et al., 1986a, 1986b). However, a few years later, we found that rats with hippocampal lesions could learn a spatial reference memory task quite well if either minimal flexibility was required in the navigational path (Eichenbaum et al., 1990) or extensive overtraining was given (Morris et al., 1990).
Using Watermaze To Study Synaptic Plasticity And Memory I presented the earliest findings at what came to be known as the “Schloss Hippocampus” meeting of 1982. This was a Hippocampus
meeting at a castle in southern Bavaria owned by the Max Planck Society at which, in the views of many, the hippocampal field was to change direction irrevocably (Siefert, 1983). Until then, work had been very much on the “septohippocampus” with particular emphasis on the cholinergic and other inputs from the mid-brain. It was at this meeting that we first heard Carol Barnes describe her tantalizing observation that the persistence of long-term potentiation (LTP) correlated with the persistence of memory in her circular arena task earlier (Barnes, 1979). While there, I met Gary Lynch who described work confirming the homosynaptic nature of the synaptic change in LTP when studied in hippocampal slices in vitro, the role of calcium in induction, and the structural changes in spines viewed at the electron microscopic level. It was immediately apparent that LTP was much more than a persistent change in synaptic efficacy induced by tetanic stimulation, as (Bliss and Lomo, 1973) had described 10 years earlier. It was also a change that was associated, at the point of induction, with an ionic current different from that used to mediate normal synaptic transmission and a change expressed in a manner that could have the very storage capacity required of Marr’s (1971) network model incorporating the Hebb synapse (Marr, 1971; Willshaw et al., 2015). The following year, Lynch and Baudry produced their remarkable Science paper, collating together all the work of the Lynch laboratory, and in which they proposed that “glutamate receptors” (sic) were inserted into membranes to express the enhanced synaptic efficacy (Lynch and Baudry, 1984). If this concept has a contemporary ring to it, bear in mind that the paper is now nearly 30 years old. It is not cited as often as it should be, perhaps because a cardinal plank of their evidence turned out to be changes in glutamate transport rather than in the expression of the synaptic receptor. However, the idea of a simple postsynaptic mechanism to express the change in synaptic weights had already emerged, albeit later to be challenged by observations of presynaptic changes. Current debates on AMPA receptor trafficking have not moved on conceptually so very far from these early ideas, even if the techniques available now are spectacular by comparison to what was around then. I resolved to go and work with Lynch and was fortunate to be able to do so in 1984, courtesy of a Medical Research Council (MRC) Fellowship scheme that released University teaching staff to focus on research for a while. In this, and many other ways, I owe a huge debt to the MRC. Lynch’s laboratory was then working on a range of projects—my first real taste of a “neuroscience laboratory”—including a serine protease inhibitor called leupeptin that inhibits the proteolytic mechanism that he and Michel Baudry had implicated in the glutamate receptor insertion process. In laboratory experiments on olfactory learning conducted in the Irvine laboratory, I had mixed success, possibly because we were using the very discrimination learning tasks that were proving insensitive to hippocampal lesions in rats and primates. Ursula Staubli was later to have success in using this drug to block LTP (Staubli et al., 1988), but its effects on learning were generally quite modest in the watermaze (Morris et al., 1987). However, while in
THE MANTLE OF THE HEAVENS Irvine and contrary to the “house rules” that reflected the friendly rivalry between the Lynch and Cotman laboratories, I discussed these experiments with Eric Harris, then a postdoctoral with Carl Cotman. He drew my attention to the recently published paper of (Collingridge et al., 1983) on the role of the NMDA receptor in LTP and the drug he used called AP5. Unfortunately, it was time to go home, but Gary and I discussed some experimental options for when I got back to my laboratory. Upon returning to St. Andrews, Jeff Watkins at Bristol University kindly made available a small supply of the racemic mixture of an NMDA antagonist (DL-AP5) and I began work. At that point, no one knew whether AP5 would work in vivo or, indeed, be very effective in crossing the blood–brain barrier. Its chemical structure did not augur well in this regard. Accordingly, using the same osmotic mini pump procedure with intracerebral administration that had been tried in Irvine with leupeptin, I did some acute in vivo experiments on dentate LTP. These were exactly as Bliss and Lømo had described, but now in the rat rather than the rabbit. After chronically infusing DL-AP5 or saline for several days, the blockade of LTP in vivo was complete, across a range of test pulse intensities, and without any apparent effect on baseline synaptic transmission. I was amazed and excited. Obviously, the next step was to try this in swimming rats and to my delight, Elizabeth Anderson and I found that rats treated with the drug were unable to learn the reference memory spatial version of the watermaze (Fig. 1C). Those given saline or the inactive isomer, L-AP5, were unimpaired. Strangely, we did not work with D-AP5 at that stage. I cannot remember why. Concerned that the deficit with DL-AP5 might be sensory in nature, I deliberately tried a simple discrimination task of the kind that animals with hippocampal lesions can learn and we observed, now with a mounting sense of disbelief, that they could. Both behavioral experiments were replicated “blind” with respect to drug assignment. Thus, chronic intraventricular infusions of DL-AP5 at a dose sufficient to block LTP in vivo without affecting fast synaptic transmission in the hippocampus caused an apparently selective impairment of hippocampal-dependent place navigation. The animals could see, could move around properly, and could learn another difficult task, but they could not find their way in a task that needed place cells and apparently required NMDA receptor-dependent LTP. Gary Lynch came to St. Andrews to help write the paper that was published in 1986 (Morris et al., 1986a, 1986b). Graham Goddard sent me a letter (such were the days!) indicating that his acceptance of our results required a certain “ suspension of disbelief,” but his News and Views piece in the journal was generous. The ECONOMIST magazine identified our work as an important. In the same year, Bruce McNaughton took a complementary step forward by also establishing the causal role of activity-dependent synaptic enhancement in learning in a different way—showing that prior physiological saturation of LTP impaired subsequent spatial learning (McNaughton et al., 1986). In 1986, I moved to Edinburgh.
