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The operation of Trepan, from Illustrations of the Great Operations of Surgery: Trepan, Hernia, Amputation, Aneurism and Lithotomy, by Charles Bell, 1815. (John Martin Rare Book Room at the University of Iowa’s
Hardin Library for the Health Sciences.)
Trepanation, or trephination (both derived from the Greek word trypanon, meaning “to bore”) is perhaps the oldest form of neurosurgery. The procedure, which is called a craniotomy in medical terminology, involves the removal of a piece of bone from the skull, and it has been performed since prehistoric times. The oldest trepanned skull, found at a neolithic burial site of Ensisheim in France, is more than 7,000 years old, and trepanation was practised by the Ancient Egyptians, Chinese, Indians, Romans, Greeks and the early Mesoamerican civilizations. The procedure is still performed today, for both medical and non-medical reasons.
The trepanned skulls found at prehistoric European sites contained round holes, which varied in size from just a few centimetres in diameter to nearly half of the skull. They are most commonly found in the parietal bone, and also in the occipital and frontal bones, but rarely in the temporal bone. In the earliest European trepanned skulls, the holes were made by scraping the bone away with sharp stones such as flint or obsidian; later, primitive drilling tools were used to drill small holes arranged in circles, after which the piece of bone inside the circle was removed. The late Medieval period saw the introduction of mechanical drilling and sawing instruments, whose sophistication would continue to increase for several hundred years.
There is a great deal of speculation about why ancient civilizations used trepanation, as it was – and still is – carried out in the absence of head trauma. However, it is almost certain that all those who used it did so because they somehow linked the brain with behaviour. Some anthropologists suggest that trepanation was performed as part of tribal or superstitious rituals. Other researchers believe that the procedure was used as a treatment for conditions such as headaches, epilepsy, hydrocephalus and mental disorders. These were presumably attributed to possession by evil demons, such that a hole in the skull would have provided the spirits a passage for escape. Although the reasons for trepanning and the instruments used for the procedure differ with time and from culture to culture, the result is always the same: a hole in the head, usually made when the individual was fully conscious and, often, unanaesthetized.
The popular delicacy foie gras (which is French for “fat liver”) is produced in a way that animal rights activists insist is barbaric. Ducks and geese are force-fed corn mash twice a day, through a tube that is inserted into the oesophagus. The birds are slaughtered 2-3 weeks later, and their engorged livers are then removed, to be sold whole or for use in making pâté, mousse or parfait.
But it seems that the slaughtered birds may be the ones who have the last laugh – researchers from the University of Tennessee Graduate School of Medicine, in collaboration with a group from Uppsala University in Sweden, have found a potential link between foie gras consumption and the development of a number of amyloidogenic diseases. The findings are published online this week, in the Proceedings of the National Academy of Sciences.
The amyloidogenic diseases include Alzheimer’s Disease, variant Creutzfeldt-Jakob Disease (vCJD), tuberculosis, diabetes and rheumatoid arthritis. They are termed “amyloidogenic” because they all involve a process called amyloidosis, whereby genetic mutations lead to the synthesis of abnormally folded and insoluble proteins which accumulate within or around cells and interfere with their function. In all the amyloidogenic diseases, the mutated proteins are believed to accumulate by a process called nucleation (or “seeding”).
The researchers purchased fresh duck- and goose-derived foie gras from three different commercial retail outlets in the United States and France. Histochemical analyses involving staining the liver tissue with amyloid protein-specific antibodies indicated that amyloid proteins were present in all the samples. And electron microscopy revealed that the fibrils which are characteristic of amyloidogenic diseases were indeed present in all the foie gras samples tested.
Then, mice carrying a mutated version of a gene that made them predisposed to amyloidosis were injected with amyloid protein extracts from the foie gras, or were fed with the foie gras for 8 weeks. Afterwards, the animals were dissected and examined, and it was found that 62% of the mice had amyloid deposits in the heart, kidneys, liver, and intestines. The amyloid proteins in the fois gras had significantly hastened the process of amyloidosis in those animals injected with or fed the food. The amyloidogenic effect of the foie gras was reduced, but not competely abolished, by cooking the livers in the way recommended by the suppliers. By contrast, no amyloid deposits were found in control animals that were not fed the foie gras.
