More than 20/20: the contrast sensitivity function

Contrast sensitivity assesses the ability to detect spatial or temporal contrast (e.g., achromatic luminance difference between areas or over time), which is one of the most fundamental functions of the human visual system. Many of our daily-life activities rely on this ability, such as finding an object, seeing stairs, noticing a moving car, driving, etc. The contrast sensitivity function (CSF) is the function to depict contrast sensitivity across a range of low to high spatial frequencies. The CSF provides comprehensive information about the contrast processing ability of the visual system on different sizes of objects. Contrast sensitivity deficits have been observed in older adults (1,2) and in patients with ophthalmic conditions such as amblyopia (3,4), glaucoma (5-7), diabetic retinopathy (8,9), multiple sclerosis (10), etc. It is a better predictor of daily visual performance and it correlates better with progression of ophthalmic conditions than the usual high-contrast visual acuity measurement does (11-13).

Despite its importance and relevancy, it is not yet a common clinical practice to measure contrast sensitivity, unlike the visual acuity measurement. A few barriers may have prevented a wide use of contrast sensitivity in clinic. First, although it is most informative to obtain the CSF rather than just the contrast sensitivity of a narrow range of spatial frequencies, it is time consuming to measure the whole CSF. As mentioned in the work by Rosa and Aleci [2022], there are clinical contrast sensitivity measurement tools available to obtain contrast sensitivity for only a narrow range of spatial frequencies, which is more feasible in clinic (14). However, the results from a narrow range of spatial frequencies often do not provide enough information about the patient’s visual system and visual function. In addition, different ophthalmic conditions affect contrast sensitivity at different spatial frequency ranges and it is sometimes on an individual basis. It would be more helpful to obtain the whole CSF, in which case more efficient clinical tools would be needed. Second, clinical tools with high reliability and repeatability are still lacking (15). Third, the concepts of contrast threshold, contrast sensitivity, and the CSF, are found to be not easy to understand as compared to the concept of visual acuity, and not easy to explain to patients. More patient and clinician education may be needed in order for contrast sensitivity and the CSF to be measured and interpreted accurately. The work by Rosa and Aleci provides an overview and easy-to-understand summary of the concepts and several clinical psychophysical contrast sensitivity tests available on the market, which is educational and will be helpful for eye care professionals to read about. As suggested by the authors, indeed, assessing contrast sensitivity should be a more common practice in clinic.

It is also noted by Rosa and Aleci that all the currently available clinical contrast sensitivity assessment tools measure contrast sensitivity at an overall perceptual level (14). On the other hand, it is well known from anatomical and physiological studies that, the different visual pathways in the primate visual system have very distinct contrast processing characteristics (16,17). For example, the magnocellular (MC) pathway has high contrast gain, responses fast to contrast increases and shows response saturation at relatively low contrasts; whereas the parvocellular (PC) visual pathway shows linear and shallower increase of response to incremental contrast across a larger range of contrast. Pokorny and Smith [1997] developed a group of psychophysical paradigms based on physiological findings to measure contrast sensitivity at the level of the MC and PC visual pathways (18). Studies using these paradigms have reported that different ophthalmic conditions and aging have differential impacts on these two visual pathways (19-22). There are advantages to measure contrast sensitivity at the level of visual pathway than at an overall perceptual level under certain circumstances. In particular, it may be more helpful for disease diagnosis and management to measure contrast sensitivity at the visual pathway level for certain ophthalmic conditions, such as glaucoma, optic neuritis, amblyopia, etc.

As more research are on the way and more technologies are applied into the development of contrast sensitivity measurement tools (23-25), it is expected that better methodologies and clinical tools will be available in the near future for more effective and efficient measurement of contrast sensitivity and the CSF in clinical settings. Our society has a large aging population, a better assessment of this fundamental visual function will facilitate diagnosis and management of ophthalmic conditions, which will benefit not only individual patients but also the whole society collectively.

Acknowledgments

Funding: None.

Footnote

Provenance and Peer Review: This article was commissioned by the editorial office, Annals of Eye Science. The article did not undergo external peer review.

Conflicts of Interest: The author has completed the ICMJE uniform disclosure form (available at https://aes.amegroups.com/article/view/10.21037/aes-22-73/coif). The author has no conflicts of interest to declare.

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