Wavelength dependency of the critical flicker-fusion frequency: therapeutic 40 Hz light source in Alzheimer’s disease

The use of controlled temporal modulated light increased in prominence since the LEDs invention, e.g., in computer monitors and light therapy devices. The benefits of using LEDs compared to old incandescent light bulbs range from environmental load to the precise control of the spectral properties and the ability to accurately control the temporal modulation of the light. The nature of LEDs also allows LED lamps to be switched on and off faster than ordinary incandescent light bulbs. The driver frequency of most LEDs (~25 kHz) is so high that the modulation of firing rates of the retinal neurons cannot time dissolve the flicker at this frequency. But in some cases, the function of the LED is to provide temporal modulation at frequencies much lower, in the range of 24-48 Hz. Knowing the Critical Flicker-Fusion Frequency (CFF), the frequency at which temporally modulated light becomes steady, is therefore important. Potential treatments of Alzheimer’s disease are currently being examined in humans using both stroboscopic and invisible spectral flickering light, using a 40 Hz temporal modulation. Ultimately, the CFF dependency on color, luminance, viewing angle and background lighting needs to be taken into account when designing and developing 40 Hz light sources for potential therapeutic use within the field of Alzheimer's disease. Here, we present a potential benefit of using the staircase method with a 2-alternative forced choice to determine the CFF. Specifically, we show a portable experimental setup that may be used directly to optimize light therapy for patients with Alzheimer’s disease.