Thus, the consumption of foie gras is potentially hazardous to those who are genetically predisposed to (i.e. have a family history of) amyloidogenic disorders. Discounting the consumption of infected brain tissue (during, for example, the ritual of mortuary cannibalism), this is the first time that a dietary component has been implicated in the amyloidogenic diseases.
Reference:
Solomon, A., et al. (2007). Amyloidogenic potential of foie gras. PNAS doi: 10.1073/pnas.0700848104. [Full text]
From Myolgie complette en couleur et grandeur naturelle, by Jacques Fabian Gautier d’Agoty, 1746. Reproduced with permission from the W. W. Kellogg Health Sciences Library at Dalhousie University, Halifax, Nova Scotia, Canada.
(via a fantastic new blog called Morbid Anatomy)
Synaesthesia is a condition in which there is increased connectivity between the areas of the brain that process information received from each sense organ. This leads to a mingling of the senses: for example, sounds may elicit perceptions of colour in a synaesthete who has increased connectivity between the brain’s visual and auditory pathways.
The first scientific description of the condition was provided by Francis Galton in the 1880s. But one form of the condition, called mirror-touch synaesthesia, was described for the first time only two years ago. People with this type of synaesthesia experience tactile sensations when they observe another person being touched. And a new study published online in Nature Neuroscience shows that mirror-touch synaesthetes are more empathetic than non-synaesthetes.
The study was led by Jamie Ward, of the Department of Psychology at University College London, who first described mirror-touch synaesthesia, and gave the condition its name, in 2005. Together with Michael Banissy, one of his graduate students, Ward recruited 10 synaesthetes who claimed to experience tactile sensations when they observed someone else being touched. In a series of experiments, the authenticity of the synaesthetes were verified. The participants were touched on the cheeks or the hands. At the same, they observed someone else being touched, either on the same part of the body that they were touched, or elsewhere. They were asked to report where they were touched, while ignoring their observations of someone else being touched.
It was found that the synaesthetic participants were much faster than the non-synaesthetes at reporting being touched when their observations corresponded to their own experiences. But the researchers were more interested in the situations when the observations of the synaesthetes did not correspond to their own sensations. They had reasoned that synaesthetes should find it harder than non-synaesthetes to distinguish between the sensations elicited by actually being touched and those elicited by observing someone else being touched. And this was indeed found to be the case: the synaesthetic participants made more “mirror-touch errors” than non-synaeshetic controls. That is, they often reported being touched on the same part of the body as those people they observed, even if they were touched on part of the body and person they observed was touched on a different part.
Banissy and Ward then measured the empathy quotients of their synaesthetic and non-synaesthetic participants. Both the experimental and control groups were asked to respond to a list of statements designed to measure their emotional and social skills. It was found that there was a strong correlation between mirror-touch synaesthesia and empathy – the synaesthetes responded more positively than non-synaesthetes to statements such as “I am good at predicting how someone will feel” and “I get upset when I see people suffering on news programmes.”
A region of the brain called the somatosensory cortex receives inputs from; the body is mapped onto this part of the brain, such that when one is touched, the subregion of the somatosensory cortex corresponding to that part of the body becomes active. It is activity in the somatosensory cortex that leads to the sensation of being touched, and it is now known that observing another person being touched also activates the somatosensory cortex. And several years ago, a neuroimaging study conducted by Ward and his colleagues showed that this region of the brain is hyperactivated in mirror-touch synaesthetes when they observe someone else being touched.
