Contrary to the intense debate about brain oxygen dynamics and its uncoupling in mammals, very little is known in
birds. In zebra finches, picosecond optical tomography (POT) with a white laser and a streak camera can measure in vivo
oxy-hemoglobin (HbO<sub>2</sub>) and deoxy-hemoglobin (Hb) concentration changes following physiological stimulation
(familiar calls and songs). POT demonstrated sufficient sub-micromolar sensitivity to resolve the fast changes in
hippocampus and auditory forebrain areas with 250 μm resolution. The time-course is composed of (i) an early 2s-long
event with a significant decrease in Hb and HbO<sub>2</sub>, respectively -0.7 μMoles/L and -0.9 μMoles/L (ii) a subsequent
increase in blood oxygen availability with a plateau of HbO<sub>2</sub> (+0.3μMoles/L) and (iii) pronounced vasodilatation events
immediately following the end of the stimulus. One of the findings of our work is the direct link between the blood
oxygen level-dependent (BOLD) signals previously published in birds and our results. Furthermore, the early
vasoconstriction event and post-stimulus ringing seem to be more pronounced in birds than in mammals. These results in
bird, a tachymetabolic vertebrate with a long lifespan, can potentially yield new insights for example in brain aging.
A new ultrafast Diffuse Optical Tomography (DOT) has been developed for real time <i>in vivo</i> brain metabolism monitoring in songbird. The technique is based on space resolved time of flight measurements of the photons across the brain tissues. A three dimensional reconstruction of the brain activity is foreseeable by means of a double space and time sampling of the reflectance signal. The setup and the treatment procedure are described in depth and promising preliminary results showing the response of brain tissues to hypercapnia stimulations (increase of CO<sub>2</sub>) are presented.
In the field of biophotonics the main goals are the control and processing of <i>in vivo</i> biological tissues and the monitoring of biomolecule dynamics. Two particular “pitfalls” are present: the dynamic multiscale organization and the photostress of the medium. Until now the state of the art of the pico-femtosecond systems designed to these applications shows that the changing laser technology has been only used as an add-on. Our approach is based on a bottom-up procedure and on the medium-centered knowledge. The range of neurobiological applications of ultrafast photonics extends from TRP (time-resolved propagation) to linear and non-linear TRE (time-resolved emission). The device combines a one kilohertz chirp pulse amplification laser system and a single shot streak camera. For discrete wavelength applications (TRE), the set-up is a SHG/OPG/OPA<sup>3</sup>/SHG design. In the case of TRP, the beam is focused into pure water to generate a white light continuum. After propagation through tissue, a single-shot streak camera with single photo-electron counting capability performs the picosecond time-resolved spectroscopy of the collected photons. Depending on the acceptable level of photostress, the integration time can extend from 33ms up to several minutes with a real-time control of the jitter and time drifts. The meaning of the TRE spectro-temporal image is particularly detailed in the 450-480nm excitation window in regards to the contributions of mitochondrial flavoproteins. This optical system fulfills the reliability and the sensitivity, conditions required for measuring opto-electronic quantities from freely moving animal at low irradiation.
Our purpose is to spectrally probe the main brain absorbers. The determination of their spatial distribution remains a challenge. According to anatomical data, the proposed 3D model of the rat pial-cortical vascular networks is divided into three parts: (1) the pial vessels could be approximated by a dense layer of around 250 micrometers depth; (2) the penetrating vessels repartition is described as periodic hexagonal prisms with three modules; (3) the capillary network is modelized using a periodic tiling of polyhedron with a density of 817mm.mm<SUP>-3</SUP> and a branching pattern of 10000mm<SUP>-3</SUP>. With anaesthetized rats under stereotaxic conditions, in vivo time-resolved brain spectroscopy experiments are presented. The setup is designed to allow broadband time-resolved spectroscopy using a streak camera. A femtosecond white light continuum is produced by focusing 800nm pulses (0.5mJ, 1kHz, 150fs) in an adapted third order non linear medium. In the case of water, the spectrum expands over 380-780nm with an efficiency of 20 percent. Mathematical homogenization techniques could be applied to the radiative transfer equation with this geometrical vascular architecture and might be useful to analyze in depth time-resolved spectroscopy of such complex media.