The word “noise” comes from the latin nausea, meaning disgust or annoyance. But in the phenomenon known as stochastic resonance, noise can actually be a good thing: it can serve as a signal amplifier in thresholded systems.
This phenomenon is not nearly as arcane as it sounds. The image above (borrowed from Stein, Gossen & Jones, 2005), clearly shows how two very weak signals can look very similar (top row), but with additional noise, the characteristics of these signals can be more easily dissociated (bottom row; now you can see one signal is generated by a sine, and one generated by a square wave).
Some theories posit that “neural noise” may be precisely tuned to serve a similar function in the brain. We know that the brain is a noisy system, and that neurons themselves are thresholded, so the possibility seems alluring. On the other hand, too much noise might “drown out” the true signals, preventing their accurate extraction. There are also more complex reasons to believe that correlated noise (again, often observed in neural systems) can have unexpected effects on information processing efficiency (based on information theoretic analyses). Together, these factors require that if noise is used adaptively to create stochastic resonance in neural circuits, the noise itself would require rather precise tuning.
These arguments focus on noise that is intrinsic to the brain. However, some recent work has shown that even environmental noise can have some positive benefits on the disordered cognitive system, perhaps by “retuning” maladaptive noise levels by introducing noise to the brain through the perceptual system, and thereby improving stochastic resonance phenomena.
In a doctoral dissertation (and now a newly published paper), Göran B. W. Söderlund describes how high intensity, broadband noise may actually alleviate some of the distractibility inherent to disorders like ADHD. Interpreted loosely, the idea is that those with ADHD are chronically understimulated by both their environment and their internal cognitive representations, leading them to search almost incessantly for more stimulating things (environments or thoughts), which takes the behavioral form of distractibility. This chronic cognitive understimulation might be overcome by introducing additional noise to the cognitive system through the perceptual system.
To test this idea, 42 children (21 with ADHD) between 9 and 12 years of age completed both a low-load and a high-load memory task. These tasks were fairly unusual as memory tasks go: they involved subjects listening to a series of actions (and then performing them in the low-load condition) and then recalling them after a delay period. Each subject completed both task under conditions of no noise, or 80 db white noise.
The results showed that children with ADHD improved on their recall in the low-load task, whereas noise had no effect on the performance of healthy children. In contrast, on the high-load task, noise actually decreased recall performance of healthy children, and had no effect on ADHD children.
This result provides some support for the idea that chronic understimulation is a causal symptom of ADHD, and that it can be at least partially remedied in terms of performance on simple recall tasks with the administration of loud white noise. Critically, this effect is limited to low-load memory tasks; if the effect were due merely to “drowning out distractions” one would expect the same benefit on the high-load task. Instead, there is no observed benefit there, suggesting that the mechanism may indeed involve stochastic resonance.
Intruigingly, this is to my knowledge the first demonstration that stochastic resonance from the perceptual system may have beneficial effects by carry-over into the more cognitive domains of encoding and recall. Stochastic resonance is more typically observed in sensory discrimination and detection experiments.
On the other hand, there’s plenty of reason for doubt here, too. Other studies have shown that background music is also beneficial in ADHD. The phenomenon observed here might therefore have less to do with stochastic resonance per se as providing some peripheral but continuous source of excitation to the chronically understimulated. It is therefore confusing that the authors chose a silent control, when a simple musical background might have ruled out this alternative explanation.