The Dynamics of Auditory Streaming: Psychophysics, Neuroimaging, and Modeling

Listening to a speaker or a melody in the presence of competing sounds crucially depends on our brain’s sophisticated ability to organize a complex sound mixture changing over time into coherent perceptual objects or “streams”, which generally correspond to sound sources in the environment (Bregman 1990). The acoustic factors governing this “auditory streaming” are well established (Carlyon 2004), and various theories have been proposed to explain auditory stream formation (Anstis and Saida 1985; Beauvois and Meddis 1991; Bregman 1990; Hartman and Johnson 1991; McCabe and Denham 1997; van Noorden, 1975). However, it remains unclear how and where auditory streaming is achieved in the brain. A common limitation of early studies that tried to find the neural correlates of auditory streaming is that the neural response patterns corresponding to the different states of perceptual streaming were evoked by physically different stimuli (Alain et al. 1998; Fishman et al. 2001, 2004; Naatanen et al. 2001; Sussman et al. 1999). This makes it difficult to determine whether the neural response patterns reflect perception per se or they simply reflect the physical properties of the stimuli. To overcome this difficulty, recent studies have taken advantage of the fact that the segregation of sounds into streams typically takes several seconds to build up (Carlyon et al. 2001; Cusack 2005; Gutschalk et al. 2005; Micheyl et al. 2005). Under appropriate conditions, a physically unchanging sequence of alternating tones initially tends to be heard as a single coherent stream, and after several seconds it appears to split into two distinct streams (Anstis and Saida 1985). This makes it possible to compare neural responses corresponding to different percepts without introducing any physical change in the stimulus. Based on this approach, the neural correlates of auditory streaming have been found in the primary auditory cortex (Micheyl et al. 2005), the non-primary auditory cortex (Gutchalk et al. 2005), and the intraparietal