As neuroscientists strive to demystify the workings of the brain, the pathways responsible for activating and deactivating consciousness have continued to elude them. However, a Stanford University-led team of researchers believe they may have now pinpointed the part of the brain responsible for switching awareness on and off. If confirmed by future studies, this research could lead to the development of novel treatments for those suffering from consciousness-related disorders, such as excessive sleeping or even comas.
The study – published in the journal eLife – was inspired by two previous cases in which doctors were able to alter the awareness of patients. The first occurred in 2007, when a man who had become minimally conscious following a brain injury was restored to full cognizance thanks to electrical stimulation of a brain region called the thalamus. This was followed up by a second case last year, when scientists in Washington DC managed to deactivate an epileptic woman’s consciousness, essentially putting her to sleep, by stimulating her claustrum, which is known to communicate with the thalamus.
However, other than these two successes, most attempts to activate or deactivate consciousness have ended in failure, according to New Scientist. This is largely because the technique for doing so is rather imprecise, and normally involves electrical stimulation of broad regions of the brain rather than specifically targeted neurons – mainly because the particular neurons involved in this process are not known.
In an attempt to solve this puzzle, the team sought to control the neurons of the thalamus in the brains of rats, and then observe the wider effects of this stimulation by performing functional magnetic resonance imaging (fMRI) scans.
To do so, they used a technique called optogenetics, which involves implanting genes for light-activated proteins into the rats' neural cells, thus enabling these neurons to become stimulated by flashes of light.
The team discovered that stimulating these neurons at a rate of 10 times a second caused the rats to lose consciousness, while raising this stimulation to between 40 and 100 times a second woke them up.
Observing the effects of this stimulation in the fMRI machine, they found that the neurons of the thalamus communicated with a brain region called the zona incerta (ZI), which acts as a relay between the thalamus and the cortex. Within the ZI are inhibitory neurons that reduce activity in the cortex when activated, thereby switching consciousness off.
When the thalamus was stimulated at low levels, it caused the inhibitory cells in the ZI to become activated, although increasing this stimulation was found to block their activity, thus explaining the effects observed during the experiment. As such, the team concluded that the ZI acts as a switch that regulates sensory processing throughout the cortex.
Lead researcher Jin Hyung Lee told IFLScience that this research "has immediate implications towards treating patients with traumatic brain injuries, [who are] disabled mainly by the fact that their alertness level is reduced." As such, she explains that the next step is to look at how the ZI can be targeted in human trials in order to develop brain stimulation therapies.