High speed imaging

We are collaborating with Istvan Gyongy, Srinjoy Mitra and Robert Henderson in the School of Engineering in Edinburgh to develop applications of single photon avalanche diode (SPAD) technology to imaging of neural activity.

Rationale

Recent progress towards understanding the biological basis for cognition and its disorders has been driven by advances in molecular tools for labelling and manipulation of defined populations of neurons. However, brain circuits operate at a millisecond time scales and our ability to resolve this activity is limited.

Electrophysiological methods have the required temporal precision, but do not reliably identify multiple individual neurons within large populations, a pre-requisite for many important questions. In contrast, imaging approaches based on detection of intracellular Ca2+ signals can track activity in large populations of neurons but give only an indirect readout of neuronal activity with limited temporal resolution.

New genetically encoded voltage indicators (GEVIs) address many of the shortcomings of Ca2+ imaging. Crucially, they report both action potentials and subthreshold electrical activity in defined neurons with millisecond resolution. However, application of GEVIs will require a new generation of cameras with frame rates sufficient to monitor millisecond scale changes.

Goals

We are developing and validating neuroscience applications of SPAD sensor technology developed by our collaborators. SPADs are electronic devices that when activated by a single photon cause an avalanche of electrons and a large electric current. Because SPADs detect the time at which individual photons arrive, they are well suited to extremely high speed and low light imaging. In contrast, standard camera sensors must bin photons across a time window, which limits their sensitivity and temporal resolution. In our prototype SPAD-based cameras, the sensor chip is a similar size to sensors used in miniature microscopes we currently use for Ca2+ imaging in behaving rodents. It is therefore physically feasible to use SPADs to image activity even in freely behaving animals.

Progress

We published the first proof-of-principle demonstration that SPAD-based cameras can detect neuronal activity reported with GEVIs (Tian et al., 2022). This required introducing viruses encoding GEVIs into a mouse brain, generate known activity patterns in neurons expressing the GEVIs and using SPAD cameras to image the signal from the GEVIs (see figure).

We are now evaluating new generation SPAD devices and working to miniaturise SPAD cameras for imaging in freely moving animals.