Summary: A newly identified subtype of fast-spiking interneuron fires with highly regular timing and may act as a neural metronome, driving perceptually important brain oscillations independently of the local field potential and direct sensory input.
Source: Brown University
Overview: Researchers at Brown University have identified a distinct class of neurons in the brain’s touch-processing area that fire very regularly—about 40 times per second—suggesting these cells could provide a stable timing signal for sensory perception. By recording the fast electrical spikes of individual neurons in rodents, the team found that this subtype of inhibitory cell keeps an internal rhythm that correlates with improved detection of faint touches to the whiskers.
Gamma-band brain rhythms—oscillations near 40 Hz—have been studied in humans and animals for decades and are thought to play a role in coordinating activity across brain regions. Previous work from the same laboratory showed that enhancing natural gamma rhythms in rodents improved their sensitivity to subtle whisker stimulation. However, the functional role of gamma oscillations has been controversial. Some scientists propose gamma rhythms act as a unifying timing reference across brain areas, while others believe they are simply a byproduct of neural computations.
In this study, Christopher I. Moore, professor of neuroscience and associate director of the Carney Institute for Brain Science, and doctoral student Hyeyoung Shin report a population of neurons that could reconcile these views. These neurons, which the authors call gamma regular nonsensory fast-spiking interneurons (grnsFS), spike rhythmically at gamma-range intervals yet do not change their firing pattern in response to sensory input. This independence from immediate sensory drive sets them apart from other gamma-associated signals that typically vary with stimulation.
The discovery arose from experiments designed to probe what distinguishes successful from unsuccessful detection of faint whisker deflections. Shin used precise mechanical stimulation to move whiskers at the threshold of detection while recording from single neurons in the rodent primary somatosensory (barrel) cortex. Unlike many studies that average activity across many cells, she analyzed the behavior of individual neurons and uncovered three distinct subtypes among fast-spiking inhibitory interneurons.
One-third of these interneurons fired with unusually regular intervals—around the gamma frequency—and this regularity predicted whether the animal would detect the subtle touch. These regularly spiking cells were synchronized with one another, suggesting a coordinated timing function across the local circuit. Other interneuron subtypes either fired irregularly or changed their activity in response to whisker stimulation.
“There’s this funny thing where neuroscientists will go into a brain, and once they find a cell that responds to the outside world, they study it,” Moore said. “If it doesn’t respond to the outside world, they don’t know what to do with it and ignore it.”
By focusing on single-cell spike timing rather than averaged signals, Shin was able to detect these metronome-like neurons that conventional approaches might overlook. Her preparation included a thorough literature review and computational modeling to inform the interpretation of fast-spiking neuron behavior.
The paper, published in the journal Neuron, reports that grnsFS spike more regularly than would be expected from a Poisson-like process, and that increased regular gamma-range spiking in these cells predicts successful detection at threshold. Notably, the gamma-band power of the local field potential (LFP) did not align with the grnsFS spiking and in this study had an inverse relationship with detection performance. This implies that a specific cell subtype, rather than the broader LFP gamma signal, may provide a stable timing reference for perception.
Human brains also exhibit gamma rhythms, and the authors plan to investigate whether analogous metronome-like interneurons exist in primates and humans. Future directions include mapping these neurons in other brain regions and testing whether artificially enhancing their synchrony—using genetic or optical methods—improves perceptual sensitivity.
Although grnsFS are newly described, disturbances in the larger class of fast-spiking interneurons have been associated with neurodevelopmental and psychiatric conditions such as autism, schizophrenia, and attention-deficit/hyperactivity disorder. The discovery of this specific subtype raises the possibility that dysfunction in metronome-like interneurons could contribute to perceptual or cognitive symptoms in these disorders, a hypothesis that will require extensive further study.
Funding: This research was supported by the National Institutes of Health (grant R01NS045130), the Carney Institute for Brain Science, and a fellowship from the Fulbright Program.
Source:
Brown University
Media Contacts:
Mollie Rappe – Brown University
Image Source:
The image is in the public domain.
Original Research: Closed access
“Persistent Gamma Spiking in SI Nonsensory Fast Spiking Cells Predicts Perceptual Success” by Hyeyoung Shin and Christopher I. Moore. Published in Neuron. DOI: 10.1016/j.neuron.2019.06.014
Abstract (summarized):
The study identifies a unique subtype of fast-spiking interneurons in primary somatosensory cortex that fires regularly at gamma-range intervals and is not driven directly by sensory input. Increased regular gamma spiking of these nonsensory interneurons predicts successful detection of weak stimuli, whereas broadband gamma power in the local field potential does not predict perceptual success and does not coherently reflect grnsFS spiking. These findings indicate a distinct interneuron subtype mediates perceptually relevant oscillations independently of the LFP and immediate sensory drive.