Fig. 1

Presentation of scotopically matched scenes on an LED-lit wall. (a) Left, LED-lit wall and door device used for the three-alternative forced-choice design. A mix of light intensities for the “door” (three stripes at right, center or left) and “wall” (elsewhere) is calculated to be visible to cone-based vision but unable to be differentiated with rod-based scotopic vision. Right, spectral content of the LED lights compared to the spectral sensitivities of the rod (scotopic, V’_ʎ) and cone (photopic, V_ʎ) vision. All curves are normalized to unity at maximum. The relative intensities of the three lights are set such that they have the same effectiveness for rods (lower left) but substantially different effectiveness when perceived by cones (lower right). These differences permit differentiation of door and wall, and enhance visual orientation performance. (b) Rod vision is expected to perceive the scene as uniform stripes with no features (upper panels), and cone vision is expected to perceive the “door” as green stripes on a blue background (lower panels). (c) Appearance of unmatched control trials that should be visible to either rod or cone systems. (d) Departures of more than approximately 1.5 dB (0.15 log) from the scotopically matched mix of lights result in successful door-wall differentiation in normal subjects. (e) Representative examples of scotopically-matched test trials interleaved with unmatched control trials. For a three-alternative forced-choice setup, a subject with rod vision but no cone vision would be expected to get 33 % correct for matched-stimuli on average, whereas a subject using cone vision would be expected to get 100 % correct. Unmatched control presentations with door dimmer (1) or brighter (2) than wall are used to determine whether the subject has any vision (rod or cone or both) under the testing conditions