Neuroscience 4 QuantumBrain

So the first term has finally come to an end with the handing in of my (rushed) dissertation. The project was pretty open ended and, as with any science, changed significantly since my post describing what I “would” be doing.
So here’s a wee description of what I was looking at and how I did it.

The population code is basically the ultimate goal for neuroscientists and as such is probably the furthest from our reach. By population code, I mean how the entire population of neurons, or a subset of the population, encodes sensory stimuli. Several techniques are available to do this, on of which is using an array of electrodes, for example a 8×8 grid of them. These have excellent temporal resolution but lack spatial information. As such, complex algorithms lacking in accuracy must be used to determine from where the voltage signals came from. Another method (and the method I employed) is 2-photon calcium imaging. This requires a dye which is injected into the brain area of interest. The dye is taken into the cell where it is cleaved such that it cannot escape. It has a high affinity for calcium with which it binds. Synchronous binding of calcium and excitation by laser light causes the dye to fluoresce which can be detected using photomultiplier tubes. Since large intracellular increases in calcium concentration are indicative of neuronal action potentials, this provides a good measure of neuronal activity and, as such, a movie of the dye-labelled brain can be taken to capture the neural activity. What’s more, the technique is much less intrusive than multi-electrode recording, has much better spatial accuracy and can have up to equal temporal accuracy whilst allowing measurement of up to hundreds of cells at any one time. This is important as there are large correlations between neurons in terms of their activity (intuitively so as this is how the brain encodes sensory stimuli).

So the questions I ultimately wanted to ask is whether the optic tectum, the main visual processing area of Xenopus tadpoles, exhibits any functional architecture, by which I mean do different neurons encode different features of visual stimuli and how are these neurons organised in the tectum? Why the Xenopus tadpole? Well, it’s a model organism which has been investigated in many areas of biology to a large degree. Also, it develops ex utero (outside of the womb) and thus provides us a rare opportunity to explore how its neural development from the outset.

A great deal of literature is out there showing that functional architectures exist in many animals, from primates to cats to ferrets to zebrafish (which are remarkably similar to tadpoles in their larval stage), at different degrees of complexity.

However, calcium imaging of Xenopus tadpoles has never been used to explore the population code. My project then needed to perfect the art of imaging in this species which, as I’ve experienced, is not an easy task.

Thus my project ended up crunching through methodological procedures including: 1) bulk loading of the fluorescent dye into the whole brain; 2) movie processing; 3) composition of analytical tools (using Matlab) which can extract functional information; 4) invesitgation of the effects of cell excitability on neural activity discrimination and 5) investigation of response characteristics to different visual stimuli. These tasks out of the way, the lab is now equipped to explore the neural ode in populations of 100s of neurons. Very exciting!

Unfortunately, my time in the lab has ended and I’m now starting my next lab rotation with Dr. Simon Stringer, composing computational models which explore the properties of self organising maps in neural networks and how the structure which creates them allows for transform invariant recognition of complex objects in cluttered visual environments. I’ll report on this in a few months time!