The Memory of a Smell

English

 

From Proust’s Madeleines to the overbearing food critic in the movie Ratatouille who’s transported back to his childhood at the aroma of stew, artists have long been aware that some odors can spontaneously evoke strong memories. Scientists at the Weizmann Institute of Science have now revealed the scientific basis of this connection. Their research appeared in the latest issue of Current Biology.
 
Graduate student Yaara Yeshurun, together with Profs. Noam Sobel and Yadin Dudai of the Institute’s Neurobiology Department, thought that the key might not necessarily lie in childhood, but rather in the first time a smell is encountered in the context of a particular object or event.  In other words, the initial association of a smell with an experience will somehow leave a unique and lasting impression in the brain.
 
To test this idea, the scientists devised an experiment: First, in a special smell laboratory, subjects viewed images of 60 visual objects, each presented simultaneously with either a pleasant or an unpleasant odor generated in a machine called an olfactometer.  Next, the subjects were put in an fMRI scanner to measure their brain activity as they reviewed the images they’d seen and attempted to remember which odor was associated with each.  Then, the whole test was repeated – images, odors and fMRI – with the same images, but different odors accompanying each.  Finally, the subjects came back one week later, to be scanned in the fMRI again. They viewed the objects one more time and were asked to recall the odors they associated with them.
 
The scientists found that after one week, even if the subject recalled both odors equally, the first association revealed a distinctive pattern of brain activity. The effect was seen whether the smell was pleasant or unpleasant. This unique representation showed up in the hippocampus, a brain structure involved in memory, and in the amygdala, a brain structure involved in emotion. The pattern was so profound, it enabled the scientists to predict which associations would be remembered just by looking at the brain activity within these regions following the initial exposure. The scientists could look at the fMRI data on the first day of the experiment and predict which associations would come up a week later.
 
To see if other sensory experiences might share this tendency, the scientists repeated the entire experiment using sounds rather than smells; they found that sounds did not arouse a similar distinctive first-time pattern of activity. In other words, these results were specific to the sense of smell. ‘For some reason, the first association with smell gets etched into memory,’ says Sobel, ‘and this phenomenon allowed us to predict what would be remembered one week later based on brain activity alone’.
 
Yeshurun: ‘As far as we know, this phenomenon is unique to smell. Childhood olfactory memories may be special not because childhood is special, but simply because those years may be the first time we associate something with an odor'.
 
Prof. Noam Sobel’s research is supported by the Nella and Leon Benoziyo Center for Neurosciences; the J&R Foundation;  the Eisenberg-Keefer Fund for New Scientists;  and Regina Wachter, New York, NY
 
 
The Weizmann Institute of Science in Rehovot, Israel, is one of the world's top-ranking multidisciplinary research institutions. Noted for its wide-ranging exploration of the natural and exact sciences, the Institute is home to 2,600 scientists, students, technicians and supporting staff. Institute research efforts include the search for new ways of fighting disease and hunger, examining leading questions in mathematics and computer science, probing the physics of matter and the universe, creating novel materials and developing new strategies for protecting the environment.
 
Weizmann Institute news releases are posted on the World Wide Web at

http://wis-wander.weizmann.ac.il, and are also available at http://www.eurekalert.org.

Life Sciences
English
  • Noam Sobel
  • fMRI

Combining Minds

English
 

Dr. Elad Schneidman. A neuron collective

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The scope of the brain’s activities presents scientists with a seemingly impossible challenge: Each of its 100 billion nerve cells – neurons – is in contact with about 10,000 others. They communicate through a series of electrical signals that get translated, at the synapses between them, to chemical signals. To interpret these “conversations,” we would need a sort of “bar code scanner” that could read the electrical signal patterns. But even for a relatively small group of cells – just 100 – the number of possible combinations of signals is 1030 (that’s one followed by 30 zeroes) – more than all the stars in the universe.

 
Clearly, mapping the brain neuron by neuron is an impossible feat; thus, many scientists choose to look at the small picture – interactions between small groups of neurons. But Dr. Elad Schneidman of the Institute’s Neurobiology Department adopts the lessons from small networks and develops new methods for combining them to decipher the workings of larger ones. He takes as his starting point groups of tens or hundreds of interconnected neurons, working upward from the observed relations between pairs and triplets of cells. “The most interesting activities involve hundreds or thousands of neurons. With today’s technology, we can investigate groups of 100 to 200 – enough to allow us to explore the fundamental design principles of brain networks,” he says. And his results are providing insight that couldn’t be obtained with individual-neuron methods. In his postdoctoral research, for instance, he showed that, even though the typical correlation between pairs of neurons is weak, the combined effect of these correlations in the group is strong. “It’s like peer pressure: One opinion doesn’t carry much sway, but the joint effect of many weak signals together can control the group as a whole.”
 
