| Crick, F., Koch, C. A framework for consciousness Nature Neuroscience 2003 (6)2:119-126 [pdf] |
| Here we summarize our present approach to the problem of consciousness. After an introduction outlining our general strategy, we describe what is meant by the term 'framework' and set it out under ten headings. This framework offers a coherent scheme for explaining the neural correlates of (visual) consciousness in terms of competing cellular assemblies. Most of the ideas we favor have been suggested before, but their combination is original. We also outline some general experimental approaches to the problem and, finally, acknowledge some relevant aspects of the brain that have been left out of the proposed framework. |
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| Yang, Z., Purves, D. A statistical explanation of visual space Nature Neuroscience 2003 (6)6:632-640 |
| The subjective visual space perceived by humans does not reflect a simple transformation of objective physical space; rather, perceived space has an idiosyncratic relationship with the real world. To date, there is no consensus about either the genesis of perceived visual space or the implications of its peculiar characteristics for visually guided behavior. Here we used laser range scanning to measure the actual distances from the image plane of all unoccluded points in a series of natural scenes. We then asked whether the differences between real and apparent distances could be explained by the statistical relationship of scene geometry and the observer. We were able to predict perceived distances in a variety of circumstances from the probability distribution of physical distances. This finding lends support to the idea that the characteristics of human visual space are determined probabilistically. |
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| Lewicki, M. Efficient coding of natural sounds Nature Neuroscience 2002 (5)4:356-363 [pdf] |
| The auditory system encodes sound by decomposing the amplitude signal arriving at the ear into multiple frequency bands whose center frequencies and bandwidths are approximately exponential functions of the distance from the stapes. This organization is thought to result from the adaptation of cochlear mechanisms to the animal's auditory environment. Here we report that several basic auditory nerve fiber tuning properties can be accounted for by adapting a population of filter shapes to encode natural sounds efficiently. The form of the code depends on sound class, resembling a Fourier transformation when optimized for animal vocalizations and a wavelet transformation when optimized for non-biological environmental sounds. Only for the combined set does the optimal code follow scaling characteristics of physiological data. These results suggest that auditory nerve fibers encode a broad set of natural sounds in a manner consistent with information theoretic principles. |
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| Todorov, E., Jordan, M.I. Optimal feedback control as a theory of motor coordination Nature Neuroscience 2002 (5):11 [pdf] |
| Motor coordination the marshalling of redundant actuators in the service of a desired behavioral outcome is among the most important and least understood facets of motor function. Models that focus on mechanisms for achieving behavioral goals often fail to account for experimental data on movement variability and the exploitation of redundancy. Models that focus on variability and redundancy often fail to explain how goals are achieved in the first place. Here we show that not only are variability and goal achievement compatible, but indeed that allowing variability in redundant dimensions is the optimal strategy in the face of uncertainty. Our approach is based on stochastic optimal control theory, which provides, for a given task, the feedback control law that maximizes expected performance. This control law does not enforce a "desired trajectory" an approach that we show to be suboptimal but instead corrects only those deviations that interfere with the task goals. We find that the resulting behavior exhibits goal-directed adjustments, synergies, controlled parameters , simplifying rules , and discrete coordination modes none of which are built in a priori. Experimentally, we investigate a range of motor tasks and report patterns of variability in close agreement with the model. |
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| Todorov, E. Optimality principles in sensorimotor control Nature Neuroscience 2004 (7)9:907-915 [pdf] |
| The sensorimotor system is a product of evolution, development, learning and adaptation which work on different time scales to improve behavioral performance. Consequently, many theories of motor function are based on optimal performance : they quantify task goals as cost functions, and apply the sophisticated tools of optimal control theory to obtain detailed behavioral predictions. The resulting models, although not without limitations, have explained more empirical phenomena than any other class. Traditional emphasis has been on optimizing desired movement trajectories while ignoring sensory feedback. Recent work has redefined optimality in terms of feedback control laws, and focused on the mechanisms that generate behavior online. This approach has allowed researchers to fit previously unrelated concepts and observations into what may become a unified theoretical framework for interpreting motor function. At the heart of the framework is the relationship between high-level goals, and the real-time sensorimotor control strategies most suitable for accomplishing those goals. |
