In the Auditory Cognitive Neuroscience Laboratory, the research topics we investigate address how people perceive sound in complex environments. An example of a complex environment we encounter in everyday life is a noisy restaurant or a crowded park. In situations like this, many objects are producing sound at the same time. The auditory system must therefore decide which sound events arise from the same object, and which arise from different objects. Without this type of sorting process, we would be unable to perceive the patterns of speech and music, or to notice changes in scenes of environmental sounds. In particular, sounds from different objects might be erroneously grouped together, creating sound patterns that make no sense.

Auditory Scene Analysis
Auditory Scene Analysis is the set of internal processes that segregate and group sounds according to the physical sources that produce the sounds (Bregman, 1990). Traditionally, auditory scene analysis has been explained in terms of peripheral processing mechanisms (e.g., Hartmann & Johnson, 1991) that do not require attention or knowledge. This peripheral channeling theory is supported by evidence that segregation is most robust when sounds differ in spectral distribution and thereby activate distinct portions of the cochlea.

More recent research using psychophysical techniques and direct brain measurements have revealed that auditory scene analysis can arise from processing at non-peripheral stages. In particular, perceptual qualities that are computed in the central auditory system can lead to perceptual segregation, demonstrating that peripheral cue extraction is not necessary for segregation to occur (Vliegen et al., 1999). Furthermore, attention has been shown to have a large influence on whether participants perceive segregated tone patterns (Carlyon et al., 2001). This suggests that segregation occurs as late as secondary auditory cortex, which is the earliest processing stage that is robustly influenced by attention (Petkov et al., 2004).

Our data have contributed to this debate by demonstrating the presence of multiple levels of processing in auditory segregation. In particular, event-related potentials (ERPs) localized to auditory cortex demonstrated that frequency-based segregation is not influenced by attention, whereas the buildup process involved in switching from one to two streams of tones is influenced by attention (Snyder et al., 2006). A separate series of experiments demonstrated an effect of context on auditory segregation that arises from representations with broad frequency tuning and sensitivity to rhythmic patterning (Snyder et al., 2008; Snyder, Carter et al., 2009; Snyder & Weintraub, in press). These results suggest that the effect arises from stages later than the auditory periphery, in contrast to other known effects of context on streaming (Anstis & Saida, 1985). These findings suggest a hierarchical model of streaming that includes important processing at multiple levels of the auditory system (for a review, see Snyder & Alain, 2007b).

Change Deafness
Because of the dynamic nature of sound, memory is a critical aspect of accurately perceiving auditory scenes. However, multiple lines of research have shown that auditory memory failures are quite prominent in laboratory experiments testing change detection or recognition memory (for a review, see Snyder & Gregg, in press). We are using ERPs to understand why people have such difficulty in change detection tasks, which results in the phenomenon known as “change deafness”. ERPs can tell us the extent to which change detection failures result from sensory and cognitive stages of processing, and can therefore inspire targeted strategies for improving human performance in complex sound environments typical of everyday situations.

Auditory Scene Analysis and Aging
During the course of normal aging, hearing becomes more difficult especially in noisy situations. Part of the reason for this is that hearing acuity declines with aging, especially in the high-frequency range. However, this is probably not the only factor. One possibility we have explored is whether difficulties in segregating sound might play a role in age-related hearing difficulties. Using psychophysical and ERP measures, we determined that normal aging results in impaired perceptual segregation of sounds played at exactly the same time (Snyder & Alain, 2005) but aging does not impair perceptual segregation of interleaved tone sequences (Snyder & Alain, 2007a). We also demonstrated the existence of rapid auditory plasticity in young adults (Alain et al., 2007) and in older adults (Alain & Snyder, in press) during segregation of simultaneous sounds, raising hopes that intensive training in older adults may be beneficial for improving listening skills in noisy acoustic environments.

Rhythm Perception and Production
Our previous research on rhythm used behavioral and ERP methods to determine what information is used to extract the underlying beat in music (Hannon et al., 2004; Snyder & Krumhansl, 2001; Toiviainen & Snyder, 2003), how culture affects the ability to synchronize with music (Snyder et al., 2006) and what neural processes underlie our anticipation of the beat in music (Snyder & Large, 2005). We are continuing this work, using a variety of measures to understand the neural basis of beat perception and what auditory and motor brain regions are involved in rhythm perception and production.

Auditory Processing in Schizophrenia
Schizophrenia is characterized by auditory hallucinations, deficits in auditory discrimination and categorization, reduced gray matter volume in auditory cortex, and reduced electrophysiological responses in primary and secondary auditory cortices. By measuring ERP correlates of auditory inhibition in schizophrenia patients, we are examining the role of physiological inhibition in auditory processing deficits in schizophrenia. We are also testing patients with schizophrenia on auditory scene analysis tasks to determine whether and at what level of processing their impairments in auditory processing have an effect on auditory perceptual organization.

References (see Publications for references to our own papers)
Anstis, S., & Saida, S. (1985). Adaptation to auditory streaming of frequency-modulated tones. Journal of Experimental Psychology: Human Perception and Performance, 11, 257-271.

Bregman, A. S. (1990). Auditory scene analysis: The perceptual organization of sound. Cambridge, MA: MIT Press.

Carlyon, R. P., Cusack, R., Foxton, J. M., & Robertson, I. H. (2001). Effects of attention and unilateral neglect on auditory stream segregation. Journal of Experimental Psychology: Human Perception and Performance, 27, 115-127.

Hartmann, W. M., & Johnson, D. (1991). Stream segregation and peripheral channeling. Music Perception, 9, 155-184.

Petkov, C. I., Kang, X., Alho, K., Bertrand, O., Yund, E. W. and Woods, D. L. (2004). Attentional modulation of human auditory cortex. Nature Neuroscience, 7, 658-663.

Vliegen, J., Moore, B. C. J., & Oxenham, A. J. (1999). The role of spectral and periodicity cues in auditory stream segregation, measured using a temporal discrimination task. Journal of the Acoustical Society of America, 106, 938-945.