A new study challenges a leading explanation for why auditory stimulation, such as pink noise, can improve cognitive performance in people with traits of attention deficit hyperactivity disorder. The research found that both random noise and a non-random pure tone had similar effects on a brain activity measure linked to neural noise, which contradicts key assumptions of the prominent moderate brain arousal model. These findings were published in the Journal of Attention Disorders.
For years, scientists have observed that listening to random auditory noise, like white or pink noise, can benefit cognitive functioning in individuals with ADHD or elevated traits of the condition. The moderate brain arousal model was proposed to explain this phenomenon. This model is built on two primary assumptions. First, it suggests that ADHD is associated with lower-than-optimal levels of internal neural noise.
Second, it proposes that external random noise boosts this internal neural noise through a mechanism called stochastic resonance, improving the brain’s ability to process signals. However, these foundational ideas had not been sufficiently tested, particularly because most studies lacked a direct measure of neural noise or a proper non-random sound condition to isolate the effects of stochastic resonance.
Joske Rijmen and her colleagues at Ghent University aimed to directly test these two core assumptions of the moderate brain arousal model. They designed an experiment to measure neural noise directly while participants listened to different types of sound. The researchers wanted to see if ADHD traits were indeed linked to lower neural noise at baseline. They also sought to determine if the effects of sound on brain activity were specific to random noise, as the theory of stochastic resonance would predict.
To conduct their investigation, the researchers recruited 69 neurotypical adults. Participants first completed the Adult ADHD Self-Report Scale, a questionnaire used to assess the number and frequency of symptoms associated with the condition. This allowed the scientists to examine ADHD as a spectrum of traits rather than a simple diagnostic category.
Each participant then underwent a resting-state electroencephalogram, a non-invasive procedure that records the brain’s electrical activity. While their brain activity was monitored, participants sat with their eyes closed for three distinct two-minute periods: one in silence, one while listening to continuous pink noise (a random signal), and one while listening to a continuous 100 Hz pure tone (a non-random signal).
The research team analyzed the electroencephalogram data by focusing on a specific feature known as the aperiodic slope of the power spectral density. This measure reflects background brain activity that is not part of rhythmic brain waves and is considered a direct index of neural noise. A steeper slope in this measurement corresponds to less neural noise, while a flatter slope indicates more neural noise. By examining how this slope changed across the different sound conditions and in relation to participants’ ADHD traits, the scientists could test the predictions of the moderate brain arousal model.
The study’s findings presented a direct challenge to the model’s first assumption. During the silent condition, the researchers found a relationship between ADHD traits and the aperiodic slope. Individuals who reported more traits of ADHD tended to have a flatter slope. This finding suggests that they had more background neural noise, not less. The result is the opposite of what the moderate brain arousal model predicted and aligns with other recent studies that have also found evidence for increased neural noise in older children and adolescents with ADHD.
The results also contradicted the model’s second assumption regarding the mechanism of stochastic resonance. When participants with elevated ADHD traits listened to pink noise, their aperiodic slope became steeper. This change signifies a reduction in their neural noise. This outcome is contrary to the model’s suggestion that random noise should increase neural noise in this group.
Most significantly, the researchers found that the non-random pure tone had a virtually identical effect on brain activity as the pink noise. Listening to the 100 Hz tone also led to a steeper aperiodic slope, or a decrease in neural noise, in participants with higher levels of ADHD traits. The fact that a non-random sound produced the same effect as a random sound strongly questions the idea that stochastic resonance, which requires a random signal, is the necessary mechanism behind the benefits of auditory stimulation. If stochastic resonance were the driving force, only the pink noise should have produced this effect.
The authors propose that an alternative explanation may be needed. Rather than relying on stochastic resonance, both types of sound might have a more general effect on brain arousal. This idea is more consistent with the state regulation deficit account of ADHD, which suggests that individuals with the condition have difficulty regulating their arousal levels to match situational demands.
According to this view, any form of additional stimulation, not just random noise, could help modulate arousal to a more optimal state. The researchers also noted the puzzling observation that stimulation appeared to decrease brain arousal in individuals with higher ADHD traits. They speculate this might relate to difficulties these individuals have in achieving a truly restful state, and the continuous sound may have helped them to calm or regulate their brain activity.
The study has some limitations that the authors acknowledge. The research was conducted with neurotypical adults who varied in their traits of ADHD, so the findings need to be replicated in a group of individuals with a formal clinical diagnosis. Another point is that the brain activity was measured during a resting state, not while participants were engaged in a cognitive task where the benefits of noise are typically observed.
Future research should explore whether these same brain activity patterns occur during tasks that require attention and focus. Investigating these effects in a clinical sample of people with diagnosed ADHD will be an important next step to confirm these conclusions.
The study, “Pink Noise and a Pure Tone Both Reduce 1/f Neural Noise in Adults With Elevated ADHD Traits: A Critical Appraisal of the Moderate Brain Arousal Model,” was authored by Joske Rijmen, Mehdi Senoussi, and Jan R. Wiersema.
