Manipulating consciousness using deep brain stimulation

Studies in rats have shown that they could be either woken up or put in an unconscious state from altering their brain activity by changing the firing rates of neurons in the central thalamus. The NIH funded study was published in eLIFE.

Located deep inside the brain the thalamus acts as a relay station sending neural signals from the body to the cortex. Damage to neurons in the central part of the thalamus may lead to problems with sleep, attention, and memory.

Previous studies have suggested that stimulation of thalamic neurons may awaken patients who have suffered a traumatic brain injury from minimally conscious states.

The researchers flashed laser pulses onto light sensitive central thalamic neurons of sleeping rats, which caused the cells to fire.

High frequency stimulation of 40 or 100 pulses per second woke the rats. In contrast, low frequency stimulation of 10 pulses per second sent the rats into a state reminiscent of absence seizures that caused them to stiffen and stare before returning to sleep.

Using functional magnetic resonance imaging (fMRI) they were able to show that high and low frequency stimulation put the rats in completely different states of activity. Cortical brain areas where activity was elevated during high frequency stimulation became inhibited with low frequency stimulation.

Electrophysiological studies confirmed this by showing that neurons in the somatosensory cortex fired more during high frequency stimulation of the central thalamus and less during low frequency stimulation.

This study besides demonstrating the power of using imaging technologies to study the brain at work, takes a big step towards understanding the brain circuitry that controls sleep and arousal.

How can changing the firing rate of the same neurons in one region lead to different effects on the rest of the brain?

Further experiments suggested the different effects may be due to a unique firing pattern by inhibitory neurons in a neighbouring brain region, the zona incerta, during low frequency stimulation. Cells in this brain region have been shown to send inhibitory signals to cells in the sensory cortex.

Electrical recordings showed that during low frequency stimulation of the central thalamus, zona incerta neurons fired in a spindle pattern that often occurs during sleep. In contrast, sleep spindles did not occur during high frequency stimulation.

Moreover, when the scientists blocked the firing of the zona incerta neurons during low frequency stimulation of the central thalamus, the average activity of sensory cortex cells increased.

Although deep brain stimulation of the thalamus has shown promise as a treatment for traumatic brain injury, patients who have decreased levels of consciousness show slow progress through these treatments.

Reference:

Liu et al. “Frequency-selective control of cortical and subcortical networks by central thalamus,” eLIFE, December 10, 2015. DOI: dx.doi.org/10.7554/eLife.09215

Cover image: Tuning consciousness in the brain: Scientists studied how the thalamus tunes brain activity during different states of consciousness in ratsLee lab, Stanford University, CA