Contrast Sensitivity

Anatomy and Physiology

Neural Mechanisms

Campbell and Green defined the neural mechanism for CS. They proposed different visual channels for handling the information about various bands of spatial frequencies.[8] This indicates that the retina is non-uniform. The fovea is the part of the retina specialized for high spatial frequencies. Only low-frequency channels represent the retinal periphery.[9]

For coarse gratings,  the central and peripheral retina has equal contrast sensitivity per unit area of the retina, but the more the area stimulated greater the CS. In peripheral retinal diseases, the CS is reduced, and low-frequency grating use help to provide a quick check of peripheral retinal function.[10]

Campbell and Robson proposed that independent mechanisms in the nervous system exist that are highly sensitive to a limited range of spatial frequencies. There is selective orientation and transfer of interocular adaptation effect in the visual cortex at the site of neurons. This helped explain the vital role of these interactions in complex image recognition and magnification generalization.[11]

M and P Pathways

The ganglion cells are subdivided into

• P cells- These cells have a high spatial resolution
• p cells- These cells are small and slow conducting cells that give input to the parvocellular layer of the lateral geniculate body.
• M cells- These cells have higher CS and temporal resolution and lower spatial resolution.
• m cells- These are large and flat conducting axons that provide input to the magnocellular layers[12]

Channel Theory

The cerebrospinal fluid (CSF) is susceptible to particular spatial frequency and is a function of several independent and parallel detecting mechanisms. The visual system is split up into several spatial frequency components separately, and the information is further transmitted into various channels in the cortex, where it is transformed.[13]

The visual system is composed of 4 to 6 spatial frequency channels. The CSF channels depend on a series of ganglion cells with varied receptive fields so that they have maximum sensitivity to various spatial frequencies. A partial response is obtained from the ganglion cells when the stimulus size is smaller than the central receptive field. When the stimulus size is larger than the central receptive field, the response from the ganglion cells is reduced.[14]

Indications

• People applying for jobs like armed forces, driving, pilots, merchant navy, lab technicians, medical professionals, etc
• People opting for athletics as a career
• Qualitative vision loss
• Retinal or optic nerve disease
• Visual pathway diseases
• Glaucoma
• Ocular hypertension
• Retrobulbar neuritis
• Multiple sclerosis
• Amblyopia
• Diabetes mellitus
• Pituitary adenoma
• Congenital dyschromatopsia
• Optic neuritis
• Cataract
• Higher-order aberrations
• Age-Related Macular Degeneration
• Trauma
• LASIK
• PRK

Equipment

Contrast Sensitivity Measurement

When the grating frequency is presented to the subject, and the contrast below is resolution is not possible, it denotes the contrast threshold, and the reciprocal of this threshold is CS.[15]

The formula for CS measurement-

Michelson’s Formula

(L– L)/ (L+ L) where L denotes the luminance of the photocells scanning across the gratings. Lis the luminance on the lighter surface, and Lis the luminance on the darker surface.[16]

Weber’s Formula

It is defined as (LL)/ L, where b is the background luminance and t is the target luminance. This formula is used to measure the contrast of the letters.[17]

Variables in CS measurement

• The average amount of reflected light depends on the darkness of ink and the illumination of paper.
• The amount of darkness relative to the white background is labeled as a contrast.
• The distance between the cycles per degree of visual angle or grating period.[18]

Methods of Contrast Sensitivity Measurement

Various methods have been described for CS measurement. Bodis Wollner described the CS measurements in day-to-day clinical practice. This was labeled as a visuogram to describe the CS curve. The CS deficit is expressed in decibels, and there are three types of deficits.

• High frequency- which is characterized by an increased loss at high frequency
• Level loss type- same loss at all kinds of spatial frequencies
• Selective loss type- Spatial frequency deficits in a narrow band

Various Recommended Methods for Contrast Sensitivity Measurement

• Simple plates
• Cathode ray tube display
• Letter acuity charts
• Laser interferometer (LI)
• Visual field testing using low contrast rings
• Pattern discrimination test
• Forced choice printed test
• Visual evoked cortical potentials
• Checkerboard pattern reversal
• Two alternative forced-choice tests[19]

Arden Gratings

In 1978, Arden introduced a booklet of seven plates having one screening plate (No.1) and six diagnostic plates (No. 2 to 7). The contrast changes from top to bottom plates, and it covers 1.76 log units approximately. The patient is examined with the plates at a distance of 57 cm with a spatial frequency varying from 0.2 cycles/degree to 6.4 cycles/degree having a double frequency of the previous one. The patient is scored from 1 to 20 depending on the number of the plate covered. The upper limit of 82 is established for normal subjects, consisting of six plates having an interocular difference of less than 12.

