A New Technique for Functional Imaging in Songbirds and Beyond

Abstract

The quantitative study of cerebral hemodynamics requires measurement methods with high spatial and temporal resolution. In the best case, these methods are noninvasive or even contact free. Mottin et al (2010, this issue) propose picosecond optical tomography, or POT, to quantitatively image hemodynamics in a contact-free way. POT relies on white laser light spectral absorption in the near-infrared measured with a streak camera. The tens-of-femtosecond temporal resolution of the streak camera enables depth-specific imaging of oxy- and deoxy-hemoglobin concentration with submillimeter resolution. The authors validate their technique on a songbird model of auditory perception and show results consistent with previous findings that were obtained using functional magnetic resonance imaging (MRI). Furthermore, they shed new light onto brain oxygen recoupling in these tachymetabolic animals, results that would be difficult to obtain with conventional functional MRI. The songbird model is an interesting choice for demonstrating the validity of POT, as songbirds provide a versatile model for studying neuronal plasticity (Brenowitz, 2004). This includes the ability to generate a significant amount of new neurons in adult life (Nottebohm, 2002), as well as the rare trait of vocal learning (Mooney, 2009). Songbirds produce behavioral patterns of high complexity (song) that can be precisely recorded and related to neuronal dynamics from time scales of milliseconds to a time spanning several generations (Hahnloser et al, 2002; Feher et al, 2009). It might be possible that POT adds information to more classical methods of birdsong research based on electrical recordings or functional MRI by virtue of its quantitative nature. For example, could it be used to get a better picture of how the song system works as a network of connected song nuclei by quantitatively analyzing the hemodynamic response dynamics? Besides further applications in songbird research, these advantages could also play out in functional studies of neurovascular plasticity, as has been observed for example after focal cerebral injury (Whitaker et al, 2007), or in neurovascular repair mechanisms after induction of small-scale cortical vascular lesions (Nishimura et al, 2006) that are believed to have a function in aging-related cognitive decline. As they point out at the end of the article, this is a field worth studying for Mottin et al too, by using their technology to investigate a proposed link between brain aging and changes in vascular reactivity found by functional MRI. However, it remains to be seen if POT can compete with the very powerful imaging techniques already used in this field, such as multiphoton microscopy and voltage sensitive dye imaging (Brown et al, 2009). Finally, the authors propose a way to expand POT into a true two-dimensional imaging method (so far it can only image line segments) by sweeping the optical field over the sample. In the long term, functional imaging methods like this could, once developed for human applications, become more relevant clinically (Prakash et al, 2009). As functional MRI maps of the human cortex display a considerable amount of interindividual variability, it is necessary to map, for example, eloquent sensorimotor areas before or during brain tumor surgery. Often, a combination of electrocortical stimulation mapping on the exposed brain and functional MRI is used to identify eloquent sensorimotor areas and thus to reduce surgical risks (Holodny, 2008). It can be envisioned that optical imaging methods based on POT could be used in the operating room to rapidly map individual eloquent cortical areas during surgery.