The selection and design of vision assessment tests must take into account the expected level of function but at the same time cover a wide level of function to be suitable for pre-implant and post-implant assessment. As noted, prosthetic vision in the near future is likely to be fairly crude, but it will exceed the visual function with which the subject presents. For preoperative assessment, tests should be designed for those with extremely limited vision. However, the prostheses may result in large improvements, so measures must be able to assess the higher function as well. There is no consensus on which aspects to measure, but the battery must be sufficiently brief to assure patient comfort. Tests that are inexpensive and easy to administer are most likely to gain wide acceptance.

20.4.1  Candidate Measures

Light perception is the ability to tell whether one is in light or darkness, or more specifically, whether the visual field is light or dark. This level of vision is well below the target for outcomes in implant trials. However, measurement of light perception, is invaluable in pre-operative testing, particularly for determining (1) whether the visual pathway proximal to the implant is functional and (2) and to define minimal light levels for ­further testing. Dagnelie [25] has suggested measuring light perception (detection) as a threshold task; at what level can the individual first detect light (threshold) and out to what range? The idea behind this test is that, considering the vast range of light levels over which the visually normal person can function, determining the operating range may have a greater ability to classify severe vision loss than almost any other measure. Knowledge whether an individual has light perception prior to implantation is necessary for comparison to post-implant visual function gain. However, vision at the level of light perception is of very limited value in terms of the recipient’s daily function.

Light projection (or localization) is the ability to indicate from which direction a light originates. Assuming clear ocular media, light projection in the eye follows the laws of geometric optics, so that the retinal location of the illuminated area (nasal vs. temporal, up vs. down) is predictable. However, in implant recipients who have no vision outside the areas driven by the implant, perception of location is likely to be driven by prosthesis location with respect to the fovea or fixation location, at least initially. The recipient will, presumably, learn to remap visual space based on the implant’s location and function and to “fit” visual space into this area.5

5 A difficult aspect of any vision task using a target of limited spatial extent will be locating the target in visual space. This is most difficult for devices with which field of view does not follow eye movements, which are currently the most common. Individuals can learn to suppress eye movements in favor of head movements, but this is difficult and perhaps inadequate. Some prosthesis developers have addressed this problem by yoking the external camera or its image to eye movements or implanting the photo-detector/camera in the eye (e.g., [22, 37]). Though prostheses are placed to tap into foveal processes, the (retinal) prosthesis may be displaced from the fovea. Prosthesis wearers must unlearn the tendency to move the eyes to foveate. For prostheses in which the receiving and stimulating elements are co-located in the parafovea, the recipient may need eccentric viewing training, currently used for patients with age-related macular degeneration with absolute scotomas that involve the fovea [72].

­resolution improves with contrast. As noted earlier, visual acuity has for some time been the predominant outcome measure in intervention studies, and is the primary visual descriptor of participants in vision studies, and of populations [24]. At the coarse end, visual acuity is clinically described in terms of whether or not the individual can detect hand motion (HM) or count fingers (CF) at a specified test distance.

It is preferable, of course, to use targets with more precisely controlled and defined specifications. Gratings and optotypes are the most common types of acuity targets. Grating targets measure the minimum separable resolution whereas optotype acuity is a form of recognition acuity. The relative value of measuring grating and optotype acuity is a matter of some debate, and also a matter of circumstances. Grating acuity can be quantitatively related to optotype acuity in visually normal individuals, but this association breaks down when disease is present (e.g., [32, 36, 110]). When using grating stimuli, aliasing associated with under-sampling or other distortions associated with abnormal retina or the prosthesis a concern and can lead to an over-estimate of resolution. Aliasing is the situation in which a high spatial frequency target is miss-perceived as a stimulus of lower spatial frequency or a distorted grating [17, 94, 95, 113, 114].

