Brightness Perception, Convergence

What do photoreceptors contain in their outer segments that is crucial for light absorption?

Discs with visual pigments.

What is the pigment in the outer segments of rods called?

Rhodopsin.

What are the two molecules that compose rhodopsin?

Retinal and scotopsin.

What process occurs when a photon of light hits rhodopsin?

Isomerization.

What happens to retinal and scotopsin in the absence of light?

They return to their original bent shape through regeneration.

What is brightness a perceptual correlate of?

Luminance.

How is luminance measured?

By counting the number of photons.

What does light/dark adaptation refer to?

A change in brightness sensitivity to accommodate the current level of illumination.

What is the fast phase of dark adaptation characterized by?

A rapid increase in sensitivity within the first 0-10 minutes.

At what time during the fast phase does the sensitivity plateau?

At about 10 minutes.

How much more light-sensitive are the eyes after 10 minutes in darkness?

100x more light-sensitive than the start.

What occurs during the slow phase of dark adaptation?

A slow but steady increase in sensitivity continuing after 10 minutes.

How much more light-sensitive are the eyes after 20 minutes of dark adaptation?

100,000x more light-sensitive.

What hypothesis did Rushton propose regarding darkness adaptation?

The fast-phase kink in the function is caused by different regeneration rates of cones and rods.

What characterizes the cone-only darkness adaptation test?

It shows the fast phase of darkness adaptation.

What is an example of a test subject used in the rod-only darkness adaptation?

Rod monochromats.

What is the conclusion of Rushton’s study on dark adaptation?

Dark adaptation is mediated by both cones and rods in two phases.

What is the field of neurocomputation concerned with?

How neuron systems interact to produce behavior.

What are the two types of interactions involved in neurocomputation?

Spatial interactions and temporal interactions.

What does convergence in neurobiology refer to?

Multiple neurons sending signals to a single neuron.

What is an example of convergence related to rods?

Many rods being attached to one ganglion cell.

What is the threshold of ganglion cells determined by?

The number of photons collected by photoreceptors.

How do cone ganglion cells differ from rod ganglion cells in terms of wiring?

Cones have 1-1 wiring to ganglion cells, whereas rods do not.

What are receptive fields?

Regions of the visual field that a given cell 'sees'.

How are receptive fields studied experimentally?

By inserting an electrode into a brain cell and moving a stimulus around.

What happens when a stimulus is outside a ganglion cell's receptive field?

The neuron fires at baseline.

What is the optimal stimulation for maximum firing in a receptive field center?

Only the center stimulated results in faster firing rates.

What is the relationship between neural convergence and receptive field size?

Greater neural convergence results in larger receptive fields.

Which ganglion cells have larger receptive fields, those connected to rods or cones?

Ganglion cells connected to rods.

What is the organization of receptive fields?

Center-surround antagonistic organization.

What firing rate results from stimulating only the surround of a receptive field?

Slower firing rate.

What does it mean if the entire receptive field is stimulated?

The neuron fires at baseline firing rate.