34.18 Vision and Visual Perception

34.18.2 Distinguish between rod and cone cell functions and distribution in the retina.

Rods

• In dim light, we use our rods, which cannot work in bright light.
• Rods outnumber cones (120 million rods and about 6 million cones in each retina) and they amplify a light signal much more than cones.  
• Another retinal mechanism that helps us to see in dim light or to see a tiny amount of light in the dark is the convergence of rod cell signals onto other retinal neurons.
• Many rods, up to 150, contact onto one another, where the signals are pooled and reinforced, which increase’s the ability of the brain to detect a small amount of light.
• This convergence amplifies weak signals, but spatial resolution is lost because rod responses are averaged. That is, we cannot see fine detail using rods.
• In order for our eyes to make the transition to dim light, rods must adapt after being saturated with light in brighter conditions.
• The rods take about ten minutes to half adapt and 30 minutes to fully adapt to the dark.

Cones

• Our vision in bright or moderate light is completely mediated by cones, which provide color vision, black and white vision, and high acuity, the ability to discern fine detail. 
• Cones are spread throughout the retina but are especially concentrated in a central area called the macula. 
• At the center of the macula is the fovea, where only cones (no rods) are found, and these are densely packed. 
• When we want to read or inspect fine detail, we move our heads and eyes until the image of interest falls onto the fovea. 
• Because the fovea lacks rods, it is easier to see in dim light by looking to the side of an object instead of directly at it. 
• You can test this by looking to the side of a faint star so that its image falls on rods, rather than on the fovea where it probably will not register. 
• When you look directly at the faint star, it disappears.
• Cones mediate day vision and rods take over in dim light and at night. 
• Both rods and cones can operate at the same time under some conditions.  
• In dim or dark conditions, rods are most sensitive, but cones respond to stimuli that are sufficiently bright.