Contrast Sensitivity Studies and Test- A Review

Contrast sensitivity (CS) expresses the quality of human vision. This article
presents the main points for the interpretation of CS and what structures of the eye
and brain contribute to achieving it. There is still information on the ways and types of
CS clinical examination, factors and conditions affecting it, and the latest technological
developments in its measurement.


Introduction
Visual acuity (V.A.) is the measure of the spatial resolution of the eye (the clarity or sharpness of vision) and its measurement determines the highest spatial frequency or smallest detail that the eye can perceive at high levels of contrast [1,2]. But V.A. is measured on a fixed target (optotype) and it may be the basic assessment of vision but does not fully meet the daily requirements of the human visual function, which comes into contact with a variety of stimuli of varying intensity. Contrast sensitivity (C.S.) is the ability of the eye to detect small changes in illumination at targets that do not have clearly defined limits [3]. Measuring C.S. is just as important as V.A.
and is now universally accepted as complementary [1]. as it reflects the quality of vision and in many cases declines earlier, while V.A. remains normal (6/6 or better) [3]. C.S. defines the threshold between visible and non-visible, which has both elementary and clinical significance in the science of vision. From the point of view of physics, most light stimuli can be analyzed in a series of sine waves (Fourier analysis -1822) [4][5][6]. To compare the performance of optical systems, a commonly used measure is the modulation transfer function (MTF). For example, the MTF of a lens is its ability to transfer contrast at a particular resolution from the object to the image. So MTF is a way to incorporate resolution and contrast into a single specification. As line spacing decreases on a line test target, it is difficult for the lens to transfer this decrease in contrast while MTF decreases. However, MTF does not correspond fully to analyze the contrast of the image created in the human visual system [7].
A more complete description of the spatial resolution of the optical system of the eye is given by the curve of the spatial contrast sensitivity function (CSF) [2,8]. The CSF curve describes the size of the object in relation to the spatial frequency (c/deg) of a sine wave [8]. and this corresponds to the actual V.A. of the person. [2].
Similarly, the graph of the time frequency as a function of pulse light (flashing) is given [2]. The first scientist that calculated the optical contrast sensitivity as a function of spatial frequency was Schade (1956) [9]. One of the most important terms for studying CSF is spatial frequency. The number of cycles of an image falling over a certain spatial distance, usually one degree of angle of vision, is the spatial frequency [4]. It is the measure that determines the clarity and gradation of the bright and dark areas. The representation consists of a light and a dark region, for example in a sine wave. [4].
The high spatial frequency representation comprises many narrow lines, that means, many circles within each degree of angle-cycles per degree (cpd), while at low frequencies the lines widen, so the cycles are less per degree. [4]. Spatial frequency can also be set with respect to the observation distance, so as the distance decreases each line increases, so the frequency decreases. [4]. The highest spatial frequency of human perception is 30 c/deg and corresponds to approximately V.A. 6/6. [6]. CS depends on the function, anatomy   The ganglion cells are divided into subgroups according to how they respond to light ((bright light or dark) -(response in the presence or absence of light)) and depending on the area of the lateral geniculate body they end up into two types of cells [10,11].
(Magni and Parvi / Large and Small). A third subclass of ganglion cells, which respond to S-cones, has recently been discovered, which percepts blue yellow. P cells make up 90% of the optic nerve and come from most of the macular fovea centralis. They have a small receptive field (excitation field from the photoreceptor region) and respond better to high spatial frequencies and color perception, but with a reduced CS. ( [10,12,13]. The M cells are few in number (10% of the optic nerve) with a large but receptive field and thus have a reduced spatial resolution but have an increased CS. [10,12,13].     However, the first to make substantial subjective measurement of CS using sinusoidal grating patterns were Green and Campell in 1965 (Journal of Physiology). [1,9]. Furthermore, the Arden grating chart of contrast sensitivity, which consisted of vertical gratings of different spatial frequency, has been used in clinical practice since 1976. [14]. Robson test is easy to use as it resembles the V.A. measurement that most patients are familiar with, its fast and with good repeatability. [5,15]. It is ideal for cataract detection, screening of patients with intraocular lenses and for checking drivers for impaired vision. [5].

