Absolute threshold
Encyclopedia
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.
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.
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.
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 charge
s 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.
Neuroscience
Neuroscience is the scientific study of the nervous system. Traditionally, neuroscience has been seen as a branch of biology. However, it is currently an interdisciplinary science that collaborates with other fields such as chemistry, computer science, engineering, linguistics, mathematics,...
and psychophysics
Psychophysics
Psychophysics quantitatively investigates the relationship between physical stimuli and the sensations and perceptions they effect. Psychophysics has been described as "the scientific study of the relation between stimulus and sensation" or, more completely, as "the analysis of perceptual...
, an absolute threshold is the smallest detectable level of a stimulus
Stimulus (physiology)
In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....
. 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.
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.- 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 foveaFoveaThe fovea centralis, also generally known as the fovea , is a part of the eye, located in the center of the macula region of the retina....
(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.
Hearing
The absolute threshold of hearing is the minimum sound level of a pure tonePure tone
A pure tone is a tone with a sinusoidal waveshape.A sine wave is characterized by its frequency, the number of cycles per second—or its wavelength, the distance the waveform travels through its medium within a period—and the amplitude, the size of each cycle...
that an average ear with normal hearing
Hearing (sense)
Hearing is the ability to perceive sound by detecting vibrations through an organ such as the ear. It is one of the traditional five senses...
can hear with no other sound present. The absolute threshold relates to the sound
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
that can just be heard by the organism.
The threshold of hearing is generally reported as the RMS
Root mean square
In mathematics, the root mean square , also known as the quadratic mean, is a statistical measure of the magnitude of a varying quantity. It is especially useful when variates are positive and negative, e.g., sinusoids...
sound pressure
Sound pressure
Sound pressure or acoustic pressure is the local pressure deviation from the ambient atmospheric pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water...
of 20 µPa (micropascals) = 2×10−5 pascal
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
(Pa). It is approximately the quietest sound a young human with undamaged hearing can detect at 1,000 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
. The threshold of hearing is frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
dependent and it has been shown that the ear's sensitivity is best at frequencies between 1 kHz and 5 kHz.
Odor
The odor detection threshold is the lowest concentration of a certain odorOdor
An odor or odour is caused by one or more volatilized chemical compounds, generally at a very low concentration, that humans or other animals perceive by the sense of olfaction. Odors are also commonly called scents, which can refer to both pleasant and unpleasant odors...
compound that is perceivable by the human sense of smell
Olfaction
Olfaction is the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates...
. The thresholds of a chemical compound is determined in part by its shape
Molecular geometry
Molecular geometry or molecular structure is the three-dimensional arrangement of the atoms that constitute a molecule. It determines several properties of a substance including its reactivity, polarity, phase of matter, color, magnetism, and biological activity.- Molecular geometry determination...
, polarity
Chemical polarity
In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...
, partial charge
Partial charge
A partial charge is a charge with an absolute value of less than one elementary charge unit .-Partial atomic charges:...
s and molecular mass
Molecular mass
The molecular mass of a substance is the mass of one molecule of that substance, in unified atomic mass unit u...
. 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.
See also
- LimenLimenIn physiology, psychology, or psychophysics, a limen or a liminal point is a threshold of a physiological or psychological response.Liminal, as an adjective, means situated at a sensory threshold, hence barely perceptible....
- Psychometric functionPsychometric functionA psychometric function describes the relationship between a parameter of a physical stimulus and the responses of a person who has to decide about a certain aspect of that stimulus. Usually these sensory decisions take the form of a two-alternative forced choice . The psychometric function usually...
- Sensory thresholdSensory thresholdSensory threshold is a theoretical concept used in psychophysics. A stimulus that is less intense than the sensory threshold will not elicit any sensation...