The Hidden Disparities in Neurophysiological Research: Exploration of Race and Sex

Introduction

Neurophysiological measures play a crucial role in medical and research settings, offering valuable insights into brain function, disease progression, and cognitive states (Parker & Ricard, 2022). Among the most commonly used neurophysiological technologies are electroencephalography (EEG), which tracks electrical activity in the brain with high temporal resolution using electrodes; functional near-infrared spectroscopy (fNIRS), which measures changes in cortical responses by analyzing light reflectance, offering high spatial resolution; and transcranial Doppler ultrasound (TCD), which uses ultrasound to assess cerebral blood flow velocity in the brain (Penner et al., 2023; Tripp & Warm, 2007).

Despite their widespread application, a critical question remains: are these tools truly equitable and effective for all populations? It is not enough for these technologies to be widely available; they must also produce accurate, unbiased results across diverse groups. If research using these tools fails to be generalizable, the very populations that could benefit most from neurophysiological assessments may be the ones left without reliable care. Addressing these disparities is essential to ensuring that neurophysiological assessments are truly inclusive and equitable.

Background

Unfortunately, for some underrepresented populations, these neurophysiological tools are not immune to systemic biases. Many studies have highlighted significant challenges that limit the effectiveness of these technologies, particularly concerning race and sex (Choy et al., 2022; Itoh et al., 1993; Nickerson, 2024). These studies highlight that while EEG, fNIRS, and TCD are valued for their cost-effectiveness and non-invasive nature, they are not exempt from disparities in their application. Physical characteristics that vary across racial and ethnic groups—such as hair type, skin pigmentation, and skeletal structure—can influence the success of these tools, often making them less effective for certain populations (Choy et al., 2022; Nickerson, 2024; Yücel et al., 2024).

These biases are further exacerbated within research environments, where there is a significant overrepresentation of Caucasian participants. Many studies utilizing these tools are conducted at universities where White populations are disproportionately represented (Ellsworth et al., 2022; Nickerson, 2024). As a result, the lack of diverse representation, combined with the existing limitations of neurophysiological tools, further alienates underrepresented groups from benefiting from these technologies.

Given these challenges, this paper seeks to highlight the factors contributing to the failure of effective neurophysiological tool application within underrepresented populations. It will examine the difficulties that hinder the accuracy and accessibility of these technologies, particularly at the intersection of race and sex. Furthermore, the paper will explore current strategies and technological advancements aimed at addressing these disparities, including efforts to improve tool design and novel methodological approaches. Lastly, potential recommendations will be discussed to promote inclusivity and ensure that neurophysiological research and clinical applications serve diverse populations equitably.

Approach

A literature review was conducted to investigate the factors contributing to the failure of EEG, fNIRS, and TCD technologies in underrepresented populations. The review specifically focused on studies addressing issues related to impedance or insonation with these neurophysiological tools. Relevant literature was identified through comprehensive searches using Google Scholar, EBSCOhost, and the Texas Tech University library website. The review aimed to identify critical research gaps that hinder the accurate and equitable use of these technologies across diverse demographic groups. It also examined ongoing efforts to mitigate these disparities, including innovations in device design and methodological improvements. By analyzing both historical and contemporary studies, the review sought to uncover root causes and explore potential solutions. Additionally, insights from researchers were incorporated to provide a deeper understanding of the practical challenges encountered during data collection.

Outcome

A review of existing research highlights several key factors that contribute to the failure of neurophysiological tool applications in underrepresented populations. One major barrier is hair type and style, which affects the use of commercially available headsets designed for EEG, fNIRS, and TCD (Yücel et al., 2024). Within research, these headsets are essential for maintaining access to specific brain regions with consistent signal quality throughout a study. However, researchers have struggled to properly fit these caps on individuals with thicker hair, often resorting to larger cap sizes when available, but still failing to achieve proper contact with the specific regions of interest (Adams et al., 2024; Etienne et al., 2020; Nickerson, 2024). African American men and women, in particular, face significant challenges, as common hairstyles such as cornrows, dreadlocks, and wash-and-go styles create additional barriers in data collection.

Beyond hair-related challenges, skin pigmentation also plays a role in the efficacy of neurophysiological measures, particularly with fNIRS (Yücel et al., 2024). As darker skin absorbs more light than lighter skin, this leads to difficulties in obtaining accurate cortical response readings. This can negatively impact data collection, such that darker-skinned individuals are potentially excluded from fNIRS-based studies.

Skeletal differences also influence the effectiveness of these technologies. With EEG, variations in skull thickness can impact signal acquisition, with thicker skulls leading to higher rates of application failure (Choy et al., 2022). Even in studies focusing on bald African American men—where hair was not a barrier—skull thickness remained a significant limiting factor. Similarly, in TCD research, the ability to insonate the medial cerebral artery (MCA) is influenced by the thickness of the transtemporal window, an acoustic window where bone density is lower, allowing for ultrasound penetration (Tripp & Warm, 2007). Studies have found that African Americans, particularly women, tend to have higher bone density in this region, which leads to lower success rates for insonation compared to other racial groups, especially Caucasian men (Itoh et al., 1993; Marinoni et al., 1997).

Recent studies have led to a growing push to improve the inclusivity of neurophysiological tools. One significant advancement is the development of SEVO electrodes—clip-like electrodes designed specifically for use with cornrowed hair, allowing for improved electrode contact with the scalp (Etienne et al., 2020). Similarly, a novel fNIRS headset has been created, with input from members of the Black community, to accommodate hairstyles commonly worn by Black individuals, such as braids, dreadlocks, and afros (Nickerson, 2024).

In addition to hardware advancements, some researchers have also focused on increasing awareness and accessibility. Specifically, concerted efforts have been made to help train researchers properly using headsets on participants with textured hair (Adams et al., 2024).

Conclusion

Although significant strides have been made toward addressing inequities in neurophysiological research, much work remains. These disparities are rooted in complex, intersecting factors that require a multifaceted response—combining technological innovation, methodological reform, and systemic shifts in research practices. One critical step forward is the consistent reporting of participant demographics, particularly race and ethnicity, which remains underreported in studies using neurophysiological tools. Greater transparency in this area would not only illuminate existing disparities but also help refine exclusion criteria and improve the generalizability of findings.

Future studies should further investigate barriers like hair-related impedance challenges. Nickerson (2024) showed that adjusting setup expectations and gently working with hair textures improved fNIRS signal acquisition among Black female participants. Expanding this approach to tools like EEG and TCD, along with developing racially inclusive setup protocols, could greatly enhance data quality and participant inclusion. Additionally, reassessing headset designs to better accommodate diverse hair textures, head shapes, and cultural hairstyles remain an important area for future research.

Additionally, within biomedical engineering spaces, further investigations into the effects of skin pigmentation and skull thickness on neurophysiological tools are needed. These studies would be instrumental in informing the development of more adaptive hardware that could potentially improve overall accuracy and inclusivity in data collection.

Lastly, active collaboration with underrepresented communities is essential to further progress. When researchers engage directly with affected populations, they can develop technologies and research practices that better meet the needs of diverse groups (Nickerson, 2024; Parker and Ricard, 2022). Through continued efforts in research, innovation, and policy changes, the goal is to ensure that neurophysiological research and clinical applications become more accessible, equitable, and representative of the populations they are meant to serve.