The somatosensory cortex and the areas surrounding it (including the primary motor cortex) are hypothesized to be a major component of the brain’s “mirror system”. The mirror system is composed of neurons which fire not just when one is performing a particular action, but also when one observes another performing that action. Thus, it is believed that these cells are involved in “mirroring” the behaviour of others so that the brain can generate simulations of their experiences. It has further been suggested that the mirror system is crucial for the acquisition of behaviors that are learnt through imitiation, such as language, and that it is impaired in conditions such as autism.
Banissy and Ward show for the first time that the sensations elicited in mirror-touch synaesthetes while observing someone else being touched are indistinguishable from those felt when they are actually touched. The researchers had no difficulty in recruiting participants for their study, and all those involved were actually unaware that they had the condition – they believed that their synaesthetic experiences were completely normal. Mirror-touch synaesthesia may, therefore, be relatively common. Perhaps the condition has gone by another name; the somatosensory hyperactivation that occurs when observing others may cause feelings of discomfort, or even pain, when observing someone being hurt. Perhaps, mirror-touch synaesthesia is in fact the condition formerly known as “squeamishness.”
References:
Banissy, M. J. & Ward, J. (2007). Mirror-touch synesthesia is linked with empathy. Nature Neurosci. doi: 10.1038/nn1926. [Abstract]
Blakemore, S. -J., et al. (2005). Somatosensory activations during the observation of touch and a case of vision-touch synaesthesia. Brain 128: 1571-1583. [Abstract]
The name Ivan Pavlov should ring a few bells for anybody who has studied psychology. About 100 years ago, Pavlov, a Russian physiologist, performed a series of classic experiments which showed that dogs could be taught to respond in a particular way to a given stimulus. Pavlov was therefore the first to demonstrate a form of behaviour modification called classical conditioning.
Aversion therapy, a form of behaviour modification used by psychiatrists, is based on classical (or Pavlovian) conditioning. For example, in dealing with a patient prone to alcoholism, the therapist might repeatedly pair alcohol with an emetic drug (one that induces vomiting), in the hope that the association between the two would elicit in the patient a feeling of sickness that leads to the avoidance of alcoholic drinks. In the U.S., aversion therapy was used to “treat” homosexuals and sexual deviants until rather recently: the American Psychological Association declared that it was dangerous and ineffective in 1994.
Classical conditioning and aversion therapy are effective in modifying behaviour. However, if the association between the two stimuli is not constantly reinforced, the elicited response becomes weaker and eventually disappears, in a process called extinction. Pavlov’s method has been demonstrated not only in humans and in dogs, but also in an invertebrate – the sea slug Aplysia californica. And now, researchers from Tohoku University in Sendai, Japan show that such conditioned responses can also be elicited in an insect – they have created Pavlov’s cockroach.
Just as Pavlov conditioned his dogs to salivate in response to the sound of a bell, the Japanese team conditioned their cockroaches to salivate when presented with a particular smell. They exposed the cockroaches to vanilla and peppermint odours. During exposure to one of them, the insects were also presented with a sucrose solution. After repeated pairings, they learnt to associate that odour with the sucrose solution, so that, when the odour was placed on its antennae, it would salivate in anticipation of a sugary drink. Exposure to the other odour, however, elicited no such response. The researchers even measured the amounts of saliva secreted by their experimental cockroaches in response to the odour associated with the sucrise solution: 100-200 nanolitres of saliva per second.
In the cockroach, the salivary glands are innervated by three types of neurons, each of which releases a different neurotransmitter – one type, called SN1, synthesizes and releases dopamine; the second, termed SN2, uses GABA; and the third uses serotonin. The authors had previously shown that these salivary neurons fire in response to conditioned stimuli, and that their activity caused the cockroach to salivate. And last year, researchers from the University of Edinburgh showed that cells in the hypothalamus of the rat fire in response to both food and anticipation of it. But little else is known about the neural bases of classical conditioning. Significantly, this study shows that the cellular mechanisms for controlling salivation, and perhaps the other autonomic functions, in the cockroach are somewhat similar to those of vertebrates. So researchers may have a simple animal model for those same mechanisms in the mammalian brain.