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| Paz, R. & al Preparatory activity in motor cortex reflects learning of local visuomotors skills Nature Neuroscience 2003 (6)8:882-890 [pdf] |
| In humans, learning to produce correct visually guided movements to adapt to new sensorimotor conditions requires the formation of an internal model that represents the new transformation between visual input and the required motor command. When the new environment requires adaptation to directional errors, learning generalizes poorly to untrained locations and directions, indicating that such learning is local. Here we replicated these behavioral findings in rhesus monkeys using a visuomotor rotation task and simultaneously recorded neuronal activity. Specific changes in activity were observed only in a subpopulation of cells in the motor cortex with directional properties corresponding to the locally learned rotation. These changes adhered to the dynamics of behavior during learning and persisted between learning and relearning of the same rotation. These findings suggest a neural mechanism for the locality of newly acquired sensorimotor tasks and provide electrophysiological evidence for their retention in working memory. |
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| Koch, C., Segev, I. The role of single neurons in information processing Nature Neuroscience 2000 (3):1171-1177 [pdf] |
| Neurons carry out the many operations that extract meaningful information from sensory receptor arrays at the organism s periphery and translate these into action, imagery and memory. Within today s dominant computational paradigm, these operations, involving synapses, membrane ionic channels and changes in membrane potential, are thought of as steps in an algorithm or as computations. The role of neurons in these computations has evolved conceptually from that of a simple integrator of synaptic inputs until a threshold is reached and an output pulse is initiated, to a much more sophisticated processor with mixed analog-digital logic and highly adaptive synaptic elements. |
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| Critchley, H.D. & al neural systems supporting interoceptive awareness Nature Neuroscience 189-195 [pdf] |
| Influential theories of human emotion argue that subjective feeling states involve representation of bodily responses elicited by emotional events. Within this framework, individual differences in intensity of emotional experience reflect variation in sensitivity to internal bodily responses. We measured regional brain activity by functional magnetic resonance imaging (fMRI) during an interoceptive task wherein subjects judged the timing of their own heartbeats. We observed enhanced activity in insula, somatomotor and cingulate cortices. In right anterior insular/opercular cortex, neural activity predicted subjects' accuracy in the heartbeat detection task. Furthermore, local gray matter volume in the same region correlated with both interoceptive accuracy and subjective ratings of visceral awareness. Indices of negative emotional experience correlated with interoceptive accuracy across subjects. These findings indicate that right anterior insula supports a representation of visceral responses accessible to awareness, providing a substrate for subjective feeling states. |
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| Crick, F., Koch, C. A framework for consciousness Nature Neuroscience 2003 (6)2:119-126 |
| Yang, Z., Purves, D. A statistical explanation of visual space Nature Neuroscience 2003 (6)6:632-640 |
| Lewicki, M. Efficient coding of natural sounds Nature Neuroscience 2002 (5)4:356-363 |
| Monnier, P., Shevell, S.K. Large shifts in color appearance from patterned chromatic backgrounds Nature Neuroscience 2003 (6)8:801-802 |
| Bizzi, E., Tresch, M.C., Saltiel, P., d'Avella, A. New perspectives on spinal motor systems Nature Neuroscience 2000 (1):101-107 |
| Todorov, E., Jordan, M.I. Optimal feedback control as a theory of motor coordination Nature Neuroscience 2002 (5):11 |
| Todorov, E. Optimality principles in sensorimotor control Nature Neuroscience 2004 (7)9:907-915 |
| Paz, R. & al Preparatory activity in motor cortex reflects learning of local visuomotors skills Nature Neuroscience 2003 (6)8:882-890 |
| Martin, K.A.C. The Pope and grandmother - a frog's-eye view of theory Nature Neuroscience 2000 (3)supp:1169 |
| Koch, C., Segev, I. The role of single neurons in information processing Nature Neuroscience 2000 (3):1171-1177 |
| Critchley, H.D. & al neural systems supporting interoceptive awareness Nature Neuroscience 189-195 |