Cambridge Low Contrast Gratings

It has a set of ten plates consisting of grating in a spiral-bound booklet. The test is performed at a distance of 6 meters with the booklet hung on the wall. The pages are shown in pairs, one above the other. It shows that one page in each pair has gratings and another page is blank, but the mean reflectance of the pages is the same. The test subject is asked to choose the pages from the top or bottom containing gratings. The pages are shown to the subjects in order of descending contrast, and are subject is stopped when the first error is made. A total of four descending series are shown in each eye separately. When an error is made on plate 5, a score of 6 is given. Depending on the total score from the four series, the CS is calculated from the conversion plate.[20]

Pelli- Robson Contrast Sensitivity Chart

This chart has letters that subtend an angle of 3 degrees at a distance of 1 meter. The chart is printed on both sides; the other side has letter sequences in a different order. The letters are arranged in a series of two triplets in each line. The contrast reduces from one triplet to the next. The log sensitivity also varies from 0 to 2.25. While examining the patient, the chart can be hung on the wall so that the chart’s center coincides with the patient’s eye. The chart is brightly illuminated so that the luminance varies from 60 and 120 cd/m, which is equal to the exposure of 1/15 to 1/30 s at f/5.6 with an ASA of 100.

The luminance is recorded with the assistance of a light meter. While recording CS with the chart, the patient is seated at a distance of 1 meter. The subject is asked to read or point out the letter on the chart, beginning from the upper left corner and going horizontally across the line. The patient can also guess when the letters are invisible. The test is terminated if the patient wrongly guesses two out of three letters. The patient’s CS is indicated by the best triplets, for which two out of three letters are named correct.[21]

Mars Letter Contrast Sensitivity Test 

This chart looks similar to the Pelli-Robson test,  but the chart is comparatively smaller and is viewed at a closer distance.[22]

Vistech Chart

This consists of sine-wave gratings and is conducted at a distance of 3 meters from the patient. In this, the contrast is tested at several spatial frequencies, and the patient has to pinpoint the orientation of the grating, either vertical or 15 degrees or anticlockwise.

Functional Acuity Contrast Testing

Dr. Arthur Ginsburg first described functional Acuity Contrast Testing (FACT). This includes a chart of sine-wave grating with varying frequencies. With the help of this chart, five spatial frequencies and nine levels of contrast can be tested. The contrast of the letter change in the row and decreases from left to right. The spatial frequency increases from top to bottom, and the test is performed at a distance of 10 feet. The patient sees the grating in each row like A, B, C, and D and then reports the orientation right, up, or left. The last grating for each spatial frequency is then plotted for the CS curve.[23]

Regan Low Contrast Sensitivity Letter Charts

This consists of three-letter charts printed on white cardboard, having a contrast of 97%, 7%, and 4%. It is tested at 3 meters with eight letters in each line. The patient is told to start from the top of the row until they can no longer identify the letter on the line. The letter size goes on reducing, and the contrast remains the same. Regan, in 1988 said that this has a role in detecting early vision loss in glaucoma and diabetes.[24]

Spaeth Richman Contrast Sensitivity Test

Spaeth Richman contrast sensitivity test (SPARCS) is a computerized test to assess CS. It can be accessed online with good internet access. The patient being tested is provided an identification number and instructed regarding the test. The test can be performed in 5 to 10 minutes for each eye and measures the central and peripheral CS. This test can also be employed in illiterate patients as this is based on gratings.[25]

Personnel

The paramedical staff, the mid-level ophthalmic personnel, the optometrist, or the refractionist trained in CS assessment can help test the subjects. CS is measured by log contrast. The lesser the score, the poorer the CS.

