Research moves on all the time, and new findings confirm and expand on old ones, or sometimes modify or contradict them. Here I aim to provide a guide to some of the most interesting new discoveries and ideas relevant to consciousness and its evolution, and to any particularly interesting new books on the subject. There will also be the occasional thought that has occurred to me since the book went to press.
Ann. Rev. Psychol. Annual Review of Psychology
Curr. Biol. Current Biology
Nat. Neurosci. Nature Neuroscience
Nat. Rev. Neurosci. Nature Reviews: Neuroscience
PNAS Proceedings of the National Academy of Sciences
TICS Trends in Cognitive Sciences
TINS Trends in Neurosciences
# Indicates latest additions
The study of consciousness has reached a level which offers plenty to write about and new books on the subject appear regularly. From Michael Gazzaniga, the eminent neuroscientist particularly associated with the study of split brains and divided consciousness comes The Consciousness Instinct: Unravelling the mystery of how the brain makes the mind Farrar, Strauss & Giroux 2018. The philosopher Daniel Dennett, author of several books on consciousness, has recorded his latest thoughts in From Bacteria to Bach and Back W. W. Norton & Co. 2017. Especially interesting are his ideas about Richard Dawkin’s ‘memes’ – of which languages, Dennett points out, are outstanding examples. And since I particularly enjoyed Stanislas Dehaene’s Consciousness and the Brain: Deciphering How the Brain Codes Our Thoughts (Viking Press 2014) I shall include it here, although it dates back to a little before The Ancestry appeared. A new book from another famous name in consciousness studies, Christof Koch, The Feeling of Life Itself: Why Consciousness is Widespread but Can’t be Computed is due out in October.
It’s known that the report from a sensory area of neocortex must activate prefrontal cortex if it is to achieve consciousness. However, similar levels of input don’t consistently produce the same result. After studying the success or decay of weak messages a team in Stanley Dehaene’s laboratory confirmed that the variation is attributable to variation in the pre-stimulus state of the brain. B. Vugt et al. The threshold for conscious report: Signal loss and response bias in visual and frontal cortex Science Vol.360. p.537 4 May 2019
The whole point of a brain, of course, is to promote, in response to sensory input, the actions most likely to lead to the survival of its owner and/or the reproduction of the genes that animal carries. Not surprising, then, if our perceptions are shaped by the potentials for action the current environment offers. Ingeniously recording brain activity while their subjects perceived and navigated virtual architectural scenes, some promising an easy or at least a possible way through, some obviously impassable, Djebbara et al. found that even the early activity in sensory brain areas varied according to the potential for movement. Z. Djebbara et al. Sensorimotor brain dynamics reflect architectural affordances PNAS Vol. 116 p.14769 16 July 2019
In broad terms there are three different classes of synapse (junction) between neurons. The transmitter that is released may be excitatory – increasing the chances of the next cell firing – or inhibitory, reducing them. Or it may be something more properly called a neuromodulator, which modifies the effect of transmitters. There are quite a few types of transmitter, however, and many types of neuromodulator. There are also several different types of receptor for each of them, so the possible effects are numerous.
The larger and more complicated the brain the longer the chains of excitatory and inhibitory neurons. An excitatory cell may excite an inhibitory one, causing it to inhibit another inhibitory one, thereby relelasing the next neuron in line from inhibition …… clearly the possibilities are enormous, and they have been increasingly exploited as brains grew larger.
In complex brains especially, a delicate balance is needed between excitation and inhibition. R. Rubin et al. Balanced excitation and inhibition are required for high-capacity, noise-robust neuronal selectivity PNAS Vol. 114 E9366 31 October 2017 But there is a puzzle here, since some of the synaptic connections of cortical neurons, which have been thought to store the results of learning by growing stronger through use, have turned out to be distinctly volatile. G. Mongillo et al. Intrinsic volatility of synaptic connections – a challenge to the synaptic trace theory of memory Curr.Op. Neurobiol. Vol. 46 p.7 October 2017 However the same authors have found that, although excitatory synapses significantly outnumber inhibitory ones, the latter play a bigger role in storing information. As long as the overall balance between excitation and inhibition is not disrupted the network can operate efficiently, despite the inconsistency of excitatory synapses. G. Mongillo et al. Inhibitory activity defines the realm of excitatory plasticity Nat. Neurosci. Vol. 21 p.1463 October 2018.
