A pair of independent studies using photonics to investigate the flow of blood in the brain may enable a better understanding of neurological disorders such as Alzheimer’s disease and strokes. In the work described in the Dec. 25 online edition of Nature Neuroscience, researchers from the University of Rochester Medical School in New York used in vivo two-photon microscopy to investigate the rapid dilation of blood vessels in the somatosensory cortices of mice during neural stimulation (above). Their findings indicate that astrocytes — cells that physically contact both synapses and cerebral blood vessels — mediate vasodilation in response to neural activity. The researchers suggest that the astrocytes’ dysfunction might lead to the death of neurons and, thus, might play a role in disorders such as Alzheimer’s disease.Reporting in the February issue of Public Library of Science Biology, investigators at the University of California, San Diego, in La Jolla, the Veterans Affairs Medical Center in San Diego and Vanderbilt University in Nashville, Tenn., propose that the architecture of the vasculature of a region of the brain affects the susceptibility of that region to damage by stroke. Using in vivo two-photon microscopy of the parietal cortices of rats, they monitored blood flow before, during and after a targeted clot (below). They caused the animals to experience microstrokes by photoexciting a dye to damage a blood vessel, inducing the localized clot to block flow. They discovered that the flow of blood reversed in vessels downstream from the blockage, compensating for the loss of flow, which may be a result of the redundant architecture of the vasculature in that region of the brain.