Flavoprotein autofluorescence imaging of visual system activity in zebra finches and mice

Large-scale brain activity patterns can be visualized by optical imaging of intrinsic signals (OIS) based on activity-dependent changes in the blood oxygenation level. Another method, flavoprotein autofluorescence imaging (AFI), exploits the mitochondrial flavoprotein autofluorescence, which is enhanced during neuronal activity. In birds, topographic mapping of visual space has been shown in the visual wulst, the avian homologue of the mammalian visual cortex by using OIS. We here applied the AFI method to visualize topographic maps in the visual wulst because with OIS, which depends on blood flow changes, blood vessel artifacts often obscure brain activity maps. We then compared both techniques quantitatively in zebra finches and in C57Bl/6J mice using the same setup and stimulation conditions. In addition to experiments with craniotomized animals, we also examined mice with intact skull (in zebra finches, intact skull imaging is not feasible probably due to the skull construction). In craniotomized animals, retinotopic maps were obtained by both methods in both species. Using AFI, artifacts caused by blood vessels were generally reduced, the magnitude of neuronal activity significantly higher and the retinotopic map quality better than that obtained by OIS in both zebra finches and mice. In contrast, our measurements in non-craniotomized mice did not reveal any quantitative differences between the two methods. Our results thus suggest that AFI is the method of choice for investigations of visual processing in zebra finches. In mice, however, if researchers decide to use the advantages of imaging through the intact skull, they will not be able to exploit the higher signals obtainable by the AFI-method.