Auditory Inference and Long-Term Modulation of Excitation and Inhibition

Auditory event-related potential (ERP) analysis typically involves extraction of responses time-locked to each sound averaged together by auditory event type. In oddball sequences, a repetitious standard sound is occasionally interrupted by a low probability deviant. ERP component differences between standard and deviant ("oddball") events support the inference that humans rapidly and automatically extrapolate from patterns to predict the most likely structure and properties of upcoming sounds. A sound sequence composed of repeating patterns at multiple timescales was used to determine whether there is evidence of response normalization over long timescales, that is, reduced differences between standard and deviant responses over time. Two sounds (30 and 60 ms pure tones) were organized into four blocks in which the two sounds alternated in tone probability as rare deviants (p = 0.125) or common standards (p = 0.875). Continuous EEG was collected from 32 participants who heard four occurrences of this four-block alternating-oddball sequence. We extracted five-tone epochs centered on the deviant, exploiting a design principle of oddball sequences used in ERP studies that deviants are always separated by occurrences of the repeating standard, thus ensuring in this case that every deviant was preceded and followed by a minimum of three standards. The five-tone epoch analysis revealed that the occurrence of a deviant triggered a prolonged negative shift in the ERP that extended across the whole response of the deviant and the subsequent standard. The amplitude of this "deviant complex" (over 120-600 ms post deviant) and a mean "standard complex" amplitude (480-0 ms pre-deviant) were analyzed alongside conventional ERP components, assessing them for change over time. Results revealed convergence of the standard and deviant responses, such that differences reduced over long timescales. Findings are discussed with respect to being potential indices of excitatory/inhibitory homeostasis and evidence of pattern prediction over several 10s of minutes.