Blood contrast agents enhance intrinsic signals in the retina: evidence for an underlying blood volume component.

To examine the extent to which neurovascular coupling contributes to stimulus-evoked intrinsic signals in the retina. The retinas of five adult cats were examined in vivo. Animals were anesthetized and paralyzed for imaging stability. The retinas were imaged through a modified fundus camera capable of presenting patterned visual stimuli simultaneous with a diffuse near infrared (NIR). Injections of nigrosin increased signal strength by as much as 36.3%, and indocyanine green (ICG) increased signal magnitudes by as much as 38.1%. In both cases, intrinsic signals maintained a colocalized pattern of activation corresponding to the visual stimulus presented. The time course of the evoked signals remained unaltered. The spectral dependency of signal enhancement mirrored the absorption spectra of the injected dyes. The data are consistent with a neurovascular coupling effect in the retina. Patterned visual stimuli evoke colocalized NIR reflectance changes. The patterned decrease in reflectance was enhanced after nigrosin or ICG was injected into the systemic circulation. These findings suggest stimulus-evoked changes in blood volume underlie a component of the retinal intrinsic signals.