Abstract
Rapidly repeated flashes are seen as steady light, equivalent in sensory brightness to the same amount of light uniformly distributed in time. This is Talbot's law, and the outcome is what would be expected from a simple train of events copying the output of the photoreceptors.
But as soon as flash rate is reduced below the fusion point for steady sensation, the flashes begin to produce an average impression which is greater than that expected from photochemical considerations. As the rate is reduced more and more, the flashes produce an effect more and more nearly equivalent to steady light of the same physical intensity, and finally surpass it, reaching a maximum when the rate is reduced to the neighborhood of 8 or 10 per second. With still slower rates, the flashes approximate equivalence to steady light, with the recognized exception that single isolated flashes always appear brighter than steady stimulation of the same intensity.
The phenomenon of enhancement was first observed in 1864 by Brücke with rotating black and white sectored discs, but was not plotted for various rates of intermittency. The phenomenon shall be called the Brücke effect, though we used flashes instead of the revolving discs.
The significance of the Brücke effect is that a series of momentary stimuli is more effective for sensation than steady stimulation of the same strength. In order to account for this, the responding system must act very different from what would be expected of photoreception in the sense cells. Since the nervous system is capable of rhythmicity and periodic facilitation, we might look immediately to it. And since the maximum Brücke effect occurs at flash rates of 8 to 10 per second, such a periodicity should be sought to account for it.
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