Using advanced computational tools borrowed from computer science and physics, Schneidman analyzes brain networks, breaking them down into their components and looking for patterns that can clarify the rules and regulations of collective behavior for large groups of nerve cells. For example, in a series of experiments conducted with Dr. Ronen Segev of Ben-Gurion University of the Negev, the electrical responses of a network of nerve cells in the retina of a salamander were recorded. The scientists showed long movie clips of the salamander’s natural environment to a group of 100 of these cells and measured their reaction, obtaining a bar code readout of patterns of electrical signals. After uncovering some of the basic rules of population activity, they then demonstrated that this allows for direct “reading” of the neuron code: Schneidman and his collaborators reconstructed the instantaneous gray level of a small part of the simple movie directly from the activity patterns of the neurons – a feat that is unattainable with single-cell methods.
 
The collective-as-a-complex-sum-of-its-parts approach may help to explain a puzzling phenomenon: that of the brain’s ability to deal with its own internal “noise.” Not only are single nerve cells notoriously inconsistent in their reactions to the same stimulus, but communication between cells is surprisingly unreliable, as well.
 
Schneidman believes that nature didn’t make the effort to create precise, noise-free nerve cells because getting them to act as a group is a “cheaper,” more efficient solution, and one that has other benefits: Such redundant activity, in addition to neutralizing noise, creates a sort of “backup” that keeps the activity going even when individual brain cells die. Furthermore, such noise may facilitate the use of trial-and-error tactics for learning.
 

The principles Schneidman is uncovering for the group activities of nerve cells may be valid for other systems. “These mathematical rules and models cross the borders between scientific fields,” he says. “Different physical and biological phenomena – from the collective actions of atoms in a magnetic field to cooperative food searching strategies of groups of animals – share similar organizing principles.” For example, he is working on mathematical models that elucidate the group behavior of shoals of fish seeking food or flies zeroing in on a food source. These models, he hopes, may provide insight into more complex group behavior: how individuals influence each other, the role of “free will” within the group and its limits, group learning, how groups deal with communal crises and more. Schneidman hopes that uncovering principles of group behavior will help shed light on little-understood aspects of social conduct in many kinds of creatures, including those that may possibly be the hardest to pin down – human beings.

 

Dr. Elad Schneidman’s research is supported by the Nella and Leon Benoziyo Center for Neurological Diseases; the Clore Center for Biological Physics; the Carl and Micaela Einhorn-Dominic Brain Research Institute; the Eisenberg-Keefer Fund for New Scientists; and the Peter and Patricia Gruber Award.

 

From Computers to Brains


Dr. Elad Schneidman was born in Jerusalem in 1969. He studied for a B.Sc. in physics and computer science through the Hebrew University’s “Amirim” program for outstanding students before serving in the IDF as a research and development officer. He then worked for a number of high-tech companies and, in 2001, completed his doctorate in the Hebrew University’s Interdisciplinary Center for Neural Computation. After five years of postdoctoral research at Princeton University, Schneidman returned to Israel and joined the Weizmann Institute’s Neurobiology Department. He is married to Hadas Mechoulam, an ophthalmologist, and they have three children.
 
 
Dr. Elad Schneidman.
Life Sciences
English
  • Brain research
  • Elad Schneidman
  • Neural code
  • Neurobiology

Screen Saver

English
Malach and his research team. Brain at work, even while resting

 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
What's happening in a brain that's disengaged from any focused task? The owner of that brain might be under the impression that when his feet are up and his eyes are closed, most mental activity comes to a standstill; but research at the Weizmann Institute shows that, in truth, the nerve cells in his brain – even those in the visual centers – are still humming with activity.

Previous research conducted by Prof. Rafael Malach and research student Yuval Nir of the Neurobiology Department used functional magnetic resonance imaging (fMRI) to measure brain activity in active and resting states. The results of their study and other similar studies around the world were as surprising as they were controversial: The magnitude of brain activity for the various senses (sight, touch, etc.) in a resting brain is quite similar to that of one exposed to a stimulus. Each sense exhibits a distinctive pattern, spread around different areas of both hemispheres in the brain; the fMRI images show activity in all the areas, whether an outside stimulus is present or not. But fMRI is an indirect measurement of brain activity; it can't catch the nuances of the pulses of electricity that characterize neuron activity.

Together with Prof. Itzhak Fried of the University of California at Los Angeles and a superb team in the EEG unit of the Tel Aviv Sourasky Medical Center, the researchers found a unique source of direct measurement of electrical activity in the brain: data collected from epilepsy patients who underwent extensive testing, including measurement of neuronal pulses in various parts of their brain, in the course of diagnosis and treatment.

An analysis of these data showed conclusively that electrical activity does, indeed, take place even in the absence of stimuli. On the other hand, the nature of the electrical activity differs between the two states. In results that appeared recently in Nature Neuroscience, the scientists showed that the resting activity consists of extremely slow fluctuations, as opposed to the short, quick bursts that typify a response associated with a sensory percept. This difference may explain why, even though our sensory neurons are constantly active, we don't experience non-existent stimuli – hallucinations or voices that aren't there – during rest. Moreover, according to the research, the rest-time oscillations appear to be strongest when we sense nothing at all – during dream-free sleep.

