What are sleep spindles called when they are preceded by sharp slow waves that can be elicited by sound?

Sleep spindles are bursts of oscillatory brain activity that occur most commonly during NREM sleep, particularly during stage 2. When these spindles are preceded by sharp slow waves that may be elicited by auditory stimulus or other external factors, they are specifically referred to as K complexes. K complexes play a critical role in consolidating sleep and protecting sleep from external disturbances by suppressing the processing of sensory information, which is evident in their characteristic waveform consisting of a sharp negative peak followed by a slower positive component.

The context of the question indicates an interactive aspect of the brain's response to stimuli while in a sleep state, showcasing the complex nature of sleep architecture. Understanding the nature of K complexes can provide insights into how the brain responds to external stimuli and maintains sleep continuity. This makes it clear why K complexes are the correct answer in this scenario.