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Neural activity in the auditory system decreases with repeated stimulation, matching stimulus probability in multiple timescales. This phenomenon, known as stimulus-specific adaptation, is interpreted as a neural mechanism of regularity encoding aiding auditory object formation. However, despite the overwhelming literature covering recordings from single-cell to scalp auditory-evoked potential (AEP), stimulation timing has received little interest. Here we investigated whether timing predictability enhances the experience-dependent modulation of neural activity associated with stimulus probability encoding. We used human electrophysiological recordings in healthy participants who were exposed to passive listening of sound sequences. Pure tones of different frequencies were delivered in successive trains of a variable number of repetitions, enabling the study of sequential repetition effects in the AEP. In the predictable timing condition, tones were delivered with isochronous interstimulus intervals; in the unpredictable timing condition, interstimulus intervals varied randomly. Our results show that unpredictable stimulus timing abolishes the early part of the repetition positivity, an AEP indexing auditory sensory memory trace formation, while leaving the later part (ϳϾ200 ms) unaffected. This suggests that timing predictability aids the propagation of repetition effects upstream the auditory pathway, most likely from association auditory cortex (including the planum temporale) toward primary auditory cortex (Heschl's gyrus) and beyond, as judged by the timing of AEP latencies. This outcome calls for attention to stimulation timing in future experiments regarding sensory memory trace formation in AEP measures and stimulus probability encoding in animal models.

Auditory change detection has been associated with mismatch negativity (MMN), an event-related potential (ERP) occurring at 100-250 ms after the onset of an acoustic change. Yet, single-unit recordings in animals suggest much faster novelty-specific responses in the auditory system. To investigate change detection in a corresponding early time range in humans, we measured the Middle Latency Response (MLR) and MMN during a controlled frequency oddball paradigm. In addition to MMN, an early effect of change detection was observed at about 40 ms after change onset reflected in an enhancement of the Nb component of the MLR. Both MMN and the Nb effect were shown to be free from confounding influences such as differences in refractoriness. This finding implies that early change detection processes exist in humans upstream of MMN generation, which supports the emerging view of a hierarchical organization of change detection expanding along multiple levels of the auditory pathway.

No abstract