In-vivo Optical Measurement of Neural Activity in the Brain

Abstract

The optical neural recording techniques are promising tools in recent years. Compared to the traditional electrophysiological recording, the optical means offer several advantages including no inclusion of electrical noise, simultaneous imaging of a large number of neurons, or selective recording from genetically-targeted neurons. Overall the optical neural recording technique comprises the intrinsic and the extrinsic optical recordings. The methods for intrinsic neural recording employ the change of optical properties in brains such as blood flow/oxygenation, cellular volume change, or refractive index change without addition of external indicators. Those properties can be detected using various optical techniques including laser Doppler flowmetry (LDF), near-infrared (NIR) spectrometer, functional optical coherence tomography (fOCT), and surface plasmon resonance (SPR). The extrinsic monitoring techniques use fluorescence signals reflecting neuronal activity via chemical or genetic modification of the neuronal cells. Two most popular activity-dependent fluorescent probes, calcium indicators and voltage-sensitive fluorescent proteins will be examined in this review. The principles, the instrumentations and in vivo applications of those optical signal measurements are described.

Keywords: calcium indicators; extrinsic optical recording; intrinsic optical recording; optical neural recording; voltage-sensitive fluorescent protein (VSFP).

Fig. 1

Detection methods for intrinsic optical signal recording [18, 36, 37, 75, 76]. The diagram illustrates the relationship between brain activity and optical measurements based on intrinsic optical contrast. The term physiological parameter is used to indicate a physiological quantity which is measured by its correspondence to an optical parameter. [Hb] indicates the concentration of hemoglobin and cytochrome-c-oxidase, respectively.