Campimetry
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Country central and peripheral vision in his left eye.
Contents [hide ]
1 Concept 2
Psychophysics: detection and perception
2.1 Psychophysics of sensation.
2.2 Psychophysics of perception.
3 Threshold, Sensitivity
Stimulation and Clinical Use 4
4.1 Perimeters Perimeter 4.2 Validation 4.3 Conditions
exploratory. Variability 5 Interest
perimetry in glaucoma perimetry 6
6.1 Summary
7 References 8 See also
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Concept [edit ]
The visual field, or more modern perimetry, is exploring psychometric visual field, ie the universe of light that is capable of covering one eye remained stationary. The visual field is divided into central (covered by the foveal gaze) and the remaining space or peripheral field.
perception.
visual field center is much more perfect than it collects in the peripheral high number of sensitivities (light, color and shape). Within this space there light stimuli such as light, shadow, contrast, borders, colors, movements, etc. This mosaic of light through the optics of the eye, focuses on the retina and stimulates their cone photo-receptors (whether it is day) and intermediate neurons. For the optic afferent all information in the pre-coded walking ganglion to the brain, where it is processed (perceived) [11] .
is possible to measure this process, resulting in stimulation and testing their effects. There are two procedures: one goal, electrophysiology, measuring in the laboratory the bioelectric potentials generated by stimulation [1] [12] and one subjective, psychometrics, a measure of sensation through psychophysical thresholds.
Psychophysics.
The psychometrics is the measure of "subjective" and connects the changes in the physical properties of light stimulation (world) with the conscious and explicit findings that the subject makes them (inner world). The psychophysical findings are presented as mathematical functions the relationship between the physical parameters of the stimulus (actual) and subjective estimation of these values \u200b\u200b(apparent). No one can know how others see them, but the interesting thing is the power psychophysics figures measure something as evanescent as the feeling.
Psychophysics: detection and perception [edit ]
Psychophysics of sensation. [Edit ]
fechneriana In psychometrics or classical only interested in the relationship between sensory stimulus and response or feeling it evokes, conceived as something like a machine reflexogenic sleepy you wake only to appropriate sensory stimuli (external psychophysical). The sensation would be an elemental psychic event resulting from a change in the environment, captured as such by the sensory systems (retina + optic tract). It is also called detection. Regardless of the structure of the stimulus, classical psychophysics is only interested in a free variable: the sensitivity of the observer.
But the contents of psychophysics have evolved with the knowledge of neuroscience (inner psychophysics). [2] [3]
Psychophysics of
perception. [Edit ]
The perception may be defined as "validation of sensory images through internally generated sensory information." [3] perception would not be a mental most elaborate of the feeling among many other feelings, background and expectations of the subject. The perception would be feeling + interpretation (thalamocortical interaction). [3] involved in the perception of the individual's mental state: attention, adaptation, memory, motivation, and other processes Cognitive . The psychometrics of perception would be interested in quantifying two variables possible at the moment: on the sensitivity and other criteria that the subject used to make your thinking or perceptual interpretation. [4] Therefore
thalamocortical representation invalidates the assumption that only translate the perception of reality conveyed by the sensory afferent system, base and foundation perimeter paradigm.
Threshold, stimulation and Sensibility [edit ]
threshold concept. The psychometrics of the visual field thresholds used fechnerianos light sensitivity differential. The differential threshold is the minimum increase in the magnitude of the stimulus required for the subject is aware that there has been a change. To an observer
perfect (ideal) the threshold would be the point where the stimulus can therefore be detected as not detected. But humans are not perfect observers and thresholds are defined in probabilistic terms: as a rule, half of the time that this same stimulus, would be perceived and the other half did not. Thus, the threshold would be the point where 50% of the time a stimulus is detected.
In perimetry threshold is reached by the method of limits, up or down in a phased manner light intensity of the stimulus to be perceived or perceive stop.
stimulation. In use perimetry (white-white stimulus) stimulus is circular, fixed size and brightness (ΔL) variable (in steps) and is in a dome for half a second and a surprise in multiple predetermined locations of space on a background of fixed luminosity (L).