Malach compares the slow fluctuation pattern to a computer screen saver. Though its function is still unclear, the researchers have a number of hypotheses. One possibility is that neurons, like certain philosophers, must "think" in order to be. Survival, therefore, is dependent on a constant state of activity. Another suggestion is that the minimal level of activity enables a quick start when a stimulus eventually presents itself, something like a getaway car with the engine running. Nir: "In the old approach, the senses are 'turned on' by the switch of an outside stimulus. This is giving way to a new paradigm in which the brain is constantly active, and stimuli change and shape that activity."

Malach: "The use of clinical data from a hospital enabled us to solve a riddle of basic science in a way that would have been impossible with conventional methods. These findings could, in the future, become the basis of advanced diagnostic techniques." Such techniques might not necessarily require the cooperation of the patient, allowing them to be used, for instance on people in a coma or on young children.  
 

 

Replay

 
In research that appeared in Science, Malach, research student Hagar Gelbard-Sagiv with Dr. Michal Harel of the Institute's Neurobiology Department, and Fried with postdoctoral fellow Dr. Roy Mukamel of UCLA, showed how remembering, at least a few minutes after the original event, is something like a rerun of that event.

Working with epilepsy patients who had thin electrodes implanted in their brains for medical purposes, the scientists were able to measure the electrical activity of individual neurons. The participants were shown a series of film clips of everything from The Simpsons and Seinfeld episodes to historical events and classic movies. As they watched, their brain's electrical activity was being measured, particularly groups of neurons in several brain areas associated with memory. These nerve cells showed preferences for some clips over others by increasing their activity; the researchers were able to connect specific patterns of electrical activity to the clips that elicited them.

A few minutes later, while their brain activity was still being measured, the subjects were invited to think about the clips they had viewed, and to report each time a new clip came to mind. The scientists found that the patterns of brain activity in remembering were so similar to those observed during the original viewing that they were able to tell which clip the subject was recalling – about a second and a half before he or she said it aloud. "It's possible that it takes a second and a half – not a short period by brain activity standards – for the subject to be able to articulate his memory. On the other hand, it might be that part of that time is spent in bringing that memory to the surface, while the conscious brain is still unaware of the event it will recall," says Gelbard-Sagiv.

Prof. Rafael Malach's research is supported by the Nella and Leon Benoziyo Center for Neurological Diseases; the Carl and Micaela Einhorn-Dominic Brain Research Institute; Vera Benedek, Israel; the Benjamin and Seema Pulier Charitable Foundation, Inc.; and Mary Helen Rowen, New York, NY. Prof. Malach is the incumbent of the Barbara and Morris Levinson Professorial Chair in Brain Research.
 
Standing: (l-r) Eran Dayan, Dr. Son Preminger, Dr. Guido Hesselman and Lior Fisch. Sitting: (l-r) Roye Salomon, Michal Ramot, Ido Davidesko, Prof. Rafael Malach and Michal Harel. Constant activity
Life Sciences
English
  • Brain research
  • Neurobiology
  • Rafael Malach
  • Sensory stimulus

When a Woman Is More Like a Man

English
 
Tali Kimchi. The genetics of sexual behavior
 
 
 
 
 
 

 

 
 
 
 
 
Men and women might appear to come from different planets, but in reality, male and female brains are largely similar. So similar, in fact, that at the flip of a single genetic switch, calm, nurturing female mice can be turned into aggressive, rump-sniffing Casanovas – according to a study conducted by Dr. Tali Kimchi, who recently joined the Weizmann Institute's Neurobiology Department.

While carrying out postdoctoral research at Harvard University in the laboratory of Prof. Catherine Dulac, Kimchi found that the behavior of female mice crucially depended on pheromones, the subtle scents that animals secrete to communicate with one another and attract the opposite sex. When Kimchi deprived these mice of just one gene – one responsible for picking up the pheromone signal –   the mouse moms started neglecting their pups and failed to protect them. At the same time, they began to display typical male sexual behaviors: They sniffed and chased potential mates, tried to mount them with pelvis-thrusting movements and even emitted high-pitched courtship whistles, of which females were previously thought incapable.
 
This macho display by the female mice suggests that their brains are wired for both female and male behaviors, and that pheromones probably suppress the male-typical behaviors and turn on the female-typical behaviors. These findings were reported in Nature, which ran a News and Views commentary entitled "Females Can Also Be from Mars."
 
At the Weizmann Institute, Kimchi will continue to explore the role of pheromones in masculine and feminine behaviors. She will investigate the hormonal, genetic and neurochemical basis of pheromone activity while putting emphasis on both social and reproductive behaviors.

In her new lab, genetically modified mice will be housed in large, enriched enclosures and be monitored by several different types of surveillance equipment, including infrared cameras to closely observe the social interactions of the mice night and day, and microphones to eavesdrop on their "conversations." In contrast to reality TV shows, which usually block out intimate moments, this "Big Brother" setup will pay special attention to the sexual and reproductive activity of the mice. The scientists will strive to reveal the factors controlling such behaviors as making the distinction between males and females, fighting over desirable mates and taking care of pups.

