Light and photobiomodulation
Dose-response relationship
A dose-response relationship describes how the size of an effect changes as the dose changes. In photobiomodulation the relationship is biphasic rather than straightforward: a low dose stimulates, a higher dose does less, and too much can abolish or reverse the effect.
Also called dose response, biphasic dose response, Arndt-Schulz curve, hormesis
The general idea. Establishing that more of an agent produces more of an effect, up to a point, is one of the oldest tests of whether an effect is real rather than coincidental. A demonstrated dose-response is evidence for causation.
Why light is different. The relationship is not a rising line. Huang, Chen, Carroll and Hamblin (Dose-Response, 2009) reviewed the field and concluded that “a biphasic dose response has been frequently observed where low levels of light have a much better effect on stimulating and repairing tissues than higher levels of light”. The curve is often drawn as the Arndt-Schulz curve: it rises, peaks, and falls back towards or below baseline. The authors also note how many settings there are to choose among: wavelength, fluence, power density, pulse structure and timing.
Practical consequences.
- A longer session is not necessarily a stronger one.
- Sitting closer to the LEDs is not necessarily better.
The same reasoning appears in ozone therapy discussions of oxidative preconditioning: a controlled small stress is proposed to be beneficial where a large one is not.
Why it matters here
It is why a longer or brighter session is not automatically a better one.