When a stimulus of sufficient energy produced in a dome affects the corresponding area of \u200b\u200bthe retina observer neurons modulate the content of the information to the thalamus. But the brain does not depend on continuous input signal the external world, but can also generate them (noise). [5] neuronal activity is inherently noisy and subject to variability: never the same pattern of bioelectrical activity even to the same stimulus. [6] in a situation exploratory When the signal generated by the stimulus has exceeded the cerebral background noise detection occurs, but it is the brain who "decide" as a binary response (SI-NO).
The subject speaks or not presented stimulus perception by pressing a button or not. The subject's answer "yes" or "no" to each stimulation, which is registered on the computer perimeter is therefore not only a complex system made up of two successive processes: a) the first is a process of detecting the signal that indicates the degree of patency of the neuro-sensory system (sensitivity) and b) the second is a process of making decision. Iterative bias or trend of these decisions is the "standard" natural or brain-specific bias (criterion β) [13] . As in other orders of things, the conservative or cautious approach show a natural tendency toward specific and NO or impulsive liberal approach towards the SI. The best approach would be the intermediate or symmetrical.
psychometric function can be studied experimentally properly trained rhesus macaques. The visual fields of monkeys are very similar to those of humans and had good reliability. [7] perimetry in children also requires special training. [8]
sensitivity. Obviously a large circular stimulus and bright will be more readily perceived that a little bit light stimulus. A small and very light stimulus may amount to a large and very bright stimulus (spatial summation). Differential light sensitivity of the visual system is in reciprocal proportion to the threshold obtained: differential light sensitivity
Threshold = 1/luminosidad
For reasons practices are not used luminance units (apostilbos [14] ) (cd / m²). The unit of differential light sensitivity is logarithmic (decibel light [15] ). The dB is not uniform for all units but dependent on the technical conditions of brightness of each brand of perimeter. [9]
A stimulus of standard size with minimal light stimulus perceived as a threshold could amount to 30-34 dB (depending on the range of each brand of product) and means a very high sensitivity of the retinal area where incident. Perceived stimulus threshold with the maximum illumination provided by the device means very low sensitivity of 0 dB. A low sensitivity means that an error has clinically neuronal (agony or death). Then there will be a spatial correlation between functional impairment and structural damage or lesions. This would be the main conjecture of the visual field and its application to the clinic. The form of visual field that gets the device in grayscale is not only a sensitivity map.
In summary, there are two different dimensions not to be confused:
a) The perceived by the subject (inner world). The brain can receive, via an afferent pathway damaged, imperfect or faded stimulation (lower contrast) of a parcel outside the universe . But it makes an internal representation (perceptual) that seeks to hide it so the subject has no awareness of their absence (as also happens with the blind spot).
b) What we perceive others (world). Instrumental measurement of this representation expressed by the subject itself shows a scotoma measurable decibel units. In turn, the numerical comparison with a normal statistical area given the size of the defect.
Clinical use [edit ]
Perimeter
automatic. The subject pressed each time a stimulus perceived
Perimeters [Edit ]
Main article: Perimeter
The instrument used for visual field testing is called campimeter or perimeter. There are several proprietary business models available. In its design, involving engineers, computer programmers, physicists and statisticians. In clinical evaluation involving psychologists, ophthalmologists and optometrists [10] and [16] .
now used increasingly computerized automatic machines [11] provided algorithms that filter and purify errors or strategies that shorten the scan time, avoiding fatigue where possible. Calculated fast strategies, by proximity, the threshold "expected" of each stimulus so that, without affecting the accuracy, significantly reducing the phasing and total test time. [12] Top Brands are provided with statistical packages (Perisuite ©, StatPac ©) taken from normal populations grouped by age, for comparison purposes (defects).
