Yavuz Odabasi shared a link. Yesterday . Terry Sejnowski & Tobi Delbruck: “The Language of the Brain” “We can make sense of the world because our brains attend to the exquisite timing of impulses that flow through billions of neurons.” ‘Scientific American’, October 2012, pp. 54-59. *** “How does the brain pull all this off, given that the complexity of the networks inside our skull—trillions of connections among billions of brain cells—rivals that of the Internet? One answer is energy efficiency: when a nerve cell communicates with another, the brain uses just a millionth of the energy that a digital computer expends to perform the equivalent operation. Evolution, in fact, may have played an important role in pushing the three-pound organ toward ever greater energy efficiencies. “Parsimonious energy consumption cannot be the full explanation, though, given that the brain also comes with many built-in limitations. One neuron in the cerebral cortex, for instance, can respond to an input from another neuron by firing an impulse, or a ‘spike,’ in thousandths of a second—a snail’s pace compared with the transistors that serve as switches in computers, which take billionths of a second to switch on. The reliability of the neuronal network is also low: a signal traveling from one cortical cell to another typically has only a 20 percent possibility of arriving at its ultimate destination and much less of a chance of reaching a distant neuron to which it is not directly connected. “Neuroscientists do not fully understand how the brain manages to extract meaningful information from all the signaling that goes on within it. The two of us and others, however, have recently made exciting progress by focusing new attention on how the brain can efficiently use the timing of spikes to encode information and rapidly solve difficult computational problems. This is because a group of spikes that fire almost at the same moment can carry much more information than can a comparably sized group that activates in an unsynchronized fashion. “Beyond offering insight into the most complex known machine in the universe, further advances in this research could lead to entirely new kinds of computers. Already scientists have built ‘neuromorphic’ electronic circuits that mimic aspects of the brain’s signaling network.... CELL CHATTER “Like many other neuroscientists, we often use the visual system as our test bed, in part because its basic wiring diagram is well understood. Timing of signals there and elsewhere in the brain has long been suspected of being a key part of the code that the brain uses to decide whether information passing through the network is meaningful.... “Brain cells receive all kinds of inputs on different timescales. The microsecond-quick signal from the right ear must be reconciled with the slightly out-of-sync input from the left.... The signals most important for this discussion, though, are the spikes, which are sharp rises in voltage that course through and between neurons.... A neuron fires a spike after deciding that the number of inputs urging it to switch on outweigh the number telling it to turn off. When the decision is made, a spike travels down the cell’s axon (somewhat akin to a branched electrical wire) to its tips. Then the signal is relayed chemically through junctions, called synapses, that link the axon with recipient neurons. “In each eye, 100 million photoreceptors in the retina respond to changing patterns of light. After the incoming light is processed by several layers of neurons, a million ganglion cells at the back of the retina convert these signals into a sequence of spikes that are relayed by axons to other parts of the brain, which in turn send spikes to still other regions that ultimately give rise to a conscious perception. Each axon can carry up to several hundred spikes each second, though more often just a few spikes course along the neural wiring. All that you perceive of the visual world—the shapes, colors and movements of everything around you—is coded into these rivers of spikes with varying time intervals separating them. ... “For years investigators have used several methods to interpret, or decode, the meaning in the stream of spikes coming from the retina. One method counts spikes from each axon separately over some period: the higher the firing rate, the stronger the signal. The information conveyed by a variable firing rate, a rate code, relays features of visual images, such as location in space, regions of differing light contrast, and where motion occurs, with each of these features represented by a given group of neurons. “Information is also transmitted by relative timing—when one neuron fires in close relation to when another cell spikes. Ganglion cells in the retina, for instance, are exquisitely sensitive to light intensity and can respond to a changing visual scene by transmitting spikes to other parts of the brain. When multiple ganglion cells fire at almost the same instant, the brain suspects that they are responding to an aspect of the same physical object. Horace Barlow, a leading neuroscientist at the University of Cambridge, characterized this phenomenon as a set of ‘suspicious coincidences.’ Barlow referred to the observation that each cell in the visual cortex may be activated by a specific physical feature of an object (say, its color or its orientation within a scene). When several of these cells switch on at the same time, their combined activation constitutes a suspicious coincidence because it may only occur at a specific time for a unique object. Apparently the brain takes such synchrony to mean that the signals are worth noting because the odds of such coordination occurring by chance are slim. ... INTO THE CORTEX “New evidence adds proof that the visual cortex attends to temporal clues to make sense of what the eye sees. The ganglion cells in the retina do not project directly to the cortex but relay signals through neurons in the thalamus, deep within the brain’s midsection. This region in turn must activate 100 million cells in the visual cortex in each hemisphere at the back of the brain before the messages are sent to higher brain areas for conscious interpretation. “We can learn something about which spike patterns are most effective in turning on cells in the visual cortex by examining the connections from relay neurons in the thalamus to cells known as spiny stellate