MIT Reports to the President 1997-98
To study the brain and its complexities, the Department of Brain and Cognitive Sciences combines the experimental technologies of molecular and systems neuroscience with the theoretical power of computational neuroscience, while our cognitive science program focuses on the mind-brain connection.
FACULTY RESEARCH DEVELOPMENTS
LEARNING AND MEMORY
Tomaso Poggio's research in the Center for Biological and Computational Learning examines learning as a gateway to understanding intelligence in man and machines. During the last year, his group established the foundation of a unified theory for supervised learning that connects regularization, Support Vector Machines and Basis Pursuit Denoising. A surprising mathematical result they obtained suggests an equivalence, under certain conditions, between generalization power and sparsity of the representation. The new work on sparse representations led to the development of an architecture that can 'learn' to perform rather difficult object detection tasks in images, such as detection of faces or detection of people in isolated cluttered images.
Earl Miller's laboratory provided several insights into the neural basis of memory and cognition in the primate prefrontal cortex. One study explored the role of prefrontal neurons in selecting the sensory information that is fully processed and reaches awareness. It is well established that our cognitive functions are severely limited in capacity; we can only think about a few things simultaneously. This experiment identified neural mechanisms in the prefrontal cortex that select the information relevant for guiding thoughts and actions.
Guosong Liu studies how synaptic activity regulates the strength of interconnections between neurons in the central nervous system and what role activity plays in the process of synapse formation, elimination, and consolidation. In the last year, he focused on uncovering the molecular and cellular events in the process of synaptogenesis. His laboratory found that influx of Ca2+ through neural activity plays a critical role in the maturation of presynaptic terminals. Furthermore, the proper level of neural activity is essential for postsynaptic receptor clustering, because both increase and removal of neural activity can block clustering.
VISION AND MOTOR CONTROL
During the past year, there were three major accomplishments in Barton Anderson's laboratory. First, he developed the first quantitative theory of the information available from binocular occlusion junctions, and experimentally demonstrated that the human visual system uses this information. Second, he demonstrated that the mechanisms responsible for synthesizing coherent contours from moving image sequences exhibit a strong velocity dependence. All prior work and theory assumed that these completion mechanisms can be understood with static geometric principles, which his data shows is incorrect. Third, he developed a general theory of how the human brain computes lightness, opacity, and depth from stereoscopic images, and experimentally demonstrated how these computations are related in the human brain.
Several projects initiated in Mary Potter's laboratory within the last year concerned the perception and encoding of meaningful material presented in a continuous sequence, as is characteristic of normal experience in which we look around or read with three or four fixations per second, or listen to continuous speech at three or more words per second. They showed that people have little difficulty in understanding sentences that switch several times between auditory and visual modalities of presentation, demonstrating that language processing is fundamentally amodal and abstract. In a study of attention switching, they found evidence that changing the nature of the attentional task, or having to note target events that appear in a rapid temporal sequence, create distinct kinds of attentional deficits.
Emilio Bizzi's laboratory provided a novel perspective on how the vertebrate nervous system produces movement. They demonstrated that the neural circuitry within the frog spinal cord produces motor responses to hindlimb cutaneous stimulation by the combined recruitment of a small number of distinct muscle groups. Such a muscle group, in which the activation level of a set of muscles is specified together, has been termed a "muscle synergy." Laboratory results provided direct support for their proposal that such spinally organized muscle synergies might underlie the production of movement.
Ann Graybiel's laboratory succeeded in applying ensemble recording techniques to ask what neural processing goes on in the basal ganglia as animals become conditioned in a T-maze task. They found striking changes in the responses of neurons as the animals learn to run the maze. During this time the neurons acquire novel responses very early in the task. The researchers suggest that the animals are building up expectancy signals in the basal ganglia and that these allow the basal ganglia to work in cooperation with the neocortex in a feed-forward loop.
Gerald Schneider obtained evidence that a particular kind of interneuron in the mammalian brain has plastic potential in the adult mammal. The neuron type is defined by the presence of a molecule (GAP-43) that in many systems is prominent only in the developmental period. His laboratory demonstrated that short-axon interneurons sprout after a brain lesion that causes a loss of axonal input in part of the thalamus of adult hamsters. The lesion is followed by a progressive filling in of the denervated areas by GAP-43 enriched neuronal processes, believed to be the axons of the interneurons. Similar cell types are prominent in parts of the human neocortex believed to play major roles in higher cognitive functions.
Mriganka Sur's laboratory identified signaling mechanisms that are important for the activity-dependent development of connections in the visual system of the mammalian brain. Synchronous neuronal activity is used widely in the developing brain for tuning synapses and their strength. A class of glutamate receptor, the NMDA receptor, is crucial for detecting this activity. Downstream of this receptor, the laboratory demonstrated the role of nitric oxide, a diffusible messenger, and cGMP, a cyclic nucleotide, as key components mediating the structural changes that follow synaptic plasticity.
Steven Pinker's laboratory studied the language development of almost seven hundred monozygotic and dizygotic twins, and found evidence of the heritability of several major milestones of language development: vocabulary growth, first word combinations, and first creative grammatical errors. The hope is that these results will be among the first studies in a new field, cognitive genetics, that will combine cognitive science and psycholinguistics with genetics to seek the genetic roots of higher mental processes.
The research in Edward Gibson's laboratory over the past year concentrated on investigating the relationship between the language comprehension mechanism and the available computational resources in working memory. Reading-time and questionnaire experiments revealed that language processing is highly locality-based: the greater the distance between an incoming word and the most local word to which it connects, the greater the integration cost, as measured by reaction times and complexity ratings. The resulting theory of sentence understanding accounts for a large array of phenomena across constructions in many languages.
Graduate students have been very successful in winning competitive fellowships to finance their education. These awards include the Clare Boothe Luce Foundation Fellowship, the John Merck/MIT Graduate Fellowship in Informatics, the Poitras Predoctoral Fellowship, and two Howard Hughes Medical Institute Predoctoral Fellowships. In addition, two students won Glenn/American Federation for Aging Research Scholarships for their summer projects involving research on the Biology of Aging.
The number of students who have chosen a major in the department increased from 74 last year to 91 this year. The department also continues to draw large numbers of undergraduates into UROP positions for course credit and employment in research projects. In addition, the Committee on Curricula accepted our proposed revisions for the new undergraduate major program. The first of the new courses, Statistical Methods, was very well received in the Spring semester. Additional new courses, including Neural Plasticity in Learning and Development, Language Acquisition, and Visual Cognition, will be offered in the coming academic year.
FACULTY HONORS AND AWARDS
Suzanne Corkin - Named Neuropsychologist of the Year by the Massachusetts Neuropsychological Society
Ann Graybiel - President, International Basal Ganglia Society & Board of Scientific Councilors, NIMH & Board on Neuroscience and Behavioral Health, Institute of Medicine
Guosong Liu - Selected as the Edward J. Poitras Assistant Professor in Human Biology and Experimental Medicine
Earl Miller - John Merck Foundation Scholar
Steven Pinker - Elected to the American Academy of Arts and Sciences & Los Angeles Times Book Prize in Science and Technology & Literary Lights Award, Boston Public Library & Books to Remember (25 Best of 1997), New York Public Library
Tomaso Poggio - Elected to the American Academy of Arts and Sciences & Member, Daimler-Benz Circle Member Group, 1997 & Honorary Chair, International ICSC/IFAC Symposium on Neural Computation, & Technical University of Vienna
Mriganka Sur - Board of Editors, The MIT Press
MIT Reports to the President 1997-98