Brain dynamics encode the spectrotemporal boundaries of auditory objects

Perception of objects in the scene around us is effortless and intuitive, yet entails profound computational challenges. Progress has been made in understanding some mechanisms by which the brain encodes the boundaries and surfaces of visual objects. However, in the auditory domain, these mechanisms are poorly understood. We investigated differences between neural responses to spectrotemporal boundaries in the auditory scene. We used iterated rippled noise to create perceptual boundaries with and without energy transients. In contrast to boundaries marked by energy transients, second-order boundaries were characterized by an absence of early components in the event-related potential. First-order energy boundaries triggered a transient evoked gamma-band response and a well-defined P90 component of the event-related potential, whereas second-order boundaries evoked only the later N1 component. Furthermore, the N1 component was delayed when evoked by second-order boundaries and theta-band electroencephalography activity at this latency exhibited significant phase lag for second-order compared to first-order boundaries. We speculate that boundaries defined by sharp energy transients can be registered by early feed-forward mechanisms. By contrast, boundaries defined only by discontinuities at discrete frequency bands require integration across the tonotopic representation of the frequency spectrum and require time-consuming interaction between auditory areas.