Normalization Through Lateral Inhibition & Enhancing Contrast

How does normalization let us view light differences?

• we perceive the difference in luminance, rather than the absolute luminance (we discount the illuminant)

  • General principle: subtracting and dividing by average luminance

  • e.g. a tree outside is X brightness and the average brightness is M

    • we perceive \frac{X-M}M for the brightness of the tree

• normalized through lateral inhibition

How does normalization through lateral inhibition work? What does this do?

• active neurons suppress activity of neighbouring neurons

• enhances contrast and improves detection of edges and fine details

What is the receptive field?

• region on the retina in which stimuli influence neuron’s firing rate

What is the center-surround organization of ganglion cells?

• ganglion cells activated by either light in the center, or surrounding, of receptive field depending on the type of cell

ON-center/OFF-surround cells: activated by light in the center and inhibited by light in the surround of the receptive field

OFF-center/ON-surround cells: activated by light in the surround and inhibited by light in the center of receptive field

How are photoreceptors, bipolar cells, and ganglion cells organized?

• center-surround organized

• center photoreceptors are connected directly to bipolar cells

  • photoreceptors in the surround send signals through horizontal cells

• ON bipolar cells are excited by light in center (inhibited by light in surround)

• OFF bipolar cells excited by light in surround (inhibited by light in center)

How do photoreceptors behave in the light/dark?

In dark:

• in dark, photoreceptors constantly release glutamate (excitatory neurotransmitter)

• keeps depolarized → graded potential

  • increases likelihood of action potential

• photoreceptors in the surround release glutamate to horizontal cells which inhibit neighbouring photoreceptors (including photoreceptors in center)

  • photoreceptors in center are inhibited, reducing glutamate release

In light:

• light inhibits release of glutamate

  ↳ hyperpolarizes photoreceptor

What happens to bipolar cells when photoreceptors release glutamate?

• ON bipolar cells don’t like glutamate

  • glutamate → decreases firing

    • reduce firing in the dark

• OFF bipolar cells like glutamate

  • glutamate → increases firing

What happens when light falls on the center, but not surround for ON-center cell?

• light hyperpolarizes photoreceptors (-)

  • stops releasing glutamate

• ON-center bipolar cells→ depolarized (+)

• ON-center RGCs increase firing (+)

What happens when light falls on center, but not surround for OFF-center cell?

• OFF-center bipolar cells hyperpolarized (-)

  • OFF-center RGCs decrease firing (-)

What happens when light falls on surround but not center for ON-center cell?

• absence of light depolarizes photoreceptor (+) causing glutamate release

• ON-center bipolar cell is hyperpolarized(-)

• ON-center RGCs decrease firing (-)

What happens when light falls on surround but not center for OFF-center cell?

• OFF-center bipolar cell is depolarized (+)

• OFF-center RGCs increase firing (+)

What are mach bands/the mach effect?

• mach effect is optical illusion that makes us see more contrast than there actually is due to lateral inhibition

• bars being closer together causes contrast to be emphasized due to more/less inhibition based on where receptive field is on bars

What is the hermann grid?

• black and white checkered box, but in the intersection of white lines, “grey dot” appears, but disappears when focused on

• appears because spots in between the black squares (ganglion 3) receives more inhibition and appears darker

  • disappears when focused on because receptive fields are smaller in the fovea