Inhibitory neurons come in several different varieties, each defined by containing some protein that the others don’t. The significance is gradually being unravelled. M. E. Hern & R. A. Nicoll Somatostatin and parvalbumin synapses on to hippocampal pyramidal cells are regulated by distinct mechanisms PNAS Vol. 115 p.589 9 January 2018. Meanwhile a study of cell types in the visual cortex and part of the motor cortex of mice has shown that the excitatory neurons differ more noticeably between the two areas than the inhibitory ones do. B. Tasic et al. Shared and distinct transcriptomic cell types across neocortical areas Nature Vol. 563 p.72 1 November 2018
How learning happens is one of the most interesting questions in neuroscience, and prompts a great deal of research. Much of it focuses on the hippocampus, the seahorse-shaped structure nestling in what’s known as medial temporal cortex – the bit that curls inward on either side of the brain. It was here it was discovered that neuronal connections strengthen with use and weaken with idleness, something that also happens in other brain areas, with varying timescales and varying degrees of permanence. Brief synaptic potentiation keeps a flow of impulses circulating (if prefrontal cortex has become involved) back around the parts of the sensory brain where it was initiated, allowing a succession of sensory inputs to be preserved for long enough for patterns spread out over time to be perceived. Most importantly this allows us and other animals to build up a map of the environment and move around it in a purposeful manner.
In building a map through exploration it’s important, of course, to preserve the order in which sensory events occur. A recent study investigated this aspect of the hippocampus and closely linked adjacent structures further – N. C. Heyworth & L. R. Squire The nature of recollection across months and years and after medial temporal lobe damage PNAS Vol. 116 p.4619 5 March 2019. Subjects with damage to the medial temporal lobe were taken on a 25 minute guided walk during which eleven planned events occurred, and their recollections of the walk were compared with those of controls immediately afterward, after one month, or up to two and a half years later. The most outstanding difference was that the memory-impaired patients reported the incidents of the walk in random order, whereas the control group preserved the order.
Another important function to which the hippocampus contributes is recording what is where. C. E. Connor & J. J. Knierim Integration of objects and space in perception and memory Nat. Neurosci. Vol. 20 p.1493 November 2017. This is one of several articles in an issue focussed on spatial cognition.
Subjective experience suggests that the experiences and discoveries we think about later, or dream about in subsequent sleep, are the things more likely to stick in long-term memory. Sure enough, many experiments have shown that the exchanges between hippocampus and neocortex which occur during new experience may be repeated during periods of idleness and in sleep, and do indeed serve to fix the new information in long-term memory. The replays take the form of condensed, high-frequency bouts of neuronal firing termed ripples, which consolidate the connections initiated earlier. Prolonged ripples create the sturdiest records. A. Fernandez-Ruiz et al. Long-duration hippocampal sharp wave ripples improve memory Science Vol. 364 p.1082 14 June 2019. Studies in young rats show how these replays of neuronal activity develop as the brain matures. L. Muessig et al. Co-ordinated emergence of hippocampal replay and theta sequences during post-natal development Curr. Biol. Vol.29 p.834 4 February 2019.
# Memory is selective, however, and it turns out that there is an editing mechanism which can suppress hippocampal activity. S. Izawa et al. REM sleep-active MCH neurons are involved in forgetting hippocampus-dependent memories Science Vol. 365 p.1308 2o September 2019
Hippocampal sharp wave ripples, meanwhile, also accompany recollection, occurring just before a memory is successfully accessed. A. P. Vaz et al. Coupled ripple oscillations between the medial temporal lobe and neocortex retrieve human memory Science Vol. 363 p.975 1 March 2019. Moreover exceptionally detailed recollection of autobiographical events has been found to correlate with exceptionally strong connectivity between prefrontal cortex and hippocampus. V. Santangelo et al. Enhanced brain activity associated with memory access in highly superior autobiographical memory PNAS Vol. 115 p.7795 24 July 2018.
Obviously the first step in creating a coherent idea of surrounding space is to take eye movements into the account, a point considered by M. Nau et al. How the brain’s navigation system shapes our visual experience TICS Vol.22 p.810 September 2018 I suspect that damage to the hippocampal formation, leading to difficulty in connecting the views gained from a series of fixations, may account for the difficulty some dementia sufferers have in finding their way around even in homes they have lived in for decades.
