Population imaging discrepancies between a genetically-encoded calcium indicator (GECI) versus a genetically-encoded voltage indicator (GEVI)

Abstract

Genetically-encoded calcium indicators (GECIs) are essential for studying brain function, while voltage indicators (GEVIs) are slowly permeating neuroscience. Fundamentally, GECI and GEVI measure different things, but both are advertised as reporters of “neuronal activity”. We quantified the similarities and differences between calcium and voltage imaging modalities, in the context of population activity (without single-cell resolution) in brain slices. GECI optical signals showed 8–20 times better SNR than GEVI signals, but GECI signals attenuated more with distance from the stimulation site. We show the exact temporal discrepancy between calcium and voltage imaging modalities, and discuss the misleading aspects of GECI imaging. For example, population voltage signals already repolarized to the baseline (~ disappeared), while the GECI signals were still near maximum. The region-to-region propagation latencies, easily captured by GEVI imaging, are blurred in GECI imaging. Temporal summation of GECI signals is highly exaggerated, causing uniform voltage events produced by neuronal populations to appear with highly variable amplitudes in GECI population traces. Relative signal amplitudes in GECI recordings are thus misleading. In simultaneous recordings from multiple sites, the compound EPSP signals in cortical neuropil (population signals) are less distorted by GEVIs than by GECIs.

Studying Synaptically Evoked Cortical Responses ex vivo With Combination of a Single Neuron Recording (Whole-Cell) and Population Voltage Imaging (Genetically Encoded Voltage Indicator)

“…Brain slices were prepared from transgenic animals expressing a type of a GEVI called chi-VSFP-Butterfly ( Mishina et al, 2014 ). In these animals, all cortical pyramidal neurons express GEVI, which increases our chances of capturing population voltage responses ( Zhu et al, 2021 ). One pyramidal cell per brain slice was patched (whole-cell) in a current clamp mode.…”

“…One possible reason is a slow decay dynamic of the GEVI used (chi-VSFP). Slow decaying optical indicators exaggerate temporal summation, or generate an impression of the ongoing summation of electrical signals when there is none ( Zhu et al, 2021 ).…”

Spatiotemporal patterns of spontaneous brain activity: a mini-review

“…GEVI signals largely represent the synaptic input arriving into a neural circuit whereas GECI signals present the action potential output generated in the neural circuit. 38 …”

“…The typical spatial resolution for population activity is 50 to . 38 The temporal resolution of calcium imaging is limited by calcium ion dynamics in the neurons and is usually too slow for following brain oscillations at frequencies , a limitation that is not shared by voltage imaging. Calcium indicators, however, usually produce larger fractional changes in fluorescence, whereas voltage-sensitive dyes provide smaller signals.…”

A positively Tuned Voltage Indicator Reveals Electrical Correlates of Calcium Activity in the Brain

“…However, GECI signals lack precision for timing spikes, as somatic calcium continues rising after the AP peak due to delayed inactivation of voltage-gated calcium channels and continued calcium diffusion from initial influx points into the cytosol (3,4). This spatiotemporal smoothing effect, together with >200-ms half-extrusion times for calcium, prevents GECIs from accurately discerning closely spaced APs (2). Lastly, calcium levels do not respond appreciably to subthreshold and hyperpolarizing changes in transmembrane voltage, and thus do not reveal subthreshold dynamics such as theta rhythms and hyperpolarizing events.…”