In neuroscience and psychophysics, an absolute threshold is the smallest detectable level of a stimulus. However, at this low level, subjects will sometimes detect the stimulus and at other times not. Therefore, an alternative definition of absolute threshold is the lowest intensity at which a stimulus can be detected 50% of the time. The absolute threshold can be influenced by several different factors, such as the subject's motivations and expectations, cognitive processes, and whether the subject is adapted to the stimulus.
The absolute threshold can be compared to the difference threshold, which is the measure of how different two stimuli have to be in order for the subject to notice that they are not the same.
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.
- 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 high 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 (510 nm).
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.. 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 .
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.
The absolute threshold of hearing is the minimum sound level of a pure tone that an average ear with normal hearing can hear with no other sound present. The absolute threshold relates to the sound that can just be heard by the organism.
The threshold of hearing is generally reported as the RMS sound pressure of 20 µPa (micropascals) = 2×10−5 pascal (Pa). It is approximately the quietest sound a young human with undamaged hearing can detect at 1,000 Hz. The threshold of hearing is frequency dependent and it has been shown that the ear's sensitivity is best at frequencies between 1 kHz and 5 kHz.
The odor detection threshold is the lowest concentration of a certain odor compound that is perceivable by the human sense of smell. The thresholds of a chemical compound is determined in part by its shape, polarity, partial charges and molecular mass. The olfactory mechanisms responsible for a compound's different detection threshold is not well understood, as such, these thresholds cannot yet be accurately predicted. Rather, they must be measured through extensive tests using human subjects in laboratory settings.
On the topic of absolute threshold and the absolute minimum threshold: Absolute threshold in neuroscience, psychology, and psychophysics, is considered the lowest possible level of a detected stimulus. According to the seventh edition of Psychology, Themes and Variations, absolute threshold is “for a specific type of sensory input and is the minimum stimulus intensity an organism can detect,” (120). An example of this would be an odor test. The least amount of the odorous object necessary to still make people smell the odor would be the absolute threshold. So, absolute threshold is the least amount of what we can detect and respond to. Thus, from this it can be concluded that the absolute minimum threshold is the measure of that lowest frequency that organisms still detect for a specific sensor.
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