Absolute threshold
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In neuroscience and psychophysics, an absolute threshold is the smallest detectable level of a stimulus. For example, in an experiment on sound detection, researchers may present a sound with varying levels of volume. The smallest level that a participant is able to hear is the absolute threshold (often 1000 kHz)[1].
[edit] 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.[2][3]
- Dark Adaptation – 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 the fovea (the most sensitive point of the eye), where there is a highly density of cone 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.[citation needed]. Threshold cannot be determined on perfect sensation or lack thereof, due to fluctuations in the threshold. Thus, there is no set brightness seen by the viewer, and no intensity just lower than this that no flash is seen. To activate a bipolar, multiple rods must be stimulated, e.g., as 1940’s experiments have determined, that eleven quanta, one for each single rod, is necessary to trigger light perception. Another experiment shows that a 60 percent frequency may represent 2,500 quanta, one per rod, among a patch of 2 million rods. Therefore only one rod out of many is perceived at the threshold, and in a short stimulus, rods cannot absorb more than a single photon of light. (A quantum or photon is stated as Planck's constant and the frequency of light .[4]
Fluctuations in retinal response are driven by fluctuations of the amount of energy of the stimulus and not the change of retinal sensitivity. For example, if an average number of photons is given, the actual quantum content received and frequency of vision are similar. [5]
[edit] References
- ^ absolute threshold — Encyclopedia.com: Dictionary of Psychology: http://www.encyclopedia.com/doc/1O87-absolutethreshold.html
- ^ 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.
- ^ "eye, human."Encyclopædia Britannica. 2008. Encyclopædia Britannica 2006 Ultimate Reference Suite DVD
- ^ "eye, human."Encyclopædia Britannica. 2008. Encyclopædia Britannica 2006 Ultimate Reference Suite DVD
[edit] See also
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