A Luminance Noise Approach to Target Post-Receptor Visual Pathways; Application to Optic Nerve Disease

Contrast sensitivity (CS), which is the ability to detect small differences in luminance, is a fundamental task of the visual system and loss of CS is an important marker for many disease states. Although clinically-applicable methods are available to assess CS, there are limitations to these standard methods. For example, they are unable to determine which of the two primary post-receptor visual pathways, the magnocellular (MC) and parvocellular (PC) pathways, mediates CS for a given task. Furthermore, standard clinical measures of CS cannot provide insight into mechanisms underlying CS abnormalities. In this thesis, a test of CS using letter stimuli will be developed and applied to address these issues. To permit the development of this new CS test, the following series of questions will first be resolved and an optimized CS test will then be applied to patients who have optic nerve dysfunction to better understand their CS deficits. Question 1: Which optotypes should be used in tests of letter CS? Letters are relatively complex targets that are not all equally identifiable. Aim 1 will define the optimal letter set for use in CS testing. Question 2: Can luminance noise, defined as decrements and increments of luminance added to a stimulus (e.g. ‘tv snow’), be used to target the MC and PC pathways? Although there is preliminary evidence that the temporal characteristics of noise can be manipulated to target these post receptor pathways selectively, the specific characteristics of noise needed to do so have not been fully defined. Aim 2 will determine the spatial and temporal characteristics of noise capable of targeting the MC and PC pathways. Question 3: What is the optimal letter size for use in targeting the MC and PC pathways using static and dynamic luminance noise? Aim 3 will determine the range of letter sizes that can be used to target selectively the MC and PC pathways. Question 4: What is the optimal letter duration for use in targeting the MC and PC pathways in noise-based CS tests? Aim 4 will evaluate the ability of static and dynamic noise to target the MC and PC pathways across a series of letter exposure durations. Once the test is developed and finalized, it will be applied in Aim 5 to a small sample of patients who have optic neuritis (ON); the results from the patients will be compared to visually-normal control subjects. Thus, this dissertation will develop a clinically applicable, noise-based test of letter CS that can independently assess function within the MC and PC pathways. This will permit evaluating potential deficits within the MC and PC pathways, as well as provide preliminary insight into potential factors that limit CS in patients who have ON.