Discovering the Brain (1992) / Chapter Skim
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8 Learning, Recalling, and Thinking
8 Learning, Recalling, and Thinking
Pages 123-144
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From page 123... ...
Chapter 7 offered some clues about the ways scientific investigation—from the molecular level to studies of the alert, behaving animal- has begun to define in physical terms an abstract quality such as "attention." Similiar techniques and approaches are being applied to other mental functions, too, even those as seemingly intangible as learning, remembering, or thinking about the outside world. Learning and memory, which for many years were considered central problems in psychology, the social sciences, and philosophy, have recently assumed~greater importance in the area of neurobiology, itself a confluence of several lines of in123
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From page 124... ...
vestigation. Neuroscientific interest in learning and memory has recently increased for two reasons, according to psychiatrist Eric Kandel, a senior scientist in the Howard Hughes Medical institute at Columbia University.
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From page 125... ...
A major set of elements in this reflex are sensory neurons in the siphon skin, which perceive the stimulus; motor neurons in the gill, which contract the muscle and cause the gill to
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From page 126... ...
A single sensory neuron and a single motor neuron, when implanted in a glass dish with a suitable nourishing culture, form functional interconnections. When a facilitating neuron is added or the cells are exposed to serotonin (the transmitter released by the facilitating neuron)
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From page 127... ...
laboratory has replicated in cell culture the same conditions that in the living animal lead to protein synthesis and neuronal growth: a motor neuron, a sensory neuron (injected with a fluorescent dye to make imaging possible afterward) , and exposure to serotonin repeated four or five times.
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From page 128... ...
What they inhibit, apparently, is the growth and proliferation of signal-transmitting elements on the axons of sensory neurons. By this reasoning, the effect of the cell-adhesion molecules would have to be held in abeyance at some point, to allow the sensory neurons to strengthen and increase their synaptic connections with the motor neurons.
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From page 129... ...
Even the finding that a neurotransmitter such as serotonin is not restricted to moment-by-moment signaling but can actually be a factor that initiates neuronal growth in the case of long-term memory adcTs to an impression of the two contexts conjoining, with neurotransmitters sometimes acting as growth factors. THE WORLD IN THE FRONT OF THE BRAIN Short-term and long-term memory are not the only forms in which the brain stores information.
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From page 130... ...
For instance, information gathered through the sense of smell undoubtedly plays a much larger role in the mental representations of a dog than in those of a bird, which relies much more on its excellent vision (both in detail and in color) to help it recognize its kin, observe the territories of its rivals, and seek out fooc!
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From page 131... ...
Recordings of this kind have become possible only in the past decade or so; those from GoldmanRakic's laboratory show several very interesting things. First, the neuron under study, in the prefrontal cortex, holds to a steady level of activity when the target light appears.
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From page 132... ...
These mental representations in the prefrontal cortex are too limited to be directly responsible for an animal's complex behavior. Goldman-Rakic and her colleagues believe that this representational knowledge does guide behavior in colIaboration with other areas particularly the parietal cortex and that the larger network very probably represents the neural circuit
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From page 133... ...
in the prefrontal cortex, for example, nerve fibers containing the neurotransmitter dopamine are found in especially high concentration, and researchers have wondered for some time what role dopamine might play in prefrontal circuits of information. The evidence gathered on this point over the past few years has begun to make clear the enormous extent to which dopamine shapes not only our physical functioning in the world but also our ability to process new information, to associate ideas effectively, and even to maintain a sense of well-being in balance with realistic perceptions.
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From page 134... ...
in a test to see whether interference with the D-1 receptors would have any effect on cognitive function, Goldman-Rakic's research team injected a compound that blocks the D-l receptor sites in the prefrontal cortex of monkeys trained in the delayed-response test described earlier. About 20 minutes after the injection, the animals showed an impairment of working memory, moving their eyes to the wrong location when the trial included a delay; but they responded correctly in a "sensory-guided" version of the task, in which the target light was left on as a guide.
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From page 135... ...
The prefrontal cortex, with its importance for cognition, shows a form of dysfunction when tested in patients suffering from schizophrenia. (An often disabling mental illness, schizophrenia interferes with the capacity for logical thought and greatly disturbs the emotions and social behavior; see Chapter 4 for a discussion of current theories about the importance of dopamine levels in schizophrenia.)
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From page 136... ...
The research being conducted in animals and humans is mutually helpful, offering the prospect over the next decacle of significant advances in a neuroscientific account of the workings of the prefrontal cortex including a cellular explanation of this area's memory functions. A view shared currently by Goldman-Rakic and many colleagues is that the main function of this greatly enlarged part of the brain, so recently evolved in the primate line, is to guide behavior by means of mental representations of stimuli, rather than by the stimuli themselves.
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From page 137... ...
But if the amount of memory were tremendously expanded, it would be possible to store many more solutions in other words, to auaress many more and different kinds of problems. It is hardly a revelation at this point that the human brain
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From page 138... ...
Research conducted on the simpler nervous system of invertebrates, as well as on nonhuman primates, other vertebrates, and humans, has indicated how learning brings about structural changes in nerve cells and how the neurons in turn form regions, which take part in networks. The networks are organized into distributed systems, which collaborate with other systems, both sensory and associative, to produce the total working effect.
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From page 139... ...
Hebb postulated two active elements (the pre- and postsynaptic terminals) , but the nervous system in the marine snail appears to include a third element, the facilitating neuron that enhances the excitability of the sensory neuron.
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From page 140... ...
testing ground for neuroscientists' ideas about how the brain functions. Theoretical models of memory, in particular, cannot be tested adequately on a digital-computer simulation of a few hundred model neurons, because the living brain works on such an enormously larger scale.
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From page 141... ...
This reduction is made possible by duplicating the central processor many times, even thousands of times, within the same computer, so that signals have less distance to travel. Even so, extrapolating from the recent rate of increase and from today's highest known speeds of computer processing, Terrence Sejnowski estimates that an artificial device approximating the human brain cannot be expected before at least the year 2015.
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From page 142... ...
This still leaves unexplored vast areas of human experience, such as perception; but as described earlier in this chapter, computer and mathematical modeling on one side, and more detailed neurobiological examination on the other side, are making inroads in this area too. Edelman and his colleagues have used an anoroach they 1 1 ~ call synthetic neural modeling to build an automaton that is able to explore its environment by simulated vision and touch; moreover, it can categorize objects on the basis of its perceptions, and its responses draw on previous experiences with
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From page 143... ...
For example, the built-in principle that light is "better" than no light serves to direct and refine the system's eye movements toward a target. Just as in living neurons, the enhanced connection provides a stronger response the next time that particular neural pathway is active.
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From page 144... ...
ACKNOWLEDGMENTS Chapter 8 is based on presentations by Gerald Edelman, Patricia Goldman-Rakic, Eric Kandel, and Terrence Sejnowski.
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