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A Scandanavian Encounter The field of neuroscience was beginning to coalesce, not just in America, but elsewhere. In 1990, I found myself at a meeting in Stockholm of the then “European Neuroscience Association” (which later became the Federation of European Neuroscience Societies—FENS). Wandering the posters, I spied a young couple (a very young couple!) standing in front of a lovely pink poster about the impact of dorsal versus ventral lesions of the hippocampus on spatial learning. Only dorsal lesions worked. We had a great discussion and I was simply bowled over by the enthusiasm of these two young folks. They were of course, Edvard and May-Britt Moser, then working as graduate students with Per Andersen in Oslo. While encouraging them to publish it, which they did (Moser et al., 1993a, 1993b), there was—I tentatively suggested—a little problem. Perhaps dorsal aspiration lesions would have de-afferented the ventral hippocampus from vital inputs from the septum, whereas ventral hippocampal lesions would not have the same effect on the dorsal hippocampus. The dorsal/ventral dissociation may be misleading. As I was by then exploring the impact of fiber-sparing ibotenate lesions, introduced to this approach by Len Jarrard, I suggested we repeat the study in Edinburgh using this lesion technique. This confirmed their earlier result and revealed that a “minislab” of just 20% of the dorsal hippocampus was sufficient for allocentric spatial learning (Moser et al., 1995). Edvard and May-Britt Moser came to Edinburgh as postdoctoral fellows in 1994 and stayed until 1996. They brought two very young daughters and a large blanket, and the former played on the latter in the lab. They had completed studies in Oslo revealing a reciprocal impact of brain temperature on field-potentials and population spikes in freely moving rats (Moser et al., 1993a, 1993b), but wanted to do further work exploring the possible relationship between activity-dependent synaptic plasticity and learning. It so happened that the finding that LTP saturation impaired learning (McNaughton et al., 1986) was proving controversial and we wondered if the conflict in the literature was due to insufficient LTP induction in some studies. We devised an electrode array that spanned the entire angular bundle of the perforant path and switched around the anode and cathode on successive episodes of highfrequency LTP-inducing stimulation. The effect was a more complete LTP saturation in most animals. Strikingly, animals in which LTP was successfully saturated failed to learn the watermaze, whereas those in which there was some residual plasticity could do so (Moser et al., 1998). Toward the end of their stay in Edinburgh, Edvard and May-Britt secured the offer of two positions at the Norwegian University of Science and Technology in Trondheim and were resolved to start their own laboratory. However, they were both keen to learn the technique of multiple single-cell recording in hippocampus and suggested going to John O’Keefe’s laboratory for the last part of their stay in Britain. We all thought this was a great idea and off they went. In truth, their short stay in O’Keefe’s laboratory proved fundamental for all that followed Hippocampus
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and was, after their earlier work with Per Andersen, another defining moment in their career—arguably more so than their time in Edinburgh. Once in Trondheim, they set up a laboratory, first in Psychology and later the Medical Technical Institute and began to study place cell activity in spatial learning tasks including the watermaze. A range of important studies were published at that time, notably Vegard Brun’s study revealing virtually no effect of CA3 lesion on place cell fidelity in CA1 (Brun et al., 2002). An important further development was the creation of the Centre for the Biology of Memory (CBM) to which they invited a number of international staff for short periods every year over a 10-year period. These included Carol Barnes, Bruce McNaughton, Randalf Menzel, Ole Paulsen, Alessendro Treves, Menno Witter, and, fortunately, myself. Our visits to Trondheim were very exciting, and each of us would be pounced by the young Masters and PhD students who would show us their data. We would also present seminars, and engage in discussions and collaborations between our respective laboratories. It was an exciting time. One of these young students, a former marine biologist, was Marianne Fyhn. I was fortunate to be in Trondheim soon after Edvard, MayBritt, Marianne, and Sturla Molden started single-unit recording the medial entorhinal cortex in a particularly large square arena. One day, Marianne came along quite excited by some data suggesting that cell firing formed a pattern. Entorhinal cells did not just fire in one position, but in several positions across the arena that did not look random. Edvard and MayBritt studied the data and suggested that she try rotating the cue card at the side of the arena. An hour or two later, she came back, and showed us that the pattern had now rotated by the same angle as she had rotated the card. It was a magic moment, Edvard and May-Britt were mesmerized by this additional observation, and so they embarked on the first systematic series of experiments identifying the “grid cell” pattern of medial entorhinal neurons (Fyhn et al., 2004; Hafting et al., 2005). I was not involved at all beyond having been a privileged early observer, and my interests were by then developing toward the study of associative “schemas” in collaboration with Emma Wood (in Edinburgh) and Menno Witter (in Amsterdam, but also part of CBM). However, it is important to note that just as John O’Keefe had the wisdom to realize he had discovered something important, Edvard and May-Britt did also—and they began a series of quantitative studies of the phase, scale, and orientation of grid cells. These revealed that all phases are represented within small cell clusters of entorhinal neurons in a “salt-and-pepper” organization (Hafting et al., 2005) and that the scale of the grid cell map increases metrically as recordings are made from more ventral locations of the medial entorhinal cortex and that the transitions between these are step-like (Stensola et al., 2012). More recent work has revealed that different grid maps display rigidity as one moves from one recording room to another, that they develop during infancy over a specific time course, and that the small set of frequently observed orientations of grid maps may arise because they reflect not just Hippocampus
