Which statement about the lateral superior olive (LSO) is true?

• A. Receives excitatory input from the ipsilateral ear and inhibitory input from the contralateral ear.
• B. Detects ITD as a time difference
• C. Receives inhibitory input from the ipsilateral ear and excitatory from contralateral
• D. Processes spectral cues

Answer: (A)

The lateral superior olive specializes in coding interaural level differences, not timing cues. It achieves this by comparing excitation coming from the ear on the same side with inhibition arriving from the opposite ear. Specifically, the LSO receives excitatory input from the ipsilateral cochlear nucleus (the ear on the same side) and inhibitory input from the contralateral ear via the medial nucleus of the trapezoid body, which provides glycinergic inhibition. This arrangement lets the LSO signal how much louder one ear is compared to the other, which is most useful for localizing high-frequency sounds where the head shadow creates strong level differences.

That’s why the statement describing excitatory input from the ipsilateral ear and inhibitory input from the contralateral ear best captures the LSO’s role. It’s not about detecting interaural time differences (that’s the medial superior olive’s job in low frequencies) and it’s not primarily about processing spectral cues, which involve other structures shaped by the pinna and spectral filtering rather than the LSO’s ILD computations.