Lateralization and Binaural Interaction of Middle-Latency and Late-Brainstem Components of the Auditory Evoked Response

Abstract

We used magnetoencephalography to examine lateralization and binaural interaction of the middle-latency and late-brainstem components of the auditory evoked response (the MLR and SN10, respectively). Click stimuli were presented either monaurally, or binaurally with left- or right-leading interaural time differences (ITDs). While early MLR components, including the N19 and P30, were larger for monaural stimuli presented contralaterally (by approximately 30 and 36 % in the left and right hemispheres, respectively), later components, including the N40 and P50, were larger ipsilaterally. In contrast, MLRs elicited by binaural clicks with left- or right-leading ITDs did not differ. Depending on filter settings, weak binaural interaction could be observed as early as the P13 but was clearly much larger for later components, beginning at the P30, indicating some degree of binaural linearity up to early stages of cortical processing. The SN10, an obscure late-brainstem component, was observed consistently in individuals and showed linear binaural additivity. The results indicate that while the MLR is lateralized in response to monaural stimuli-and not ITDs-this lateralization reverses from primarily contralateral to primarily ipsilateral as early as 40 ms post stimulus and is never as large as that seen with fMRI.

Keywords: SN10; auditory brainstem response; binaural interaction; interaural time difference; lateralization; middle-latency response.

Fig. 1

Paradigm and cortical source waveforms. A Stimulus conditions and experimental design. B Grand-average source locations/orientations overlaid onto BESA’s template brain/head surface. C Auditory cortex source waveforms for the grand average (all conditions collapsed; top panels) as well as each condition separately (bottom panels). Five main peaks of the MLR are visible: P13, N19, P30, N40, and P50. The solid (dashed) traces show the waveforms for the monaural (binaural) conditions. The response to the diotic click stimulus is not shown, after it was found that it did not differ from either of the remaining binaural conditions. C _Mon contralateral monaural stimulus, I _Mon ipsilateral monaural stimulus, C _Bin contra-leaded binaural stimulus, I _Bin ipsi-leading binaural stimulus.

Fig. 2

Cortical amplitude (A) and latency (B) quantifications for each of the four component pairs (for amplitude) or components (for latency). Error bars denote standard error of the mean across listeners. Legend as in Fig. 1. For display purposes, values were collapsed across hemispheres but analyzed separately statistically.

Fig. 3

Midbrain source analysis and amplitude/latency quantifications. A Average source orientation. B Grand-averaged source waveforms collapsed across all conditions (top panel) and for each of the five stimulus conditions separately (bottom panel). The SN10 is clearly visible as a negative-going deflection at ∼10 ms. R _Mon right-ear monaural stimulus, L _Mon left-ear monaural stimulus, R _Bin right-leaded binaural stimulus, L _Bin left-leading binaural stimulus. Question marks denote components of questionable source origin. C, D SN10 amplitude and latency quantifications for all conditions. Legend same as in B. Error bars denote standard error of the mean across listeners.

Fig. 4

Binaural difference (BD) responses for the cortical (A) and subcortical (B) sources and their amplitude quantifications (C, D). The binaural response is the average of the two binaural ITD conditions (C _Bin and I _Bin for A, R _Bin and L _Bin for B), and the monaural sum response is the sum of the two monaural conditions (C _Mon + I _Mon for panel A, R _Mon + L _Mon for panel B). Note that both the peak amplitudes shown in panel C and the associated statistical tests were computed after collapsing across the two hemispheres.

Fig. 5

Cortical source waveforms and binaural difference responses after applying a 20-Hz HPF. A Grand-average (top panels) and individual condition (bottom panels) source waveforms (B). Corresponding binaural difference responses.