The Sounds of Silence: the brain attends to quiet gaps in its inputs
Friday 5 May 2017: Neuroscientists at NeuRA and UNSW Sydney, Australia have shown that the brain may ignore “chatter” in its inputs, and instead use the periods of quiet to make judgements about the environment. The study, published today in Current Biology, flies in the face of the conventional view in which neural activity is the main driver of human perception.
The scientists, Dr Ingvars Birznieks and Dr Richard Vickery, have developed a novel way of controlling the neural information presented to the brain in their study. Using very brief mechanical taps delivered to the fingertips, each tap generated only one nerve impulse in each activated neuron. By varying the pattern of taps, they were able to control the pattern of nerve impulses generated in the experimental subjects. This enabled them to test several ideas describing how these nerve impulses encode information about the environment.
Sense of touch relies on skin vibrations set up as skin ridges on the fingertips scan over surfaces. However, how these vibrations are decoded is not well understood. Previous theories have suggested that the number of nerve impulses are counted as an index of vibration frequency, or that somehow a periodic regularity in the impulse patterns is detected. The research findings from this study showed conclusively that neither of these ideas could be correct.
Birznieks and Vickery generated bursts of nerve impulses, grouped as though caused by successive skin ridge contacts. They showed that subjects rated the frequency independently of the number of impulses in the burst. The period between successive bursts also did not correlate with the reported frequency.
Dr Birznieks said, “instead, it was the silent period between bursts that best explained the subjects’ experiences.”
“We were hoping to disprove one of the two competing theories, but to show they were both incorrect and find a complete new coding strategy totally surprised us,” said Dr Birznieks.
Dr Vickery tells that after the initial discovery, both researchers were excited but sceptical, and spent several years trying to disprove these findings before having confidence to publish their findings to the scientific community. The interest is these findings is not just that they represent a new way of understanding how the brain deciphers the barrage of neural impulses presented to it.
A better understanding of these coding strategies will help researchers build better brain machine interfaces. These next-generation interfaces will allow more sophisticated control of a range of devices for applications including tele-surgery, control of prostheses, and interfacing to drivers via their steering wheel.
Media: Katrina Usman