Mesopic mechanism is explained from three viewpoints below. First, three pathways from the retina to the LGN (lateral geniculate nucleus), MC-pathway, PC-pathway, and KC-pathway, are described. The fact that rods share the common route with cones in the MC-pathway in a sophisticated manner is shown, and that might be a cause of instability of mesopic vision. Next, variation of dark adaptation curve is indicated. Although dark adaptation curve is known to show double phases, single phase curve appears if the experimental condition is sensitive only to cones or rods. Thus the shape of dark adaptation curve corresponds to the functional light level and retinal distribution of cones and rods. Finally, the scotopic and photopic spectral sensitivity functions are compared. Scotopic spectral sensitivity obtained as the radiant energy at the cornea agrees to the CIE V (λ), and that at the retina after taking the absorption of ocular media and transferred to quantum unit matchs well with the absorption spectra of frog visual pigment, indicating simplicity of scotopic mechanism conveyed by rods only.