Kimchi works with dark-furred mice bred from animals caught in the wild. These are in many ways closer to nature than their laboratory-bred counterparts, even though they are much smaller, quicker and more difficult to study. She focuses on female mice, which pose a particular challenge as their behavior is less predictable than that of the males. For example, a male mouse can almost always be counted upon to attack a rival male or try to mount a female in heat, whereas it's hard to predict how a female mouse will act in many situations.
 
Kimchi's research aims at casting new light on the biological roots of behaviors typically viewed as masculine or feminine. For more than half a century, scientists have believed that males and females were set apart mainly by sex hormones that controlled hard-wired brain circuits. Recent studies by Kimchi and others, however, suggest a different view: Gender-specific behaviors stem from small subsets of neurons "switched" on or off by pheromones and other sensory signals. Interestingly, the macho females in Kimchi's studies had normal female hormones and estrous cycles.
 
A long-term goal of this project is to discover new genes and neuronal circuitry that govern the physiology and reproductive behavior of mammals. Kimchi's findings might, for example, improve our understanding of mental disorders believed to be related to a person's gender, such as autism, sexual dysfunction, and mood and anxiety-related disorders.  

 

A Head Start in Science
 

When Dr. Tali Kimchi went for walks with her parents at the age of four, she would occasionally stop them so that she could observe rows of ants crossing the road. For as long as she can remember, she always wanted to study animal behavior and other natural phenomena.
 
During her Ph.D. studies in Tel Aviv University's Department of Zoology under the guidance of Prof. Joseph Terkel, she provided fascinating insights into the navigational behavior of the blind mole rat, a 25-cm-long rodent that lives underground, has no eyes and looks like a furry sausage with sharp front teeth. Kimchi found that in digging its tunnels in search of food and mates,the mole rat chooses its course by using the Earth's magnetic field like a compass. When this blind rodent encounters an obstacle, it always burrows its tunnel along the shortest possible detour. In doing so, Kimchi discovered, the mole rat relies on a peculiar natural "radar": It bangs its head against the tunnel wall to create vibrations, detects the reverberations with its feet and processes these signals to identify the optimal route for digging.

 
Dr. Tali Kimchi's research is supported by the Carl and Micaela Einhorn-Dominic Brain Research Institute; Rina Mayer, Israel; and Esther Smidof, Switzerland.
Dr. Tali Kimchi. Behavioral switches
Life Sciences
English
  • Brain research
  • Sex›Sex role
  • Tali Kimchi

Smelling Like a Rose

English
 
Is the distance between the smell of almonds and that of bananas greater than the distance between the smell of almonds and that of roses? A new method, developed by research student Rafi Haddad under the supervision of Prof. Noam Sobel of the Neurobiology Department and Prof. David Harel of the Computer Science and Applied Mathematics Department, accurately measures, for the first time ever, the distance between different types of odors. Thus, for example, the distance between the smell of almonds and that of bananas is one and a half times greater than the distance between the smell of almonds and that of roses.
 
The new Weizmann technique offers scientists an important research tool for selecting appropriate odorants in olfaction experiments. It also provides the first steps toward understanding the laws that govern the sense of smell.
 
In order to develop their method, the researchers chose about 250 odorants usually used in olfaction experiments and generated for each molecule a long list of chemical characteristics. They then created a multidimensional map of chemical characteristics and placed each odorant in the appropriate location on the map.
 
The scientists used the map to predict neural responses to the 250 odors and compared these predictions to published experimental data on the actual neural responses of various lab animals to these odors. The result: a striking similarity between the predicted calculation and the actual measured responses. These results suggest that the differences between smells are universal and depend on the chemical and physical characteristics of the odor, not on each person’s individual experience or preference – a finding that contradicts the widespread opinion that smell is subjective.  

Prof. Noam Sobel’s research is supported by the Nella and Leon Benoziyo Center for Neurosciences; the J&R Foundation; the Eisenberg-Keefer Fund for New Scientists; and Regina Wachter, New York, NY.

Prof. David Harel’s research is supported by the Arthur and Rochelle Belfer Institute of Mathematics and Computer Science; and the Henri Gutwirth Fund for Research. Prof. Harel is the incumbent of the William Sussman Professorial Chair of Mathematics.
Life Sciences
English
  • Computer Science and Applied Mathematics
  • David Harel
  • Noam Sobel
  • Smell

The Power of Emotion

English
Dr. Rony Paz. Emotional context of a memory
 
 
H
 
Do you remember where you were when the first man landed on the moon? Can you recall the day your first child was born? And what did you eat for breakfast last Monday? Many people can still answer the first questions in detail, even when those events happened close to 40 years ago, but they have trouble answering the last. This phenomenon is well known to science. “The emotional context affects memory and learning,” says Dr. Rony Paz, who recently joined the Weizmann Institute’s Neurobiology Department. “Our memories are more easily recalled, and are more vivid, when they are tied to strong emotions. Unfortunately, this means memories connected to traumatic events may be especially powerful.”
 
