20.4.1.4  Depth Perception

Stereopsis is the perception of depth produced by binocular retinal disparity. Stereopsis subserves fine depth discriminations for near objects. Prosthesis recipients will not have stereopsis. These individuals will necessarily rely on monocular depth cues including those mentioned previously (occlusion and motion parallax), as well as lighting and shadows, linear perspective, texture gradient, height in field, to name but a few.

Color is one of the most compelling aspects of vision, adding beauty to our visual experience as well as aiding in detection and identification of objects. Some reports indicate that induced phosphenes are of many colors [51, but cf. 31]. Nonetheless, due to the complexity of color-conferring circuitry and crudeness of available stimulation methods, veridical color vision will not be afforded by prostheses in the foreseeable future.

20.4.2  Tests Used in Prosthesis Trials

In the absence of a well-described, validated set of tests, measuring visual function has been and still is up to the ingenuity of those working on the projects. Dobelle (2000) measured visual acuity in three recipients of their 64-element cortical implant and reported acuity and visual field size for one [31]. For this subject acuity measured using tumbling E’s and Landolt C’s show excellent agreement (20/1,200). Second Sight Medical Products (Sylmar, CA) has published a report on visual function in three recipients implanted with their 16-electrode prosthesis [117] based on three to four alternative tests (locate and count objects such as common household items, discriminate among those items, determine the orientation of a large L, and identify the direction of object motion). A grating orientation discrimination task was used to measure acuity. Kiser et al. used standard static (HFA to test central 30 degrees) and kinetic perimetry (Goldmann to test far periphery) methods to test vision of eight Artificial Silicon Retina implant (Optobionics Corp.) [53]. Yanai [118] found that, with repeated testing, three implanted subjects could discriminate between plate, knife or cup against a dark background.

20.4.3  Tests that Have Been Designed for Use with Prostheses

A few laboratories have developed new tests specifically for assessing prosthesis recipients, but have not yet tested recipients with them. In many instances the devices have been used in the context of simulated prosthetic (pixelized) vision (e.g., [26, 27, 87, 96]), but have not been used to assess prosthesis candidates or recipients. In addition, simulations in subjects with better vision have been used to estimate how well a person may perform using a prosthetic, with the idea that the pixels correspond to electrodes [25, 106].