Hyperacuity is the term used when a sensory performance is better than the limits set by its underlying anatomical apparatus.
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The most prominent example of a hyperacuity, and the one for which the word was coined,[1] is vernier 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,[2] pitch discrimination, without which a violin could not be played in tune, that of the cochlear neural apparatus.[3] It has been identified in many animal species, for example in the detection of prey by the electric fish,[4] echolocation in the bat,[5] and the mechanical deformation of whiskers in the rodent.[6]
In spatial vision, where it has been investigated most thoroughly, examples of hyperacuity abound.[7] They include stereoscopic acuity, detection of the curvature of borders, and of the separation, length and orientation of borders.
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 vernier 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.[8] Of some conceptual interest are age changes[9] and susceptibility to perceptual learning[10] which can help in understanding underlying neural channeling.
In clinical vision tests,[11] 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 stereopsis is a hyperacuity task. Hyperacuity perimetry is used in clinical trials evaluating therapies for retinal degenerative changes.[12]