Calcium Imaging
There is a difference in the probes used for monkeys and mice for Calcium experiments:
MOUSE PROBE
The neural probes for calcium imaging of the mouse visual cortex, consist of multiple flexible needles with many microelectrodes close together. This creates a dense 3D-grid of stimulation sites inside the visual cortex, without damaging the brain, which allows to study advanced stimulation patterns. These include bipolar current steering, where two or more local microelectrodes are used to selectively target neural populations around the electrodes, which increases the resolution of the neural stimulation.
For the calcium imaging itself, several technologies are used:
THE STIMULATOR
A custom-built neural stimulator is connected to the neural probes, and allows us to apply bipolar stimulation on an arbitrary amount of electrodes, while keeping accurate control on the charge going in and out of the brain.
CALCIUM IMAGING SETUP
The experimental set-up used for calcium imaging of advanced stimulation patterns consists out of a custom neural stimulator, that is integrated and synchronized with the two-photon imaging set-up. After implantation of the electrodes, the animal or tissue is placed underneath a two-photon microscope.
PCA
We applied Principal Component Analysis on the activated cells of 10 different mice, where current was sent into 13 different directions. This allowed to investigate the dimensionality of the current steering, being the number of independent neuronal populations to be possibly activated. This number of independent neuronal populations determines the increase in selectivity by using bipolar stimulation instead of the traditional monopolar stimulation, thus the increase in resolution of a visual prosthesis
In vivo and ex vivo calcium imaging experiments
Ex vivo and in vivo calcium imaging are powerful techniques used to study brain activity in mice by visualizing the dynamics of neuronal calcium levels, which serve as proxies for neural firing. Using 2-photon imaging, researchers can literally see the neurons fire in a field of view of about 1 mm². Both are done on brain tissue of mice that are genetically altered to express calcium indicators like GCaMP.
Ex vivo calcium imaging is typically performed on brain slices, allowing for detailed observation of individual neurons and circuits in a controlled environment outside the living organism. This approach provides high-resolution data but lacks the natural connectivity of the brain. On the other hand, in vivo calcium imaging is conducted on live animals. Through optical windows or cranial implants, researchers can monitor real-time neuronal activity, preserving the natural context of brain function. Both approaches complement each other in the project of Hyperstim, providing insights into the brain’s cellular mechanisms.