Question

Diagram and describe how lateral inhibition via horizontal cells produces the surround of a retinal ganglion cell's receptive field.

Short answer

Horizontal cells mediate lateral inhibition in the retina that plays a crucial role in the formation of a center-surround receptive field for retinal ganglion cells. In this setup, the 'center' of the receptive field is excitatory and the 'surround' is inhibitory. This forms due to the fact that horizontal cells inhibit activation of the surrounding photoreceptors when light illuminates the center of the receptive field, creating a contrast between the center and surround.

Step-by-step solution

Understanding basic components

Firstly, let's understand the basic components involved. Receptive fields describe the region of the retina where light affects a neuron's firing rate. In this context, we're looking at retinal ganglion cells. Horizontal cells are interneurons in the retina that play critical roles in the lateral inhibition, helping to sharpen our perception of boundaries in the visual scene.

Role of Horizontal cells and lateral inhibition

Horizontal cells connect photoreceptors (i.e. rods and cones) and bipolar cells. The job of horizontal cells is to regulate, modify and integrate the signals that are being passed from the photoreceptors to the bipolar cells and ultimately to the retinal ganglion cells. The horizontal cells inhibit firing of neurons in their periphery, a process known as lateral inhibition, increasing contrast and sharpness in images.

Creation of Surround

Now, the lateral inhibition plays a critical role in creating a receptive field with a center-surround structure for a retinal ganglion cell. Imagine a retinal ganglion cell that is receiving excitatory input from one group of photoreceptors (the 'center' of its receptive field). Due to lateral inhibition, the horizontal cells inhibit the activation of the surrounding photoreceptors ('surround'). Consequently, if light is cast over the entire receptive field, the inhibitory effect from the surround can counter the excitatory center, thus the overall response of the ganglion cell becomes less prompting a contrast or difference between the center and surround.

Key concepts

Retinal Ganglion Cell

A retinal ganglion cell is an essential component of the eye's neural circuitry. It acts like a reporter, relaying visual information received from the photoreceptors to the brain.
These cells are located in the retina, the light-sensitive layer of tissue at the back of the eye. When light hits the retina, it is converted into electrical signals by photoreceptors, which are then processed and transmitted by the retinal ganglion cells.
Each retinal ganglion cell has a specific role. They collect information from bipolar cells and amacrine cells, and finally send these signals along the optic nerve to the brain. This complex transmission of signals allows us to perceive images.

• Ganglion cells are divided into different types based on their response to light and contrast sensitivity.
• Their performance can be measured by checking how they respond to changes in light intensity.
• These cells are pivotal in creating our visual experiences, helping in perceiving motion, depth, and even color.

Horizontal Cells

Horizontal cells are crucial interneurons found in the retina, which perform the function of lateral inhibition. They are situated laterally, making connections between photoreceptors and bipolar cells.
Their strategic positioning allows them to mediate feedback and modulate the input received by photoreceptors from surrounding cells. By doing so, horizontal cells can enhance contrast at edges and contours of the visual scene, leading to sharper and more defined images.
Lateral inhibition is a process in which the horizontal cells inhibit the signals from photoreceptors that are not in the direct pathway of the ganglion cell.

• This inhibition leads to contrast enhancement.
• It allows the visual system to detect fine details and tiny differences in light and dark.
• Ultimately, it enables our eyes to adapt to different lighting conditions.

Receptive Field

In the realm of vision, a receptive field refers to the specific area of the retina where a change in light intensity will affect the firing of a particular neuron. Each retinal ganglion cell has its own receptive field structured in a unique manner.
This field is often composed of two parts: the center and the surround. The center is directly driven by photoreceptor input, while the surround is modulated by lateral inhibition from horizontal cells.
The arrangement of center-surround receptive fields allows for the detection of contrasts, such as light and dark spots, which in essence helps in edge detection and visual perception.

• Center-surround receptive fields help create a mechanism to measure and respond to variations in light and contrast.
• The balance between excitatory and inhibitory input determines how a ganglion cell reacts to stimuli.
• This structure makes them vital for motion detection and object recognition.

Photoreceptors

Photoreceptors are the light-sensitive cells found in the retina. They come in two primary forms: rods and cones. These cells are essential for capturing light and initiating the process of vision.
Rods are more sensitive to light and are crucial for vision in low-light conditions. Cones, meanwhile, are responsible for detecting color and fine detail and function well under brighter lighting conditions.
After detecting light, photoreceptors convert it into electrical signals and transmit this data to bipolar cells and eventually to ganglion cells. This forms the basis of the visual signal relay through the retina.

• Rods are concentrated in peripheral regions of the retina, contributing to night and peripheral vision.
• Cones are densely packed in the center of the retina (fovea) for sharp central vision.
• Each type of photoreceptor has different light sensitivities and spectral responses, aiding color differentiation and detail recognition.