Holographic Stimulation

There hasn't been a lot of activity in the past few days for some reason, which is odd because I think I have actually been writing both more regularly and substantively. Of course, it is summer time and apparently the rest of the northern hemisphere is actually enjoying some warm weather (people may complain about British weather, but it seems Germans don't have it much better), so perhaps people are just off having real life fun instead of sitting inside reading my ramblings. Also, I haven't actually written a lot about science lately, so it is entirely possible that what I think have been substantive posts have simply been amateur attempts at besting the triviality that so readily consumes a blogger's body of work. Ah well, I guess what I am really trying to say is I am intellectually vain and enjoy it when people at least appear to be reading what I write, so you should all tell your friends about this site.

In the meantime, here is a quick return to science. We had a symposium at the Institute today with two rather interesting talks, so I will give a brief summary of each of them (the first talk tonight, the second talk gets a summary tomorrow).

The first talk was by Dr. Christoph Lutz from the Université Paris Descartes. He was describing a new technique his group has developed for more effectively stimulating neurons optically. This requires a bit of background, though. One rather interesting experimental technique for analyzing neuronal properties is optical stimulation (technically called photolysis excitation or inhibition depending, naturally, on whether you excite or inhibit the neurons). I believe it is a fairly recent technique, but I might be mistaken. The basic idea is that you bind a neurotransmitter (in the case of the experiment Dr. Lutz described, they chose the most common excitatory transmitter in the brain: glutamate) to a specific molecule which essentially prevents normal interactions with the transmitter (this is called 'caging' the neurotransmitter). You then bathe the neurons (in this case, a slice of tissue from a rat hippocampus) with the caged neurotransmitter. The inactivating molecule has been specifically selected, however, such that in the presence of a specific wavelength of light it releases the neurotransmitter, thus allowing you to release a targeted dose of neurotransmitter as though you just activated a group of synapses by shining a laser onto the tissue.

What Dr. Lutz and his fellow researchers have done is extend the technique using optical techniques in holography. Up until now, experiments in optical stimulation have used a single column of laser light with various degrees of focus and targetting systems. Using a liquid crystal spatial light modulator, however, you can take a column of laser light and create multiple focus points, even at different focal lengths. Thus, Lutz was able to specifically stimulate along the length of a dendrite using a thin band of focused light without also activating the neurotransmitter farther away from the dendrite that the normal circular column of light would do (this extra neurotransmitter that is activated would then be free to diffuse through the local region, both weakly stimulating the neuronal membrane region being looked for an extended period of time after the laser light was turned off as well as possibly interacting with other nearby dendritic branches). By only stimulating the neurotransmitter directly along the length of the dendritic branch, you can more carefully localize the activated neurotransmitter to much more realistically simulate synaptic input. Alternatively, you are able to simultaneously focus light on multiple branches of a neuron's dendritic tree, allowing you to look at the interaction post-synaptic electric potentials generated to see how the signals interact.

Essentially, Lutz and his fellow researches have provided a novel application of well understood concepts in physics to design a much more powerful experimental technique for probing the properties of neurons. Since the computational power of a neuron rests in the electrochemical dynamics of its cell membrane, this expanded ability to probe the membrane's reaction to targeted chemical stimuli is likely to provide valuable information into the complicated world of neuronal computing.

Tomorrow: Robots with Organic Brains