- Absolute threshold
In
neuroscience andpsychophysics , an absolute threshold is the smallest detectable level of astimulus . For example, in an experiment on sound detention, researchers may present a sound with varying levels of volume. The smallest level that a participant is able to hear is the absolute threshold.Vision
The absolute threshold for vision was assessed in a landmark experiment by Hecht, Shlaer and Pirenne in 1942. The experiment was designed to measure the minimum number of photons detectable by the human eye, therefore various controls were implemented to ensure that this is was the case. ["Fundamentals of Sensation and Perception", Michael Levine. Oxford University Press (3rd Edition). London, 2000.] ["Visual Perception", Tom Cornsweet. Harcourt Publishing (1970). Chapters 2 and 4.]
* Dark Adaption – the participants were completely dark adapted (a process lasting forty minutes) in order to optimise their visual sensitivity.
* Location – the stimulus was presented 20 degrees to the left of the point of focus, in order for it to fall 20 degrees to the right of thefovea (the most sensitive point of the eye), where there is a highly density of rod cells.
* Stimulus size – the stimulus, a circle of red light, had a diameter of 10 minutes (1 minute=1/60th of a degree). This ensured that the light stimulus fell only on rod cells connected to the same nerve fibre (this is called the "area of spatial summation").
* Wavelength – the stimulus wavelength matched the maximum sensitivity of rod cells (510nm).It was found that the emission of only 90 photons was required in order to elicit visual experience. However, only 45 of these actually entered the retina, due to absorption by the
optic media . Furthermore, 80% of these did not reach the fovea. Therefore, it only takes nine photons to be detected by the human eye.Moreover, as the chance of any one rod receiving more than one photon is very small, we can assume that it only takes one photon to excite a rod receptor.Fact|date=May 2008References
ee also
*
Absolute threshold of hearing
*Just noticeable difference
*Limen
*Psychometric function
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