Reference:
Watanabe, H. & Mizunaim, M. (2007). Pavlov’s cockroach: Classical conditioning of salivation in an insect. PLoS ONE 2(6): e529. doi:10.1371/journal.pone.0000529. [Full text]
Sea sponges are sedentary organisms that attach themselves to the sea bed and filter nutrients from the water that they force through their porous bodies with flagella. They are the most primitive of all multicellular animals, with just four different types of cells making up partially differentiated tissues in a simply organized body.
Because of the lifestyle they lead, sea sponges do not need, and therefore lack, nerve cells, muscle cells and internal organs of any kind. However, researchers from the University of California at Santa Barbara now find that one species of sea sponge, called Amphimedon queenslandica, synthesizes many of the proteins that are essential for the cell-to-cell communication that takes place within nervous systems. These surprising findings, which are published in the open access journal PLoS One, therefore provide clues about how the first neurons may have evolved in the most ancient of animals.
Neurons are specialized to communicate with one another. The signalling between one nerve cell and another takes place at a structure called the synapse, a miniscule gap of about 40 nanometres found at the junction between adjacent cells. The gap itself is no more than a space across which chemical signals (the neurotransmitters) diffuse. The active elements of the synapse are the two apposed cell membranes – the pre-synaptic membrane, from which the chemical signals are released, and the post-synaptic membrane, which detects the signals and responds to them in the appropriate way by communicating to the interior of the cell.
Each neuron has regions of its cell membrane specialized for sending the chemical signals, and others specialized for receiving them. The signalling process is known to involve many dozens of proteins that are either embedded within, or located just inside, the membrane. Each type of protein has a specific role in the signalling process. At the pre-synaptic membrane, for example, neurotransmitter molecules are packaged in synaptic vesicles, which remain “docked” at release sites along the inside of the membrane until needed. When a nervous impulse reaches the nerve terminal, it causes the vesicles to fuse with the membrane and to release their contents into the synapse. Once a vesicle is emptied of its cargo, it goes through an elaborate process of recycling. Docking of vesicles is mediated by one family of proteins; fusion of the vesicles with the membrane is mediated by another protein family, and a number of different families of protein are involved in the vesicle recycling process.
As the neurotransmitters diffuse across the synapse, they bind to receptors, Some receptors are voltage-gated ion channels; these are barrel-shaped proteins with a central pore that spans the membrane and opens in response to binding of a transmitter molecule. When the channel opens, it allows the passage of a specific type of ion (sodium, say, or potassium) into or out of the cell. This is a typical response of a neuron – the ion movements change the voltage across the membrane, altering the pattern of signals generated by that cell. Other receptors do not contain pores but instead are specialized to interact with various other proteins inside the cell, in response to a signal. These interactions initiate biochemical reactions within the cell, or communicate the signal to the cell membrane so that the activity of a specific gene or set of genes is altered.
For synaptic signalling to be effective, it is crucial that all the proteins involved are organized correctly. At both the pre- and post-synaptic membranes, this organization is achieved by a scaffold of proteins called the pre- and post-synaptic densities (left, click to enlarge). These structures are a specialization of the cytoskeleton found just beneath the pre- and post-synaptic nerve cell membranes. The density is a highly complex network – in humans it contains perhaps several hundred different types of protein – which organizes the molecular machinery needed for a neuron to detect and respond to the chemical signals sent to it by adjacent cells.
The density also regulates the movements of the machinery within the membrane and the area immediately inside it, and, so, is a highly dynamic structure. The dynamic nature of the density is believed to be crucial the strengthening of synapses that occurs during memory formation. Shuffling of proteins within the post-synaptic density after an initial stimulus may, for example, lead to an increase in the number of receptors at a certain region of the membrane, such that the subsequent response to the same stimulus becomes more efficient. Thus the density is essential in maintaining the integrity of the synapse, and is a major player in the neuronal plasticity that underlies learning and memory.