The hippocampus allows us to find patterns in inputs spread out over extended periods of time. The most valuable pattern, for almost any animal, must be the shape of its environment, knowledge which enables it to head as directly as possible for whatever it currently needs. Another area involved in the mapping process is the adjacent entorhinal cortex, where cells fire in such a way that the animal’s position is registered in terms of a grid covering all the area within sensory reach. An important bit of data here concerns the locomotion that takes the animal from one junction on the grid to the next. A contribution to the understanding of this system comes from M. G. Campbell et al. Principles governing the integration of landmark and self-motion cues in entorhinal cortical codes for navigation Nat. Neurosci. Vol. 21 p.1096 July 2018. Another type of entorhinal cell has been found to fire when an object within the surrounding space is at a specific distance and direction. Object-vector coding in the medial entorhinal cortex O. A. Hoydal et al. Nature Vol 568 p.400 18 April 2019
Remarkably, it has been possible to create a computer programme which mastered the art of navigating around a modest area using a grid system based on the entorhinal one. A. Banino et al. Vector-based navigation using grid-like representations in artificial agents Nature Vol. 557 p.429 17 May 2018
Our view of surrounding space is often limited by buildings, hedges or other barriers to vision and locomotion. Increasing attention is being given to how the brain records such things. X. Gofman et al. Dissociation between postrhinal cortex and downstream parahippocampal regions in the representation of egocentric boundaries Curr. Biol. Vol. 29 p.2751 5 August 2019; Q. He & T. I. Brown Environmental barriers disrupt grid-like representations in humans during navigation Curr. Biol. Vol. 29 p.2718 5 August 2019
Alas, in us long-lived humans the area in which hippocampus and entorhinal cortex are found seems to be particularly vulnerable to wearing out. M. Stangl et al. Compromised grid-cell-like representations in old age as a key mechanism to explain age-related navigational deficits Curr. Biol. Vol. 28 p.1108 2 April 2018. (This too may be relevant in dementia – see earlier.)
Another contributor to navigation and the creation of the cognitive map is retrosplenial cortex, which seems to record the large-scale patterns, including those beyond sensory reach, and the overall geometry of a habitat. An addition to this picture comes from A. S. Alexander & D.A. Nitz Spatially periodic activation patterns of retrosplenial cortex encode route sub-spaces and distance travelled Curr. Biol. Vol. 27 p.1551 4 February 2019
Mapping the environment must be an ancient form of learning. There are many other sorts. A skill that evolved comparatively recently is learning how to use a flexible muscle system to the best advantage. How it happens is becoming clearer. A. J. Peters et al. Reorganisation of corticospinal output during motor learning Nat. Neurosci. Vol. 20 p.113 August 2017 And as might be expected, new patterns of neuronal activity also emerge in the brain. E. R. Oby et al. New neural activity patterns emerge with long-term learning PNAS Vol. 116 p.15210 23 July 2019
Here too sleep serves to consolidate learning, and even a very brief period of rest can help, it turns out. M. Bonstrup et al. A rapid form of offline consolidation in skill learning Curr. Biol. Vol. 19 p.30219 2019
Some forms of learning are so well understood now that scientists have been able to create a response to a previously insignificant odour by co-ordinating stimulation in mouse olfactory cortex with stimulation in two areas concerned with reward and discouragement. G. Vetere et al. Memory formation in the absence of experience Nat. Neurosci. Vol. 22 p.933 May 2019
The primary visual cortex has been most extensively studied in primates and in cats, where neurons favouring lines of a particular orientation cluster together, forming columns. A new study finds that this holds good in carnivores generally and in ungulates – but not in rats. M. Weigand et al. Universal transition from unstructured to structured neural maps PNAS Vol.114 E4057 16 May 2017 This looks like another example of the general principle that the more complicated brains get the more delicately organised they have to be.
Early studies of visual cortex, confined to recording from only a few neurons, indicated that different aspects of the visual input were processed through separate channels. Now that much more sophisticated recording equipment is available it seems that a good many neurons in V1 (in macaque monkeys) are sensitive to both colour and orientation. A. K. Garg et al. Rethinking primary visual cortex function Science Vol. 364 p.1275 28 June 2019 More is being uncovered, too, about a later stage of the pathway through visual cortex, V4. Neurons in the upper layers here extract more detail to pass on to the next processing stage, while neurons in deep layers seem to be more concerned with behavioural relevance and the possible need for attention. W. W. Pettine et al. Laminar segregation of sensory coding and behavioral readout in macaque V4 PNAS Vol. 116 p.14749 16 July 2019
A thought about colour vision: the magnificent array of colours we experience, produced by comparing the inputs of just three wavelength-sensitive types of photoreceptors and one less fussy one, is perhaps a particuarly telling example of what the extended processing of sensory input that leads to consciousness can achieve. Some invertebrates have a much wider range of colour-tuned receptors – dragonflies, for instance, and mantis shrimps. At least one variety of the latter has twelve sorts. But the shrimps don’t learn to distinguish among different blends of input. H. H. Thoen et al. A different form of color vision in mantis shrimps Science Vol. 343 p.411 24 January 2014 It seems pretty certain that where there is such a wide range of wavelength-tuned receptors each variety is dedicated to eliciting one particular hard-wired response, comparable to the male stickleback’s automatic display indicating territorial ownership at the sight of any patch of red. The visual systems of some of those brilliantly coloured little reef fish probably work the same way, with receptors pretty precisely matched to the colours displayed by the species. Shoaling with fellow members of the species is managed, I would guess, by a hardwired response to the distinctive colour patterns they display – blue and yellow stripes, red and green ones, or whatever.