Paz researches what happens in the brain when emotions and motivations meet cognitive functions. How do our emotions, whether positive or negative, reinforce our memories? How does the expectation of reward affect learning? How do emotional factors affect “rational” decisions, and vice versa: How do we use our rational thinking to control emotions? Disruptions in the delicate balance between feeling and rational thought may be involved in many psychological phenomena – from post-traumatic stress syndrome and anxiety disorders to autism and schizophrenia. Understanding how that balance is achieved might lead to a better understanding of these problems, as well as suggesting better means of treating them.
 
An evolutionarily primitive, almond-shaped structure deep in the brain – the amygdala – is a main neural circuit for processing emotions. Incoming sensory information, on the other hand, is processed in a number of brain areas, including the neocortex – the wrinkled gray matter forming the thin outer layer of the brain – and a small structure found on either side of the brain called the hippocampus. The neocortex and the hippocampus engage in a sort of dialogue, with signals traveling from the neocortex to the hippocampus and back. But when that information is tied to an emotion, the amygdala jumps into the conversation and affects the transfer of signals. “To understand exactly what role the amygdala plays in this discussion, we need to use unconventional methods to observe a whole array of brain activity at once,” says Paz. Using a combi
nation of neurophysiological and behavioral techniques as well as computational and statistical analyses, Paz’s studies capture the activities of both solitary neurons and large, multidimensional networks of brain cells, all at the same time.
 
These findings appeared in Nature Neuroscience, which described them as a “tour de force of neurophysiological behavioral research.” Paz developed his systematic approach while conducting postdoctoral research at Rutgers University, New Jersey, where he began to uncover the role of the amygdala in reinforcing memories with emotional content. By measuring electrical activity of neurons in a number of brain regions simultaneously, he discovered that the amygdala intercepts the signals, intensifying them and realigning them as they’re sent from the neocortex, so they arrive at the hippocampus strong and clear. 
 
Existing memories can also be extinguished. Paz has investigated this phenomenon, as well. “Memory extinction isn’t forgetting,” he says, “but rather new learning that alters the original memory. Essentially, we learn to ‘silence’ the response we learned earlier.” The best model to date points to a specific part of the neocortex that modulates the activities of the amygdala by depressing the emotional response, though exactly how it does this is not clear. This subduing of one part of the brain by another takes place, for instance, when we try to make a “rational” decision: The cognitive, information-processing outer layer suppresses the “gut feelings” of the emotion center. In episodes of post-traumatic stress or panic attacks, the neocortex fails to properly suppress emotion-laden memories, and details of the traumatic event surface uncontrollably.
 
Those who suffer from post-traumatic stress may also have trouble maintaining a separation between one specific event and the general class of similar events.  Generalization is a normal part of the learning process – we learn early on to lump things into categories, so that even if we’ve never seen a particular cup before, we still know we can drink from it. In other words, generalization allows us to apply past experience to unfamiliar situations. But we also learn to remember specific details (e.g., coffee tastes better in the red mug). People who overgeneralize may have difficulty separating a specific incident such as a traffic accident from the broad activity of driving, and they may therefore be more prone to developing a fear of driving after an accident. Paz is now focusing his research on the neurobiological bases of this sort of generalization when emotions or rewards are involved. This research may not only aid in understanding why some people seem more susceptible to post-traumatic stress than others; it may also provide valuable insight into how we manage to achieve a mental balance between specific details and sweeping generalizations. Understanding the mechanisms underlying this balance might also aid in the creation of machines that think and learn like humans.  
 
 Dr. Rony Paz’s research is supported by the Estelle Funk Foundation; the estate of Florence Cuevas, Mineola, NY; Mr. and Mrs. Gary Leff, Calabasas, CA; and Ms. Lois Rosen, Los Angeles, CA.
 

The Math of Neurons

 
Tel-Aviv born and raised, Dr. Rony Paz originally wanted to be a doctor. But after beginning studies at the Hebrew University Medical School, Paz switched to a double major in mathematics and philosophy, a decision that entailed going back and forth between the university’s two campuses on Mount Scopus and Givat Ram. Searching for graduate studies that would combine his two subjects, Paz learned of an interdisciplinary program in computational neuroscience, and went on to receive an M.Sc. and Ph.D. in the field. Paz, who had served in the army as head of a programming unit, held senior R&D positions in several high-tech companies while studying, designing and implementing machine-learning algorithms. He joined the Weizmann Institute’s Neurobiology Department as a senior scientist in 2007.
 
Paz is married to Netta and is father to Iddo, aged two, and Abigail, aged two months.
 
Dr. Rony Paz. Understanding the delicate balance
Life Sciences
English
  • Brain
  • Learning and memory
  • Neural pathway›Neural mechanism
  • Neurobiology
  • Rony Paz
  • Stress reaction

Observing the Law

English
 
Prof. Tamar Flash and team. Perception and power laws
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
The motions involved in writing, waving goodbye or turning a jumprope all obey the same law of movement. As far back as the 19th century, observers noted that sharper curves are tackled at lower speeds. Anyone thinking of violating this law should think again: Experiments have shown it is nearly impossible to force healthy subjects to generate arm movements that do not conform to this law, today known as the 2/3 power law of motion.
 
