Asymmetric frequency-following responses.

This study offers evidence of asymmetry in the frequency-following response (FFR), a brainstem evoked potential that mimics stimulus frequency. The FFRs to stimulation of one ear compared to the other are shown to have latency and amplitude dissimilarities, as well as considerable variations in waveform characteristics. The findings were obtained from eight subjects. Tone bursts at 500 Hz were delivered to the right ear, left ear, and binaurally at three sound levels. With few exceptions, subjects had asymmetric FFRs at every sensation level (SL); at some, the sound to the left ear evoked a larger response than the same sound to the right; at other levels, the right ear FFR was larger than the left. The results provide evidence that cortical processing asymmetries must, to some extent, be reflective of asymmetric mechanisms within the lower brain stem.