Visual Pathway Processing When Light Strikes the Periphery of an OFF-Center Photoreceptor
When light strikes the periphery of an OFF-center photoreceptor, an ON-center ganglion cell is activated (depolarized). 1
Photoreceptor Response to Light
- When light strikes a photoreceptor, it becomes hyperpolarized (not depolarized), causing a decrease in glutamate release 1
- This hyperpolarization occurs whether the light strikes the center or periphery of the receptive field
- The term "OFF-center photoreceptor" refers to the receptive field organization of the pathway, not the photoreceptor itself
Bipolar Cell Response
- ON-center bipolar cells have metabotropic glutamate receptors that are inhibited by glutamate 1
- When light causes photoreceptors to decrease glutamate release, ON-center bipolar cells are released from inhibition and become depolarized (activated) 1, 2
- When light strikes the periphery of an OFF-center receptive field, it activates the surrounding ON-center bipolar cells in that region 3
Horizontal Cell Function
- Horizontal cells provide inhibitory (not excitatory) feedback to surrounding photoreceptors 1
- They create center-surround antagonism in the visual field
- Horizontal cells do not send excitatory feedback to cells surrounding the center cone 1
Ganglion Cell Response
- ON-center ganglion cells receive excitatory input from ON-center bipolar cells 1
- When light strikes the periphery of an OFF-center receptive field, it activates ON-center bipolar cells in the surround, which then excite ON-center ganglion cells 3, 2
- OFF-center ganglion cells would not be hyperpolarized (deactivated) in this scenario, as they respond to decrements in light intensity in their center 1, 2
Center-Surround Organization
- The visual system is organized with antagonistic center-surround receptive fields
- In an OFF-center pathway, light in the center causes hyperpolarization of OFF-center bipolar cells and inhibition of OFF-center ganglion cells
- Light in the periphery (surround) of an OFF-center receptive field activates ON-center pathways, which can inhibit the OFF-center response through lateral connections 3, 4
Common Misconceptions
- The center cone would not become depolarized when light strikes it; photoreceptors always hyperpolarize in response to light 1
- Horizontal cells provide inhibitory, not excitatory feedback 1
- The term "OFF-center photoreceptor" is somewhat misleading, as the center-surround organization emerges at the bipolar cell level, not at the photoreceptor level 5