Why do the body’s movements comply with the power law? Some have argued that these forms of body movement are simply a byproduct of muscular action. Recent experiments, however, have shown that not only does the 2/3 power law constrain movement generation, it also comes into play when we visually perceive motion. In one such experiment, volunteers were asked to observe a dot on a computer screen that rotated around an ellipse according to one of three scenarios: 1: the dot travels around the ellipse at constant speed; 2: the dot speeds up at the straighter segments of the ellipse, and slows down as it rounds the bend; 3: the dot slows down in the straighter segment of the ellipse and speeds up at the bend. Contrary to intuition, the dot that appeared to the subjects to move more uniformly was not the first dot, which actually moves at a constant speed, but rather the second dot – the one that obeys the 2/3 power law. 
 
Until now, there was little evidence to suggest neural underpinnings to the 2/3 power law, and no one had been able to identify the brain areas that may be involved. Ph.D. student Eran Dayan and Prof. Tamar Flash of the Weizmann Institute’s Computer Science and Applied Mathematics Department, Nava Levit-Binnun of the Institute’s Physics of Complex Systems Department, and Dr. Talma Hendler of the Tel Aviv Sourasky Medical Center, Israel, in collaboration with Antonino Casile and Martin Giese of the Hertie Institute for Clinical Brain Research, Tubingen, Germany, have identified, for the first time, specific brain regions that are selectively activated in response to motion that obeys the 2/3 power law. These findings suggest that the body’s 2/3 power law of motion is controlled by neural mechanisms arising in the brain, as opposed to being a by-product of muscular action. The implication is that such trajectories are planned in detail in the brain prior to their execution. Their findings have recently been published in the journal Proceedings of the National Academy of Sciences (PNAS), USA.
 
In a variation on the original behavioral experiment, the team asked volunteers to observe a cloud of dots rotating around an ellipse according to the same three scenarios while fixing their eyes on a point in the middle of the ellipse. During some experiments, their eye movements were also monitored using special equipment, to eliminate the possibility that the eye movements themselves accounted for the observed results. With the help of functional magnetic resonance imaging, the scientists mapped the areas of the brain that were activated during the task. The results show that different areas of the brain are activated according to the type of motion the volunteers observe. The regions that respond to the 2/3 power law of motion are those that subserve the generation of movement, visual motion processing and action observation. The activation of these areas in response to the 2/3 power law of motion is much stronger than the response to any other form of motion.
 
Why does the brain seem to prefer the 2/3 power law of motion? And why do separate neuronal networks exist for different types of motion? Apart from the idea that the law keeps movement smooth, Flash and her colleagues have come up with a new theory: It all boils down to underlying geometrical features.
 
“After conducting various mathematical and geometrical analyses, we suggest that different areas of the brain seem to be using distinct geometries that take into account such features as curvature or straight lines, and this is why we see different brain areas responding to and generating different types of motion perception and action.”
 
A number of recent brain studies have found a strong coupling between perception and action: Seeing movement can activate the same brain areas as those activated by the physical act itself. Scientists have theorized that this dual activation helps us to learn by imitation as well as to understand the actions of others. The new findings show that this coupling extends to the most basic aspects of motion, and they identify the precise laws that govern the very flow of our movements. They suggest, says Flash, any number of future lines of research into how the brain is organized to control and perceive movement, as well as how possible impairments in these processes may be involved in neurological diseases. 
 
Brains see motion differently
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Prof. Tamar Flash’s research is supported by the Manfred D. Moross Laboratory for Vision Research and Robotics; and Sylvia and Henry Legrain, Spain. Prof. Flash is the incumbent of the Dr. Hymie Moross Professorial Chair.
 
 
(l-r) Prof. Tamar Flash, Dr. Talma Hendler and Eran Dayan. Setting the speed limit
Life Sciences
English
  • Brain
  • Computer Science and Applied Mathematics
  • Mathematical model
  • Perception
  • Physics of Complex Systems
  • Tamar Flash

Bat Memories

English
Dr. Nachum Ulanovsky. Bats in the lab
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Perhaps because bats fly at night, they easily provoke fears and conjure up eerie visions of witches and vampires. Yet in Chinese mythology bats are happy omens, portending good luck. And at the Weizmann Institute, bats promise to bring good fortune by helping reveal the secrets of human memory.
 
These chatty nocturnal mammals – who constantly communicate with one another through high-pitched shrieks and clicks – have long been used in the study of hearing. But in addition, bats have remarkable navigational and memory skills. Thus fruit bats, the most common bat species in Israel and the one studied at Weizmann, have no problem returning to the same cave, no matter how small, after covering distances of dozens of kilometers in the course of the night.
 
