Mice were anesthetized with isoflurane (5% induction, 2% maintenance). Upon obtaining surgical-plane anesthesia, the skull was exposed and a stainless steel head plate was attached over the left hemisphere using a mixture of dental cement and superglue. The head plate was secured in a holding frame, and a small (~2-mm diameter) circular cranial window was drilled in the skull above the somatosensory cortex. Approximately 300 μL of a 1-mg/mL solution of FITC (molecular weight, 2000 kDa) in saline was injected via the tail vein to allow visualization of the cerebral vasculature and contrast imaging of RBCs. Upon conclusion of surgery, isoflurane anesthesia was replaced with α-chloralose (50 mg/kg) and urethane (750 mg/kg). Body temperature was maintained at 37°C throughout the experiment using an electric heating pad. Penetrating arterioles were first identified by observing RBCs flowing into the brain (as opposed to out of the brain via venules), and capillaries downstream of arterioles were selected for study. A pipette was next introduced into the solution covering the exposed cortex, and the duration and pressure of ejection were calibrated (200–300 ms, 8 ± 1 psi; n = 59) to obtain a small solution plume (radius, ~10 μm). The pipette was maneuvered into the cortex and positioned adjacent to the capillary under study (mean depth, 73 ± 6 μm; n = 19), after which agents were ejected directly onto the capillary. Placement of the pipette in the brain as described restricted agent delivery to the capillary under study and caused minimal displacement of the surrounding tissue (Fig. 3d and Supplementary Movies 2 and 3). Spatial coverage of the ejected solution was monitored by including tetramethylrhodamine isothiocyanate (TRITC, 150 kDa; 0.2 mg/mL)-labeled dextran. RBC velocity and flux data were collected by line scanning the capillary of interest at 5 kHz. For experiments in which KIR channels or neural activity were blocked, 100 μM BaCl2 or 3 μM tetrodotoxin in aCSF, respectively, was applied to the cranial surface for a minimum of 20 min to allow penetration. Whisker stimulation was performed using a piezoelectric actuator driven by a waveform generator coupled to an amplifier (Piezo Master; Viking Industrial Products). Whiskers were stimulated at a frequency of 4 Hz for 1 min with a total deflection of 5 mm. Images were acquired through a Zeiss 20x Plan Apochromat 1.0 NA DIC VIS-IR water-immersion objective mounted on a Zeiss LSM-7 multiphoton microscope (Zeiss, USA) coupled to a Coherent Chameleon Vision II Titanium-Sapphire pulsed infrared laser (Coherent, USA). FITC and TRITC were excited at 820 nm, and emitted fluorescence was separated through 500–550 and 570–610 nm bandpass filters, respectively.