Lecture 3: Receptive Fields in the Retina - Study Notes

Basic Approach to Study Vision: Researchers record from the axons of individual ganglion cells located in the optic nerve while an anesthetized animal is presented with specific visual stimuli.
Experimental Models: The vast majority of these experiments are conducted using mice and rats.
Recording Mechanism: Microelectrodes are used to record activity from a single cell body or axon.
Extracellular Recording: * Uses a microelectrode placed outside the cell body. * Primarily captures action potentials (spikes). * Used to record from Retinal Ganglion Cells (RGCs).
Intracellular Recording: * Significantly more difficult technique compared to extracellular recording. * Records both action potentials and graded potentials, such as Excitatory Post-Synaptic Potentials (EPSPs) or Inhibitory Post-Synaptic Potentials (IPSPs). * Essential for studying cells that do not spike, such as rods and cones.
Measuring Firing Rates: * Baseline Firing Rate: In the absence of stimuli, neurons in the retina and visual system tend to fire baseline action potentials (e.g., $12$ action potentials per second). * The primary goal is to measure the change in the firing rate (increase or decrease) relative to this constant baseline in response to a signal.

Definition of Receptive Field: "The receptive field of a neuron is a region of the visual field in which the presence of a stimulus alters the firing of the neuron. The neuron responds to the stimulus only within this area."
Ganglion Cell Receptive Field Structure: * They typically possess circular receptive fields. * They are organized into two subzones in a center-surround organization. * These subzones are antagonistic: one is excitatory ( $+$ , increasing firing from baseline) and the other is inhibitory ( $minus$ , decreasing firing from baseline).
Types of Receptive Fields: * On-center Ganglion Cells: Stimulated by a light spot in the center; inhibited by light in the surround. * Off-center Ganglion Cells: Inhibited by light in the center; stimulated by light in the surround.
Spatial Summation: The larger the area of light stimulation within a specific subzone, the stronger the modulation (increase or decrease) of the neuron's firing rate.

On-center Receptive Field Dynamics: 1. Spot in Center: Increases firing rate significantly above baseline. 2. Spot in Surround: Decreases firing rate below baseline (suppression). 3. Diffuse Illumination (Center and Surround): Result in a very weak stimulus/response because the excitation in the center is cancelled out by the inhibition in the surround.
Off-center Receptive Field Dynamics: 1. Dark Spot in Center: Since it is an off-center cell, it reacts to the darkness/center absence of light with increased firing. 2. Light in Center: Inhibits the firing rate below baseline. 3. Surround Illumination: Increases firing if the surround is illuminated or the center is darkened. 4. Diffuse Illumination: Similarly produces a minimal response due to antagonistic cancellation.
Edge Detection: The center-surround organization is optimized for detecting "edges," which are defined as sharp contrasts in the visual image rather than absolute levels of light.
Overlapping Fields: There is extensive overlap between the receptive fields of adjacent ganglion cells across the retina.

Fovea vs. Periphery: * Fovea (Resolution): Contains very small receptive fields. High resolution (ability to discriminate between two close sources of light), but low sensitivity (requires stronger stimulus to detect). * Periphery (Sensitivity): Receptive fields become progressively larger. High sensitivity (can detect very weak stimuli), but low resolution (unable to discriminate between two distinct light sources if they both fall into the same large receptive field).
Direct vs. Indirect Routes: * Direct Route (Low Convergence): The center of the receptive field follows a more direct route from photoreceptors to ganglion cells. * Indirect Route (High Convergence): The surround of the receptive field follows a more indirect route involving lateral connections.
Synaptic Organization for On-center Cells: * Center Response (Direct): Typically involves photoreceptors numbered $3-5$ . * Surround Response (Indirect): Involves lateral inhibition from photoreceptors numbered $1-2$ and $6-7$ . * If only the direct route existed with no lateral connections, there would be no surround antagonism.

Luminance Contrast: Ganglion cells are not sensitive to overall luminance levels but are extremely sensitive to luminance contrasts. They can detect as little as a $2\%$ difference (contrast) in light intensity between the center and the surround.
Photosensitive Ganglion Cells: * Comprise approximately $1-3\%$ of all Ganglion Cells (GCs). * These cells are sensitive to total light intensity rather than contrast. * They send information to the brainstem to synchronize circadian rhythms. * Crucially, they are not involved in forming visual images.
Boundary Perception and Filling-In: Ganglion cells primarily detect the boundaries (edges) of objects. The brain uses this boundary information to "fill in" the rest of the object. Vision is thus an active process of construction rather than a passive recording of every pixel.
Parallel Processing Subdivisions: * P-system (Parvocellular): Responsible for the detailed analysis of stationary objects, including shape, size, and color. It provides a sustained response throughout the duration of the stimulus. * M-system (Magnocellular): Responsible for detected movement and the broad outlines of objects. It provides a transient response that occurs only when the stimulus appears or disappears.
Change Blindness: A psychological phenomenon demonstrating that we do not always pay attention to all information provided by the retina (e.g., experiments like "Whodunnit" or the "monkey business" type videos).

Is absolute light measured by these cells?: No, for the vast majority of ganglion cells, the luminance contrast between center and surround matters, not the absolute amount of light.
How does the brain know what's in the middle of a shape?: The "filling in" process is handled by the brain after the ganglion cells provide the initial edge/boundary data.
Reference Materials: Additional background can be found in the Purves textbook via the NCBI website: https://www.ncbi.nlm.nih.gov/books/NBK11052/.