In clinical practice this exploration is carried out by technical staff optometrist [13] [17] doctor's instructions. In hospital resources depend on the perimeter Address Nursing . The operator's role is to ensure that you meet all the requirements of reliability . An exploration is an exploration unreliable useless to be erased from the hard drive. You get a two or three dimensional map of differential light sensitivity for each eye whose highest point is at the center and decays towards the periphery. Three-dimensional normal field of view can be likened to an island of vision in a sea of \u200b\u200bblindness. The peak can be measured 34 dB (decibels) and 0-1 dB at the edge of the beach slope. Near the peak there is a deep well that reaches the sea: it is the blind spot, corresponding to the optic disc.
Validation of perimeters [edit ]
The refinement, efficiency and insight discriminative (diagnostic or evolutionary) of a given perimeter is determined by its sensitivity his specificity and reproducibility . Defined as sensitivity to the ability of a test to detect specific pathology and its ability to define normal (defined in%) [18] .
In the case of the disease called glaucoma in the absence of a gold standard [19] robust reference (100%) displayed both qualities are often quite similar in all brands tested perimeter for black-white (on average 87% [14] ).
The reproducibility is a third feature of the efficiency of a process meter. Can be defined as "the ability to detect a true difference between multiple measurements." Terms
exploratory. Variability [edit ]
signal integration sensory perception depends on the brain domestic context very important: the care (momentary functional intent.) [3]
The subject may or may not be familiar with the exploration (learning). But in any case must continuously maintain high degree of concentration across a sequence of tasks. Their share of attention will be worn by the need to maintain a rigid oculomotor control (stable fixation), the spatial uncertainty suggested stimulation and inhibition constant with irrelevant or dubious decisions.
perimetry is an exploration and sought power, but to be consistent and reproducible spatial conditions requires brain maturity vigilant attention by the subject to explore, and narrative skill by the browser to be impractical in children or elderly and little reliable in adolescents (ending on frontal lobe development the brain to 20 years). Various circumstances, such as the duration of the test and moods can affect results.
There are other cognitive factors, for example, has shown how, in the manner of giving the instructions, the browser may influence the natural approach explored by highly trained that is it. [15]
When he started using computerized automatic perimetry eye doctors realized that, surprisingly, even with high levels of concentration and ideal subjects, oscillations occurred at the threshold to repeat them. [16]
fluctuation or variability factor thresholds [20] (repeating in the same test or subsequent tests) is the Achilles heel of perimetry that deserves special consideration. By not prove to be the visual field a stable entity is difficult to differentiate subtle changes real.
There is a normal fluctuation of small magnitude, implicit in the very definition of threshold (50% probability).
abnormal variability depends mainly on the degree of sensitivity of departure but also of learning, fatigue, micro-fixing mistakes and bad otherwise specified. Areas with loss of sensitivity (scotomas) are potentially volatile. They are not stable gray spots in the field of vision but faded or decontrastadas areas. As mentioned above, are often not socially aware of the subject because the brain tries to "fill" [17] possibly at the expense of greater variability in bioelectrical (noise). The increase in noise leads to an increase of the signal + noise and increased uncertainty psychophysics.
Finally, the perception is the interpretation of sensed, the fluctuation of the threshold in two consecutive tests (same theoretical detection) may be a function of changes in the natural criterion of the subject explored.
perimetry Interest [Edit ]
perimetrística The scientific literature is diversified on three main aspects: a) work on the physiology and psychology of vision, b) studies of clinical (medical) and c) engineering and design instruments (inventions, validation, marketing )
.
Value space / via optical
perimetry's interest in the field of medicine is based on the following aphorism:
"If the visual pathway is responsible for carrying visual information to the brain, any damage optical path will result in the corresponding functional loss of visual perception. "Any loss of structure is a proportional loss in function."
From a medical perspective, aimed at translating the visual field, with the fidelity of a map, sensory integrity conductivity of retina and optic pathway [21] to the occipital cortex of the brain. Discover the fault of the retina and optic nerve interested in Ophthalmology and manifested in one eye as local depressions or craters (scotomas) or General (troughs or depressions) total or partial islet. The fault located further back in the optic chiasm , Special interest in Endocrinology affectations especially the pituitary and manifest as bilateral depressions not very symmetrical (quadrantanopsia or hemianopsia) called heteronymous. Failures in the visual pathway postquiasmáticas brain (compression, bleeding, demyelinated etc) interested in Neurology and translated by bilateral hemianopsia great regularity, congruence and symmetry for what are called homonyms, which corresponds to the blind hampers the opposite cerebral hemisphere.
