Ganglion Receptive Fields
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Firing Rate:
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Introduction
This activity is designed to simulate an experiment in which we map out the receptive fields of retinal ganglion cells by recording the firing rates of the cells in response to spots of light shined on the retina. Click and drag around the white circle at left, which represents a spot of light, and note the effects on the cell’s firing rate. The control panel at the top left allows you to change the size of the spot, turn the spot off and on, or change to a new receptive field.
This activity simulates a single-cell recording experiment mapping out the receptive fields of retinal ganglion cells. At the bottom left of the screen you see a black square representing a small piece of retina, with a diagram of the receptive field of a sample ganglion cell that responds to some part of this retinal patch. A spot of light shines somewhere on the retina, and the neuron’s firing rate in response to this light is given above and to the left.
Instructions
Click and drag the white circle to move the spot of light around, or click in the control panel at top left to change the size of the spot to small, medium, or large. Observe the effect that these changes have on the firing rate of the ganglion cell.
Click the SPOT OFF/SPOT ON button to turn the spot of light on or off.
Click the NEW RF button to switch to a new cell with a different receptive field.
Click the “Quiz Mode” link to try your hand at identifying the receptive fields of new ganglion cells.
Experimental Procedure
In the early 1950s, neuroscientists developed techniques for recording action potentials from single retinal ganglion cells. In these experiments, a tiny microelectrode is inserted into the retina of a laboratory animal (e.g., a cat), either into or right next to a ganglion cell. A spot of light is shined onto the animal’s retina for a brief time and the firing rate of the ganglion cell is recorded while the light is on. The spot is then moved around on the retina and made larger or smaller. By observing the response of the neuron to spots at different locations and of different sizes, the receptive field of the neuron can be mapped out.
Real neurons can fire at rates ranging from zero (never generating an action potential) to hundreds of action potentials per second. To simplify this, our demonstration represents neural firing rates on a scale from 0 to 100. When no stimulus is being applied, a neuron will fire with a resting rate of about 10 “spikes” (action potentials) per second. A spike rate below or above 10 indicates that the neuron is being inhibited or excited, respectively, by the stimulus.
Note that there is a certain amount of randomness associated with neural firing rates, so the firing rate is constantly changing regardless of whether or not the neuron is being stimulated.
What Is a Receptive Field?
Once scientists were able to record from neurons in the visual system (including retinal ganglion cells), they found that each one has a distinctive receptive field—an area of the retina in which light must fall for the neuron to increase or decrease its firing rate. One important way to categorize visual system neurons is by the characteristics of their receptive fields:
- The location on the retina where an image must fall for the neuron to respond
- The size of the retinal area to which the neuron responds
- The pattern of light that causes the best response (i.e., the highest firing rate) in the neuron
When you begin this activity, the location, size, and type of receptive field are provided. But scientists doing single-cell recording studies do not start out with the benefit of this information. Indeed, their goal is to discover the properties of a cell’s receptive field through systematic investigation.
ON-Center and OFF-Center Ganglion Cells
Researchers have discovered that most retinal ganglion cells respond to one of two patterns of light. ON-center cells respond best (i.e., with the highest firing rate) when spots of light fall on the centers of their receptive fields, and are inhibited when light falls in the surrounding area of their receptive fields. OFF-center cells show the opposite pattern of responses, being excited by light falling on their surrounds and inhibited by light falling on their centers. Click the words “ON-Center”; or “OFF-Center” to force the current cell to be one type or the other.
Ganglion cells also differ with respect to how large their center and surround areas are. Click the following words to force the current cell to have alarge or small receptive field. In general, ganglion cells closer to the retina have smaller receptive fields than ganglion cells farther out in the periphery. Note, however, that a neuron with a small receptive field may respond just as strongly as a neuron with a large receptive field. What determines the strength of response is the fit between a given image on the retina and the neuron’s receptive field, whether it be ON-center or OFF-center.
Turning Spots Off
When a spot of light is turned off, a ganglion cell’s activation briefly switches polarity, such that its excitatory region becomes inhibitory and vice versa. This means that an OFF-center cell will respond with a short but strong burst of firing when a spot of light falling in the center of its receptive field is turned off.
To see this, make sure you have an OFF-center cell (click here to change the cell to OFF-center if it isn’t already), then position a spot of light so that it completely covers its center. Next, turn the spot off with the button in the control panel. The cell’s activity will quickly jump to near 100, then fall back down to the cell’s resting rate (about 10).
What do you think will happen if you turn a spot off while it is in the center of an ON-center cell? Click here to change the cell to ON-center and see for yourself.
Quiz Mode
Imagine you are a neuroscientist recording from an unexplored retinal ganglion cell. You know the cell’s receptive field is somewhere in the black square, but you don’t know exactly where. You also want to find out the size and type of the receptive field. Move and change the size of the spot of light, observe any changes in the cell’s firing rate, and try to determine what kind of receptive field is present.
When you think you know the location, size, and type of the cell’s receptive field, position the spot of light over the center of the field and click one of the links below corresponding to the type of receptive field you think it is. You will be told whether you are right or wrong, and the receptive field will be revealed.