Human frequency following response correlates of interaural-intensity differences

The binaural interaction component (BIC) represents the difference between the sum of two independent monaural responses and the binaural response. The existence of an electrophysiologic correlate of lateralization as exhibited in BIC of the human frequency-following response (FFR) evoked by 500 Hz tone bursts and the alterations of the BIC as a function of interaural-intensity differences (IID) were investigated. The FFR BIC was demonstrated and was shown to systematically decrease with increasing IID, much like the summed monaural response. The binaural FFR response showed a smaller decrease in amplitude as a function of IID. These results indicate that an excitation-inhibitory mechanism is the basis for binaural interaction and that the BIC represents a decrease in the firing rate of neurons as a 500 Hz tone burst lateralizes from a midline position to one side. The change in the BIC with IID, and therefore with lateralization of the fused image of the 500 Hz tone burst from midline to one ear, is taken to reflect the electrophysiologic correlate of lateralization.