Think about what it is to experience the sensation of touch. Across the vast expanse of our bodies, we are constantly touching—clothing draped on skin, shifts in air pressure, the slight drift of hair as head follows gaze from browser window to coffeecup and back again—but we are not constantly perceiving. We simply couldn't process it all, the flood of nigh limitless but mostly inconsequential sensations from all corners of our bodies vying for our focus. Too much touch would make us mad.
But we seem to experience just the right amount of touch, perceiving it only past certain thresholds. The sensing is always happening, but it's only when sensation spikes enough that we're actually tasked with percieving it. This whole system is wrapped up into the general mystery of perception, or how we distill and organize information into meaningful representations of the world from sensory chaos.
Neuroscientists have now demonstrated that it's possible to tweak the aforementioned perceptual threshold in mice, adjusting upward and downward the level at which whisker stimuli yielded perceptual detections in mouse brains. According to a paper published Thursday in Science, the key is in manipulating upward and downward the activity of dendrites, the branching extensions of neuron cells responsible for receiving electrical impulses from other neurons.
The key finding of the paper, which comes courtesy of Humboldt University of Berlin neuroscientist Matthew Larkum and colleagues, is that it is indeed dendrites that are responsible for setting the threshold of touch perception (how much touching actually yields sensation). This builds on research published in 2015 suggesting that dendrites play a key role in amplifying and suppressing sensory input based on relevance, the dysfunction of which is thought to be a key feature of schizophrenia.
"There is evidence for the crucial role of feedback to primary sensory regions in perceptual processes, but it still remains to be demonstrated experimentally that perception depends on a dendritic mechanism," Larkum and co. write.
In figuring out what something does, a pretty good place to start is turning it off and back on again, if possible. This is what the group behind the current paper did with mouse whiskers, or, rather, the neurons responsible for perception related to mouse whiskers. To begin, they imaged dendritic activation based on the activity of calcium ions, the presence of which is what creates the voltage needed for a dendritic spike and the transmission of a signal. Basically, they tickled some mouse whiskers and watched for the flood of calcium characteristic of activity, and, thus, perception.
After several weeks of training mice to take some action (lick some water) in response to whisker tickling, Larkum and co. were able to come up with likely individual perceptual thresholds for a number of different animals. Next, it was a matter of plotting perceptual/dendritic activation against stimulus intensity. For the most part, increases in calcium correlated to the trained licking behavior—that is, mice responding to stimuli showed increases in dendritic signalling.
Finally, the researchers took this information and tried manually increasing and decreasing calcium levels in the mouse dendrites. "Our data show that dendritic [calcium ion] signals are correlated to animals' perceptual behavior," the authors conclude. "Indeed, the timing between dendritic [calcium] activity and licking behavior suggests a causal relationship." Boosting calcium levels enabled the mice to detect sensations at much lower levels of intensity, while limiting calcium had the opposite effect, leaving the mice relatively numb to whisker tickling.
We can assume that this carries over to human brains, though it's yet to be observed. A paper released last spring found that calcium levels in human brains can be tweaked non-invasively via transcranial magnetic stimulation, so it seems like only a matter of time before neuroscientists are hacking human touch as well.