difference­ between the contrasts among stimuli in stimulus sets decreasing down the chart. PC-based tests on the same principle would be more flexible.

20.4.1.3  Motion Perception

The ability to detect a luminance-defined, or color-defined target as moving and to judge motion speed and direction requires detection and localization over an extended retinal area, as well as interactions between neighboring areas and, intact temporal processing. Human visual motion occurs beyond the retina [12], so that intact temporal processing at earlier stages is crucial. The temporal sequence of retinal stimulation in a degenerated retina may become highly distorted, so that the signals reaching the cortex become ambiguous [25]. Temporal processing and displacement discrimination are abnormal in RP patients [3, 5]. Prosthesis processing delays and those from degenerating retina may favor slow stimuli of coarse spatial grain. Movement perception is likely to be greatly impaired in retinal degenerations [25].

In fact, perceived motion in the presence of a prosthesis is most like apparent motion, or sampled motion, which is the perception of smooth motion from sequential presentation of discrete stationary targets, in this case, electrode-generated phosphenes. The appropriate combinations of temporal and spatial interval characteristics for apparent motion have been worked out for normally sighted individuals [19]. However, the same relationships are unlikely to hold for prosthetic vision.

Bearing these considerations in mind, motion processing may thus be severely impaired in (potential) prosthesis recipients. The loss and rewiring of post-receptoral elements is an additional factor in the case of sub-retinal implants.

In patients with retinal degenerations who retain relatively good form vision, aspects of motion perception that have been assessed include judgments of motion displacement thresholds [5] and heading direction [101]. Minimum displacement thresholds are increased and maximum displacement thresholds decreased, greatly restricting the range of detectable motion in patients with RP, even when visual acuity showed only minor reductions (acuity of 20/40 or better) [5]. Patients with retinal degenerations with form vision have elevated thresholds, reduced maximum velocity and/or direction discrimination for two-dimensional (2D) motion. Yanai et al. [118], testing three RP patients implanted with a 16-electrode prosthesis, found that performance on a motion discrimination task was above chance only so long as the subjects were allowed to move their heads to scan.

2D motion perception imparts important information unrelated to perception of motion per sé. For example, motion parallax is an important depth cue, particularly for those lacking binocular vision, among whom are prosthesis recipients. Object (or person) motion facilitates detection, particularly in complex environments.

In terms of survival skills, the ability to judge motion toward or away from oneself may be more important. Our movement through an environment is guided by optic flow, perceived visual direction, and judgment of focus of expansion, among other things. Turano et al. (2005) reported a change in the ability of individuals with