The received wisdom has long been that cone receptors are tuned to different wavelengths of light while the more sensitive rod receptors respond to a wide range and record only luminance. Now it has been discovered that some species of deep-sea fish have a whole range of rod receptors tuned to different wavelengths (38 in the most extreme case) – which probably function to detect the bioluminescent signals that are quite common in the deep ocean’s inhabitants. Z. Musilova et al. Vision using multiple distinct rod opsins in deep-sea fishes Science Vol. 364 p.588 10 May 2019
It has been becoming ever clearer that activity in sensory pathways throughout the neocortex can be influenced by factors such as attention snd emotion. Now it’s been shown, in the mouse, that even neurons in primary visual cortex registering such basic visual elements as vertical or horizontal lines can fire somewhat differently depending on whether the overall pattern of input is new, or constitutes a view the mouse has seen before. A. B. Saleem et al. Coherent encoding of subjective spatial position in visual cortex and hippocampus Nature Vol. 567 p.158 10 September 2018. This intriguing result was obtained by an ingenious experiment in which the mouse ran on a revolving ball while a changing scene was projected in front of it, allowing tight control of just what the animal sees and where it thinks it’s going. The results of several such virtual reality experiments have been reviewed by Liam Drew in Nature: The mouse in the video game Vol. 567 p.158 4 March 2019. This experimental method has even been extended to study how olfaction guides locomotion. B. A. Radvansky & D. A. Dombeck An olfactory virtual reality system for mice Nature Communications Vol.9 article 839 2018.
Tiny, unnoticed eye movements called microsaccades ensure that fine features of the visual scene are at some point perfectly centred in the receptive fields of appropriate detectors in the visual system. Schelkova and colleagues confirm how helpful this is for distinguishing very small-scale stimulus-patterns. N. Schelkova et al. Task-driven visual exploration at the foveal scale PNAS Vol. 116 p.5811 19 March 2019.
The micromovements of the eye are far too small to be consciously noticed but I nurse a strong suspicion that that they are what make the eyes of mammals and birds seem lively and bright – in contrast to the glassy look of reptilian and fish eyes – and are part of what attracts infant attention to eyes. But as far as I know nobody has ever looked for microsaccades in birds, let alone in reptiles, and nobody has checked for unconscious effects on attention.
Mirror neurons, identified first in monkeys then in humans, fire both when an action is performed and when the same action is seen being performed, apparently for the same purpose. They are found in various parts of both motor and parietal cortex.
The evolution of such neurons must underpin the capacity for imitation clearly seen in many mammalian and avian species. And what massive consequences this talent has had. An individual who can imitate can learn what is worth doing without taking the risks involved in experimentation, so knowledge gained by a single adventurous animal can be passed on from generation to generation painlessly. Moreover these new behaviours can become established in a very short time, in contrast to those which emerge from genetic changes to brains or muscular systems. (Indeed the influence may work in the other direction – the learnt new behaviours can create a selective pressure which favours modifications to brain or muscle.)
The capacity for imitation is particularly valuable for infants. In its simplest form it probably involves only the release of an innate action when the infant sees the parent performing it, so that the infant learns to look for food where the parent looks, and what sort of thing prompts flight in the parent. This form seems to be present in birds. I have seen a peahen take a few pecks in a likely piece of earth, which prompted her chick to forage there while she moved on a little. Something similar has been reported in domestic chickens. I imagine it’s a very similar mechanism which often causes a human infant only a few days old to smile back at someone who smiles at them, and infant monkeys to perform similar imitations.
This form of imitation seems not significantly different from the way little fishes in shoals tend to follow any individual which makes a sudden turn, so that the change of direction rapidly sweeps though even a large group. Perhaps we should look for pretty ancient origins for the first mirror neurons. Once they were established, though, they created a situation in which parental care of the young, providing readily available models to imitate, could become increasingly valuable.
The more complex forms of imitation no doubt evolved as muscle systems grew more complex. And as brains grew larger infants could build on the sensations created by automatic imitations and develop the ideas which support conscious purpose. Thus humans and no doubt other primate infants gradually develop a conscious sense of how facial expressions coincide with emotion and mood, and how they operate as a means of communication.