genetic determinism but the cardinal axes of any local environment. It would have be wonderful if the ratio of the step-like transitions between discrete entorhinal modules observed nature’s beautiful golden ratio (1.618), but it turns out to be smaller (1.421) and for reasons which theoretical studies now suggest may enable an optimum organization. Comparative work has revealed grid cells in other mammals, including humans, with bats echolocating their way into picture via Ulanovsky’s finding of grid cells in bats in the absence of the hippocampal theta rhythm (Yartsev et al., 2011). This body of work is beautifully described in Edvard Moser’s Nobel lecture (http://www.nobelprize.org/mediaplayer/index.php?id=2415). Taken together with the earlier discovery of place cells, one can appreciate the physiological beauty of this spatial navigational system—with its array of distinct cell types, not just place and grid cells, but of head direction and boundary cells also.
SCIENTIFIC REFLECTIONS So where are we now? Hundreds if not thousands of papers later, reflecting developing concepts in both navigation and memory, technical improvements in recording technology, intriguing new experimental paradigms and, through these, new studies of both the spatial and nonspatial properties of hippocampal neurons, is everything sewn up? Far from it! There remains, in particular, a major divide in the hippocampus community between those focused on spatial memory and navigation, and those studying memory more generally (Eichenbaum and Cohen, 2014). While the Nobel Prize has been given for work in the former category, it is important to recognize a wider picture relating to hippocampal involvement in memory. There are numerous outstanding questions and space permits that I focus briefly on only three: 1. How do place cells enable navigation? 2. Do hippocampal neurons represent nonspatial and spatial attributes? 3. What is the place of context in episodic-like memory?
How Do Place Cells Enable Navigation? The bottom line of any answer to this question is: we do not know. Clearly, the existence of a medial/temporal network of spatially responsive cells in the entorhinal cortex, hippocampus, postsubiculum, and thalamus strongly supports the general concept of there being a “cognitive map.” These neurons are active when animals navigate. Maps are for navigation, ergo these cells must be the basis of spatial navigation. I can buy into this, but I am not alone in having early on raised a conundrum about the mechanistic role of place cells in navigation (Morris, 1989). Place cells are defined as cells that fire at specific locations— so one set of cells will fire preferentially at place A, another at Place B, respectively. But suppose an organism wishes to navigate from Place A to Place B while avoiding Place C. It could head off from Place A randomly and, using the cells
THE MANTLE OF THE HEAVENS corresponding to Place B, somehow recognize when it has got there, hoping not to arrive at Place C. But data from the watermaze indicate that rats can head from A (a start point at the side of the pool) to B (a hidden platform) using relatively direct paths. It follows that the animal can access information about Place B while still at Place A, but such firing appears to violate the definition of place cells as cells that fire “if and only if” the animal is at that place. One way to get round this conundrum is to suggest that place cells usually respond at their respective firing field, but may occasionally fire at other places also. David Foster addressed this issue in his PhD thesis completed in both Edinburgh and MIT, eventually developing a temporal difference algorithm to compute the path to a familiar target location in an arena not dissimilar to a watermaze (Foster et al., 2000). Recording studies by others have suggested that anticipatory firing can sometimes be observed, such as at the choice point of a maze where forward scanning of place fields further on is sometimes observed during the vicarious hesitations that occur at such locations (Johnson and Redish, 2007). However, strikingly, and using spectacular multiple single-cell recording technology that enabled simultaneous recording from circa 2001 cells, (Pfeiffer and Foster, 2013) have recently identified rapid “preplay” of spatial trajectories (i.e., a sequence of place cells) during sharp-wave ripples (SWRs). Rats were trained in a task in which they alternated between searching for reward locations located randomly at any position in a large arena and navigating from each of these to a rewarded “home” location always in the same place throughout a session. The trajectory preplay of place cell firing occurred very rapidly (in
COMMENTARY
The Mantle of the Heavens: Reflections on the 2014 Nobel Prize for Medicine or Physiology Richard G. M. Morris*
ABSTRACT: The award of the Nobel Prize in Medicine or Physiology in 2014 for the discovery of place and grid cells was both a personal award to three great scientists and also a mark of the maturity of systems neuroscience as a discipline. This article offers both personal and scientific reflections on these discoveries, detailing both how getting to know all three winners had an impact on my life and the research questions that we shared in common work together. It ends with brief C 2015 Wiley reflections on three important outstanding questions. V Periodicals, Inc.