In the new study, Kenneth Kosic and his colleagues analyzed the Amphimedon genome, and found that it contains 36 families of genes known to encode proteins of the post-synaptic density. So, even though it has no neurons, this sea sponge synthesizes an almost complete set of post-synaptic density proteins. A comparison of the DNA sequences from the 36 sea sponge genes with the homologous sequences from humans, Drosophila melanogaster (fruit flies) and Nematostella vectensis (a cnidarian with a simple nervous system, consisting of a loose network of nerves) revealed striking similarities between the genes in all four species. One gene, called dlg, encodes a crucial component of the post-synaptic density scaffold. The protein product of that gene contains a number of regions that form the protein-protein bonds that hold the scaffold together. The segment of the dlg gene encoding these binding regions was found to be highly conserved – the DNA sequences in the sea sponge gene were identical to the human sequences. This suggests that in the sea sponge these proteins interact in exactly the same way as they do in the human post-synaptic density.
Amphimedon has nearly all the components required to make a post-synaptic density; only a few of the human postsynaptic density genes are missing from the sea sponge’s genome – those encoding ion channel receptors for the neurotransmitter glutamate. (These genes are, however, present in the cnidarian, which expresses them in its simple nervous system.) In sponges, the genes are expressed predominantly in the flask cells of the free-swimming larvae, where they may be involved in sensing chemical cues found in the organism’s environment. Flask cells with post-synaptic densities may predate the first neurons. If so, the first synapses may have evolved from post-synatic densities in a process called exaptation, whereby a pre-existing structure is modified slightly to perform a new function. It is, however, also possible that flask cells evolved from simple neurons that lost some of their synaptic components.
Reference:
Sakarya O., et al. (2007). A post-synaptic scaffold at the origin of the animal kingdom. PLoS ONE 2 (6): e506. doi:10.1371/journal.pone.0000506. [Full text]
A new study into the biophysical properties of a highly reflective and self-organizing squid protein called reflectin will inform researchers about the process of “bottom-up” synthesis of nanoscale structures and could lead to the development of thin-film coatings for microstructured materials, bringing scientists one step closer to the development of an “invisibility cloak.”
The reflectin protein comes from the Hawaiian Bobtail squid, Euprymna scolopes, which is native to the Central Pacific ocean. E. scolopes leads a nocturnal existence in the shallow waters off the coasts of Hawaii and Midway Island. Like other cephalopods, this species can manipulate the sunlight falling upon it to produce rapid changes in colouration and bioluminescence in order to camouflage from predators and prey and, as was recently discovered, to communicate with each other.
These changes are produced by a number of neurally-controlled photonic structures found throughout the squid’s body. One of these, called the bilobed light organ, houses bioluminescent bacteria of the species Vibrio fischera. The squid and the bacteria have a symbiotic relationship – in return for generating light, the bacteria receive nutrients from the squid. The bacteria colonize the hatchling squid and secrete a toxin called tracheal cytotoxin. This toxin, which is a small fragment of a bacterial cell surface protein called peptidoglycan, causes whooping cough and gonorrhea in humans. But in E. scolopes, it serves a more useful function – it acts in synergy with various other substances to regulate the development of the light organ. The entire surface of the squid’s body can also be considered as a light organ, as it contains reflective tissues in the mantle. Unlike the bilobed organ, whose reflectivity is static, the organs in the skin mantle have variable reflectivity, changes in which can quickly camouflage the squid and even make it invisibile.
The light organ and the reflective tissues in the skin mantle consist largely of proteins called reflectins. These are encoded by at least six genes which appear to be unique to squid. Nearly 44% of the reflectin primary sequence (the string of amino acids encoded in the reflectin gene) is made up of aromatic amino acid residues and amino acids containing sulphur. Reflectins are insoluble, and are deposited inside the light organs as flat, stacked structures called platelets. The reflectin molecules in the platelets are organized irregularly, and the layers formed by the stack of platelets alternate between areas of high and low refractive indices, so that the stacks act as multilayer reflectors (see the image below on the right). Because of this structure, incident light is reflected and scattered in all directions. The rapid changes in colouration that are characteristic of squid are the result of rearrangements in the organization of reflectin deposits within the reflective tissues, which may occur as a result of post-transcriptional modifications to the protein. Because of their chemical composition and the way in which they assemble themselves, the reflectins have the highest refractive index of any known proteins.
