Final answer:
Rachel is experiencing spontaneous recovery, where an extinguished conditioned fear response to a spider has resurfaced after a period of no exposure.
Step-by-step explanation:
The phenomenon that Rachel is experiencing is called spontaneous recovery. This term refers to the return of a previously extinguished conditioned response after a period of no exposure to the conditioned stimulus. In Rachel's case, the fear response to a picture of a spider had been extinguished through exposure therapy, but after a week without exposure, the fear response resurfaced when she saw the picture again.
Extinction in classical conditioning occurs when the unconditioned stimulus (UCS), which naturally produces a fear response, is no longer paired with the conditioned stimulus (CS), which is the picture of a spider in this case. Over time, this leads to a decrease in the fear response. However, this process is not always permanent, and the previously extinguished response can return, demonstrating spontaneous recovery. This is a common occurrence in conditioning where a response can reappear after a time without exposure to the conditioned stimulus, despite the response having been previously diminished through extinction.