Question

The sleep spindle is an episodic rhythm with:

• A. 30 Hz frequency and 50 mV amplitude
• B. 14 Hz frequency and 50 $\mu$V amplitude
• C. 50 Hz frequency and 50 $\mu$V amplitude
• D. 60 Hz frequency and 50 mV amplitude

Hint:

Scalp EEG signal amplitudes are always measured in microvolts ($\mu$V), never millivolts (mV). A sleep spindle is a burst of activity centered around 12–14 Hz with an amplitude of roughly 50 $\mu$V.

Answer 1

The Correct Option is B

Concept: Sleep spindles are distinct, short bursts of oscillatory brain activity that serve as a key marker for Stage 2 Non-Rapid Eye Movement (NREM) sleep. These bursts are generated by interactions between the thalamus and the cerebral cortex, and they play a critical role in shutting down sensory processing to preserve a deep sleep state.

Step 1: Identifying standard physiological parameters.
In clinical sleep studies (polysomnography), a sleep spindle is defined by specific physiological boundaries:
• Frequency Range: Sleep spindles typically oscillate within a frequency band of 11 Hz to 16 Hz (most commonly centered around 12 Hz to 14 Hz), which falls within the sigma wave band.
• Amplitude Scale: Because EEG electrodes measure microvolt-level potentials through the thick barrier of the human skull, normal cortical rhythms have an amplitude on the order of microvolts ($\mu\text{V}$), typically around 50 $\mu$V.

Step 2: Evaluating the options.
Let us check the parameters across the choices:
• Options (A) and (D) specify a voltage scale of millivolts (mV). A $50\text{ mV}$ signal is far too large for scalp EEG recordings and represents a massive equipment saturation artifact, ruling them out.
• Option (C) specifies $50\text{ Hz}$, which falls into the gamma band and matches industrial power line noise interference, ruling it out.
• Option (B) specifies a frequency of 14 Hz (falling perfectly within the 11–16 Hz spindle band) and an amplitude of 50 $\mu$V, which matches normal sleep spindle characteristics. This confirms Option (B).