Carol L. Colby - US grants

The goal of the proposed experiments is to determine how many distinct visual areas exist in macaque intraparietal sulcus and annectent gyrus, and to discover how these areas are organized, topographically, connectionally and hierarchically. We hypothesize that there may be as many as four distinct areas within the intraparietal sulcus. A second hypothesis is that these areas are topographically organized, containing a systematic representation of the visual field. The third hypothesis is that the connections of these areas are topographically organized. Finally, we also intend to determine the position of each area in the hierarchy of cortical visual areas and determine whether each area belongs to one or another of the specific channels identified within the overall hierarchy. Anatomical methods will be used to outline the borders and overall topography of each area. Physiological mapping of receptive field position will be done to produce a finer-grained analysis of the visuotopic organization in each area. Finally, the connections of each area will be determined and their topographic and hierarchical organization will be analyzed. The purpose of these experiments is to provide the basis for a more precise description of the flow of information through extrastriate visual cortex. Such a description is essential to a complete understanding of the physiological mechanisms of visual perception and visually guided behavior in these higher-order areas. Ultimately, this information will be of use in the treatment of perceptual deficits after cortical damage and in the development of visual protheses.

As we move our eyes, new images are constantly presented to the brain, yet we perceive the world as remaining still. This perceptual stability is thought to depend on a convergence of visual signals and corollary discharges reflecting the generation of voluntary eye movements. Visual and motor signals together construct an internal representation of space that is constantly updated. The neural mechanisms underlying this process are beginning to be understood at the level of single neurons, through single-unit recording in monkeys. Dr. Colby's research goal is to extend these observations to humans. She will use functional magnetic-resonance imaging (fMRI) to examine activity in the human brain during spatial updating. She expects that multiple brain regions will be involved in updating of visual memory traces. The most exciting outcome of these imaging experiments will be the delineation of the complete set of brain regions involved in spatial memory and cognition. To accomplish this goal by single-unit recording alone would take several years. The promise of functional imaging is that activity throughout the brain can be observed simultaneously, allowing one to see the entire network in action. In accord with the goals of the POWRE program, this new research program will significantly enhance Dr. Colby's research capabilities by providing access to unique technological resources.

The idea that vision is an active process has recently attracted considerable interest. Nowhere is the active nature of perception more evident than in the construction of a stable image of the world. As we move our eyes, new images are constantly presented to the brain yet we perceive the world as staying still. The perception stability we experience is thought to depend on a convergence of visual signals and corollary discharges reflecting the generation of voluntary eye movements. The goal of these experiments is to understand the impact of motor action on sensory processing as it relates to spatial constancy. We will ask how a specific motor act, saccadic eye movement, affects the representation of visual stimuli in primate cortex. Neurons in monkey parietal cortex have been shown to remap the presentation of a visual stimulus when the eyes move. This surprising observation has raised numerous questions about the neural mechanism underlying spatial constancy. The proposed experiments are designed to characterize the impact of the behavioral relevance of the stimulus on remapping (Aim 1); to discovered whether similar phenomena occur at earlier stages of the visual hierarchy (Aim 2); to determine how visual information is remapped from one hemisphere to the other (Aim 3); and to explore the interactions among frontal and parietal cortical stages which may underlie the process of updating spatial representations (Aim 4). Findings from these studies will provide a deeper understanding of the natural of spatial representation in cortex. Such an understanding is necessary as a step towards designing scientifically based diagnostics and rehabilitation programs for patients who have impair spatial functioning as a result of parietal lobe damage.

DESCRIPTION (provided by applicant): Vision is an active process. We do not see the world directly; rather, we construct a representation of it from sensory inputs in combination with internal, non-visual signals. In the case of spatial perception, our representation of the visual scene takes into account our own movements. This allows us to perceive the world as stationary despite the constant eye movements that produce new images on the retina. How is this perceptual stability achieved? Our central hypothesis is that a corollary discharge of the eye movement command updates, or remaps, an internal representation when the eyes move. We have previously shown that single neurons in the lateral intraparietal area (LIP) and extrastriate visual cortex are activated by the remapped trace of a visual stimulus. These neurons fire in the single-step task, in which a saccade brings the receptive field onto a previously stimulated location. Remapping is also observed in the double-step task, in which the animal makes sequential saccades to two target locations. Our long-term goal is to discover the neural mechanisms that produce remapping. To achieve this we need to learn much more about the phenomenon and about the neural circuitry that supports it. The proposed experiments are designed to discover whether LIP neurons have equal access to visual information from the entire visual field; to determine whether remapping varies with hemifield or distance; to discover the source of remapped visual signals; and to determine the source of the corollary discharge signals used in remapping. The aim of the proposed work is to elucidate the neural circuitry that contributes to active vision.