KEY WORDS: associative memory; grid cells; place cells; spatial learning; synaptic plasticity
winners acknowledged their debt to Edward Tolman and his concept of the “cognitive map” (Tolman, 1948). Both discoveries were soon confirmed by others and later established in other species than the rat including humans. It was fitting that accounts of the Nobel Prize in the media should speak of the discovery of “the GPS system of the brain” (http://www. bbc.com/news/health-29504761), though in doing so they tied the discovery firmly to navigation rather than memory.
PERSONAL REFLECTIONS INTRODUCTION When I first heard the news of the 2014 Nobel Prize by an email from Yadin Dudai, I was delighted. There was quickly a happy buzz around the web for this was a popular choice of winners, and a sense that systems neuroscience was also being recognized. I said to myself, to others and later wrote in print: “ Three great friends, three great scientists”. Given this immediate reaction, and focusing almost exclusively on animal research, I shall present both personal and scientific reflections on the award. These merge one with another for their intersection is the joy of life as a scientist—living and breathing what we do. The discovery of “place cells” in London was a fascinating one, and John O’Keefe had the wisdom to realize he had observed something of potential importance. He followed it up and their study became his life’s work, beautifully described in his Nobel Lecture (http://www.nobelprize.org/ mediaplayer/index.php?id=2413). The later discovery of “grid cells,” by Edvard and May-Britt Moser in Trondheim, added a critical and surprisingly hexagonal metric to the puzzle about how mammals recognize their location in space and navigate through it. At the Nobel Lectures in Stockholm, which I had the enormous privilege of attending, all three
Centre for Cognitive and Neural Systems, Edinburgh Neuroscience, The University of Edinburgh, 1 George Square, Edinburgh, United Kingdom *Correspondence to: Richard G. M. Morris, Centre for Cognitive and Neural Systems, Edinburgh Neuroscience, The University of Edinburgh, 1 George Square, Edinburgh, EH8 9JZ, UK. E-mail: [email protected] Received 6 March 2015; Accepted for publication 17 March 2015. DOI 10.1002/hipo.22455 Published online 19 March 2015 in Wiley Online Library (wileyonlinelibrary.com). C 2015 WILEY PERIODICALS, INC. V
A First Meeting I first met John O’Keefe, with Lynn Nadel, in the spring of 1974. I had just finished a D.Phil. in Experimental Psychology at the University of Sussex and wanted to work on the brain—and it was suggested that I visit them at University College London. It was fascinating talking together and an encounter that was to change the direction of my scientific career. I was impressed by their repeated assertion that space mattered, an assertion that was later to find expression in the famous first three sentences of their 1978 book: “Space plays a role in all our behavior. We live in it, move through it, explore it, and defend it. We find it easy enough to point to bits of it: the room, the mantle of the heavens, the gap between two fingers, and the place left behind when the piano finally gets moved” (O’Keefe and Nadel, 1978). Their perspective on learning was refreshingly different to what I had learned both at Cambridge as an undergraduate and at Sussex as a graduate student. Not that Sussex was backward looking—far from it. It was an immensely stimulating place to study, I shared an office with Anthony Dickinson now at the University of Cambridge, and both of us were exposed to new ideas from our supervisors and others—notably sabbatical visitors, such as Herb Terrace (Columbia) and Bob Rescorla (Pennsylvania). Indeed, it was during our time as graduate students that the famous Rescorla and Wagner book chapter was published and a new
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FIGURE 1. Place cells, quantification, and the watermaze. A) The discovery of place cells was put on a firm quantitative footing in various ways, starting with O’Keefe’s own experiments, but notably by Muller et al (1987). Their multicolored pixel approach was soon imitated by others. Quantification is vital in science and a semantically clear method of visualization is an ideal and accessible method of conveying information (from Muller et al., 1987). B) The conceptual link between a “hot-spot” in a uniform arena
that corresponds to the firing field of a place cell and the location of a hidden platform in a watermaze is straightforward (Morris, 1981). C) The watermaze has been used to test numerous hypotheses, as a screening test for cognitive enhancers and to explore a range of basic science and translational issues including the role of N-methyl-D-aspartate receptors in spatial learning. (Morris et al., 1986a, 1986b). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
era of thinking about the role of “expectancy” in reinforcement for learning was ushered in (Rescorla and Wagner, 1972). The animal learning framework was, however, one in which associative processes, the fundamental glue that enabled learning to occur, operated in a kind of vacuum. There was little sense then, as I remember it, that learning happened in a context, still less a spatial context. So O’Keefe and Nadel’s challenge to this framework in the form of dissociable “locale” and “taxon” processes was refreshingly new. John showed me his in vivo recording laboratory and was then trying to be responsive to the skepticism of others who were questioning the existence of place cells. This was over 40 years ago, methodology was more limited than now, and he was planning a study with a still camera that, with a long shutter time, would simultaneously capture the movements of the animal (a flashing light of one color on the animal’s head) and of the