Visual System and Retinal Receptive Fields

Practice questions covering the mechanics of single-cell recordings, the organization and function of retinal ganglion cell receptive fields, and the parallel processing systems of the visual pathway.

What is the basic approach to studying vision using single-cell recordings?

Recording from the axons of individual ganglion cells in the optic nerve while an anesthetized animal is presented with various visual stimuli.

What is the difference between extracellular and intracellular recording?

Extracellular recording uses a microelectrode outside the neuron to measure action potentials, whereas intracellular recording involves puncturing the cell membrane to measure both action potentials and graded potentials ($EPSPs$ or $IPSPs$).

Which cells in the retina are the only ones to fire action potentials?

Retinal ganglion cells.

What is meant by the "baseline firing rate" of a neuron?

The constant rate at which neurons spike even when no visual stimuli are present.

Who were the researchers that discovered the properties of receptive fields in the visual system and won a Nobel Prize for their work?

Hubel and Wiesel.

What is the definition of a receptive field?

A region of the visual field in which the presence of a stimulus alters (increases or decreases) the firing rate of a neuron.

Describe the organization of a retinal ganglion cell's receptive field.

They possess circular receptive fields with a center-surround organization, divided into an inner center and a larger outer surround with opposing properties.

How does an "on-center" ganglion cell respond to light in its center versus its surround?

Light in the center increases the firing rate (excitation), while light in the surround decreases the firing rate (inhibition).

What happens when a stimulus covers the entire receptive field (both center and surround) of a ganglion cell?

The firing rate remains at baseline because the excitatory and inhibitory responses cancel each other out in a $1:1$ ratio.

What is "rebound excitation"?

A burst of action potentials that occurs immediately after a neuron's activity has been suppressed and the suppression is removed.

Why is the center-surround organization useful for vision?

It makes the cells excellent edge detectors, highlighting sharp contrasts or transitions from light to dark in the visual image.

What does the visual system care about more: absolute luminance or luminance contrast?

Luminance contrast, which is the difference in light intensity between the center and the surround.

How do receptive fields change as you move away from the fovea?

Receptive fields are very small in the fovea for high resolution and become progressively larger as you move away.

Compare the direct and indirect routes in the retina regarding resolution and sensitivity.

The direct route (low convergence) provides high resolution for discriminating detail, while the indirect route (high convergence) provides high sensitivity for detecting faint light.

Which retinal cells are responsible for those lateral connections that help build the surround of a receptive field?

Horizontal and amacrine cells.

What is the function of the $1-3\%$ of "photosensitive" ganglion cells that detect absolute luminance?

They project to the brainstem to synchronize circadian rhythms (detecting day and night) and are not involved in forming visual images.

What are the characteristics of the P-system (parvocellular) retinal ganglion cells?

They are small, process detailed analysis of stationary objects (shape, size, color), and provide a sustained response during stimulus presentation.

What are the characteristics of the M-system (magnocellular) retinal ganglion cells?

They are large, detect movement and broad outlines, and provide a transient response when a stimulus appears or disappears.

What does the "change blindness" experiment reveal about visual attention?

It reveals that the brain processes very little of what comes through the eyes; about $75\%$ of people fail to notice large changes in their visual environment when their attention is focused elsewhere.