Spiking information, often from a cell-attached recording, can be used to refine the spike inference model and thereby optimize AP detection. Second, there is no ground truth spiking information available for most neurons in a population. In contrast, the AP to calcium-dependent fluorescence transfer function is typically characterized with a soma filling the field of view of the microscope, to maximize photon flux from the soma and thereby signal-to-noise ratio. First, population imaging studies necessarily employ a large field of view containing many neurons. Inferring the underlying AP train or firing rate from calcium imaging remains challenging for several reasons. Yet undetected APs are common in population imaging experiments ( Theis et al., 2016 Berens et al., 2018). Using a contemporary GECI such as GCaMP6s, fluorescence changes associated with isolated spikes (action potentials, APs) in vivo can be detected when imaged at sufficiently high spatiotemporal resolution ( Chen et al., 2013) ( ). This optical approach is minimally invasive and enables simultaneous measurement of activity from hundreds or even thousands of neurons at single-cell resolution, over multiple sessions. Genetically encoded calcium indicators (GECIs) are widely used with two-photon laser scanning microscopy to report neuronal activity within local populations in vivo ( Luo et al., 2018). In addition, the data provided will be useful as a reference for the development of activity sensors, and to benchmark and improve computational approaches for detecting and predicting spikes. These findings are intended to serve as a guide for interpreting calcium imaging studies that look at neurons in the mammalian brain at the population level. ![]() This is an important caveat that researchers need to take into consideration when interpreting calcium imaging results. These experiments revealed that, while the majority of time periods containing multi-spike neural activity could be identified using calcium imaging microscopy, on average, less than 10% of isolated single spikes were detectable. used mice that had been genetically modified to produce a calcium indicator in neurons of the visual cortex and simultaneously obtained both fluorescence measurements and electrical recordings from these neurons. To shed some light on this, Huang, Ledochowitsch et al. ![]() However, this is extremely challenging experimentally, so this type of data is rare. The only way to directly measure this relationship is by using calcium imaging and electrical recording simultaneously to record activity from the same neuron. ![]() ![]() In order to interpret fluorescence data, it is important to understand the relationship between the fluorescence signals and the spikes associated with individual neurons. However, calcium fluorescence and spikes do not translate one-to-one. Using calcium indicators, it is possible to simultaneously record hundreds or even thousands of neurons. That change can be observed with specialized microscopes know as two-photon fluorescence microscopes. Another way to study neuronal activity is by using molecules that change how they interact with light when calcium binds to them, since changes in calcium concentration can be indicative of neuronal spiking. One way to do that is by recording electrical activity with microelectrodes. Studying the spiking patterns of neurons in the brain is essential to understand perception, memory, thought, and behaviour. Neurons, the cells that make up the nervous system, transmit information using electrical signals known as action potentials or spikes.
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