Illuminating the brain through a new channel

Ruth Litovsky, PhD
Ruth Litovsky, PhD

As scientists seek to understand more about the brain and how it functions, neuroimaging modalities such as magnetic resonance imaging (MRI) provide integral tools in this pursuit. However, an MRI is not possible for all study participants such as individuals with a cochlear implant or other implanted medical devices that would be susceptible to an MRI’s powerful magnet. A new study from the Binaural Hearing and Speech (BHS) Lab at the Waisman Center examines functional near-infrared spectroscopy (fNIRS) – a neuroimaging option that is safe for individuals with metal implants – and validates a technique that makes this neuroimaging tool a more effective option. The paper “Comparing fNIRS signal qualities between approaches with and without short channels” was published December 2020 in the journal PLOS One.

The BHS Lab, which is led by Waisman investigator Ruth Litovsky, PhD, professor of communication sciences and disorders, studies binaural hearing in individuals with and without bilateral cochlear implants.

‘Denoising’ the data

“fNIRS uses light sources, which are safe to use with patients who have metal in their bodies,” says Xin Zhou, PhD, the study’s lead author and a researcher in the BHS Lab. When light sources – or “channels” – produced by fNIRS are used to measure changes in oxygenated (HbO) and deoxygenated (HbR) hemoglobin, or protein molecules in red blood cells, these channels tend to pick up superfluous information, or “noise.” Such noise distorts the data.

Xin Zhou, PhD
Xin Zhou, PhD

“fNIRS is very sensitive to the responses in extracerebral tissues and physiological signals and comparatively less sensitive to changes in hemoglobin in the deeper cerebral tissue,” the authors write. “Therefore, removing the extracerebral components and physiological signals in the fNIRS signals is crucial for improving the fidelity of measures of neuronal activity.” The larger goal is to have a cleaner signal that focuses on brain activity as opposed to other body tissue activities that occur within the cranium, Zhou says. “This is extremely important if we are to obtain clean signals that specify which brain regions are active, when they are active, and the magnitude of activity.”

Researchers typically “denoise” regular channels, which are roughly 3 centimeters in length, by shortening these channels to about 8 millimeters. This way, they limit the amount of information that the channels pick up. There has been, until now, no data suggesting that this is a particularly accurate way to measure blood flow. In the paper, Zhou, Litovsky, fellow Waisman researcher Gabriel Sobczak, and Collette M. McKay, PhD, of the Bionics Institute of Australia fill in this unexplored knowledge gap. This study adds legitimacy to a way in which fNIRS is often used in research.

“The concept of including shorter channels for fNIRS recording is not novel,” says Zhou. “However, in the literature, limited information has been reported about the shorter channel measures.” Confirming that such signals produce accurate results not only benefits the Waisman Center’s BHS Lab, but the scientific imaging community as a whole.

Reliable results

The researchers discovered that the shorter channels provided clearer data and should therefore be preferred to regular channels in a research setting.

“We think our study and data can help anyone who is doing fNIRS research but has not been using shorter channels as they are not sure about the efficiency of including the shorter channel subtraction method,” Zhou says. “We hope that future studies will use our method to improve the way that data are obtained and disseminated.”

The next step for the BHS Lab is to expand the methods to children and individuals with intellectual disabilities. “Some future studies aim to understand how brain development impacts children’s ability to think and learn,” Zhou says. “We are designing developmental studies that will also be applied to children with developmental disabilities and deafness.” “We are also exploring opportunities for using fNIRS to assess brain development in regions involving sensation, perception and cognition in individuals with intellectual and developmental disabilities.”

Zhou says that she and her colleagues are proud that their work is embraced by the Waisman Center. The fNIRS instrument was recently acquired through a joint effort by the Office of the Vice Chancellor for Research and Graduate Education and the Waisman Center and is part of the Brain Imaging Core. The BSH Lab is the first lab at Waisman to use fNIRS in their research. “We have a few fNIRS projects in our lab and are collaborating with other teams at the Waisman Center,” she says.

Litovsky is excited by the ability of the Brain Imaging Core to support this technology and hopes that fNIRS will be embraced by a growing number of researchers.