The talent for imitation becomes even more useful in social species, and seems to need no further development. Malaysian sun bears normally lead solitary lives, apart from a period of about three months when a mother is rearing a couple of infants. However some young bears that were being rehabilitated before return to the forest readily played together in a social fashion, and numerous instances were observed of two sorts of open-mouth expression – something that sounds rather like the chimpanzee play-face – being reciprocated. D. Taylor et al. Facial complexixty in sun bears: exact facial mimicry and social sensitivity Scientific Reports 9 article 4961 2019 This report seems, incidentally, to add to the evidence that many large-brained mammals can switch between social and solitary life styles as the circumstances make appropriate. (Coyotes provide a particularly notable example.)
As well as mirror neurons for action there are also emotional mirror neurons – empathy neurons – which are active both when emotion is experienced and when someone else is seen experiencing it. These too must contribute enormously to infant education, demonstrating what is to be feared, what can be approached confidently. They’ve now been identified in rats as well as primates. M. Carillo et al. Emotional mirror neurons in the rat’s anterior cingulate cortex Curr. Biol. Vol. 19 p.30322 2019 There is evidence, too, of their existence in at least some birds. J. E. C. Adriaense et al. Negative emotional contagion and cognitive bias in common ravens (Corus corax) PNAS Vol. 116 p.11547 4 June 2019
There is evidence, incidentally, that motor imitation is a built-in urge that has to be suppressed. By adulthood the instinct is normally completely inhibited and must be released from inhibition when wanted, but damage in a certain brain area can affect the ability to control it. Patients instructed not to copy an experimenter’s actions have difficulty in complying. It’s obvious that emotional mirror neurons work in the same way – it’s all to easy to catch an emotion or mood, and it takes practice and self-control to avoid infection.
On being social
In social animals both sorts of mirror neuron are obviously also useful in adulthood, serving to spread useful information through the group. And if the group must deal with a predator or an attack from a rival group their best chance lies in sharing the same emotion and acting in unison – either all standing up to the threat or all running away. A mixed reaction reduces the chances of survival for everybody. The most successful group leader, meanwhile, is likely to be the animal which can not only defeat all rivals but can also powerfully activate emotional and mirror neurons in others when needed. This ability, which also implies a capacity for attracting attention, is surely the basis of that indefinable quality charisma, and the leader who has it generally makes for a more successful group.
Our capacity for living together in large groups, sharing our ideas, combining our efforts, and submitting to some form of central governance, has clearly contributed greatly to our success as a species. But the very characteristics that made the vast and rapid expansion of our numbers possible carry dangers in the modern world, for what works well in small groups isn’t always beneficial in large societies. Emotional mirror neuerons at play in a crowd can produce mass hysteria. Motor mirror neurons can prompt people to do things they wouldn’t if they stopped to think about it. Charisma doesn’t necessarily coincide with wisdom. Now that we are organised into very large societies and equipped with the means of destroying ourselves completely (along with many other species) one of the challenges is to keep mirror neurons under control.
The knowledge stores
As we learn about the world around us, and about our own capacities, the knowledge is stored in the neocortex, neatly classified into different areas, just as different subjects are stored in different sections of a library. But the brain also constructs a complex network of connections between the areas, so that the record of a visual stimulus-pattern is linked to records of the sounds connected with it, of any emotions it might evoke, and so forth. In humans there is also likely to be a link to a word. Registering a stimulus-pattern in any one area can thus prepare the way for related input elsewhere, or for producing the appropriate word. And as we learn to recognise particular stimulus-patterns in ever greater detail the neuronal response becomes ever more finely tuned, the pathway of excitation increasingly refined by new inhibitory connections.
Particularly complex stimulus-patterns carrying several forms of significance activate several areas. Faces are a notable example, analysed separately for identity, expression, orientation, significance – and new face-sensitive areas keep being discovered. Landi and Freiwald have found (in rhesus monkeys) two in which, as blurred faces gradually beame clearer, the response to familiar ones ramped up quite suddenly, parallelling the way a face can suddenly be recognised. The response to unfamilar faces grew more steadily, as in other face-sensitive areas. S. M. Landi & W. A. Freiwald Two areas for familiar face recognition in the primate brain Science Vol. 357 p.591 11 August 2017
J. S. H. Taylor et al. have analysed the areas involved in recognising written words. Mapping visual symbols onto spoken language along the ventral visual system PNAS Vol. 116 p.17723 3 September 2019
The neocortical areas which store knowledge of such important things as faces, bodies and landmarks are pretty consistently sited. How does this orderly arrangement come about? Several explanations have been put forward, not all mutually exclusive. My own proposition is that these categories of stimulus-pattern are the ones for which the infant has genetically determined responses – imitation for faces, bodies and voices, grabbing for grabbable objects, and so on – and the hardwired action is reported to a predestined part of the neocortex, which is thereby sensitised to the sensory input which evoked the action. This hypothesis doesn’t rule out other factors which might be at work, such as the suggestion that the reward infants gain from looking at faces and engaging in social interaction with them may play a role. L. J. Powell et al. Social origins of cortical face areas TICS Vol. 22 p.752 1 September 2018
But the importance of action in perception seems to be underlined by an experiment showing that accuracy in making a quick estimate of the number of visual objects in a display is greatly influenced by the number of fixations made, suggesting that “numerical estimation ability is closely tied to the mechanisms that control eye movements and attention.” S. J. Cheyette & S. T. Piantadosi A primarily serial, foveal accumulator underlies approximate numerical estimation PNAS Vol. 116 p.17729 3 September 2019
Certainly the development of these areas is not dependent on the sensory channel through which the input comes. The language areas of congenitally deaf people who communicate by means of visual sign language are similarly situated to those of people who use spoken language. In like manner, it’s now been found, the congenitally blind store knowledge of faces, bodies, objects and scenes in much the same way as the sighted, although their knowledge must be gained entirely through other senses. J. van den Hurk et al. Development of visual category selectivity in ventral visual cortex does not require visual experience PNAS Vol. 114 E4501 30 May 2017 (This fits well with the long-ago discovery that the blind can draw recognisable pictures of things like tables that they know through touch.)
