Intensity and inter-stimulus-interval effects on human middle- and long-latency auditory evoked potentials in an unpredictable auditory context.

It is not known how Auditory-Evoked Responses (AERs) comprising Middle Latency Responses (MLRs) and Long Latency Responses (LLRs) are modulated by stimulus intensity and inter-stimulus interval (ISI) in an unpredictable auditory context. Further, intensity and ISI effects on MLR and LLR have never been assessed simultaneously in the same humans. To address this important question, thirty participants passively listened to a random sequence of auditory clicks of three possible intensities (65, 75, and 85 dB) at five possible ISI ranges (0.25 to 0.5 s, 0.5 to 1 s, 1 to 2 s, 2 to 4 s, 4 to 8 s) over four to seven one-hour sessions while EEG was recorded. P0, Na, Pa, Nb, and Pb MLR peaks and N1 and P2 LLR peaks were measured. MLRs P0 (p = .005), Pa (p = .021), and Pb (p = <.001) were modulated by intensity, while only MLR Pb (p = <.001) was modulated by ISI. LLR N1 and P2 were modulated by both intensity and ISI (all p values < .001). Intensity and ISI interacted at Pb, N1, and P2 (all p values < .001), with greater intensity effects at longer ISIs and greater ISI effects at louder intensities. Together, these results provide a comprehensive picture of intensity and ISI effects on AER across the entire thalamocortical auditory pathway, while controlling for stimulus predictability. Moreover, they highlight P0 as the earliest MLR response sensitive to stimulus intensity and Pb (~50 ms) as the earliest cortical response coding for ISIs above 250 ms and showing an interdependence between intensity and ISI effects.

Differential deviant probability effects on two hierarchical levels of the auditory novelty system.

Deviance detection is a key functional property of the auditory system that allows pre-attentive discrimination of incoming stimuli not conforming to a rule extracted from the ongoing constant stimulation, thereby proving that regularities in the auditory scene have been encoded in the auditory system. Using simple-feature stimulus deviations, regularity encoding and deviance detection have been reported in brain responses at multiple latencies of the human Auditory Evoked Potential (AEP), such as the Mismatch Negativity (MMN; peaking at 100-250ms from stimulus onset) and Middle-Latency Responses (MLR; peaking at 12-50ms). More complex levels of regularity violations, however, are only indexed by AEPs generated at higher stages of the auditory system, suggesting a hierarchical organization in the encoding of auditory regularities. The aim of the current study is to further characterize the auditory hierarchy of novelty responses, by assessing the sensitivity of MLR components to deviant probability manipulations. MMNs and MLRs were recorded in 24 healthy participants, using an oddball location paradigm with three different deviant probabilities (5%, 10% and 20%), and a reversed-standard (91.5%). We analyzed differences in the MLRs elicited to each of the deviant stimuli and the reversed-standard, as well as within deviant stimuli. Our results confirmed deviance detection at the level of both MLRs and MMN, but significant differences for deviant probabilities were found only for the MMN. These results suggest a functional dissociation between regularity encoding, already present at early stages of auditory processing, and the encoding of the probability with which this regularity is disrupted, which is only processed at higher stages of the auditory hierarchy.