In the new study, which is published in Nature Materials, Rajesh Naik and his colleagues at the Air Force Research Laboratory in Dayton, Ohio inserted the gene encoding reflectin 1a into Escherichia coli bacteria. The recombinant protein synthesized by the E. coli cells was isolated and purified, and the self-assembly of the molecules was investigated under different conditions. The photonic properties of the structures were then investigated, and transmission electron microscopy was used to examine the structures formed in detail.
Naik’s group found that the reflectin molecules formed a number of different types of structure, depending on the conditions in which they were assembled. In solutions of very low concentration, the reflectin molecules spontaneously precipitated to form nanospheres with diameters of 50-1,000 nanometres (nm, billionths of a metre), while with higher concentrations they formed . At low concentrations in non-reducing conditions (that is, in the absence of spare electrons), the precipitated nanospheres were optically clear – light could pass unhindered through them. But in reducing conditions (with spare electrons available) precipitation led to the formation of filamentous structures. (These fibres different from the fibres associated with aggregation of abnormally folded proteins in neurodegenerative disorders such as Alzheimer’s Disease and the transmissible spongiform encephalopathies, which are formed by crystallization and not by precipitation.) And when left at 4°C for several weeks, these filaments formed a webbed structure that assembled itself into ribbons.
The researchers then sought to process the recombinant reflectin protein into films and fibres. They used a technique called flow-coating: small amounts of protein solution were added onto a silicon wafer substrate, and the edge of a blade was used to spread the solution across the surface of the wafer. This cast a thin film of the protein across the surface of the wafer. By altering the concentration of the solution used, films of different thickness were formed. The thickness of the film was found to determine the wavelength of light reflected by it. For example, exposure to water vapour dramatically increased film thickness from ~120 nm to ~207 nm. As a result, the wavelength of light reflected by the film changed from 760 nm (which corresponds to red light) to around 400 nm, which gave rise to a blue reflectance. When the water vapour was removed, the film became thinner and then began to reflect red light once again. In this way, the researchers formed films that reflected every colour in the visible light region of the electromagnetic spectrum; They also made gradient films whose thickness differed, so that a rainbow of colours was reflected along the length of the film.
The reflectin solution on the silicon wafer substrate was then dipped into an ionic solution (i.e. one containing positively or negatively charged atoms) called BMIM. This resulted in the formation of striped patterns of reflectin protein on the wafers. The patterns had highly regular spacing which extended unblemished for distances of up to several millimetres. The researchers found that the spacing between the stripes depended upon the velocity of dipping – the greater the velocity, the smaller the space between each stripe. These striped patterns are what materials scientists call diffraction gratings – reflective or transparent elements that split incident light into its constituent wavelengths and are used in a variety of optical devices. Light reflection by the reflectin stripes was observed even while the silicon wafers were still submerged in the solvent, and the wavelength of light scattered could be changed by increasing or decreasing spacing between the stripes.
The researchers had provoked nature into initiating some of her own mechanisms of nanofabrication. The reflectin molecules assembled themselves into nanometre-sized spheres and striped microstructures with photonic properties that could be manipulated. Knowledge of how the self-assembly of reflectins can be manipulated will inform researchers who are trying to synthesize supramolecular nanostructures from the “bottom up”. But a better understanding of these mechanisms and of the properties of reflectin has another potential application: the synthesis of small-scale materials for use in the development of invisibility cloaks.
To this aim, various research groups are using so-called metamaterials, whose nanoscale properties alter the way in which their surfaces reflect visible light. Objects are visible because light bounces off them; in theory, a material which could cause incident light to pass round it could be used in a cloak that covers objects and renders them invisible. The military is, of course, very interested in developing such a material. As the current research was partly funded by DARPA (the research and development arm of the Pentagon), the development of an invisibility cloak could well be the ultimate aim of the project.