neurons in a middle layer of the visual cortex. In 1994 Kevan Martin, now at the Institute of Neuroinformatics at the University of Zurich, and his colleagues reconstructed the thalamic inputs to the cortex and found that they account for only 6 percent of all the synapses on each spiny stellate cell. How, then, everyone wondered, does this relatively weak visual input, a mere trickle, manage to reliably communicate with neurons in all layers of the cortex? “Cortical neurons are exquisitely sensitive to fluctuating inputs and can respond to them by emitting a spike in a matter of a few milliseconds. In 2010 one of us (Sejnowski), along with Hsi- Ping Wang and Donald Spencer of the Salk Institute and Jean- Marc Fellous of the University of Arizona, developed a detailed computer model of a spiny stellate cell and showed that even though a single spike from only one axon cannot cause one of these cells to fire, the same neuron will respond reliably to inputs from as few as four axons projecting from the thalamus if the spikes from all four arrive within a few milliseconds of one another. Once inputs arrive from the thalamus, only a sparse subset of the neurons in the visual cortex needs to fire to represent the outline and texture of an object.... “In the 1960s David Hubel of Harvard Medical School and Torsten Wiesel, now at the Rockefeller University, discovered that each neuron in the relevant section of the cortex responds strongly to its preferred stimulus only if activation comes from a specific part of the visual field called the neuron’s receptive field. Neurons responding to stimulation in the fovea, the central region of the retina, have the smallest receptive fields—about the size of the letter e on this page. Think of them as looking at the world through soda straws. In the 1980s John Allman of the California Institute of Technology showed that visual stimulation from outside the receptive field of a neuron can alter its firing rate in reaction to inputs from within its receptive field. This ‘surround’ input puts the feature that a neuron responds to into the context of the broader visual environment. “Stimulating the region surrounding a neuron’s receptive field also has a dramatic effect on the precision of spike timing.... “The timing of spikes also matters for other neural processes. Some evidence suggests that synchronized timing—with each spike representing one aspect of an object (color or orientation)— functions as a means of assembling an image from component parts. A spike for ‘pinkish red’ fires in synchrony with one for ‘round contour,’ enabling the visual cortex to merge these signals into the recognizable image of a flower pot. ATTENTION AND MEMORY “...The activity of cortical neurons that receive visual input is influenced not only by those inputs but also by excitatory and inhibitory interactions between cortical neurons. Of particular importance for coordinating the many neurons responsible for forming a visual perception is the spontaneous, rhythmic firing of a large number of widely separated cortical neurons at frequencies below 100 hertz. “Attention—a central facet of cognition—may also have its physical underpinnings in sequences of synchronized spikes. It appears that such synchrony acts to emphasize the importance of a particular perception or memory passing through conscious awareness. Robert Desimone, now at the Massachusetts Institute of Technology, and his colleagues have shown that when monkeys pay attention to a given stimulus, the number of cortical neurons that fire synchronized spikes in the gamma band of frequencies (30 to 80 hertz) increases, and the rate at which they fire rises as well.... “Neural activation of the gamma-frequency band has also attracted the attention of researchers who have found that patients with schizophrenia and autism show decreased levels of this type of signaling on electroencephalographic recordings. David Lewis of the University of Pittsburgh, Margarita Behrens of the Salk Institute and others have traced this deficit to a type of cortical neuron called a basket cell, which is involved in synchronizing spikes in nearby circuits. An imbalance of either inhibition or excitation of the basket cells seems to reduce synchronized activity in the gamma band and may thus explain some of the physiological underpinnings of these neurological disorders. Interestingly, patients with schizophrenia do not perceive some visual illusions, such as the tilt illusion, in which a person typically misjudges the tilt of a line because of the tilt of nearby lines. Similar circuit abnormalities in the prefrontal cortex may be responsible for the thought disorders that accompany schizophrenia. “When it comes to laying down memories, the relative timing of spikes seems to be as important as the rate of firing. In particular, the synchronized firing of spikes in the cortex is important for increasing the strengths of synapses—an important process in forming long-term memories. A synapse is said to be strengthened when the firing of a neuron on one side of a synapse leads the neuron on the other side of the synapse to register a stronger response.... “Some of the strongest evidence that synchronous spikes may be important for memory comes from research by György Buzsáki of New York University and others on the hippocampus, a brain area that is important for remembering objects and events. The spiking of neurons in the hippocampus and the cortical areas that it interacts with is strongly influenced by synchronous oscillations of brain waves in a range of frequencies from four to eight hertz (the theta band).... These theta-band oscillations can coordinate the timing of spikes and also have a more permanent effect in the synapses, which results in long-term changes in the firing of neurons.” --------------- Terry Sejnowski is an investigator with the Howard Hughes Medical Institute and is Francis Crick Professor at the Salk Institute for Biological Studies, where he directs the Computational Neurobiology Laboratory. Tobi Delbruck is co-leader of the sensors group at the Institute of Neuroinformatics at the University of Zurich. papers.cnl.salk.edu/PDFs/The%20Language%20of%20the%20Brain%202012-4327.pdf papers.cnl.salk.edu/PDFs/The%20Language%20of%20the%20Brain%202012-4327.pdf papers.cnl.salk.edu88
Posted on: Wed, 29 Jan 2014 16:38:19 +0000
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