Receptive Field of Retinal Ganglion Cell
When light strikes the back of the eye it might be transduced (changed into a neurological event) by a photoreceptor such as a rod or a cone. The photoreceptors do not send information directly to the brain. Rather, the photoreceptors (indirectly through the bipolar and amacrine cells) communicate with the retinal ganglion cells (RGC) which project their axons toward structures in the brain such as the lateral geniculate nucleus and the superior colliculus. All of the visual information that leaves the eye does so via the retinal ganglion cells.
The retinal ganglion cells filter, or change, the information received from the photoreceptors. The photoreceptor information is basically a picture of the visual field -- information about the color, intensity and spatial location of the light is preserved. The retinal ganglion cells change this visual information by picking out various spatial frequencies. Stimuli such as a line or an edge contain high spatial frequencies, while stimuli such as a uniformly painted wall (which is usually a little darker in some locations than in others because of the location of a light source -- in my office which has most of its lights in the ceiling, the top of the wall is darker than the bottom of the wall) will contain lower spatial frequencies. The retinal ganglion cells pick out the various spatial frequencies and transmit that information toward the brain.
Each retinal ganglion cell has a receptive field -- a small part of the retina to which it is sensitive. A given retinal ganglion cell only responds to light that is on its receptive field. If the light is not on the retinal ganglion cell's receptive field, it will not respond to the light.
The receptive field of most retinal ganglion cells consist of two concentric regions known as the center (a somewhat circular region) and the surround (a somewhat circular region all around the center.) These two regions behave antagonistically to each other. If the center is excited by light, then the surround will be inhibited by light. Such a retinal ganglion cell would be called an on-center, off-surround cell because light in the center makes the neuron more likely to produce an action potential while light in the surround decreases the likelihood that the neuron will produce an action potential. There are also off-center, on-surround retinal ganglion cells.
Based on the center-surround antagonism, an on-center off-surround retinal ganglion cell would be most active if all of the center of its receptive field was brightly illuminated while all of the surround of its receptive field was in complete darkness. This, however, is not a likely stimulus in the real world (the tittle (dot) of an i printed with white chalk on a blackboard might be an example.) Thus, evolution probably did not come to this organization of the receptive field in order to see tittles.
There is another class of stimuli that cause such receptive fields to respond vigorously and which are abundantly common in the everyday world -- lines, edges or changes in contrast. Unless you are in a raging blizzard (sorry if you are), there are likely lots and lots of edges and lines in your visual field. Edges usually arise when one object (e.g. your computer monitor) stops and another object (e.g the wall behind it) begins in the visual field. Unless the two objects are exactly the same color and illuminated in exactly the same way, there will be an edge or line where the monitor stops and the wall begins in the visual field. As you read this text, there are many such edges -- where each letter starts or ends forms an edge with the white background. Some retinal ganglion cells are tuned to pick out such edges and carry information about those edges to the brain.
In this activity you will investigate how an on-center, off-surround retinal ganglion cell responds as you move an edge over its receptive field. In the activity there are two circles which represent the center and surround of the receptive field. By convention, a "+" indicates that that part of the receptive field is excited by light and a "-" indicates that that part of the receptive field is inhibited by light. Thus, in this activity, the receptive field is an on-center, off-surround field.
The amount of excitation is related to the amount of the center that is in the light and the brightness of the light. The greater the amount of the center that is in the light, the more excited the retinal ganglion cell will be. The brighter the light, the more excited the retinal ganglion cell will be. The amount of inhibition is related to the amount of the surround that is in the light and the brightness of the light. The greater the amount of the surround that is in the light, the more inhibited the retinal ganglion cell will be. The brighter the light, the more inhibited the retinal ganglion cell will be. Because neurons produce action potentials even when there is no stimulus, there is a certain amount of randomness added to the excitation and inhibition.
When the amount of excitation sufficiently exceeds the amount of inhibition, the retinal ganglion cell will send an action potential down its axon. This is represented by a red line (spike) in the spike train. The spike train shows the activity of the retinal ganglion cell across time. The current moment in time is represented on the right edge of the spike train and previous moments in time are represented as you move to the left in the spike train. If the spike train was printed on paper, the oldest action potentials would be on the left and the newest action potentials would be on the right. This is consistent with how culture that read from left to right tend to think about time.
Remember that we (and the visual system) are not interested in a single action potential / spike. Rather, we are interested in the rate of action potentials. Because there is a certain amount of randomness in the process (neurons continue to produce action potentials at a low rate even when there is no stimulation), you need to look at the spike train across time to see how the neuron responds to any given stimulus. That is, move the edge and then wait a few seconds to see the record of the neural activity in the spike train.
In this activity you can move the edge by dragging the two red dots around. You can also change the color of the two sides of the edge by adjusting the sliders -- moving the slider to the left will make the area darker while moving it to the right will make the area lighter.
Some Things To Try:
Try to predict how the retinal ganglion cell will respond before doing each of these things.
Note that the maximum rate of neural activity is greatly slowed in this demonstration compared to real life. Neural events tend to happen very quickly in real life and this can make it harder to see what is going on.
Spike Train: Use Sounds