References:
Kramer, R. M., et al. (2007). The self-organizing properties of squid reflectin protein. Nature Mater. doi: 10.1038/nmat1930. [Abstract]
Crookes, W. J., et al. (2004). Reflectins: The unusual proteins of squid reflective proteins. Science 303: 235-238. [Full text]
Researchers at the Hebrew University of Jerusalem have developed a miniature image-guided robot for use during keyhole neurosurgery.
The MiniAture Robot for Surgical Applications (MARS) was designed and developed by Leo Joskowicz and his colleagues at the Hebrew University’s Computer Aided Surgery and Medical Image Processing Laboratory. It measures 5 x 5 x 8.5 cm and weighs just one quarter of a kilogram.
The MARS device can be mounted on the patient’s head using a clamp, or attached directly to the skull with two small screws. It positions itself with respect to the structures being operated on. It is programmed to do so using data obtained pre-operatively by computed tomography (CT) or functional magnetic resonance imaging (fMRI). The robot can then be used by surgeons to target neuroanatomical structures. This targeting is at least as accurate, if not more so, than that of existing neurosurgical robots – structures in the brain can be targeted to within 1 mm.
This will provide improved dexterity for surgeons as they guide their instruments towards target structures in the brain. MARS is already being used for spinal and orthopaedic surgery, and has proved to be safe. It could also be used for procedures such as brain biopsies, the draining of cerebral hemorrhage, and the insertion and placement of catheters or implants containing electrodes.
Recall of a particular memory often involves its selection from several memories which are similar to it. Although these other memories are irrelevant, their similarity to the “target” memory leads to competition for retrieval, placing demands on the cognitive mechanisms by which the correct memory is selected.
A new study by researchers at Stanford University now shows that forgetting may be part of the process of remembering. Anthony Wagner and his colleagues at the Stanford Memory Laboratory provide evidence that the active suppression by the brain of competing memories is essential for proper memory function. Their findings – that to remember something involves forgetting something similar – have been published online in Nature Neuroscience.
In the study, 20 participants performed a simple memory test. They were first asked to study and remember a series of three word pairs. Two of these word pairs (ATTIC-junk and ATTIC-dust) were similar and could easily be confused with each other, while the third (MOVIE-reel) was completely unrelated. The participants were asked to examine one of the two similar word pairs (ATTIC-dust) again. They were then presented with retrieval cues (the capitalised words) to help them recall all three pairs.
The researchers had hypothesized that during retrieval the similarity of the two word pairs containing the word ‘ATTIC’ would lead to competition between the two, and that the presentation of one of the pairs a second time would cause the memory of the other to be suppressed. This was indeed the case – the participants had greater difficulty retrieving the ATTIC-dust word pair than in recalling the other two word pairs. For example, the unrelated word pair, MOVIE-reel, was recalled 15% more of the time than was ATTIC-dust.
With practice, the participants became more adept at retrieving one word pair and suppressing the memory of the other. Using functional magnetic resonance imaging (fMRI) the researchers showed that this “increased forgetting” of the competing word pair was closely correlated with changes in the activity of a region in the prefrontal cortex called the anterior cingulate cortex (ACC), is known to be involved in detecting similarities in, and conflict between, similar (or competing) information. It was found that activity in the ACC decreased with each successive trial.
This retrieval-induced forgetting has its benefits – the suppression of competing but irrelevant memories reduces the demands placed on the cognitive control mechanisms mediating selection of memories for retrieval. The process by which the relevant memory is selected for retrieval therefore becomes more efficient. Thus, forgetting may be crucial to remembering.
Reference:
Kuhl, B. A., et al (2007). Decreased demands on cognitive control reveal the neural processing benefits of forgetting. Nat. Neurosci. doi:10.1038/nn1918. [Abstract]
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