Background: Previous human auditory brainstem response (ABR) studies have suggested that the right ear auditory network preferentially processes a spectrotemporally complex speech syllable and the left ear auditory network preferentially processes temporally devoid spectral stimuli. Human cortical studies also suggest lateralization effects to spectral versus temporal stimuli. However, it remains unclear if the reported brainstem lateralization effects may be due to the spectrotemporal content or the higher order lexical content of the evoking speech stimulus. Also, the lateralization effects observed at the cortical level in late evoked auditory potentials are based upon responses obtained well after the stimulus has arrived to the auditory cortices (~100 ms). Lateralization effects to spectrotemporally complex stimuli are unknown upon first arrival to the auditory cortices or in the auditory middle latency Pa response which occurs approximately 30 ms post-stimulus.
Purpose: The purpose of this study was to gain a better understanding of how the human auditory processing system encodes spectrotemporally complex acoustic stimuli from subcortical levels to first arrival at the bilateral cortices.
Research Design: This study is a comparative analysis of both brainstem frequency following responses (FFRs) and cortical auditory middle latency responses (AMLRs) to spectrotemporally complex speech and spectrally complex nonspeech stimuli evoked from right and left ear stimulation in normal hearing adult females.
Study Sample: ABR and AMLR responses elicited by a spectrotemporally complex speech stimulus /da/ and a spectrally complex nonspeech stimulus were obtained in a group of twenty right-handed normal hearing adult females.
Data Collection and Analysis: Electrophysiological brainstem FFRs and AMLRs were recorded using a 40 ms synthesized speech syllable /da/ presented both forwards and backwards in addition to a 40 ms complex tone. Monaural ipsilateral FFRs and AMLRs were obtained with insert earphones at an intensity of 80 dB SPL.
Results: There were no significant differences in the right or left ear evoked FFRs to the complex tone or the speech stimulus played either forwards or backwards. However, the left ear AMLRs to the speech syllable played both in the forwards and backwards mode were significantly earlier than those obtained from the right ear.
Conclusions: The results from this study do not support previous findings of a subcortical right ear advantage (REA) for any portion of the synthetic syllable /da/ and suggest that the subcortical neural network does not preferentially process short duration spectrotemporally complex acoustic stimuli differently based upon the spectral, temporal or lexical content of the stimulus. However, the AMLR results suggest that the neural mechanisms generating the AMLR Pa response react earlier to the speech syllable played both forwards or backwards during left ear stimulation. It may be deduced that the earlier Pa responses to left ear stimulation are due to the prosodic acoustic features of both the forwards and backwards syllable that are absent in the spectrally complex tone.