Dr. Nachum Ulanovsky, who recently joined the Weizmann Institute as a senior scientist in the Neurobiology Department, has pioneered the use of bats in the study of learning and memory. According to him, bats are an excellent research model in this area not only because of their impressive spatial memory but also due to their highly developed senses and unique behaviors. “Most studies of memory-related brain activity have been done with rats and mice,” he points out, “and it’s important to perform a ‘reality check’ to see whether those findings are relevant for other mammals. By comparing different animals we can find the features that all mammals have in common, and such features can help us understand our own memory.”
 
Ulanovsky is particularly interested in the brain area called the hippocampus, a bunch of cells on either side of the brain that is responsible for spatial navigation and episodic memory. As opposed to motor memory (remembering how to ride a bike) or factual memory (knowing the name of the queen of England), episodic memory deals with day-to-day events and enables us to remember what we did yesterday, for instance, or whom we met two days ago.
 
The central role of the hippocampus in episodic memory is mainly known from the classical case of a patient who in the 1950s had his hippocampus removed and lost the ability to remember new events as a result. However, it is still unknown which nerve cells or neuronal networks are involved in episodic memory, and that is precisely what Ulanovsky seeks to find out by using bats.
 
He intends to study the brain activity of bats as they fly or crawl. In the “flight room” in his lab, bats will be outfitted with advanced telemetry equipment that transmits information about the activity of individual neurons or neuronal networks as the bat performs certain tasks in flight. The relatively large Israeli fruit bat is perfect for such studies: At 150 grams it can fly while carrying about 9 grams of equipment. Telemetry systems are generally heavy, but this miniature device – a world first – has been developed in the past two years by an American company in collaboration with Ulanovsky. The nerve cell signals will be picked up by another kind of innovative equipment developed in the 1990s: tetrodes – micro-electrodes that have four wires instead of one – enabling a more precise recording of the activity of individual nerve cells. Ulanovsky started using these technologies during his postdoctoral studies at the University of Maryland, where he made a number of significant discoveries about the bat hippocampus.
 
An additional room in Ulanovsky’s lab will be devoted to the study of crawling bats, the goal being to find out how the brain processes and remembers sounds over time. Yet another experimental room will be devoted to behavioral studies. To avoid disrupting their natural behavior, the bats will all share one large, cave-like space with rough-hewn rocks in the ceiling.
 
Ulanovsky also conducts field studies in collaboration with the Hebrew University of Jerusalem. The bats’ bodies are outfitted with the smallest GPS system in the world – 10 grams including the battery – which was specially developed for this research.
 
These studies promise to shed new light not only on human memory but also on diseases involving the hippocampus, including epilepsy and such neurodegenerative diseases as Alzheimer’s. And while the bats continue to chat away in a language we don’t understand, other discoveries might emerge from the study of their outstanding capabilities. 
 
Dr. Nachum Ulanovsky’s research is supported by the A.M.N. Fund for the Promotion of Science, Culture and Arts in Israel; and the Chais Family Fellows Program for New Scientists.
 
Lessons from a fruit bat
 
 

The Return Home

Dr. Nachum Ulanovsky immigrated to Israel with his parents from Moscow in 1973 as a four-month-old baby. The family settled in Rehovot, where Nachum took part in youth activities at the Weizmann Institute. “I feel now as if I’ve returned home,” he says. “New buildings and new roads have been built at the Institute; only the swimming pool has remained the same.” Ulanovsky enrolled in Tel Aviv University’s physics faculty at age 16. He served in the intelligence corps of the Israel Defense Forces and during his army service started taking courses in neurobiology. He then earned a Ph.D. in neural computation from the Hebrew University of Jerusalem. He lives on the Weizmann campus with his wife and three children, has a passion for outdoor activities, such as hiking, sea kayaking, scuba diving and canyoneering, and is a certified rappelling instructor.

 
Dr. Nachum Ulanovsky. Neurons in flight
Life Sciences
English
  • Bats
  • Learning and memory
  • Nachum Ulanovsky

Memory’s Gatekeeper

English
Dudai and his team. Active memory supression
 
 

 

Everyone has repressed memories.Though most of the time we’re unaware of it, that action can spare us emotional pain and embarrassment, and even preserve our internal world view. The failure of this normal mechanism, however, may result in emotional and cognitive problems. What prevents certain memories from surfacing while others flood our consciousness at will? Where is the elusive gatekeeper that keeps some memories firmly outside the doors of our awareness? Prof. Yadin Dudai, Head of the Neurobiology Department, and his research team recently shed light on memory repression and pinpointed an area in the brain that acts as memory’s gatekeeper.
 
Their study used an intriguing phenomenon, dubbed “post-hypnotic amnesia” – a transient, controlled loss of specific memories following a hypnotic session. The research, which relied on advanced brain imaging techniques, was recently featured in Neuron. Dudai and research student Avi Mendelson, together with medical hypnosis experts Dr. Yossi Chalamish and Prof. Alexander Solomonovich of the Wolfson Medical Center, chose as their research subjects people who are especially susceptible to hypnotic suggestion. In these volunteers they were able to induce post-hypnotic amnesia: While under hypnosis, the subjects were told to forget, upon exiting the hypnotic state, particular events they had experienced a week earlier. When a specific signal agreed upon by the subject was given later, the memory loss was reversed, allowing the subject to recall the repressed memories. 
 
