Introduction
This activity simulates a single-cell recording experiment mapping out the receptive fields of retinal ganglion cells. On the left there is 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. The retinal ganglion cell appears as a large circle with a smaller circle in its center. A spot of light shines somewhere on the retina, and the neuron’s firing rate in response to the light is given above and to the left in a black box with vertical green lines drifting to the right; each green line represents a neural spike. The firing rate of the neuron is listed as a number. At the beginning of the simulation, the spike rate hovers around 10, with some variation between 8 and 12.
The user can click and drag around the white circle, which represents a spot of light, and note the effects on the cell’s firing rate. The control panel at the top allows the user to change the size of the spot, turn the spot off and on, or change to a new receptive field.
There are two types of receptive fields shown in the activity: on-center cells and off-center cells. The on-center cells have an inner region that is excited by light and an outer region that is inhibited by light. The off-center cells have an inner region that is inhibited by light and an outer region that is excited by light. When the spot of light is entirely inside the inhibitory region of one of the cells, it causes the spike rate to drop from about 10 spikes per second to near 0. The more of the inhibitory region that is covered, the lower the spike rate goes. When the spot of light is positioned entirely within the excitatory region of one of the cells, the spike rate increases dramatically. The more of the excitatory region that is covered, the higher the firing rate, up to a maximum of 100.
When the spot of light covers both an inhibitory and excitatory region of the receptive field at the same time, the spike rate depends on the relative proportion of the inhibitory and excitatory regions that are covered. When the spot of light covers the entire receptive field, including the entirety of both the excitatory and inhibitory regions, the spike rate stays at about 10, which is the same amount seen when there is no light shining on the receptive field at all.
Ganglion Receptive Fields
The 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 projected 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 of light 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:
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The location on the retina where an image must fall for the neuron to respond
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The size of the retinal area to which the neuron responds
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The pattern of light that causes the best response (i.e., the highest firing rate) in the neuron
When the user begins this activity, the location, size, and type of receptive field are provided. But scientists doing single-cell recording studies do not start out knowing 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 inhibited by light falling on their centers and excited by light falling on their surrounds.
Ganglion cells also differ with respect to how large their center and surround areas are. In this simulation, there are small and large receptive fields used. In general, ganglion cells closer to the retina have smaller receptive fields than ganglion cells 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, large or small.
Turning Spots Off
When a spot of light is turned off, the ganglion cell’s activation momentarily switches polarity, such that its excitatory region becomes briefly 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 on the center of its receptive field is shut off.
In the simulation, if the center of an OFF-center cell is completely covered by a spot of light and that spot is turned off, activity will quickly jump to near 100, then settle 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? The firing rate of the cell is quite high – near 100 – when the spot of light is on. When it is turned off, the firing rate drops all the way down to 0 for a moment before returning to the resting firing rate of about 10.