Hyperacuity (scientific term)
Encyclopedia
Hyperacuity is the term used when a sensory performance is better than the limits set by its underlying anatomical apparatus.
acuity in vision where a misalignment of two abutting lines can be detected with a precision up to ten times better than the optical resolving power
of the eye and the spacing of the anatomical receptors cells (retinal cones), for which the term visual acuity
is reserved. But there are many other examples where the organism’s performance substantially surpasses the grain of the concerned receptor cell population. The normal human has only three kinds of color receptors in the retina, yet in color vision
, by subtly weighing and comparing their relative output, one can detect thousand of hues. Braille
reading involves hyperacuity among touch receptors in the fingertips, pitch discrimination, without which a violin could not be played in tune, that of the cochlear neural apparatus. It has been identified in many animal species, for example in the detection of prey by the electric fish
, echolocation
in the bat, and the mechanical deformation of whiskers in the rodent.
In spatial vision, where it has been investigated most thoroughly, examples of hyperacuity abound. They include stereoscopic acuity
, detection of the curvature of borders, and of the separation, length and orientation of borders.
pair is misaligned. The figure on the right depicts schematically, against an outline of the mosaic of retinal cones in the human fovea, the images of (upper) a two-bar pattern at resolution limit, exemplifying visual acuity
, and (lower) two vernier
lines at the misalignment detection limit, a hyperacuity.. Spacing of the receptor cells is one of the factors that sets a lower bound to the resolution limit; the activation of the middle of three adjoining columns of cones, separately emerging, must be detectably different from its neighbours.
On the other hand, alignment discrimination is achieved by neural circuits in the brain which identify the location of the bars in the lower part of the figure. Several cells are activated to different extents, and a "center of gravity" of the joint activity distribution is arrived at with a precision that is considerably better than the location signature of the individual receptor cells. That the hyperacuity apparatus involves signals from a range of individual receptor cells, usually in more than one location of the stimulus space, has implications concerning performance in these tasks. Low contrast, close proximity of neighboring stimuli (crowding), temporal asynchrony of pattern components are examples of factors that cause performance deficits. Of some conceptual interest are age changes and susceptibility to perceptual learning which can help in understanding underlying neural channeling.
is a hyperacuity task. Hyperacuity perimetry
is used in clinical trials evaluating therapies for retinal degenerative changes.
Hyperacuity in Various Sense Modalities
The most prominent example of a hyperacuity, and the one for which the word was coined, is vernierVernier
Vernier may refer to:Secondary adjustment devices for forms of "fine tuning":*Vernier scale on measuring instruments*Vernier thruster, on spacecraft*Vernier throttle, on aircraft*Vernier sight, on target riflesPeople:...
acuity in vision where a misalignment of two abutting lines can be detected with a precision up to ten times better than the optical resolving power
Resolving power
Resolving power may refer to:* Angular resolution* Spectral resolution* Resolution...
of the eye and the spacing of the anatomical receptors cells (retinal cones), for which the term visual acuity
Visual acuity
Visual acuity is acuteness or clearness of vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain....
is reserved. But there are many other examples where the organism’s performance substantially surpasses the grain of the concerned receptor cell population. The normal human has only three kinds of color receptors in the retina, yet in color vision
Color vision
Color vision is the capacity of an organism or machine to distinguish objects based on the wavelengths of the light they reflect, emit, or transmit...
, by subtly weighing and comparing their relative output, one can detect thousand of hues. Braille
Braille
The Braille system is a method that is widely used by blind people to read and write, and was the first digital form of writing.Braille was devised in 1825 by Louis Braille, a blind Frenchman. Each Braille character, or cell, is made up of six dot positions, arranged in a rectangle containing two...
reading involves hyperacuity among touch receptors in the fingertips, pitch discrimination, without which a violin could not be played in tune, that of the cochlear neural apparatus. It has been identified in many animal species, for example in the detection of prey by the electric fish
Electric fish
An electric fish is a fish that can generate electric fields. It is said to be electrogenic; a fish that has the ability to detect electric fields is said to be electroreceptive. Most electrogenic fish are also electroreceptive. Electric fish species can be found both in the sea and in freshwater...
