In the Hecht-Schlaer-Pirenne 1942 experiment, which lighting system and photoreceptors were investigated?

• A. Scotopic/rods
• B. Photopic/cones
• C. Mesopic/rods and cones
• D. Ultraviolet/photoreceptors

Answer: (A)

In low-light conditions, vision relies mainly on rods, which is called scotopic vision. The Hecht-Schlaer-Pirenne experiment was designed to probe how sensitive human vision is when the eye is dark-adapted, i.e., under dim illumination, to see which photoreceptors are driving detection. They used a very dim monochromatic test light presented to a dark-adapted observer and measured the threshold for detection. The findings showed that detection thresholds and the time course of sensitivity recovery fit the behavior of the rod system, not cones, indicating rod-mediated (scotopic) vision in those conditions. This work helped establish how rods support vision in the dark and how their sensitivity changes with adaptation, including the role of rhodopsin regeneration.

So the lighting system studied was scotopic (low-light), and the photoreceptors involved were rods. The other options describe conditions and receptors that are not what this experiment examined: bright-light (photopic) conditions involve cones, a mix of rods and cones (mesopic) would require intermediate lighting, and ultraviolet photoreceptors aren’t part of normal human vision in the tested range.