Simultaneously-evoked auditory potentials: a novel paradigm for measuring auditory-evoked electroencephalographic activity at successive levels of the auditory neuraxis.

Recent electrophysiological work has evinced a capacity for plasticity in subcortical auditory nuclei in adults (Skoe & Kraus, 2010). Similar plastic effects have been measured in cortically-generated auditory potentials (e.g., Bosnyak, Gander, & Roberts, 2007; Näätänen, 2008) but it is unclear how the two interact. Here we present SEAP (simultaneously-evoked auditory potentials), a novel paradigm designed to concurrently elicit electrophysiological brain potentials (EEG) from inferior colliculus (IC), thalamus, and primary and secondary auditory cortices. We use a specially designed stimulus consisting of a carrier frequency (500 Hz), amplitude-modulated at the sum of 37 and 81 Hz (depth 100%). We have shown that it elicits a 500 Hz frequency-following response (FFR; generated in IC), 81 (subcortical) and 37 (primary auditory cortex) Hz steady state responses, and mismatch negativity (when there is an occasional change in carrier frequency; secondary auditory cortex). Furthermore, cortical and subcortical processes are linked as the amplitude of the FFR predicts the amplitudes of the 37 and 81 Hz responses. SEAP offers a new window into the dynamics of encoding along the auditory pathway as well as a new and inexpensive tool with which to measure plasticity at multiple levels of the auditory neuraxis in individuals.