Optical coherence tomography (OCT) enables um-resolution and high-speed imaging of tissue structure, facilitating a number of basic and clinical studies in ophthalmology, cancer biology, and neuroscience. Through the proposed K99/R00 program, the candidate will develop novel OCT-based technologies for um-resolution imaging of tissue dynamics, especially in the brain cortex of a living animal. In detail, the candidate will develop three technologies for imaging various vascular and cellular dynamics occurring in the rodent cerebral cortex: in vivo imaging of the motion of neuronal intracellular organelles with single-cell resolution (Specific Aim 1a), simultaneous imaging of blood flow speed over hundreds of capillaries with single-capillary and 1-s resolution (Specific Aim 1b), and imaging of fast optical signals of neuronal activity with single-cell and ms resolution (Specific Aim 2). Thes technologies will be generally useful for a range of neuroscience and pathophysiology studies that benefit from direct visualization of those tissue dynamics with high spatiotemporal resolution. The proposed K99/R00 program will focus on using the technologies to propose and demonstrate the concept of neuro-capillary coupling. This concept will challenge the current paradigm, neurovascular coupling, for understanding the brain's energy supply regulation and for interpreting hemodynamics-based human brain mapping data. Recently, blood flow regulation at the capillary level has been suggested in vitro as mediated by pericytes, but not demonstrated in vivo. Further, cortical capillary flow dynamics is also suggested to relate with pathophysiology. Therefore, the proposed concept will improve our understanding of blood flow regulation and thus offer new opportunities for developing therapeutic approaches to a range of disorders of the brain including stroke and Alzheimer's disease. In detail, using the technologies developed in Specific Aims 1 and 2, the candidate will test three hypotheses for demonstrating and characterizing neuro-capillary coupling in vivo (Specific Aim 3): (H1) Capillaries regulate blood flow in response to neuronal activation in the somatosensory cortex, directly proving the capillary control of flow; (H2) Neuro-capillary coupling leads to an early capillary network flow homogenization, identifying the role of the capillary flow regulation; and (H3) Neuro-capillary coupling exhibits a microscopic spatial correlation between excited neurons and responding capillaries, revealing the characteristics of neuro-capillary coupling. The proposed research project will enable the candidate to gain further research experience and scientific knowledge in the field of biomedical optics and neuroimaging. Along with the research project, the proposed career development programs including course work and seminars will assist him in achieving his career goal: to establish an independent research program in a biomedical engineering or applied physics department.