By custom and economy are often referred patients to ophthalmology for the practice of perimetry. Keep in mind that many programs and strategies of commercial automated perimeters are prepared only for Glaucoma .
perimetry in glaucoma [edit ]
Main article:
Glaucoma Eye disease definition is the ultimate perimeter chronic glaucoma. [18]
The gold standard for glaucoma diagnosis turns out to be somehow a tautology precisely because the glaucoma is defined perimeter. Unlike other diseases, glaucoma has no no marker or extraperimétrico or independent gold standard (as it might be a serological test).
perimeter definition of glaucoma is extremely vague or contrived, especially when it concerns people allegedly suspicious. Often resort to expert opinion based on a few principles heuristic. The value of the chosen indicators and cutoffs, are global indexes or spatial patterns of sensory loss (mean defect, variance, clusters etc.) Depends on the kind of scope, methodology or approach the group of experts.
functional or visual field progression in glaucoma occurs in the long term in three phases: hidden, and critical thresholds increased. [19]
a) Initially there is a hidden period in which the most valid perimetry remained normal (silent). This is what is called a latent class: there is a real disease, but is undetectable. If glaucoma is a disease of perimeter definition how to pretend to shorten the pre-perimeter? : Increasing the sensitivity.
The effect is developing new criteria surround stimulation using qualitatively different: [14] colored stimuli with variable contrast sinusoidal structure, blinking, moving etc. Is a stimulation addressed to the state and their difficulties dying neuronal axonal transmission. [20]
experimental procedures which are diagnostic validation will necessarily speculative and tautological to populations at this stage of glaucoma.
1) The determination of isochronous discriminative value (sensitivity and specificity in the present) is achieved by matching with an alleged gold standard surround more laborious (full threshold strategy) but also ambiguous (the test to validate could and should be more valid than its referent).
2) The determination of the predictive value [22] (sensitivity and specificity projected into the future) can not be achieved due to lack of an independent gold standard in glaucoma. Experimenting with using Bayesian mathematics prevalence. [21]
b) The second phase or increased thresholds (diffuse or focal defects in the visual field) is shown inducing intermittent diagnostic confusion. The defects appear and disappear with great variability. According to the therapeutic effectiveness defects may take several years to consolidate and identifiable patterns.
At any time during this stage will raise the issue of the progressive / stable disease (or whatever it is, progression YES-NO) for therapeutic impact. In an essay clinical series involving consecutive perimeter concept of progression (YES-NO) parameter would be but a "soft" because it is a more than a magnitude. If the progression is a metric defined in the protocol is an agreement of experts and also is involved in the uncertainty of increasing variability.
With these reservations, perimeter consecutive series, held every six months and analyzed with special statistical programs for some brands of circumferences (analysis of events and trends) [23] , indicate whether the disease progresses or remains stable (overall as focally). [22]
The validation of these programs progression detectors suffers from the same constraints with a much greater variability. In the interest of reproducibility is more practical to always use the same perimeter and the same exploratory strategy of confidence, trying to contain the maximum information in minimum time (eg Octopus TOP 32 ©, © Humphrey SITA standard).
c) After this stage, the disease enters a critical stage of glaucoma evolved with progressive deterioration despite treatment, proper tension, which may result in blindness. [23]
Summary [edit ]
remain perimeter important issues to be resolved:
1.Encontrar and debug causes instability of repeated threshold (variability). 2.Homogeneizar
accurately defined perimeter (normal / abnormal) of glaucoma and quantification of the concept of progression.
3.Establecer the predictive ability of the test perimeter. Research 4.La
perimeter (inventions) only has meaning beyond the existing efficiency when searching for more sensitive procedures (a situation that will only with the discovery of an independent gold standard) and procedures with less variability.
( http://es.wikipedia.org/wiki/Campimetr% C3% ADa )
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