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Visual Coding and Cortical Plasticity: From Synapse to Perception
Summary: Yang Dan's laboratory studies how visual information is encoded in the activity of cortical neurons and how cortical circuits are shaped by experience. Using both bottom-up and top-down approaches and a combination of electrophysiology, imaging, and computational techniques, her group aims to understand neural processing at multiple levels, from synaptic learning rules to cortical microcircuitry, and from network dynamics to animal behavior.
The long-term goals of our research are (1) to decipher the neural code in the mammalian visual system and to dissect the underlying neuronal circuitry; and (2) to elucidate how cortical circuits are dynamically modified by sensory experience, and how these modifications contribute to learning and memory. We use a multidisciplinary approach, combining advanced computational analyses and experimental investigations that involve a broad spectrum of techniques.
Neural processing occurs at multiple levels—from subcellular dendritic compartments, to local circuits of interconnected neurons, to large ensembles over extended brain areas. Although the traditional approach of studying each level in isolation has been fruitful in previous decades, some of the artificial divides have begun to hinder our progress. A key feature of our research program is the integration of studies at multiple organizational levels of the nervous system—from synapse, to network, to behavior. Our expertise in computational analyses and modeling plays an important role in bridging our experimental studies at different levels.
Microcircuitry, Receptive Field, and Plasticity in the Visual Cortex
An essential step in understanding sensory processing is to elucidate the intricate synaptic circuitry underlying the neural representation of various sensory features. Among all sensory cortical areas, the primary visual cortex (V1) is the best understood regarding the neuronal receptive field properties. Thus it provides a unique opportunity for investigating the underlying circuitry.
Previous studies of cortical microcircuitry have focused mainly on its physical structure, defining the rules of connectivity in terms of laminar location and neuronal morphology. However, in building models of cortical circuitry underlying the receptive field properties, it becomes clear that the rules of connectivity must also be specified in terms of the functional properties of the pre- and postsynaptic neurons. Our strategy for studying cortical circuitry and plasticity is to emphasize the neuronal response properties and to analyze synaptic connectivity with respect to the visual feature selectivity of each neuron in the circuit. We are currently combining in vivo two-photon Ca2+ imaging and intracellular (whole-cell) recording with computational analysis of cortical receptive fields. In collaboration with other researchers, we will also use viral vector-mediated gene delivery to manipulate the activity of specific cell types and to test their contributions to visual cortical receptive field properties.
Population Neural Dynamics in Visual Coding and Memory Storage
In addition to the microcircuitry within cortical columns that gives rise to the basic receptive field properties, there are also extensive horizontal connections between columns, which mediate distinct functions. These horizontal connections are highly susceptible to activity-dependent synaptic modifications, and they play important roles in both developmental circuit refinement and adult learning and memory. Due to their broad spatial distributions, the horizontal connections can coordinate the activity of large neuronal populations and mediate long-range perceptual interactions between different parts of the visual scene. To study visual coding and plasticity at the neuronal ensemble level, we have established multielectrode recording and voltage-sensitive dye-imaging techniques to analyze large-scale spatiotemporal activity patterns.
Our experiments have revealed a prevalence of spontaneous and visually evoked activity waves propagating over large areas of the adult visual cortex, presumably mediated by horizontal connections. Intriguingly, visually evoked activity patterns appear to reverberate in subsequent spontaneous waves. These initial observations have led us to investigate the interactions between visual experience and spontaneous cortical waves in visual processing and perceptual learning. Mechanistically, the reverberation may result from activity-dependent plasticity of horizontal connections. Functionally, such reverberation of recent sensory experience may play important roles in memory consolidation. We are using a combination of voltage-sensitive dye imaging and multielectrode recording to test these hypotheses.
Visual Coding in Awake Animals
Although anesthetized animals have been useful for studying early sensory processing, our ultimate goal is to understand brain functions in awake animals under natural behavioral states. We have recently extended our studies of visual processing to awake animal models, both within my lab and through collaboration with others. This allows us to study visual processing in higher cortical areas (beyond V1), to perform simultaneous measurements at the perceptual and physiological levels to establish their causal relationship, and to examine the effects of top-down modulations on visual cortical response properties.
We have developed visual psychophysical paradigms for the rat. Our experiments showed that some of the visual illusions found in human subjects are also present in rats, making them a valid model for studying the neural mechanisms for these perceptual effects. To establish a direct link between the neuronal response properties and the visual behaviors, we are making chronic multielectrode recordings from rat visual cortex during the tasks.
Over the past several years, we have also established collaborations to study visual coding in awake behaving primates. We are combining single-unit recording experiments with advanced computational analyses to study the neuronal response properties in cortical areas beyond V1, which are known to be more complex and nonlinear. Our goal is to elucidate feature selectivity at higher stages of the visual pathway, which remains a formidable challenge in systems neuroscience.
This laboratory is currently supported by the National Eye Institute, the National Science Foundation, the Li Ka Shing Foundation, and the Mary Elisabeth Rennie Endowment Fund.
Last updated January 14, 2009
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