The output from these areas which deal in subjects such as faces is passed on to yet other neocortical areas which record the asociations among inputs from different senses, such as one towards the front of temporal cortex which responds not only to the appearance which defines an individual but also to the name that goes with it, and any other known aspects of the person concerned. Y. Wang et al. Dynamic neural architecture for social knowledge retrieval PNAS Vol. 114 E3305 18 April 2017
Interesting ideas and new discoveries about the past continue to emerge. It seems that the transition from single-celled animals to multicellular ones may have been less complicated and occurred more often than once thought. E. Pennisi The power of many Science Vol. 360 p.1388 29 June 2018
Our knowledge of the weird and wonderful multicellular animals that had evolved by the time of the Cambrian period will be increased by the discovery of an extensive deposit of remarkably well preserved fossils – including, unusually, some of soft-bodied animals – on a river bank in China’s Hubei province. It will be fascinating to correlate them with the long-known fossil riches of the Burgess Shale in Canada, also from half a billion years ago. And now this Canadian fossil-bearing shale has been found to extend for kilometres beyond the original site, and more exciting finds are appearing. J. Sokol Cracking the Cambrian Science Vol. 632 23 November 2018
Darwin’s essential insight is simple enough. Those organisms best fitted to meet the challenges of survival are likely to breed the most prolifically, and with many descendants comes the best chance that some will adapt to new circumstances and giving birth to new species. But changing environments and competition for food and living space among the evolving species create all sorts of complexity. Pringle et al. explored one of the outstanding questions in a long-term experiment making use of 12 small Bahamian islands on which the top predator was a semi-terrestial insect-eating lizard. They introduced either a ground-dwelling lizard capable of eating smaller lizards as well as arthropods, or a tree-dwelling species, or both, while 4 islands were left as controls. When all three species were present numbers of both smaller species declined, but the experimenters deduced that this was more because of the competition for living space than because of predation by the carniverous lizards. The original inhabitants learnt to make more use of the trees, forcing the specialists in arboreal living to retreat to higher levels. R. M. Pringle et al. Predator-induced collapse of niche structure and species coexistence Nature Vol. 570 p.58 6 June 2019
An important aspect of the evolution of Homo sapiens has been an increasing ability to get on with each other, leading to the possibility of ever larger social groupings, which permit the exchanges of ideas that can inspire major technological advances. All sorts of genetic modifications have contributed to this development – a reduction in the tendency to be aggressive to strangers, an increase in facial expressiveness and so on. Many of the qualities selected for parallel those selected by us in the domestication of other species, prompting the suggestion that we have in effect domesticated ourselves. An interesting treatment of this subject is B. Hare Survival of the friendliest: Home sapiens evolved via selection for prosociality Ann. Rev. Psychol. 2017
Why should there be two locomotory centres in the brain? I hypothesised in the book that they might govern slightly different forms of locomotion, serving different purposes, such as gentle exploration and hasty escape. New research suggests that I was partially right. The different forms are not quite what I guessed at – and there are probably more than two such centres. V. Caggiano et al. Midbrain circuits that set locomotor speed and gait selection Nature Vol. 553 p.455 25 January 2018.