Dudai and his team produced a 45-minute documentary, which they then showed to the participants. A week later, the same participants returned for the second half of the experiment. One by one, they entered the functional magnetic resonance imaging (fMRI) scanner and underwent hypnosis. Once under, they were told to forget the film upon waking and were also instructed as to the signal. The subjects were brought out of hypnosis and, while still being scanned in the fMRI, were given a quiz that included questions about both the film and the room in which the film was screened. They were then given the signal to remember and asked to retake the quiz. A control group followed the same routine, but did not undergo post-hypnotic amnesia.
 
The hypnosis subjects could not recall the details of the film the first time they took the quiz, though they did remember the context in which the film was screened. The second time, after receiving the agreed-upon signal, they were able to recall the film as well. In other words, their memory of the film had not been lost, only repressed. In analyzing the fMRI data, the scientists noted that some parts of the brains of those experiencing post-hypnotic amnesia had depressed levels of activity in specific brain circuits, but, interestingly, in one small area the activity was significantly heightened. This area is part of a larger area called Brodmann area 10, which is involved, among other things, in memory retrieval. They concluded that the area they identified “vetoes” the recall of specific items in long-term memory. Further research along these lines might shed light on common memory problems such as functional amnesia or cognitive and behavioral pathologies associated with the overrepression of certain memories.     
 
Prof. Yadin Dudai’s research is supported by the Norman and Helen Asher Center for Brain Imaging; the Nella and Leon Benoziyo Center for Neurological Diseases; the Carl and Micaela Einhorn-Dominic Brain Research Institute; the Irwin Green Alzheimer’s Research Fund; Mr. Rowland Schaefer, New York, NY; and the Sylvia and Martin Snow Charitable Foundation. Prof. Dudai is the incumbent of the Sara and Michael Sela Professorial Chair of Neurobiology.
 
Prof. Yadin Dudai, Uri Nili, Avi Mendelson, Nahum Stern, Rachel Ludmer, Dr. Yossi Chalamish and Efrat Furst. Hypnotic scan
Life Sciences
English
  • Brain research
  • Learning and memory
  • Neurobiology
  • Yadin Dudai

Common Scents

English

Prof. Noam Sobel. Physical laws underly smell perception

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Most would agree that roses smell good; old sweaty socks do not. What makes one smell pleasant and another odious? Is our perception of smell based on a physical reality, such as the chemical makeup of odor molecules, or is it a subjective, learned experience?
 
Scientists at the Weizmann Institute of Science and the University of California at Berkeley have now discovered that knowing the molecular structure of a substance can help predict whether we will find its smell delightful or malodorous.
 
Other senses can be explained by physical laws. When we see red, for instance, it is because light is reflected off an object in a certain wavelength, while hearing is based on the varying frequencies of sound waves. But until now, there was no known physical factor that could explain how the brain senses odors. The new study conducted by Prof. Noam Sobel of the Institute’s Neurobiology Department and his colleagues, published in the Journal of Neuroscience, demonstrates that real physical laws underlie our perception of smell.
 
To identify the general principles by which our sense of smell is organized, the researchers began with a database of 160 different odors that had been ranked by 150 perfume and smell experts according to a set of 146 characteristics (sweetish, smoky, musty, etc.). These data were then fed into a statistical program that analyzed the variance in perception among the smell experts. The scientists found that the data fell along an axis – described as the “pleasantness rating” of the odors – running from “sweet” and “flowery” at one end to “rancid” and “sickening” at the other. The distribution along this axis, they discovered to their surprise, closely matches the variation in chemical and physical properties from one substance to another. The researchers found they could build a model to predict, from the molecular structure of a substance alone, how pleasing its smell would be perceived to be.
 
To double-check their model, Sobel and his team had experimental subjects assess for pleasantness 50 odors they had never encountered before. They found that the ratings of their test subjects fit closely with the rankings shown by their model.
 
In other words, they were able to predict the level of pleasantness quite well, even for unfamiliar smells. They noted that although preferences among smells are commonly supposed to be culturally learned, their study showed that the responses of American subjects, Jewish Israelis and Muslim-Arab Israelis all fit the model’s predictions to the same extent.
 
Sobel: “Our findings show that the way we perceive smells is at least partially hard-wired in the brain. Although there is a certain amount of flexibility, and our life experience certainly influences our perception of smell, a large part of our sense of whether an odor is pleasant or unpleasant is due to a real order in the physical world. Thus we can now use chemistry to predict how the smells of new substances will be perceived.” 
 
Prof. Noam Sobel’s research is supported by the Nella and Leon Benoziyo Center for Neurosciences; the J&R Foundation; and the Ben and Joyce Eisenberg Foundation.

 

Prof. Noam Sobel. Hard-wired for smell
Life Sciences
English
  • Neurobiology›Neurobiology
  • Noam Sobel
  • Smell

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