Optotype acuity has won out in clinical settings. Common optotype targets are simple shapes, letters, numbers, the tumbling E (formerly “illiterate E”), and Landolt rings (also called Landolt C’s). The smallest optotype target size (in terms of visual angle) that the patient can identify is determined [98]. For the tumbling E targets, the observer’s task is to indicate in which of the four cardinal directions direction the “tines” are pointing. The Landolt C target is a circle with a gap in it. The gap is presented in four or eight locations (the four cardinal plus the four obliques) and the observer’s task is to indicate the location of the gap in each ring.

Provided that the subject is required to continue to attempt to identify or guess until some criterion is reached (e.g., three out of five optotypes are identified incorrectly), acuity measures are criterion and bias free. It is recommended that acuity be scored letter-by-letter rather than line by line [15].

Though standard, commercially available letter charts such as the ETDRS acuity chart [34] or the Bailey-Lovie Chart [16] were not designed to measure extremely poor acuity, the lower end of their range can be extended into the range of interest by simply decreasing the test distance. At the standard (20 ft. or 6 m) test distance, the largest letters on the Bailey-Lovie Chart correspond to an acuity of 20/125; at a 10 ft. test distance 20/250, and down to 20/2,500 at 1 ft.

Tests specifically designed to measure acuity for low vision are discussed in a later section. An important gain from using optotypes to measure acuity is that the presence of measurable optotype acuity provides evidence of form vision capability.

20.4.1.1  Contrast Sensitivity (Contrast Detection)

Contrast (of a grating; Peak-to-peak contrast or Michelson contrast) is defined as

Cm = (Lmax −Lmin ) / (Lmax +Lmin ).

Contrast can vary from 0 to 1, and is more often specified as a percentage (0–100%). Contrast sensitivity is the inverse of contrast at threshold. Within the linear systems approach to vision, a description of an individual’s contrast sensitivity function (CSF, i.e., the minimum contrast required to see a grating, measured as a function of spatial frequency, or bar width) provides a means of knowing the visual system’s response to any stimulus defined by luminance contrast. For practical purposes, though, contrast sensitivity testing is typically limited to a single large (relative to acuity) target size, specifying one point on the CSF near the peak of the CSF.

In clinical settings, optotype measures are more commonly used than grating targets [9]. Unlike grating stimuli, optotype targets, such as those on the PelliRobson Chart [76] are specified in Weber contrast, which is defined as

Cw = (Lmax −Lmin ) / Lmax .

Note that the two measures are the same if the mean grating luminance is Lmax/2. Contrast sensitivity deficits are present in RP patients, even in those with normal or near-normal acuity [4, 7].

Despite a strong correlation between contrast sensitivity and visual acuity, one cannot predict one from another on an individual basis [44], and therefore, both should be measured. An individual with very poor acuity, but fairly good contrast sensitivity and fields of reasonable size will have no trouble navigating and moving through the environment but probably will not be able to read well or at all. The converse is also true. An individual with a very small visual field, good contrast sensitivity and good acuity will have great difficulty moving about the world or finding targets; however, once the targets are “found” (are placed within the functional field) they will have no trouble identifying the target or reading print.

Reports suggest that contrast sensitivity better predicts performance than other measures (e.g., acuity). Associations have been reported between contrast sensitivity and reading performance [57, 111], ambulation mobility [38, 45, 55, 66, 99], driving [115, 116], face recognition [75, 109], and tasks of daily living [79, 80, 109].

20.4.1.2  Contrast Discrimination

Most natural images contain both high and low contrast. In scenes, features to be detected are frequently observed in the presence of other supra-threshold (visible) background structures. Detection of such features requires contrast discrimination, which is necessary for the subsequent process of object recognition. Contrast discrimination is impaired in RP patients, even those who have good acuity to moderately reduced contrast sensitivity [6].

No simple chart or other test of contrast discrimination is available though, in principle, one could be developed fairly easily. Such a chart might consist of sets of stimuli each with at least two elements ranging in contrast, with the patient’s task being, for example to identify the stimulus with the highest contrast, with the