Popular Tests for CS
The patient's CS is determined by the last triplet's letter in which he or she will be able to read two of them. [4]. Striped or sinusoidal tests consist of bright and dark areas called circles. [4]. The most popular is the Vision Contrast Test System (VCTS) or Vistech, which was introduced in the 1980s [18]. It consists of circular gratings in five rows and nine columns, with spatial frequencies of 1.5, 3, 6, 12 and 18 c/deg and the contrast decreases from left to right. [17].    The difference is that it is held within one meter of the subject and may require a correction of approximately + 0.75D [4,19]. As with other eye examinations, in the case of CS, measurements must be made under specific lighting conditions. If this is not done, the test will produce results that are not true. As a general rule, ambient lighting should be within 10-30% of the average brightness of the target [21]. However, each test is different and test conditions should be suggested by the manufacturer / researcher promoting it. .and in some patients before even reducing their field of vision

Studies on CS Related to Diseases
[37]. Optical neuritis, usually due to multiple sclerosis, affects the optic nerve and consequently the ganglion cell axons and the three pathways of transport of stimuli to the brain (M / P cellular and the S cone response group) [11]. Mostly, color perception is affected by optic neuritis rather than CS, especially if the P pathway is affected [11,38,39]. Therefore, the important role of the M pathway in CS is confirmed .

Other Factors That Affect CS
Intraocular lens used either in cataract surgery or in refractive surgery does not reduce CS [34,40]. Especially in the case of cataracts they certainly improve CS postoperatively, and the possible differences that can be observed in CSF measurements are mainly due to their materials [41] and whether they are spherical or aspherical [42]. In refractive surgery (PRK and Lasik) there is a change in CS postoperatively, mainly at medium and high frequencies, even if V.A. reaches 6/6, [43] and when the pupil's diameter is about 7mm the problem is aggravated by the increasing aberrations [44][45][46]. The problem is now improved with PRK in combination with mitomycin, thinner flaps and Lasik wavefront, but compared to intraocular lenses CS postoperatively is slightly reduced. [46][47][48]. The purpose of orthokeratology is good daytime vision, without any aids such as glasses or contact lenses. [49]. The application is successful in low and medium myopia. [49]. Specially designed RGP contact lenses are used throughout the night (worn during sleep) to eliminate myopia [49]. The disadvantage of this method is the high order aberrations and the reduction of mesopic CS, thus making night vision particularly difficult [49]. Another factor that affects CS is severe dry eye [50]. In addition to being an unpleasant condition for the patient, in combination with the glare it can reduce the quality of one's vision. The administration of artificial tears usually leads the CS to normal levels [50]. It has been observed in clinical studies that schizophrenia reduces the CS of the sufferer and is associated with deficits in the Magno tract. [51]. Studies have shown a decrease in the CS curve, but no direct interaction with M-pathway, and may be due to the medication patients receive [52].
The immune system of HIV patients is malfunctioning, so they are affected by other viruses that the healthy body would have been able to deal with. Ophthalmologically, abnormalities in CS and color vision are often present in HIV patients [53].

Developments
New technologies for examining CS are starting to emerge, mainly taking advantage of modern electronic devices. Quick-CSF (Quick CSF) promises complete testing in a matter of minutes (2-5min) with fully automated means (handled via a tablet). test [54][55][56][57][58][59][60]. Finally using the well-known web browser, Firefox, an innovative CSF Nearby SPARCS test, uses a computer screen and the gray gradations it can perform Figure 8.

Discussion
Vision is one of the most demanding functions of the human brain. The introduction of visual stimuli into the eye and their path from its anatomical structures to its retinal photoreceptors is a condition that is interdependent on many factors. Subsequently, transferring stimuli from the retina to the occipital lobe but also analyzing and deciphering them is a complicated process involving many parts of the brain [61][62][63][64]. The case of CS is even more complicated as the roles of the anatomical structures of the optic nerve and the brain that contribute to it have not been fully defined [11]. Each eye is also a separate case, with its own visual