Introduction
The figure at left shows a schematic drawing of a neural circuit for motion detection. The goal of the circuit is to enable motion detector cells (green squares) to respond to motion of a specific direction and speed. Input to the circuit comes through the photoreceptor cells (blue circles) that normally respond to static spots of light, but by connecting them to two types of intermediate cells (red octagons and purple triangles), motion detection becomes possible.
The white circle represents a spot of light, which can be moved to the right or left by clicking the triangle “play” button near the bottom of the screen. Click the double-triangle “fast-forward” or “rewind” buttons to start the spot moving automatically rightward or leftward, and click on the words “Slow,” “Ideal,” or “Fast” to control the speed of the spot’s movement.
Start with the spot of light on the far left, set the speed of motion to “Ideal,” and click the rightward double-triangle to start the spot moving right. You will see that the green cell in the bottom-left corner of the diagram fires continuously while the green cell in the upper-right corner does not fire at all. If you then click the other double-triangle to start the spot moving back to the left, you will get the opposite result; the upper-right green cell will fire continuously while the lower-left cell remains quiet.
To understand how this all works, read below for descriptions of how each of the cells in the network function. The description of the green cells discusses the characteristics of the neural circuit as a whole. If this is your first time visiting this activity, begin by reading about the blue cells.
Activity Description
Move the spot of light (white circle) around to observe how the circuit as a whole responds to movement (all of the following controls are in the black control panel below the image at left):
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Click a single triangle to move the spot “manually” one position left or right (moving left from the far-left position will cause the spot to wrap around to the far right, and vice versa).
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If the white spot is moved either too quickly or too slowly, the green motion detector cells won’t fire.
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Click a double triangle to start the spot moving automatically to the left or right.
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This automatically moves the white circle at the correct speed to activate the motion detector circuit.
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Click the words “Slow,” “Ideal,” or “Fast” to change the speed of automatic movement.
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These automate the movement of the white spot of light to be either too slow, ideal, or too fast.
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When you see a cell’s outline turn from black to white, the cell is firing action potentials. Read on for a description of each of the cell types.
Blue Circles: Photoreceptor Cells
The blue circles represent photoreceptor cells. Each photoreceptor begins firing when the spot of light moves over it (note that when a neuron fires, its outline turns from black to white). The cell stops firing when the light moves, or after one second (whichever comes first). Note that nothing happens in any other cell of the circuit until a photoreceptor cell is stimulated, and that the green motion detector cell will not respond until the spot of light moves from one photoreceptor to another.
Each photoreceptor synapses with one or two red delay cells and with one or two purple multiplication cells. Each of these cells responds in a different way to the input from the photoreceptors.
To continue your exploration of the neural circuit, next read about the red cells to find out what they do.
Red Octagons: Delay Cells
The red octagons represent delay cells. As you can see, each delay cell receives input from a blue photoreceptor cell and sends output to a purple multiplication cell. Each delay cell fires whenever it is stimulated by the photoreceptor it synapses with. But, as indicated by their name, delay cells do not react immediately upon being stimulated. Rather, each delay cell begins to fire exactly one second after it is stimulated by a blue photoreceptor cell. The delay cell then stops firing exactly one second after it starts.
Set the Speed control to “Fast,” move the spot of light to the far left position and click the double-triangle on the right side of the control panel. Pay attention to the red delay cells as the spot moves quickly across the blue photoreceptors. You will see that the delay cells do indeed fire a second after their corresponding photoreceptor cells fire.
Also note that neither the purple nor the green cells fire when the spot of light moves at this “Fast” rate. To find out why, read on to learn what the purple cells do.
Purple Triangles: Multiplication Cells
The purple triangles represent multiplication cells. Note that each purple cell receives input from two neurons: one blue photoreceptor and one red delay cell. The purple cells function like AND gates in an electronic circuit. Each one will only fire if it is being stimulated by both of its presynaptic neurons at the same time.
Now you should understand why these cells do not fire when the spot of light moves quickly across the photoreceptors at the “Fast” rate. Consider what happens when we start with the spot at the far left and move it quickly to the right.
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First, a blue photoreceptor cell fires, sending input to a purple multiplication cell as well as a red delay cell. Although the purple cell is receiving input from the blue photoreceptor cell, it does not fire at this time because it is not being stimulated by the red delay cell, and it needs stimulation from both cells simultaneously to fire.
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Next, the light moves right one position and causes the next photoreceptor cell to fire. Now two purple cells are being stimulated, but again, they do not fire because they are not receiving input from their presynaptic delay cells at that time.
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The spot continues to move across to the next photoreceptor units, but no purple cells fire, for the reasons stated above.
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Once the light moves to the right of the last photoreceptor cell, one second will have passed since the light covered the first photoreceptor cell, so the delay cells will start to fire, one after another. Again, however, the purple cells will fail to fire, because now, though they are receiving input from delay cells, they are no longer getting input from the photoreceptor cells.
Finish your journey through this neural circuit and read about why the purple (and green) cells fire when the spot moves at its “Ideal” speed by clicking on one of the green cells.
Green Squares: Motion Detector Cells
The green squares represent the end products of this neural circuit—the motion detector cells. Each green cell fires whenever it is stimulated by one of its four presynaptic purple cells. As you have probably already observed, the lower-left green cell fires when the spot of light moves rightward at the proper speed, while the upper-right green cell responds to leftward motion. We will now explore what occurs when the spot starts at the far left and moves right with the speed set to the “Ideal” setting.
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First blue photoreceptor fires. It sends a signal directly to one of the purple multiplication cells for leftward motion and to one of the red delay cells for rightward motion.
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At the same time that the rightward motion red delay cell begins to fire, the spot moves over one blue photoreceptor to the right. Now the rightwards purple multiplication cell is receiving input from both the rightward red delay cell and the second blue photoreceptor cell, so it fires and in turn causes the rightward green motion detector cell to start firing. Note that none of the leftward motion purple cells are firing, so the leftward green motion detector cell does not fire.
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One second later, the spot moves over to cover cell the third blue photoreceptor cell, sending immediate stimulation to the next rightward motion purple multiplication cell. At the same time, the first red delay cell stops firing and the second red delay cell begins firing (because it was stimulated by the second blue photoreceptor cell one second previously). Now the first rightward purple multiplication cell stops firing (because it is not receiving input from either of its presynaptic cells), but the second rightward purple multiplication cell starts firing, because it is receiving simultaneous input from the second red delay cell and the third blue photoreceptor cell. This causes the rightward green motion detector cell to remain in its excited state.
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As the spot continues to move to the right, the next several rightward purple cells will become activated in turn, keeping the green rightward motion detector cell continuously activated. However, the leftward purple multiplication cells will never be stimulated simultaneously by their presynaptic neurons, so the leftward green motion detector cell will never fire while the spot moves rightwards.
Also consider what happens when the spot moves in either direction at the “Slow” speed. Neither motion detector cell will ever become activated by motion this slow because the inputs from the red delay cells and blue photoreceptor cells will be out of synch with each other because the spot of light is taking longer to move than the time delay built into the red delay cells.