Final answer:
In stimulus fading, a device is inserted into the rat's brain that is activated and controlled by flashing light. Different frequencies of light flashes can cause the expression of different types of proteins. For example, 1 flash per second can elicit the expression of one type of protein, while 100 flashes per second can elicit the expression of another type of protein.
Step-by-step explanation:
In the procedure of bringing the lever pressing response under the control of only the tone, known as stimulus fading, a miniaturized device is inserted in the nuclei of neurons transmitting the tone to the rat's brain. This device is optically activated and controlled by flashing light. By using different frequencies of light flashes, the device can cause the expression of different types of proteins by the cells in the rat's brain. For example, light with 1 flash per second (1 Hz) can elicit the expression of one type of protein, while light with 100 flashes per second (100 Hz) can elicit the expression of another type of protein.