firing of a place cell (a different color). He conveyed a sense of wanting to be rigorous about what he believed was an important discovery. The late Bob M€uller, with John Kubie and Jim Ranck, was later to take quantitative monitoring of place cells to new levels of sophistication (Fig. 1A; Muller et al., 1987). Not being an electrophysiologist, I wondered what kind of behavioral experiments on spatial learning using lesions or other manipulations could best test the idea of “place” as something not determined by local cues, unlike conventional discrimination learning, and requiring the integrity of the hippocampus. In John’s view, the sense of location was worked out by a dedicated brain system that used information from diverse sensory modalities. Lynn had a circular track in which he could deliver water reward at various places, and he went on to establish the deleterious effects of fornix lesions in this apparatus (O’Keefeet al., 1975), but I was unsure about this approach—as it did not really let the animal navigate. I
was on my way to a Research Fellowship at the University of Durham and, arriving there in 1974, I set up a large circular apparatus with an uniform floor that could be rotated, and with food delivered at various places in it that the animals had to find—but the task did not work very well. Nor was my time in Durham as enjoyable as life had been in Sussex and so I left academic life for a period. I was not long away and, in 1977, found myself fortunate to be appointed to a Lectureship in Physiological Psychology (sic) at the University of St. Andrews.
The Watermaze After I had got my first-year undergraduate lectures underway, the key research issue for me remained that place cells fired where they did irrespective of local cues. They could not be strictly sensory cells, whether unimodal or polymodal, and they had to depend on some kind of spatial memory processing. How to get at this key concept? I also wanted to study the possible relationship between learning and plasticity, rather than just spatial navigation. To achieve these aims, I reasoned that I had to get rid of local cues completely but in a true learning task and one in which I could, in addition to lesions, manipulate neuronal plasticity. The department had no animal facilities and so I was assigned laboratory space in the remarkable but somewhat antiquated Gatty Marine Laboratory located on the West Sands of St Andrews’ north facing and often bleak shoreline—where the opening shots of the film “Chariots of Fire” was soon to be filmed. It was a slightly strange place to work, quite apart from not infrequently having to battle my way down the shore path through the winds of a northerly winter gale that had blown in from Russia. Once indoors, I got to my laboratory past tank Hippocampus
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after tank of sea creatures of various shapes and forms, some of whom might have been the subject of the former Director Graham Horridge’s recently completed studies of invertebrate interneurons (Horridge, 1968). One day, it occurred to me that rats would be able to learn while swimming and that this might help solve the local cue problem. I knew rats could swim, but I wondered if they could learn to escape from water at a specific place onto a platform that was hidden beneath the water surface and so was neither visible nor audible, offered no olfactory cues, and could not be identified using somatosensory cues until the animal had already got to it. This might be the solution to the local cue problem. The first “watermaze” was built from hardboard and fiberglass yacht resin by myself with the help of Chris Barman, an animal technician. We completed it in the workshop over the weekend, these being the days when staff still had access to workshops at weekends and health and safety officers were still over the horizon. To my amazement and delight, the rats learned the task very quickly (Fig. 1B). I ran some essential control conditions and a paper on “place navigation” followed soon (Morris, 1981). The discussion in that paper reflects my “animal learning” background and while I did not pursue it, John Pierce (who was also a graduate student at Sussex) has latterly gone on to conduct imaginative studies exploring the possible associative basis of spatial learning (Pearce, 2009). The observation that spatial learning in the watermaze is severely impaired by hippocampal lesions was made a year later—in collaboration with Paul Garrud, John O’Keefe, and Nick Rawlins (Morris et al., 1982). Paul and I tracked the animals by tracing a path with a felt tip pen onto clear film that we had taped over a video monitor. A year or so later, the British Broadcasting Corporation (BBC) introduced the “BBC Computer” with 128 K of random access memory and an easily learned software language called BBC Basic. I wrote a little program and, using a commercially available tracking device that John O’Keefe had by then started to use from HVS Image C , we were soon able to follow the paths of the swimming rats V directly. By tracking the black head of a hooded rat, we even managed to do without the little “police car” lights attached to the animal. This was a step toward better objectivity for, to my knowledge, studies of spatial learning had hitherto relied on an observer reports. A series of lesion studies followed, in which we also observed deleterious effects of both fornix lesions and retrosplenial lesions encompassing the entorhinal cortex (Schenk and Morris, 1985; Morris et al., 1986a, 1986b). However, a few years later, we found that rats with hippocampal lesions could learn a spatial reference memory task quite well if either minimal flexibility was required in the navigational path (Eichenbaum et al., 1990) or extensive overtraining was given (Morris et al., 1990).
