![]() Several studies support cellular swelling as the primary mechanism underlying the scattered light change ( Cohen, 1973 Tasaki and Byrne, 1992 Yao et al., 2003), and some studies also point to swelling as a contributor to the birefringence signal ( Yao et al., 2005). In−vitro studies have shown that scattering and birefringence signals result from a combination of mechanisms including changes in refractive index, protein conformation, and other processes associated with changes in membrane potential ( Foust and Rector, 2007). Investigators have non−invasively monitored changes in blood flow and the oxy−/deoxy−hemoglobin ratio using near−infrared spectroscopy and diffuse optical tomography (for review see Villringer and Chance, 1997 Boas, 2002). Neural activity also increases metabolism and elicits slower cascades of hemodynamic events that can be monitored through light absorption by oxy− and deoxy−hemoglobin. Cellular swelling and molecular conformational changes elicit rapid changes in light scattering and birefringence concomitant with membrane depolarization ( Cohen et al., 1968 Tasaki et al., 1968 Rector et al., 2005). Neural activation initiates both fast and slow optical changes. Intrinsic optical signals have the potential to revolutionize the way neuroscientists record and image neuronal function, but low signal−to−noise ratios currently limit their usefulness. Our data recommend low coherence and portable light sources for in−vivo chronic neural recording applications. In the absence of tissue, LED noise increased linearly with intensity, while LD noise increased sharply in the transition to lasing and settled to noise levels significantly higher than the LED’s, suggesting that speckle noise contributed to the LD’s higher noise and lower SNRs. The LED data exhibited significantly higher SNRs in fewer averages than LD data in all recordings. We stimulated lobster nerves and rat cortex, then compared the root mean square (RMS) noise and signal−to−noise ratios (SNRs) of data obtained with LED, superluminescent diode (SLD) and LD illumination for different numbers of averages. ![]() A better understanding of noise sources will help improve optical recordings and make them more practical with fewer averages. ![]() To understand noise sources in optical recordings, we systematically compared instrument and physiological noise profiles in two recording paradigms. However, noise introduced by LDs may counteract the benefits of brightness when compared to low−noise light emitting diodes (LEDs). Laser diodes (LD) are commonly used for optical neural recordings in chronically recorded animals and humans, primarily due to their brightness and small size.
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