Other interesting papers on motor subjects are M. N. Economo et al. Distinct descending motor cortex pathways and their role in movement Nature Vol. 563 p.79 1 November 2018 and Z. Gao et al. A cortico-cerebellar loop for motor planning Nature Vol. 563 p.113 1 November 2018
Modern lifestyles make considerable demands on attentional systems, and the effects are receiving some interest from neuroscientists, as in M. R. Uncapher & A. D. Wagner Minds and brains of media multitaskers: Current findings and future directions PNAS Vol. 115 p.9889 2 Occtober 2010 The effect of multimedia use in early years is also increasingly studied, as in D. A. Christakis et al. How early media exposure may affect cognitive function: A review of results from observations in humans and experiments in mice PNAS Vol. 115 p.9851 2 October 2018
Attentional priorities differ, it seems, among individuals in a way that probably reflects both nature and nurture. Viewing a large series of complex scenes some people fixated earlier and longer on faces, others on text, others on food, and so on. B. de Haas et al. Individual differences in visual salience vary along semantic dimensions PNAS Vol. 116 p.11687 11 June 2019
The neuromodulator dopamine is probably one of the most ancient conveyors of messages between neurons – perhaps it even antedates neurons. I became rather obsessed with it when I first started studying the brain, until I feared I was placing too much emphasis on it, and told myself to be more objective. But it has turned out to be deeply involved in so many aspects of brain function – movement, attention, learning, reward, addiction – that it seems fair to deduce it has had a particularly vital role in brain evolution. Here are some of the latest papers.
B. Enghelhard et al. Specialised coding of sensory, motor and cognitive variables in VTA dopamine neurons Nature Vol. 570 p.509 27 June 2019; A. Mohebi et al. Dissociable dopamine dynamics for learning and motivation Nature Vol. 570 p.65 6 June 2019; M. Sheng et al. Emergence of stable striatal D1R and D2R neuronal ensembles with distinct firing sequence during motor learning PNAS Vol. 116 p.11038 28 May 2019; L. Ferren et al. Dopamine modulates the reward experiences elicited by music PNAS Vol. 166 p.3793 22 January 2019; C. E. Stelly et al. Pattern of dopamine signaling during aversive events predicts active avoidance learning PNAS Vol. 116 p.13641 2 July 2019
Serotonin (also known as 5-HT) is another extremely influential neuromodulator, and must be at least as ancient as dopamine. But the latter got a headstart with neuroscientists when it transpired that a shortage of it was a major factor in Parkinson’s disease. A shortage of serotonin would later be linked to depression, which fired up scientific interest, but the complexities here still seem even more challenging than for dopamine. However a useful review of the current state of knowledge has been published: B. W. Okaty et al. Embracing diversity in the 5-HT neuronal system Nat. Rev. Neursci. Vol. 20 p.397 April 2019
The thalamus is a central structure in the brain, part of the diencephalon (the second of the major divisions of the brain, which are based on the way the swelling at the front end of the embryonic spinal cord is initially divided). In primates and many other species the thalamus is thoroughly buried under the vastly expanded telencephalon, and was consequently very difficult to get at with recording needles until extremely fine ones were developed. For a long time it was known principally for relaying visual, auditory and somatosensory information to the necortex for further analysis, and for its contribution to shutting off access from sensory systems to the neocortex during sleep. Now a great deal more is being discovered, though the overall picture is still far from clear. A valuable exposition of the new evidence is L. Acsady The Thalamic Paradox Nat. Neurosci. Vol. 20 p.901 27 June 2017.
Here are some other recent papers: S. S. Bolkan et al. Thalamic projections sustain prefrontal activity during working memory performance Nat. Neurosci. Vol. 20 p.987 July 2017; M. M. Hakassa & S. Kastner Thalamic functions in distributed cognitve control Nat. Neurosci. Vol. 20 p.1669 December 2017; G. Pergola et al. The regulatory role of the human mediodorsal thalamus TICS Vol. 22 p.1011 November 2018.
The mammalian thalamus has many detectable specialised divisions. Those of such amphibians as have been studied are much simpler. I’ve suggested that the structure began as a sort of map, in a self-centred reference frame, of the animal’s current situation, one that came into existence at a period when all actions were pretty much hardwired. Actions would be reported to the map, and in a way that indicated the position of the stimulus that prompted them – the position in relation to the animal that is. Probably just enough visual information came from the retina to the proto-thalamus to anchor the map to the external world. (The visual data which guided the action would be dealt with by the visual centre in the lower brain which in mammals has been so greatly overshadowed by the visual cortex.) The complex functions of the mammalian thalamus could have evolved, I postulate, from this sort of beginning.