Using Watermaze To Study Synaptic Plasticity And Memory I presented the earliest findings at what came to be known as the “Schloss Hippocampus” meeting of 1982. This was a Hippocampus
meeting at a castle in southern Bavaria owned by the Max Planck Society at which, in the views of many, the hippocampal field was to change direction irrevocably (Siefert, 1983). Until then, work had been very much on the “septohippocampus” with particular emphasis on the cholinergic and other inputs from the mid-brain. It was at this meeting that we first heard Carol Barnes describe her tantalizing observation that the persistence of long-term potentiation (LTP) correlated with the persistence of memory in her circular arena task earlier (Barnes, 1979). While there, I met Gary Lynch who described work confirming the homosynaptic nature of the synaptic change in LTP when studied in hippocampal slices in vitro, the role of calcium in induction, and the structural changes in spines viewed at the electron microscopic level. It was immediately apparent that LTP was much more than a persistent change in synaptic efficacy induced by tetanic stimulation, as (Bliss and Lomo, 1973) had described 10 years earlier. It was also a change that was associated, at the point of induction, with an ionic current different from that used to mediate normal synaptic transmission and a change expressed in a manner that could have the very storage capacity required of Marr’s (1971) network model incorporating the Hebb synapse (Marr, 1971; Willshaw et al., 2015). The following year, Lynch and Baudry produced their remarkable Science paper, collating together all the work of the Lynch laboratory, and in which they proposed that “glutamate receptors” (sic) were inserted into membranes to express the enhanced synaptic efficacy (Lynch and Baudry, 1984). If this concept has a contemporary ring to it, bear in mind that the paper is now nearly 30 years old. It is not cited as often as it should be, perhaps because a cardinal plank of their evidence turned out to be changes in glutamate transport rather than in the expression of the synaptic receptor. However, the idea of a simple postsynaptic mechanism to express the change in synaptic weights had already emerged, albeit later to be challenged by observations of presynaptic changes. Current debates on AMPA receptor trafficking have not moved on conceptually so very far from these early ideas, even if the techniques available now are spectacular by comparison to what was around then. I resolved to go and work with Lynch and was fortunate to be able to do so in 1984, courtesy of a Medical Research Council (MRC) Fellowship scheme that released University teaching staff to focus on research for a while. In this, and many other ways, I owe a huge debt to the MRC. Lynch’s laboratory was then working on a range of projects—my first real taste of a “neuroscience laboratory”—including a serine protease inhibitor called leupeptin that inhibits the proteolytic mechanism that he and Michel Baudry had implicated in the glutamate receptor insertion process. In laboratory experiments on olfactory learning conducted in the Irvine laboratory, I had mixed success, possibly because we were using the very discrimination learning tasks that were proving insensitive to hippocampal lesions in rats and primates. Ursula Staubli was later to have success in using this drug to block LTP (Staubli et al., 1988), but its effects on learning were generally quite modest in the watermaze (Morris et al., 1987). However, while in
THE MANTLE OF THE HEAVENS Irvine and contrary to the “house rules” that reflected the friendly rivalry between the Lynch and Cotman laboratories, I discussed these experiments with Eric Harris, then a postdoctoral with Carl Cotman. He drew my attention to the recently published paper of (Collingridge et al., 1983) on the role of the NMDA receptor in LTP and the drug he used called AP5. Unfortunately, it was time to go home, but Gary and I discussed some experimental options for when I got back to my laboratory. Upon returning to St. Andrews, Jeff Watkins at Bristol University kindly made available a small supply of the racemic mixture of an NMDA antagonist (DL-AP5) and I began work. At that point, no one knew whether AP5 would work in vivo or, indeed, be very effective in crossing the blood–brain barrier. Its chemical structure did not augur well in this regard. Accordingly, using the same osmotic mini pump procedure with intracerebral administration that had been tried in Irvine with leupeptin, I did some acute in vivo experiments on dentate LTP. These were exactly as Bliss and Lømo had described, but now in the rat rather than the rabbit. After chronically infusing DL-AP5 or saline for several days, the blockade of LTP in vivo was complete, across a range of test pulse intensities, and without any apparent effect on baseline synaptic transmission. I was amazed and excited. Obviously, the next step was to try this in swimming rats and to my delight, Elizabeth Anderson and I found that rats treated with the drug were unable to learn the reference memory spatial version of the watermaze (Fig. 1C). Those given saline or the inactive isomer, L-AP5, were unimpaired. Strangely, we did not work with D-AP5 at that stage. I cannot remember why. Concerned that the deficit with DL-AP5 might be sensory in nature, I deliberately tried a simple discrimination task of the kind that animals with hippocampal lesions can learn and we observed, now with a mounting sense of disbelief, that they could. Both behavioral experiments were replicated “blind” with respect to drug assignment. Thus, chronic intraventricular infusions of DL-AP5 at a dose sufficient to block LTP in vivo without affecting fast synaptic transmission in the hippocampus caused an apparently selective impairment of hippocampal-dependent place navigation. The animals could see, could move around properly, and could learn another difficult task, but they could not find their way in a task that needed place cells and apparently required NMDA receptor-dependent LTP. Gary Lynch came to St. Andrews to help write the paper that was published in 1986 (Morris et al., 1986a, 1986b). Graham Goddard sent me a letter (such were the days!) indicating that his acceptance of our results required a certain “ suspension of disbelief,” but his News and Views piece in the journal was generous. The ECONOMIST magazine identified our work as an important. In the same year, Bruce McNaughton took a complementary step forward by also establishing the causal role of activity-dependent synaptic enhancement in learning in a different way—showing that prior physiological saturation of LTP impaired subsequent spatial learning (McNaughton et al., 1986). In 1986, I moved to Edinburgh.
