Visual Psychophysics in Diabetic Retinopathy

INTRODUCTION Irreversible and severe visual loss may represent the end of long lasting diabetic retin-opathy. The progression of visual impairment and the quantification of final residual visual function are currently determined by means of diagnostic tests which rely on the physiological and mathematical principles of psychophysics. The best known among these tests is the quantification of visual acuity: a classic visual function psychophysical test. Visual psychophysical tests are the cornerstone of visual function investigation, and any physical or pharmacological therapy for the treatment of diabetic retinopathy still has the maintenance (or improvement) of visual function as primary endpoint. More recently, subtle and precocious neurosensory visual abnormalities have been quantified in diabetic patients in order to detect early visual dysfunction, even before the onset of clinically detectable retinopathy. The aim of these investigations is to try to identify among diabetic subjects a population at higher risk of developing vision-threatening retinopathy [ 1 ]. Psychophysics is a science which developed as a way to measure the internal sensory and perceptual responses to external stimuli [ 2 ]. Psychophysical visual function testing 70 Midena and Vujosevic may reflect the neural activity of the whole visual pathway, but it is known that these tests are valuable clinical indicators of retinal function derangements induced by the metabolic changes secondary to diabetes mellitus. In fact, in diabetic patients, impaired vision in dim light and difficulties in recognizing the contour of objects in low-contrast conditions are common complaints even with good visual acuity and full visual fields [ 3 ]. Moreover, health-related quality of life can become affected in diabetics even prior to vision loss due to anxiety about the future and emotional reaction to diagnosis and treatment of retinopathy [ 4 ]. Visual acuity is still considered the gold standard in clinical practice of vision testing, but it does not entirely reflect functional vision. Functional vision describes the impact of sight on quality of life that represents the patient's point of view [ 5, 6 ]. This approach is better quantified using available psychophysical tests (visual acuity, color vision, contrast sensitivity, macular recovery function, perimetry, and microperimetry). The quantification of visual acuity (VA) is the best known and most widely used test for assessing the integrity of the visual function in clinical settings. It represents the ability to discriminate, at high contrast (black symbols/letters on a white background), two separated stimuli. The Snellen chart is the …