Optical imaging of neuronal activity in the living brain

Abstract

The processing of sensory information, coordination of movement, and other higher brain functions are carried out by millions of neurons that form elaborate networks. Anatomical and physiological investigations of the mammalian brain have demonstrated its extraordinary complexity. How these neurons and their intricate connections endowed the brain with its remarkable performance is an important question which can greatly benefit from the development of new technologies. Recent progress in the development and application of two optical imaging techniques to the investigation of the intact mammalian brain is described. In the first methods fluorescent voltage-sensitive dyes are used to image the flow of information from one cortical site to the next in real time. This imaging method provided information about the retinoptic organization of the cortex and its functional organization into various modules. It revealed extensive long-range interactions between these cortical modules, much larger than those predicted from retinooptic or somatotopic maps, indicating a large degree of parallel processing. The combination of optical imaging with single unit recordings permitted the visualization of coherent activity in neuronal assemblies even in cases where they are spatially mixed (real time optical imaging is illustrated with a movie). A second imaging method, which does not require dyes, is based on reflection measurements of activity dependent intrinsic signals resulting from changes in optical properties of active brain tissue. This method permitted the high resolution visualization of many elements of the functional architecture of cortex in the living brain of cats and monkeys. These two complementary optical imaging techniques are particularly attractive for providing new insights to the development, organization, and function of the mammalian brain. The second technique is also likely to have clinical application in certain neurosurgical procedures on human patients.