Prefrontal cortex and decision-making
What does the brain do when the rules that guide profitable behaviour change? An answer comes from M. Sarafyazd & M. Jazayeri Hierarchical reasoning by neural circuits in the frontal cortex Science Vol. 364 p.6441 17 May 2019
Addicts fail to control their drug use despite knowing the negative consequences. This reflects an increase in the power of ‘go’ signals from the orbitofrontal cortex to the basal ganglia and a decrease in the power of suppressive signals from other prefrontal areas. Y. Hu et al. Compulsive drug use is associated with imbalance of orbitofrontal- and prelimbic-striatal circuits in punishment-resistant individuals PNAS Vol. 116 p.9066 30 April 2019 The imbalance may be related to a less than normal growth of the myelin that insulates the nerve fibres in the former pathway during adolescence. A similar deficit in a parallel pathway from lateral and medial prefrontal cortex can be linked to impulsiveness. G. Ziegler et al. Compulsivity and impulsivity traits linked to attenuated developmental frontostriatal myelination trajectories Nat. Neurosci. Vol.. 22 p.992 June 2019
Decision-making involves ‘working memory’ in which prefrontal cortex plays a leading role. An impressive use of high-resolution functional MRI has distinguished the activity of upper layers of dorsolateral prefrontal cortex from that of lower layers during a working memory task. # E. S. Finn et al. Layer-dependent activity in human prefrontal cortex during working memory Naat. Neurosci. Vol. 22 p.1687 October 2019
The Default Mode Network
Over the past couple of decades cortical areas have been discovered which are quiet when attention is paid to the outside world and active during remembering and thinking. It’s now become apparent that they add up to “multiple interwoven networks.” # R. L. Buckner & L. M. DiNicola The brain’s default network: updated anatomy, physiology and evolving insights Nat. Rev. Neurosci. These networks must surely support, in the authors’ words “a class of processing function dependent on internally constructed rather than externally constrained representations.”
However there is intriguing evidence that the default network is not always suppressed when attention is paid to the external world. It can be active when aesthetic pleasure is derived from viewing paintings, architecture or landscape. # E. A. Vessel et al. The default mode network represents aesthetic appeal that generalizes across visual domains PNAS Vol. 116 p.19155 17 September 2019
Parts of the default mode network are also involved in mental imagery. # J. Pearson The human imagination: the cognitive neuroscience of visual mental imagery Nat. Rev. Neurosci. Vol. 20 p.624 September 2019
F. Huettig & M. J. Pickering suggest that people who learn to read fluently thereby learn to process language more quickly, gain an increased awareness of words as linguistic units, and where homophones are spelt differently can distinguish them more readily. The learning done through reading, moreover, can be done with less distraction. Literacy advantages beyond reading: Prediction of spoken language TICS Vol. 23 June 19
One of the most exciting aspects of neurobiology is the increasing understanding of mental disorders, and the promise of reducing the impact they have. In more general biology, meanwhile, a hot topic is the internal microbiome – the vast array of micro-organisms that live within and on us in profitable symbiosis. Skolnick and Greig have brought the two subjects together, suggesting that some symbiotic microbes, established in us for untold generations but sometimes lost in these days of liberal antibiotic consumption, may be essential for mental health. They cite two that contribute to the synthesis of neurotransmitters and one that assists in the excretion of mercury and possibly other heavy metals. These pathways, they say, ‘have the potential to impact systems relevant to a wide range of neurodevelopmental and psychiatric conditions including autism, depression, anxiety and schizophrenia.’ S. D. Skolnick & N. H. Greig Potential neuropsychiatric consequences of dysbiosis TINS Vol. 42 p.151 March 2019
A suggestive new idea about schizophrenia is to be found in L. Barkovitch et al. Disruption of conscious access in schizophrenia TINS Vol. 21 p878 November 2017. And it looks as if pyschopathy might be explained at least partially by a deficiency of emotional mirror neurons. L. A. Drayton et al. Psychopaths fail to automatically take the perspective of others PNAS Vol. 115 p.3302 27 March 2018 Nevertheless they are not incapable of considering the welfare of others, it was established. Hope for the addicted, meanwhile, is held out by M. Diana et al. Rehabilitating the addicted brain with trancranial magnetic stimulation Nat. Rev. Neurosci. Vol. 18 p.685 November 2017
Recovery after stroke involves incorporating new neurons in areas adjacent to the damage. The process is fuelled by corporal and the concomitant brain activity, and it turns out that brain areas compete for the new neurons (which are produced in the subventricular zone, SVZ) H. Liang et al. Region-specific and activity-dependent regulation of SVZ neurogenesis and recovery after stroke PNAS Vol. 116 p.13621 2 July 2019
Neonatal intensive care wards for premature infants tend to hum with the noise of essential machinery, and often music is played to mask the sound and provide more meaningful sensory stimulation. Lordier et al. find that music helps to establish important brain circuitry in preterm infants. L. Lordier et al. Music in premature infants enhances high-level cognitive brain networks PNAS Vol. 116 p.12103 11 June 2019