Charles D. Gilbert - US grants

Visual information is carried by the optic nerves to a major center called the lateral geniculate nucleus (LGN), which in turn projects to two major areas of the cortex of the brain, called Areas 17 and 18. When a single cell in the visual pathway responds to a visual stimulus, that stimulus is said to be in the cell's "receptive field." Past discoveries have shown that some cells in the visual cortex only respond to stimuli in particular locations and having particular properties (such as orientation of a line, or movement in a certain direction), which may be quite complex. Cortical cells were discovered to be organized in a columnar fashion, where cells within a column share sensitivity to a preferred visual stimulus. This study will examine the cortical connections in Area 17 that extend horizontally, among the columns. Cellular dye labels, autoradiography, and the new methods of optical recording will be used to visualize the anatomy of the connections. Physiological recordings in a newly developed preparation of an isolated slice of cortex will reveal how these horizontal connections function at the synaptic level. Together these studies will clarify how visual information over a wide region of the animal's visual world are integrated in the visual cortex. The results of this novel approach will be very important for visual science, and may have impact on other areas of neuroscience and information processing studies.

DESCRIPTION (provided by applicant): Adult primary visual cortex performs a high level analysis of the visual scene, mediating contour integration and surface segmentation. We propose to test the hypothesis that the primary visual cortex encodes information about complex visual shapes and that the properties of cortical neurons are experience dependent even in adulthood. Cortical receptive fields have higher order properties that represent the geometry of the visual world and these properties are mutable throughout life, allowing us to assimilate information during perceptual learning. The functional properties that show these changes are known as contextual influences, where the response of a cell to a local feature is dependent upon its global context. We will characterize the contextual influences seen in V1 and relate them to the geometry of visual scenes. We will determine the way in which visual experience further shapes the functional properties of visual cortical neurons. An important part of our study is to explore the role of top-down influences in shaping the response properties of V1 neurons. We will determine how higher order cognitive influences of attention; expectation and perceptual task interact with information about stimulus characteristics within V1. Our studies will include approaches that enable us to investigate cortical mechanisms at multiple scales, from single neurons, via single unit recordings in alert, behaving animals, to optical imaging of neuronal ensemble activity within a cortical area, to fMRI in human subjects. Psychophysical studies will help us define the nature of shape information encoded during perceptual learning, and will guide the physiological studies. Plasticity of V1 reflects an ongoing process, beginning with our early experience of the regularities of the world and continuing throughout our lives, to assimilate the specific patterns to which we become familiar. The properties of any cortical area, even in adults, are dynamic, being experience dependent and subject to top-down influences.

Tubby-like proteins (TULPs) comprise a family of proteins found in all multicellular organisms. These molecules are characterized by the presence of a conserved carboxy-terminal "tubby domain" that does not exhibit sequence homology to other known proteins. Genetic mutation of tubby or other TULPs often leads to one or more of three disease phenotypes: (1) obesity - from which the name "tubby" is derived, (2) retinal degeneration, and (3) hearing loss. The disease phenotypes associated with mutations in tubby-like proteins clearly indicate a vital role for these molecules in normal tissue function. While the expression pattern of each family member is distinctive, tubby proteins are found mainly in the nervous system, and all known human tubby proteins are expressed in the retina. Mutation of the TULP1 gene is the cause of retinitis pigmentosa type 14 (about 5 percent of inherited RP cases), and the human TULP2 gene maps within the minimal identified region for the cone-rod retinal dystrophy locus on chromosome 19. Furthermore, tubby mutants bear a remarkable similarity to several human syndromes that result in combined sensorineural hearing loss and retinal degradation, accompanied by obesity. Despite the clear medical importance of tubby-like proteins, no biochemical function has yet been ascribed to any member of this protein family. In order to identify the biochemical function of tubby and other TULPs -and to thus understand their role in disease - we will use X-ray crystallography to determine the high-resolution three-dimensional structure of tubby. We will identify a function for tubby based on structural similarities to other known proteins, by identification of chemical functionalities such as active sites or cofactors, and by the application of concomitant cell biological and biochemical studies, including cellular and sub-cellular localization studies. This is a model problem for a "structural genomics" approach to identification of function for a medically relevant protein. This approach relies on structural information, phenotype data, and classical biology approaches enabled by the availability of pure protein. This type of approach should be greatly facilitated by the recent massive expansion of the database of three-dimensional structures.

DESCRIPTION (provided by applicant): Plasticity of the adult visual cortex plays an important role in normal processes of perceptual learning as well as in functional recovery following central nervous system lesions. We propose in particular that the normal integrative processes seen in visual cortex are recruited for adaptive changes following CNS lesions. To understand the mechanisms underlying adult cortical plasticity, as well as its perceptual consequences, we will work with an experimental model involving the reorganization of cortical topography following binocular retinal lesions. We have made a computational model that relates the normal properties of visual cortical neurons to the changes occurring after retinal lesions, and further relates these changes to perceptual fill-in. In the current study we propose to test the predictions of this model, and to quantify the nature of the functional changes in V1 that ensue following retinal lesions. Monitoring the functional reorganization will be done by electrophysiological recordings in awake behaving monkeys, allowing us to follow the process of reorganization over multiple time points and over large areas of cortex. These experiments are also intended to resolve controversies concerning the prevalence, extent and nature of the remapping of cortical topography in the lesion model. To address issues of mechanism, we will explore the involvement of different components of cortical circuitry in the reorganization. This will be done by in vivo 2-photon imaging of cells, dendrites and axons labeled by neuronal infection with genetically engineered AAV virus carrying genes encoding different fluorophores. We will investigate the relative contribution of long-range horizontal connections, feedback connections from higher order cortical areas, and interlaminar connections within V1, as well as changes in dendritic morphology. With these experiments we hope to learn about the central mechanisms of recovery following retinal degenerations, such as macular degeneration and other forms of CNS damage. Our approach is also designed to further our understanding of the perceptual consequences of the cortical changes induced by retinal damage. Beyond their value in the study of lesion dependent plasticity, these studies will be of relevance to understanding the mechanism of normal experience dependent changes in the visual pathway, such as those associated with perceptual learning. The proposed experiments will help reveal the mechanisms of functional recovery following lesions and degenerative diseases of the CNS, including adult macular degeneration. The studies will also be relevant to understanding the normal experience dependent changes associated with perceptual learning, since the same circuits are likely to be involved.

? DESCRIPTION (provided by applicant): The adult cerebral cortex is capable of undergoing experience dependent functional changes. Such changes play a role in perceptual learning and recovery of function following CNS damage. The circuitry mediating adult cortical plasticity includes the plexus of long range horizontal connections formed by cortical pyramidal cells as well as the connections formed by inhibitory interneurons. The experience dependent changes in this circuitry involve the sprouting of new axonal connections and the pruning of preexisting connections. The proposed project involves investigating the molecular basis for the remodeling of axonal arbors, including signaling by neurotrophins and the caspase dependent apoptotic pathway. Using a mouse model of adult cortical plasticity involving remapping of the somatosensory cortex following whisker plucking and 2- photon in vivo longitudinal imaging of cortical axons, the role of different components in these signaling pathways in the process of axonal pruning and outgrowth will be determined. The experiments are designed to map the steps and molecular interactions leading from alterations in the patterns of neural activity to alterations in cortical circuits.