Fiber photometry emerges as a sophisticated technology meticulously designed for the precise detection of neural activity within the brain nuclei of unrestrained animals. This method entails the amalgamation of fluorescence emitted by neurons expressing genetically encoded calcium indicators (GECI) or neurotransmitter probes. (Adelsberger, 2005).

The evolution of fiber photometry calcium imaging spans nearly a decade, earning commendation from diverse laboratories for its rigorous scientific merit. Its application extends to the systematic investigation of regulatory mechanisms governing animal behavior. One reason for the growth of this technique is the ongoing development of biosensors that measure general cellular activity, or the activity of specific neurotransmitters such as glutamate, GABA, acetylcholine, serotonin, dopamine, norepinephrine, and orexin (Bruno et al., 2021).

Fiber photometry constitutes an optical methodology wherein light serves as a stimulus to initiate and quantify variations in fluorescence arising from structural alterations in an expressed biosensor. Briefly, light of a specific wavelength for excitation is transmitted through an implanted optical fiber, and the ensuing fluorescence is conveyed back via the same fiber to a photodetector. Subsequently, a digital optical intensity signal is generated, hypothesized to depict the proportional quantity of the target-bound sensor situated at the distal end of the fiber (Li et al., 2019).

The detected signal emanates from the tissue surrounding the fiber tip, with a spatial extent ranging from 50 to 400 um, thus constituting a regional or ‘bulk’ measurement. Owing to the genetic encoding of biosensors, their expression can be directed toward specific circuits and/or cell types, where stability may endure for extended durations, spanning weeks to months.

This protracted temporal capacity distinguishes fiber photometry from other in vivo techniques, enabling repetitive recordings over considerable time frames (Gunaydin, et al., 2014). Notably, this methodology has facilitated unprecedented insights into the correlation between population activity within specific cell groups and various facets of intricate behaviors, including but not limited to movement, memory, motivation, appetitive and aversive learning, among others.

As remarked by Simpson et al., 2023, Fiber photometry has gained popularity for in vivo monitoring of neural signals in behaving animals due to its practical advantages. In comparison to other methods like electrophysiology, it offers signals with molecular and cellular specificities, higher spatial resolution, and much higher temporal resolution. It outperforms microdialysis in terms of temporal resolution and can provide concurrent recordings of dopamine, revealing differences in observable temporal dynamics. Additionally, photometry is more sensitive for certain analytes/environments compared to cyclic voltammetry and allows access to molecules without electrochemical methods. Its practical benefits include less invasive surgical procedures, flexibility with lightweight optical fibers, and the availability of cost-effective “plug-and-play” systems, making it accessible for diverse researchers to study brain-behavior relationships at scale. Furthermore, fiber photometry generates relatively low-sized and less complex raw data compared to other in vivo techniques like electrophysiology and imaging.

The integration of fluorescent reporters with fiber photometry enables the monitoring of cell-type-specific neuronal activity and the assessment of specific gene expression levels (Chen et al., 2013). This experimental approach, crucial for investigating brain functions, involves studying correlations between neuronal activity and natural animal behavior. Traditional electrophysiological recording, renowned for its high temporal resolution, has provided profound insights into neural circuit functions. However, employing light-sensitive proteins for optical tagging in electrophysiological recordings is technically challenging, sensitive to noise interference during natural behaviors, and inefficient.

Fiber photometry, leveraging genetically-encoded Ca2+ indicators like GCaMP proteins, facilitates the monitoring of genetically-defined neuron populations. Numerous research groups have utilized fiber photometry to make exciting observations of neuronal activation patterns during various behaviors, including affective reward- and punishment-related behaviors (Wang et al., 2017), feeding behaviors, social behaviors (Chen et al., 2015), arousal, and long-term learning. Moreover, fiber photometry allows for the measurement of gene expression levels, as demonstrated in a study of circadian rhythms by monitoring circadian clock gene expression using bioluminescent reporters (Mei et al., 2018).