• A score of 2 means normal CS
• A score less than 1.5 indicates visual impairment
• A score of less than 1 indicates a visual disability[26]

Types of Contrast Sensitivity

Spatial

It refers to obtaining the striped pattern, which is detected at various levels of contrast and spatial frequencies. The spatial frequency is measured with the help of (Arden gratings) in which the patient is shown sine wave grating having parallel light and dark bands, and they are asked to tell the minimum level of contrast at which the bars are seen. The width of the bars is essential as it defines the spatial frequency, which denotes pairs of dark and light bars that subtend an angle of 1 degree at the eye. If the spatial frequency is high, it denotes narrow bars, and if it is low, it indicates wide bars.[27]

Temporal

Here CS is a function of time rather than spatial position. It is a time-oriented (temporal) processing in the visual system by exhibiting a uniform target field modulation sinusoidal in time.[28]

Technique or Treatment

Snellen’s Test Types Versus Contrast Sensitivity

The visual acuity tested by Snellen’s test types is of very high contrast, but in a real-world scenario, the patient may be visually handicapped due to loss of CS. In conditions with low contrast, the visual acuity becomes reduced, and the quality of vision is also reduced. The CS is considered a better predictor of visual acuity as it provides information not provided by the visual acuity alone.[29]

Contrast Sensitivity Curve

The contrast sensitivity can also be plotted as a curve, with the x-axis having a spatial frequency and the y-axis having contrast sensitivity.[30]

Contrast Sensitivity Function

Manos and Sakrison first proposed this. The graph of CS over spatial frequency range gives CS function. CS is a function of spatial frequency, denoted in cycles per degree; this is a subjective measurement of a subject’s ability to detect a low contrast stimulus, usually vertical stripes of reducing shades of black to grey. This measurement should give a more accurate representation of the eye’s visual function. The peak value range from 1 to 8 cycles/ degree, and the high frequencies are not distinguished due to a limited number of photoreceptors. Neural and optical mechanisms limit the higher spatial CS.[31]

Modulation Transfer Function

This indicates the ability of the optical system to transform minute details (spatial frequencies) from object to image. The units are denoted by the ratio of image contrast over object contrast, a function of spatial frequency. MTF is the optical component of the CS function.[32]

Gratings

There are a series of pairs of light and dark bars in this. One adjacent pair make up one cycle. The thickness is denoted by spatial frequency in cycle per degree of the visual angle of the eye. Many gratings can be accommodated within a degree of visual angle. They are broader with low spatial frequencies. The spatial phase describes the position. A change in spatial phase by 180 degrees denotes that the grating is displaced by a half cycle.[33]

Sine Wave Gratings

These lack a sharp outline, and sinusoidal grating is preferred because they are free from defocus, light scatter, aberration and diffraction. It affects only the contrast and spatial positions.[34]

Square Wave Gratings

These grating have sharp edges.[35]

Clinical Significance

Patients with CS loss can have glare and halos during the night and may need extra illumination to read an object. After straining, the eyes may have congestion; the eyes may not be able to see clothes, dishes, and small objects. The patients may not be able to notice facial gestures.

The clinical significance of CS is governed by the factors which affect the CS.

• Refractive errors - Low spatial frequencies are not affected; only high spatial frequencies are affected by refractive error.
• Cataract - Early cataract reduces CS for low spatial frequencies; this is not related to visual acuity. Brannan et al., in their analysis, found that CS reduced from 37% to 19% within six months of performing the cataract surgery.
• Age - The CS decreases with age. After the second decade, it has been documented that CS scores decline by 10% each decade. The average decline is equal to the CS range within the normal population at any age.
• Systemic disease - Contrast sensitivity is reduced in several systemic diseases listed above.[36]

Diagnostic Applications

Contrast sensitivity is labeled as a diagnostic indicator of visual function. It is reduced in various ocular and systemic conditions, as listed above. It is also known that CS (modulation transfer function) is a complete statement for the relationship between spatial frequency and visual field intricacies, the contrast needed for resolution, and stimulus luminance. The use of yellow-tinted glasses helps in improving CS. These glasses increase the sharpness perception during the daytime. These should be avoided while driving to prevent glare, and anti-reflective coating glasses can be used to reduce glare. Yellow or copper tinted lenses help improve contrast and working environment in low ambient light.[37]

Enhancing Healthcare Team Outcomes

When patients present to the outpatient department with diminished or blurred vision, they should be examined in detail to pinpoint the pathology. A reduced CS should be suspected if the patient complains of reduced visual acuity despite 20/20 vision. The optometrist plays a crucial role in assessing the contrast sensitivity of the subject with the available charts. The examining ophthalmology should also keep a high suspicion of reduced CS in these cases. In case of reduced CS, the patient should be counseled well regarding the ocular condition and all symptoms the patient can have during nighttime. The patient should be explained the prognosis based on the ocular pathology.[38]

Nursing, Allied Health, and Interprofessional Team Interventions

The nursing and allied staff help examine the patient with CS charts, explaining the prognosis and nature of underlying pathology.