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.
KEYWORDS: Light emitting diodes, Light sources, Light, Electroencephalography, Modulation, Alzheimer's disease, CIE 1931 color space, Phototherapy, Eye, Brain, Color vision, Human vision and color perception
Alzheimer’s disease (AD) not only takes an emotional toll on the individual with the disease, their families, relatives, and caretakers, it also has immense socioeconomic consequences on the health care system and society. Moreover, the socioeconomic consequences are expected to increase significantly, thus reducing the social and economic cost of AD is of high importance. Recently, exposure to 40 Hz stroboscopic light therapy, for one hour a day, resulted in slowing the progression of AD in mice and has a considerable potential for treatment in humans. However, exposure to such stroboscopic light carries its own consequences being that it is difficult to implement in a patient’s daily routine, irksome to use, and can cause visual discomfort which may result in a lack of patient adherence.
Here, we demonstrate a novel technology based on controlling multiple single-color LEDs to produce white light where its spectral composition alternates at a given modulation frequency without visible flicker. We coin this technique as Invisible Spectral Flicker (ISF). We present 40 Hz invisible spectral flicker light as a potential alternative in reducing discomfort compared to 40 Hz stroboscopic light, whilst still entraining oscillations in various areas of the brain. Furthermore, we demonstrate a distinct way to generate a 40 Hz metameric light source with the presented color mixing scheme, and validate that the CIE 1931 (x, y) coordinates match for two different spectral power distributions. Finally, we illustrate the light characteristics of seven 40 Hz color fusion light sources and two 40 Hz stroboscopic light sources. The technology presented here will lead to new, and hopefully improved, designs of light therapy systems for the treatment of Alzheimer’s and dementia.
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