The main idea is how the eye’s sensitivity to colors changes as lighting drops. In bright conditions, vision is dominated by cones and the peak luminance sensitivity sits toward longer wavelengths (green-yellow). As light fades, rods take over and their spectral sensitivity peaks at shorter wavelengths (blue-green). This makes the eye relatively more sensitive to shorter wavelengths in dim light, so colors like blue-green appear brighter compared to red objects. That’s why the Purkinje shift describes a shift toward shorter wavelengths under scotopic (low-light) conditions. The other statements don’t fit because they’d imply a shift to longer wavelengths, no shift at all, or a vague change in brightness rather than a change in spectral sensitivity.

The idea being tested is that cones have three different photopigments, each tuned to a different part of the spectrum, which together provide color vision. In humans, cones carry cyanolabe (blue-sensitive pigment), chlorolabe (green-sensitive), and erythrolabe (red-sensitive). This trio lets the visual system compare the signals from the blue, green, and red channels to distinguish colors. Rods, on the other hand, use a different pigment called rhodopsin and are responsible for vision in low light, not color vision. So the statement that cones contain cyanolabe, chlorolabe, and erythrolabe best describes cone photopigment distribution.

Convergence in the retina shapes how much light from many photoreceptors is pooled into a single bipolar cell. Rods have high convergence: many rods feed into relatively few rod-bipolar cells. This pooling boosts sensitivity in dim light, but it reduces spatial detail. Cones, especially in the fovea, connect more sparsely, often near one-to-one with bipolar cells, preserving high acuity and color discrimination. So the photoreceptor type that tends to have more photoreceptors connected to a single post-synaptic bipolar cell is rods.

Under photopic vision, cones provide high-acuity, low-convergence signaling. Each cone’s input is carried forward through pathways that preserve spatial detail, so there is nearly a one-to-one relationship between active cones and the ganglion cells that respond to them, especially in the central retina. A 5-quanta flash is sufficient to activate a cone, and that cone’s signal can drive its corresponding ganglion cell to threshold. If two flashes illuminate two separate cones, two distinct cone-to-ganglion-cell pathways reach threshold, producing two active ganglion cells. So, the number of ganglion cells reaching threshold matches the number of distinct cones activated by the flashes—two in this case.