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A Scandanavian Encounter The field of neuroscience was beginning to coalesce, not just in America, but elsewhere. In 1990, I found myself at a meeting in Stockholm of the then “European Neuroscience Association” (which later became the Federation of European Neuroscience Societies—FENS). Wandering the posters, I spied a young couple (a very young couple!) standing in front of a lovely pink poster about the impact of dorsal versus ventral lesions of the hippocampus on spatial learning. Only dorsal lesions worked. We had a great discussion and I was simply bowled over by the enthusiasm of these two young folks. They were of course, Edvard and May-Britt Moser, then working as graduate students with Per Andersen in Oslo. While encouraging them to publish it, which they did (Moser et al., 1993a, 1993b), there was—I tentatively suggested—a little problem. Perhaps dorsal aspiration lesions would have de-afferented the ventral hippocampus from vital inputs from the septum, whereas ventral hippocampal lesions would not have the same effect on the dorsal hippocampus. The dorsal/ventral dissociation may be misleading. As I was by then exploring the impact of fiber-sparing ibotenate lesions, introduced to this approach by Len Jarrard, I suggested we repeat the study in Edinburgh using this lesion technique. This confirmed their earlier result and revealed that a “minislab” of just 20% of the dorsal hippocampus was sufficient for allocentric spatial learning (Moser et al., 1995). Edvard and May-Britt Moser came to Edinburgh as postdoctoral fellows in 1994 and stayed until 1996. They brought two very young daughters and a large blanket, and the former played on the latter in the lab. They had completed studies in Oslo revealing a reciprocal impact of brain temperature on field-potentials and population spikes in freely moving rats (Moser et al., 1993a, 1993b), but wanted to do further work exploring the possible relationship between activity-dependent synaptic plasticity and learning. It so happened that the finding that LTP saturation impaired learning (McNaughton et al., 1986) was proving controversial and we wondered if the conflict in the literature was due to insufficient LTP induction in some studies. We devised an electrode array that spanned the entire angular bundle of the perforant path and switched around the anode and cathode on successive episodes of highfrequency LTP-inducing stimulation. The effect was a more complete LTP saturation in most animals. Strikingly, animals in which LTP was successfully saturated failed to learn the watermaze, whereas those in which there was some residual plasticity could do so (Moser et al., 1998). Toward the end of their stay in Edinburgh, Edvard and May-Britt secured the offer of two positions at the Norwegian University of Science and Technology in Trondheim and were resolved to start their own laboratory. However, they were both keen to learn the technique of multiple single-cell recording in hippocampus and suggested going to John O’Keefe’s laboratory for the last part of their stay in Britain. We all thought this was a great idea and off they went. In truth, their short stay in O’Keefe’s laboratory proved fundamental for all that followed Hippocampus
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and was, after their earlier work with Per Andersen, another defining moment in their career—arguably more so than their time in Edinburgh. Once in Trondheim, they set up a laboratory, first in Psychology and later the Medical Technical Institute and began to study place cell activity in spatial learning tasks including the watermaze. A range of important studies were published at that time, notably Vegard Brun’s study revealing virtually no effect of CA3 lesion on place cell fidelity in CA1 (Brun et al., 2002). An important further development was the creation of the Centre for the Biology of Memory (CBM) to which they invited a number of international staff for short periods every year over a 10-year period. These included Carol Barnes, Bruce McNaughton, Randalf Menzel, Ole Paulsen, Alessendro Treves, Menno Witter, and, fortunately, myself. Our visits to Trondheim were very exciting, and each of us would be pounced by the young Masters and PhD students who would show us their data. We would also present seminars, and engage in discussions and collaborations between our respective laboratories. It was an exciting time. One of these young students, a former marine biologist, was Marianne Fyhn. I was fortunate to be in Trondheim soon after Edvard, MayBritt, Marianne, and Sturla Molden started single-unit recording the medial entorhinal cortex in a particularly large square arena. One day, Marianne came along quite excited by some data suggesting that cell firing formed a pattern. Entorhinal cells did not just fire in one position, but in several positions across the arena that did not look random. Edvard and MayBritt studied the data and suggested that she try rotating the cue card at the side of the arena. An hour or two later, she came back, and showed us that the pattern had now rotated by the same angle as she had rotated the card. It was a magic moment, Edvard and May-Britt were mesmerized by this additional observation, and so they embarked on the first systematic series of experiments identifying the “grid cell” pattern of medial entorhinal neurons (Fyhn et al., 2004; Hafting et al., 2005). I was not involved at all beyond having been a privileged early observer, and my interests were by then developing toward the study of associative “schemas” in collaboration with Emma Wood (in Edinburgh) and Menno Witter (in