Harvard-MIT Division of Health Sciences and Technology HST.722J: Brain Mechanisms for Hearing and Speech Course Instructors: Bertrand Delgutte, M. Christian Brown, Joe C. Adams, Kenneth E. Hancock, Frank H. Guenther, Jennifer R. Melcher, Joseph S. Perkell, David N. Caplan Student Topic: Brain Attending a Cocktail Party Adrian KC Lee Brain Attending a Cocktail Party Introduction Imagine after our topic presentation, we go to a crowded pub to celebrate. You are trying hard to ignore those eighties tunes blaring out of the loudspeakers while you are chatting with your colleagues. Suddenly, this familiar melody from a Mozart’s piano concerto grabs your attention – yes, your cell phone alerts you that your partner just called for the third time… This classic phenomenon, in which our brain analyzes a scene by perceptually organizing sensory data to form auditory objects (or auditory streams), is often referred to as the “Cocktail Party Effect” (Cherry, 1953). Many cues have been identified that influence perceptual organization, but only little is known about the actual brain mechanisms underlying this phenomenon. In this proposed topic, we look at the latest development in the quest to find the neural basis for auditory stream segregation. Background – Auditory grouping mechanisms The ability to form auditory objects is important in the natural environment where sounds arriving at our ears are a resultant of all spectro-temporal components that may have arisen from different auditory events. We constantly analyze an auditory scene by trying to group related components from one source, and segregating out other frequency components that are not in the attended object (Carlyon, 2004). This ability of bringing acoustical events to the attention foreground may increase the chance of survival for a species in the animal kingdom through auditory awareness to the movement of their predators. Humans also rely on auditory grouping mechanisms for daily communication, especially in the presence of noise and other competing sources, since we need to group simultaneous components originating from a single source across frequency, as well as grouping events across time, in order to hear whole words and messages. In his seminal book, Bregman (1990) provides us with working definitions on the terminologies used in the world of auditory scene analysis. He described auditory stream segregation as: The general process of auditory scene analysis in which links are formed between parts of the sensory data. These links will affect what is included and excluded from our perceptual descriptions of distinct auditory events. HST.722 Brain Mechanisms for Hearing and Speech Page 1 of 6Student Topic: Brain Attending a Cocktail Party Adrian KC Lee If these links, which are also commonly referred to as streaming cues, are correct sensory parts across time, we refer to such perceptual grouping as sequential. However, if these sensory data coexist in time, and the formation of multiple auditory objects are as a result of perceptually partitioning of the spectral contents into distinct objects, e.g., harmonics in a vowel, it is referred to as simultaneous grouping. Apparent spatial location, onset / offset synchrony, frequency proximity, and fundamental frequency are but some of the common acoustic cues that we employed in auditory scene analysis. Experimental paradigm – Buildup of Streaming How does one systematically investigate the process of stream segregation that occurs in a complex auditory environment? A popular paradigm is to use an ambiguous auditory figure that could either be heard as one stream with a galloping rhythm (commonly labeled as “Horse”) or as two concurrent streams with two different tempi (“Morse”) (See Figure 1). The basic stimulus consists of a high tone A alternating with a low tone B, in repeated ABA_ sequences. If the frequency difference (∆f) between the A and B tones is small, then neighboring tones tend to bind together perceptually, resulting in a “Horse” rhythm. Conversely, if ∆f is large, the A and B tones are no longer bind to each other, resulting in a “Morse” rhythm. The tone repetition rate also influences the percept: the faster the repetition rate, the more binding there is between the A and B tones (van Noorden, 1975). At intermediate values of ∆f and tone repetition rate, the initial galloping “Horse” percept changes after a few seconds of listening to a “Morse” percept (Anstis and Saida, 1985; Bregman, 1978; Carlyon et al, 2001). With this systematic change in auditory percept over time, it is possible to record neural responses at various points during an ongoing sequence of sounds without any change in the evoking stimulus, and compare the neural responses associated with dramatically different percepts. HST.722 Brain Mechanisms for Hearing and Speech Page 2 of 6Student Topic: Brain Attending a Cocktail Party Adrian KC Lee Figure removed due to copyright considerations. Figure 1 For the correct parameters, these sequences are ambiguous and can be heard with one or two perceptual organizations with different rhythms: (Left): a characteristic galloping rhythm (“Horse”); (Right): 2 isochronous streams, like Morse code (“Morse”). Colored regions correspond to perceptual streams. (Taken from Fig. 2, Cusack, 2005). Key Areas of Research Many investigations into the neural correlates of auditory streaming employ the aforementioned ABA- paradigm, or the variants thereof, but their generic approaches and the specific questions they address can be divided into several distinct classes. One class of approach is to use single-unit recordings in animals to infer perceptual effects of stream segregation. Fisherman et al (2001, 2004) showed that multiunit spiking responses to tone sequences in the primary auditory cortex of awake monkeys follow the pattern that one might expect on the basis of published psychophysical data from human subjects. Bee and Klump (2004) performed similar sequential streaming experiments and recorded neural responses in the auditory forebrain of awake starlings. They concluded that while there are preattentive auditory processes, such as frequency selectivity and forward masking, that contribute to the perceptual segregation of sequential acoustic events having different frequencies into separate auditory streams, there may be additional processes that are required to account for all known perceptual