How many rods need to be activated to elicit a visual response?

How many rods need to be activated to elicit a visual response?

• A. 1
• B. 10
• C. 100
• D. 1000

Answer: (B)

Rods become useful in very dim light because their signals are integrated (converge) onto downstream neurons. In the retina, many rods feed into a relatively small number of bipolar and then ganglion cells. Because of this convergence, a single rod’s response is usually too weak by itself to push the next neuron above its firing threshold and produce a detectable signal. When a small group of rods is activated together, their inputs summate, crossing the threshold and yielding a perceptible visual response. This pooling explains why vision in low light relies on multiple rods working together rather than a single rod, even though individual rods can respond to photons.

Rods become useful in very dim light because their signals are integrated (converge) onto downstream neurons. In the retina, many rods feed into a relatively small number of bipolar and then ganglion cells. Because of this convergence, a single rod’s response is usually too weak by itself to push the next neuron above its firing threshold and produce a detectable signal. When a small group of rods is activated together, their inputs summate, crossing the threshold and yielding a perceptible visual response. This pooling explains why vision in low light relies on multiple rods working together rather than a single rod, even though individual rods can respond to photons.