Final answer:
The brain area responsible for the difference in awareness is the cerebral cortex. Consciousness levels dictate how internal and external stimuli are processed, which explains why one could notice a noise while their spouse, who is sleeping, does not.
Step-by-step explanation:
The structure of the brain responsible for the awareness of the noise produced by the fridge while your spouse is unaware is the cerebral cortex. Our consciousness dictates our awareness of internal and external stimuli, making us aware or unaware of things happening around us. When engaged in a focused activity such as studying, your cognitive resources are actively processing the environment, and unexpected noises, such as a fridge malfunctioning, can breakthrough attentional filters due to their bottom-up nature.
On the other hand, the reason your spouse is not aware of this noise can be attributed to the state their brain is in during sleep. Deep sleep involves a dramatic change in the level of consciousness, reducing awareness of external stimuli. The cognitive processes that would normally process these sounds are significantly less active, leading to a lack of response to the noise.