Amsterdam, but also part of CBM). However, it is important to note that just as John O’Keefe had the wisdom to realize he had discovered something important, Edvard and May-Britt did also—and they began a series of quantitative studies of the phase, scale, and orientation of grid cells. These revealed that all phases are represented within small cell clusters of entorhinal neurons in a “salt-and-pepper” organization (Hafting et al., 2005) and that the scale of the grid cell map increases metrically as recordings are made from more ventral locations of the medial entorhinal cortex and that the transitions between these are step-like (Stensola et al., 2012). More recent work has revealed that different grid maps display rigidity as one moves from one recording room to another, that they develop during infancy over a specific time course, and that the small set of frequently observed orientations of grid maps may arise because they reflect not just Hippocampus
genetic determinism but the cardinal axes of any local environment. It would have be wonderful if the ratio of the step-like transitions between discrete entorhinal modules observed nature’s beautiful golden ratio (1.618), but it turns out to be smaller (1.421) and for reasons which theoretical studies now suggest may enable an optimum organization. Comparative work has revealed grid cells in other mammals, including humans, with bats echolocating their way into picture via Ulanovsky’s finding of grid cells in bats in the absence of the hippocampal theta rhythm (Yartsev et al., 2011). This body of work is beautifully described in Edvard Moser’s Nobel lecture (http://www.nobelprize.org/mediaplayer/index.php?id=2415). Taken together with the earlier discovery of place cells, one can appreciate the physiological beauty of this spatial navigational system—with its array of distinct cell types, not just place and grid cells, but of head direction and boundary cells also.
SCIENTIFIC REFLECTIONS So where are we now? Hundreds if not thousands of papers later, reflecting developing concepts in both navigation and memory, technical improvements in recording technology, intriguing new experimental paradigms and, through these, new studies of both the spatial and nonspatial properties of hippocampal neurons, is everything sewn up? Far from it! There remains, in particular, a major divide in the hippocampus community between those focused on spatial memory and navigation, and those studying memory more generally (Eichenbaum and Cohen, 2014). While the Nobel Prize has been given for work in the former category, it is important to recognize a wider picture relating to hippocampal involvement in memory. There are numerous outstanding questions and space permits that I focus briefly on only three: 1. How do place cells enable navigation? 2. Do hippocampal neurons represent nonspatial and spatial attributes? 3. What is the place of context in episodic-like memory?
How Do Place Cells Enable Navigation? The bottom line of any answer to this question is: we do not know. Clearly, the existence of a medial/temporal network of spatially responsive cells in the entorhinal cortex, hippocampus, postsubiculum, and thalamus strongly supports the general concept of there being a “cognitive map.” These neurons are active when animals navigate. Maps are for navigation, ergo these cells must be the basis of spatial navigation. I can buy into this, but I am not alone in having early on raised a conundrum about the mechanistic role of place cells in navigation (Morris, 1989). Place cells are defined as cells that fire at specific locations— so one set of cells will fire preferentially at place A, another at Place B, respectively. But suppose an organism wishes to navigate from Place A to Place B while avoiding Place C. It could head off from Place A randomly and, using the cells
THE MANTLE OF THE HEAVENS corresponding to Place B, somehow recognize when it has got there, hoping not to arrive at Place C. But data from the watermaze indicate that rats can head from A (a start point at the side of the pool) to B (a hidden platform) using relatively direct paths. It follows that the animal can access information about Place B while still at Place A, but such firing appears to violate the definition of place cells as cells that fire “if and only if” the animal is at that place. One way to get round this conundrum is to suggest that place cells usually respond at their respective firing field, but may occasionally fire at other places also. David Foster addressed this issue in his PhD thesis completed in both Edinburgh and MIT, eventually developing a temporal difference algorithm to compute the path to a familiar target location in an arena not dissimilar to a watermaze (Foster et al., 2000). Recording studies by others have suggested that anticipatory firing can sometimes be observed, such as at the choice point of a maze where forward scanning of place fields further on is sometimes observed during the vicarious hesitations that occur at such locations (Johnson and Redish, 2007). However, strikingly, and using spectacular multiple single-cell recording technology that enabled simultaneous recording from circa 2001 cells, (Pfeiffer and Foster, 2013) have recently identified rapid “preplay” of spatial trajectories (i.e., a sequence of place cells) during sharp-wave ripples (SWRs). Rats were trained in a task in which they alternated between searching for reward locations located randomly at any position in a large arena and navigating from each of these to a rewarded “home” location always in the same place throughout a session. The trajectory preplay of place cell firing occurred very rapidly (in
