Inducing and visualizing brain activity

with light at the speed of light!

By Gaia Tomasello

Gaia Tomasello is PhD candidate at Polytechnique school of montréal in Biomedical Engineering


Human brain: a great mystery in the universe...

Human brain always fascinated human species, having been considered the most complicated part of our body but also the most important being, the headquarter of our emotions, memory and general behavior. Human brain is extremely mysterious because is the most unfathomable human organ due to the intrinsic complexity and sensitivity: although being the closest thing to ourselves and our mind we feel his “infinite dimensions” as the those characterizing universe itself. Furthermore recent speculations about the possible and undiscovered skills that our brain hides and the limited use of just 20% of it, encourages even more the mystery related to his comprehension and understanding. Nowadays is well established that brain activity is mainly due to complex cells in the brain named neurons communicating each others through junctions named synapses. The extremely fast and efficient way to transmit information among the incredible dense neurons wire enables us to think, to learn, to remember and to move by controlling our arms or our legs. The number of neurons is extremely high and the number of possible synapses and new generated pathways is even higher giving rise to common consideration like: “there are more synapses in our brain than atoms in universe” that is obviously untrue but can give the idea of how people like to speculate about it.

Recently incredible advances in neurophysiology and neuroscience have been achieved spreading light to fundamental neurological mechanisms and having clarified the role of the special brain regions involved in those mechanisms. Several advanced techniques are now able to map brain anatomy and monitor neural activity permitting great advances in neuroprosthetics and neurosurgery. However the urgent need of a technique able to induce fast and efficient specific activity to selected neuronshas lead to the revolutionary technique of optogenetic able today to either trigger or visualize brain activity at the speed of light! Before going to detail and further explanation, in the next paragraph I would like just to shortly mention what is at the base of neurological signal transmission.

Signal transmission in the brain: an ionic current.

We could wonder how information generated at one site of the brain can travel down to other sites and therefore how a signal can be efficiently propagated among all the infinite number of neurons. In the '50 years it was identified a voltage difference between the inside and outside of the cell, named the membrane potential. The typical resting value across a cell membrane corresponds to a negative one (-70 mV) indicating a different ions concentration across the membrane. Although lipophilic the membrane is permeable to some ions and when a certain voltage input is detected from protein—channels inserted into the membrane, a fast inward and outward ion flux across the same membrane occur. This ions flux leads to voltage fluctuations characterized from a rapid rising phase (depolarization) and a consequent rapid fall (repolarization). In particular the ions responsible of this changing voltage are sodium and potassium cations (Na+ and K+ respectively) that start to enter or exit the cell according to their gradient concentration. This ion flux generates an ionic current that, as in an electrical cable, is propagated towards the bridges connecting several neurons, named indeed axons. These "nerve impulses" are named action potentials and their temporal sequence is called its "spike train". A neuron that emits an action potential is often said to "fire". This event is the base of the generation and transmission of electrophysiological stimuli among neurons.

The revolution of Optogenetics

Due to the intrinsic nature of signal transmission among neurons, electrical stimulation or detection of brain activity has been always performed via voltage or current-clumped techniques implanting classical electrodes in the brain. The revolutionary idea of optogenetics consists instead in adopting laser techniques to induce a precise stimulus via shining light in a precise targeted regions. The possibility of using light for selectively control neural activity (action potential) patterns within subtypes of cells in the brain was articulated and have been evolved from several scientists but the first authentic and complete demonstration was due to Ed Boyden and Feng Zhang at Stanford university. In brief this technique involves the use of light to control neurons that have been genetically modified to express a light-sensitive ion channels. A particular gene-encoding virus is injected in to the animal leading therefore to a photo-induced ion channel (for example a protein named channelrhodopsin) expression able to trigger a membrane voltage via pumping photoinduced ion flux across the same cells where it is expressed. It's an extremely efficient type of neuromodulation that has enabled to control and monitor activities of individual neurons in living tissues even in freely moving animals: able for example, to simulate a tone response in mice via shining light into genetically modified cells in auditory cortex. Millisecond-scale temporal precision is central to optogenetics, which allows the experimenter to keep pace with fast biological information processing (for example, in probing the causal role of specific action potential patterns in defined neurons). The concept of converting light to membrane voltage and the adoption of selective light-driven mechanisms has opened the doors to incredible new methods to investigate fundamental neurological processes as for example the memory encoding process and the consequent learning phenomenon.

Visualizing neuron firing in to the cells

Although relatively recent (~10 years) the technique of optogenetic is already well established and adopted on daily routine in neuroscience laboratories. In particular the reason because I've chosen to write about this specific topic is due to the incredible inspiration I've got recently meeting the prof Adam Cohen, an incredible young and talented professor coming from Harvard University to give a seminar entitled “Bringing bioelectricity to light”. I was impressed from the content of the seminary and from the way this charismatic scientist has caught the attention of all the audience despite the very different background. It has me suggested to deeply think about scientific creativity, in this particular case where from existing technologies a genius can derive new revolutionary ideas and implementations. Basically the group of prof. Cohen for the first time has applied the reverse principle of optogenetics to map and visualize neurons while firing events happen. If in classical channelrhodopsin over expression it is possible to convert light to voltage membrane, in this reverse technique the voltage generated when a neuron is firing is converted into light-emission events observed with fluorescence miscroscopy. This was also achieved via genetic manipulation able to drive expression of voltage-sentitive fluorescent proteins into specific targeted neurons. These new optical approaches can finally let us to directly observe the incredible fast process of a neuron firing with bioelectrical signals propagating all the way down the axons. These not only open the doors to new frontier of knowledge in mapping brain functions but also to new therapeutic strategies. As for example via genetically engineering some mammalian tumor cells - endogenously electrically inert – and inducing therefore the capability of generating electrical spikes and collective electrical waves it is possible to trace them while they are growing. The incredible scenario opened from this technology is so wide that one can only start to speculate about possible artificial intelligence simulation thanks to the precise and deep understanding guaranteed from this extremely revolutionary technique. 

1) Hodgkin, A. L.; Huxley, A. F. The Journal of Physiology 117 (4), 500–544(1952)
2) Edward S Boyden, Feng Zhang, et a. Nature Neuroscience 8, 1263 - 1268 (2005)
3) [a]J. M. Kralj, A. D. Douglass, D. R. Hochbaum, D. Maclaurin, A. E. Cohen Nature Methods, 9, 90- 95 (2012)[b] J. M. Kralj, D. R. Hochbaum, A. D. Douglass, A. E. Cohen, Science, 333, 345-348 (2011)