MAGAZINE SOBRE HISTÒRIA (Iniciat com AUCA satírica el 1960.. en edició de M. Capdevila a la classe de F.E.N. )
"La història l'escriu qui guanya".. així.. "El poble que no coneix la seva història... es veurà obligat a repetir-la.."
|10-12-2020 (521 lectures)||Categoria: Science|
InÂ neuroscience andÂ psychophysics, anÂ absolute threshold was originally defined as the lowest level of aÂ stimulus â€“ light, sound, touch, etc. â€“ that an organism could detect. Under the influence ofÂ signal detection theory, absolute threshold has been redefined as the level at which a stimulus will be detected a specified percentage (often 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.
A landmark 1942 experiment byÂ Hecht, Shlaer, and Pirenne assessed the absolute threshold for vision. They tried to measure the minimum number ofÂ photons theÂ human eye can detect 60% of the time, using the following controls:
The researchers found that the emission of only 5-14 photons could elicit visual experience. However, only about half of these entered the retina, due to reflection (from the cornea), absorption, and other factors relating to transmittance of the ocular media. The researchers estimated that 5 to 14 of the estimated 500 rods in the test area would each absorb one photon, with a 4% chance that one rod would absorb two photons.
A second absolute threshold for vision involves the minimum photon flux (photons per second per unit area). In this case the light covers a wide field over an extended period of time instead of being concentrated on one spot on the retina in a short burst. Knowing the pupil diameter and the wavelength of the light, the result can be described in terms ofÂ luminance (~0.000001Â candela per square meter or 10âˆ’6 cd/m2) or retinalÂ illuminance (~0.00002 Trolands). By including estimates for the probability of an average photon being absorbed by an average rod cell, the threshold stimulation for the rods is approximately one photon absorption per second per 5000 rods.
In terms of total absolute power sensitivity, Denton and Pirenne in the Journal of Physiology in 1954 found that for diffuse, extended sources i.e. a relatively large (~ 45-degree wide source, as viewed by the subject) frosted glass aperture, and a long (5 second) observation and decision time, the human eye could begin to reliably distinguish the lit from unlit glass at a power level of approximately 7.6 x 10âˆ’14 watts / steradian-cm2 at the eye for green (510Â nm) light. This power level was dependent on the wavelength of the light used according to the usual luminosity curve. For white light, the absolute sensitivity found was 5.9 x 10âˆ’14 watts / steradian-cm2. This base sensitivity varied only about 0.03 log steps between monocular (one-eyed) or binocular (two eyed) vision.
In 1972 Sakitt conducted an experiment that combined elements of signal detection and threshold theory. Two key elements of the study were a high tolerance for false positives and a multiple-choice option on deciding whether or not a light was seen. In the classic studies described above, the tolerance for false positives was so low that threshold was biased upward. Based on statistical analysis of a large number of trials, 6 photons each absorbed by one rod near-simultaneously looked "very bright," 5 photons looked "bright," 4 photons "a moderate light," 3 photons "a dim light." Two observers were able to see 2 photons as "slightly doubtful if a light was seen." One observer saw a single photon as "very doubtful if a light was seen." Zero photons were seen as "did not see anything."
The absolute threshold of hearing is the minimumÂ sound level of aÂ pure tone that an average ear with normalÂ hearing can hear with no other sound present. The absolute threshold relates to theÂ sound that can just be heard by the organism.
An example of this would be hearing a watch ticking twenty feet away from you in a quiet room. The threshold of hearing is generally reported as theÂ RMS sound pressure of 20 Î¼Pa (micropascals) = 2Ã—10âˆ’5 pascal (Pa). It is approximately the quietest sound a young human with undamaged hearing can detect at 1,000Â Hz. The threshold of hearing isÂ frequency dependent and it has been shown that the ear's sensitivity is best at frequencies between 1Â kHz and 5Â kHz. Humans typically have a lower hearing threshold for their own names. Dennis P. Carmody and Michael Lewis studied this phenomenon in 2006 and found that brain regions respond to the person's name differently than they do to a random name.
TheÂ odor detection threshold is the lowest concentration of a certainÂ odor compound that is perceivable by the humanÂ sense of smell. The thresholds of a chemical compound is determined in part by itsÂ shape,Â polarity,Â partial charges andÂ 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.
The absolute threshold for touch is a bee's wing dropping onto a person's cheek from one centimeter (.5 inches) away. Different parts of the body are more sensitive to touch, so this varies from one body part to the next (20).
As people age, the absolute threshold for touch becomes larger, especially after age 65. In general, women have a lower absolute threshold and are more sensitive to touch than men. However, it also seems to vary from person to person. Even individuals experience long-term variation within their own absolute threshold for touch. This could potentially affect how sensory disorders are evaluated by medical professionals.
In 1974, Ulf Lindblom studied how the speed of a stimulus affects absolute threshold. A WaveTek stimulator was used to measure absolute threshold of touch by "tapping" a participant's finger pad with a 2Â mm diameter probe. Lindblom found that on average, there was a 27% difference in threshold level between slow and fast mechanical pulses on a participant's finger pad. The threshold for rapid pulses was 5Â Âµm, and 80Â Âµm for slow pulses. Lindblom's study shows that humans are more sensitive to fast stimulation than slow stimulation, at least for touch.
In 1999, J. A. Stillman, R. P. Morton, and D. Goldsmith performed a study testing absolute threshold of taste and found that automated testing of taste was just as reliable as traditional testing. Additionally, they found statistical significance of the right side of the tongue having a lower absolute threshold than the left side. This finding leads to the possibility that the right hemisphere of the brain is better at processing gustatory stimuli than the left. Being deprived of calories for a short time increases sensitivity to and decreases absolute threshold for sweet and salty foods. Other factors such as pregnancy and smoking can influence taste sensitivity.
Main article:Â Sensory Processing Disorder
Some people have an abnormally high or low absolute threshold for one or more senses that interferes with their quality of life. They tend to avoid stimulation, seek after it, or perhaps not notice it at all. This is a symptom of can be diagnosed as a sensory processing disorder, also known as sensory integration dysfunction, which is common in people with autism.