REMARKABLE CHANGES IN THE BRAIN SHOW ITS ABILITY TO RESPOND TO ITS ENVIRONMENT
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REMARKABLE CHANGES IN THE BRAIN SHOW ITS ABILITY TO RESPOND TO ITS ENVIRONMENT.
ORLANDO, Monday, Nov. 4 - Increasing research shows the remarkable ability of the brain, which was once thought to be hard-wired, to change in response to a variety of factors. Now new work shows that this process, known as plasticity, occurs in the mental ailment schizophrenia and even in the expectation of a reward. Other work shows that a condition known as synesthesia, in which a person's senses are mixed, creates surprising brain changes.
The new studies were reported today during the 32nd annual meeting of the Society for Neuroscience.
One research team found changes in the structural qualities of neurons in the brains of patients with schizophrenia, a mental ailment characterized by disorganized thought, hallucinations and an inability to relate to others and function in society.
In schizophrenics, brain neurons have more dendritic spines, the receiving side of the synapse, with no heads and fewer with heads. Dendrites are tree-like extensions of the neuron cell body that receive information from other nerve cells. Spines are lollipop-like protrusions from dendrites.
"The findings of alterations in spine morphology in schizophrenia suggest alterations in the nature of synaptic transmission in prefrontal cortical networks," says William Greenough, PhD, of the University of Illinois, who led the research. Synaptic transmission is the basic way in which electrical signals, or messages, are passed from one neuron to another.
"The result suggests that the neuropathology of schizophrenia includes activation of brain mechanisms that control normal processes of brain plasticity," says Greenough. Spine changes are invoked in the normal process of development and have also been seen in other pathologies such as fragile X syndrome and other forms of mental retardation.
"The abnormal spine shape very likely interferes with the normal process of synaptic transmission through which neurons communicate," notes Greenough. "This result suggests that the disorder may not be limited to specific brain chemicals or pathways."
In the new study, postmortem brain samples from prefrontal cortex, a cortical region known for its higher cognitive function of integrating a vast amount of information from all sensory modalities, were collected from 14 control subjects and 12 subjects with chronic schizophrenia.
In other research, brain plasticity work in animals shows for the first time that reward alters the basic brain function of vision. Until now, the general belief has been that the effect of rewarding stimuli is restricted only to higher level processing in the brain.
Everyday behavior is controlled to a large extent by particular sets of "satisfying," or rewarding, stimuli. Indeed, we tend to learn faster or acquire certain skills more rapidly if our responses are accompanied or followed by reward, notes Mriganka Sur, PhD, a neuroscientist at the Massachusetts Institute of Technology.
In a recent series of experiments, Sur and his colleague Valentin Dragoi, PhD, taught two adult rhesus monkeys to identify the orientation of lines, which were presented on a computer display, and studied the effect of a reward of apple juice on the brain. Individual neurons in the primary visual cortex were recorded for two to three hours.
"Specifically, when animals expected to receive a high reward they tended to discriminate orientations better, and this improvement in perceptual performance was likely to be caused by a transient increase in the orientation selectivity of visual cortical neurons that we observed," Sur says. "The study shows, for the first time, that the degree of reward affects the way in which simple visual features, such as line orientation, are encoded in the monkey brain, and, possibly, by extension, the human brain."
Sensory information within the cerebral cortex was thought to flow from lower to higher stages of processing: the lower, or earlier stages are thought to simply process elementary features of stimuli while the higher, or later, stages are believed to integrate the bottom up flow of sensory information with behaviorally relevant, internal state-dependent, components of tasks. The early cortical areas are generally believed to passively reflect changes in the environment, showing limited plasticity even to changes in sensory input.
"Our work overturns this view," Sur says. "We show that neurons in the primary visual cortex, the very first stage of visual processing within the cortex, rapidly alter their responses when non-visual inputs such as those related to reward are altered. In similar behavioral experiments, we show that visual discrimination is similarly altered. Thus, neurons early in the visual pathway can quickly acquire information about reward conditions and then change their responses (and visual perception) depending on the expectation of reward."
The results suggest that rapid plasticity in the visual cortex may combine with slower, more persistent, forms of plasticity observed in the higher brain areas in order to modulate behavioral decisions.
In the work on synesthesia - the unusual phenomenon of seeing sounds or seeing specific colors upon seeing specific numerals - researchers find that some types of this condition result from changes in wiring in a specific brain region.
V.S. Ramachandran, PhD, and graduate student Edward Hubbard, both at the University of California at San Diego, examined people with a type of synesthesia where they see numbers and days of the week and months of the year as colors. Even Roman numbers are colored, suggesting that the concept is critical for causing these synesthetes to experience colors.
Both classical neurology and recent brain imaging experiments suggest that the ability to perform mathematical calculations depends on a network of brain areas, including the fusiform gyrus where letters and numbers are recognized and the angular gyrus where abstract numerical calculations are performed.
Ramachandran and Hubbard's examination of the synesthesia group indicates that in these individuals cross-wiring occurs in the region of the angular gyrus, instead of in the region of the fusiform gyrus. Based on these results, Ramachandran and Hubbard further propose that the angular gyrus may be involved not only in numerical calculations, but also in more abstract processing of sequences generally.