, echolocation
Echolocation
Echolocation may refer to:* Acoustic location, the general use of sound to locate objects* Animal echolocation, non-human animals emitting sound waves and listening to the echo in order to locate objects or navigate...
in the bat, and the mechanical deformation of whiskers in the rodent.
In spatial vision, where it has been investigated most thoroughly, examples of hyperacuity abound. They include stereoscopic acuity
Stereoscopic acuity
Stereoscopic acuity, also stereoacuity, is the smallest detectable depth difference that can be seen in binocular vision.-Specification and Measurement:...
, detection of the curvature of borders, and of the separation, length and orientation of borders.
Analysis of Hyperacuity Mechanism
The fundamental difference between hyperacuity and the traditional concept of acuity is that for acuity the task is to resolve whether there are one or two stimuli, whereas the identification of the precise location of a stimulus within its perceptual realm is usually a hyperacuity, transcending the positional grain of the anatomical receptor cells in their sensory space. Hence the neural mechanisms by which the two kinds of task are achieved are distinct. This is best exemplified by counterposing the processing of the visual stimuli when the task is (a) resolving two stars or distinguishing between sans-serif roman I and II symbols, and (b) discrimination whether a vernierVernier
Vernier may refer to:Secondary adjustment devices for forms of "fine tuning":*Vernier scale on measuring instruments*Vernier thruster, on spacecraft*Vernier throttle, on aircraft*Vernier sight, on target riflesPeople:...
pair is misaligned. The figure on the right depicts schematically, against an outline of the mosaic of retinal cones in the human fovea, the images of (upper) a two-bar pattern at resolution limit, exemplifying visual acuity
Visual acuity
Visual acuity is acuteness or clearness of vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain....
, and (lower) two vernier
Vernier
Vernier may refer to:Secondary adjustment devices for forms of "fine tuning":*Vernier scale on measuring instruments*Vernier thruster, on spacecraft*Vernier throttle, on aircraft*Vernier sight, on target riflesPeople:...
lines at the misalignment detection limit, a hyperacuity.. Spacing of the receptor cells is one of the factors that sets a lower bound to the resolution limit; the activation of the middle of three adjoining columns of cones, separately emerging, must be detectably different from its neighbours.
On the other hand, alignment discrimination is achieved by neural circuits in the brain which identify the location of the bars in the lower part of the figure. Several cells are activated to different extents, and a "center of gravity" of the joint activity distribution is arrived at with a precision that is considerably better than the location signature of the individual receptor cells. That the hyperacuity apparatus involves signals from a range of individual receptor cells, usually in more than one location of the stimulus space, has implications concerning performance in these tasks. Low contrast, close proximity of neighboring stimuli (crowding), temporal asynchrony of pattern components are examples of factors that cause performance deficits. Of some conceptual interest are age changes and susceptibility to perceptual learning which can help in understanding underlying neural channeling.
Clinical Applications
In clinical vision tests, hyperacuity has a special place because its processing is at the interfaces of the eye's optics, retinal functions, activation of the primary visual cortex and the perceptual apparatus. In particular, the determination of normal stereopsisStereopsis
Stereopsis refers to impression of depth that is perceived when a scene is viewed with both eyes by someone with normal binocular vision. Binocular viewing of a scene creates two slightly different images of the scene in the two eyes due the the eyes' different positions on the head...
is a hyperacuity task. Hyperacuity perimetry
Perimetry
Perimetry or campimetry is the systematic measurement of differential light sensitivity in the visual field by the detection of the presence of test targets on a defined background. Visual field testing can be performed clinically with confrontational field testing keeping the subject's gaze fixed...
is used in clinical trials evaluating therapies for retinal degenerative changes.