  We report optical brain imaging using a semi-transparent organic
light-emitting diode (OLED) based on the orange light-emitting polymer (LEP)
Livilux PDO-124. The OLED serves as a compact, extended light source which is
capable of uniformly illuminating the cortical surface when placed across a
burr hole in the skull. Since all layers of the OLED are substantially
transparent to photons with energies below the optical gap of the LEP, light
emitted or reflected by the cortical surface may be efficiently transmitted
through the OLED and into the objective lens of a low magnification microscope
('macroscope'). The OLED may be placed close to the cortical surface, providing
efficient coupling of incident light into the brain cavity; furthermore, the
macroscope may be placed close to the upper surface of the OLED, enabling
efficient collection of reflected/emitted light from the cortical surface.
Hence the use of a semi-transparent OLED simplifies the optical setup, while at
the same time maintaining high sensitivity. The OLED is applied here to one of
the most demanding forms of optical brain imaging, namely extrinsic optical
imaging involving a voltage sensitive dye (VSD). Specifically, we carry out
functional imaging of the primary visual cortex (V1) of a rat, using the
voltage sensitive dye RH-1691 as a reporter. Imaging through the OLED
light-source, we are able to resolve small (~ 0.1 %) changes in the
fluorescence intensity of the dye due to changes in the neuronal membrane
potential following a visual stimulus. Results are obtained on a single trial
basis -- i.e. without averaging over multiple measurements -- with a
time-resolution of ten milliseconds.
