A well-known phenomenon is the sparkler's trail effect: A quickly moving spot of light appears as a line of light, due to the persistence of vision.
Now the question arises: What is the perceived brightness of that apparent line, compared to the perceived brightness of the spot of light?
Dear Sascha
In human visual perception the eye/brain neural complex has to process and analyse a massive flow of input sensory data related to movement, colour, object recognition and so forth. Thus, two key parts of the eye (and their associated analytical processes) have developed by means of genetic optimization. The wide-angle general view of your surroundings is captured by a relatively wide patch of sensor cells at the back of the eye known as the retina. The focal point of direct visual interest falls on the small central patch of sensor cells known as the fovea. Here minute detail is perceived by a densely packed set of ‘specialist sensors’.
As human observers we interpret the overall retinal response as an optimised continuum that depends mainly on the existing illumination. Visually, we maintain only a low resolution retinal image of our surroundings. Movement is however detectable as abrupt changes in sensor output that may attract more detailed attention.
The foveal sensation has additional abilities that as necessary, allow us to perceive and interpret your personal location in space and over time, and also that of observed objects and events. Visual image persistence is a key feature of the temporal sense that allows time for analytical interpretation, at the same time tapering off smoothly in sensitivity toward the next interpretation.
Given these capabilities and returning to your question, a stationary point of light will be interpreted as having apparent constant brightness relative to an averaged background continuum. It follows that under human sense of visual persistence, a moving light point may be perceived if you will, as a well-defined visual line in the form of a set of spatially located point identities, with sequentially decaying apparent brightness.
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