13b: Center-Surround Antagonism

What is simultaneous contrast?

Says that the apparent brightness of an object is inversely related to the luminance of its surroundings

Where are horizontall cells found?

Outer margin of the inner nuclear layer

What is unique about the structure of HCs?

Don’t have the typical differentiated pre-synaptic terminals vs post-synaptic dendrites but instead either ends of the HCs (called neurites) act as pre-and-post synaptic

Do HCs depolarize or hyperpolarize to light?

Hyperpolarize always

What glutamate receptors do HCs have? Are they sign-conserving or sign-inverting?

Ionotropic kainate/AMPA glutamate receptors

This makes them sign conserving just like OFF bipolars

What is a receptive field?

The region of space over which the cell is capable of responding to a light or dark stimulus

What limits the PR receptive field?

Width of the outer segment

Do HCs have a large/med/small receptive field? Why?

Extremely large d/t gap junctions

Will an ON bipolar cell depolarize or hyperpolarize when a spot of light hits the center of its receptive field?

Depolarize

Will an ON bipolar cell depolarize or hyperpolarize when a spot of light hits the edge (off-center) of its receptive field?

Hyperpolarize

Will an OFF bipolar cell depolarize or hyperpolarize when a spot of light hits the center of its receptive field?

Hyperpolarize

Will an OFF bipolar cell depolarize or hyperpolarize when a spot of light hits the edge (off-center) of its receptive field?

Depolarize

If both center and surround are stimulated at the same time, the response will be…

Less than the responses to each of the separate stimuli (cancel each other out!)

Horizontal cells play a key role in…

Center-surround antagonism (allow for contrast sensitivity)

What is the main action of HCs?

They release inhibitory NTs back on PRs for lateral inhibition

What NTs do HCs use?

GABA-like

What is the point of lateral inhibition?

It enhances signal detection because it reduces the signaling from “spillover” stimulation

How do HCs interact with PRs in the dark?

In the dark HCs are depolarized so release their inhibitory NT → PRs become slightly more hyperpolarized (but overall still depolarized)

Say you have 2 PRs (“A” and “B”) connected by a HC and you shine a light on PR A. How will the HC affect the signaling cascade?

PR A hyperpolarize in response to light →

HC hyperpolarize (sign conserving) →

HC stops releasing its inhibitory NT onto PR B →

PR B depolarizes even more →

The ON and OFF bipolar cells receiving input from PR B will receive a larger signal →

Lateral inhibition

Near the fovea, EVERY cone connects to what cells?

2 midget bipolars — one ON and one OFF

What is the point of lateral inhibition on our vision/the image

It hones the image (by enhancing the edges that separate highlights and shadows)

What are Mach bands?

The contrast bands going from dark to light where each band looks slightly lighter on one edge and darker on the other edge

What mechanism drives the Mach band illusion?

Center-surround antagonism

On a cellular level, the relative difference between the center luminance and surround luminance of a receptive field affects…

The bipolar cell response