The cocktail party problem: what is it? How can it be solved? And why should animal behaviorists study it?

In many animals, acoustic communication occurs in large groups or aggregations of signaling individuals (Gerhardt & Huber, 2002; Greenfield, 2005; Kroodsma & Miller, 1996; McGregor, 2005; this issue). It is important to appreciate that the sounds produced by multiple signalers in groups, as well as other biotic and abiotic sources of noise and acoustic reflections from objects in the environment, are sound pressure waves that add together to form a composite pressure waveform that impinges on a receivers’ hearing organs. Thus, signaling in groups poses a number of special problems for receivers in terms of detecting and classifying signals (Brumm & Slabbekoorn, 2005; Hulse, 2002; Klump, 1996; Langemann & Klump, 2005; Wiley, 2006). These problems, and their solutions, are our concern here. For humans, solutions to these problems rest, in part, on the auditory system's ability to parse the composite acoustic waveform generated by multiple sources into perceptually coherent representations – termed auditory objects, auditory images, or auditory streams – that represent different sound sources in the acoustic scene (Bregman, 1990; Van Valkenburg & Kubovy, 2004; Yost, 1991). One well-known example of a problem in human auditory scene analysis (Bregman, 1990) is the aptly-named cocktail party problem (Cherry, 1953), which refers to the difficulty we sometimes have understanding speech in noisy social settings (reviewed in Bronkhorst, 2000; Yost, 1997). In this review, we discuss issues relating to auditory scene analysis and the cocktail party problem that have a long history in studies of human hearing and speech communication but have received less attention in the study of animal acoustic communication. The take-home messages from this review are (i) that many of the sensory solutions to the human cocktail party problem represent potentially important mechanisms underlying acoustic communication in non-human animals and (ii) that studying these mechanisms has important implications for our understanding of animal acoustic communication. This review is organized around three questions. In the first section we pose the question: What is the “cocktail party problem”? Here, we more explicitly outline the issues at hand and show that non-human animals also encounter and solve cocktail-party-like problems. In the second section, we ask: How can the cocktail party problem be solved? In this section, we draw extensively on studies of human hearing and speech perception to illustrate the diversity of mechanisms that allow humans to perceive speech in noisy social settings. We also show how many of these same auditory mechanisms operate in non-human animals. These studies could serve as useful heuristics to guide future research on the mechanisms of animal acoustic communication. In the third and final section we ask: Why should animal behaviorists study the cocktail party problem? Here, we raise a number of basic and applied issues to suggest the potential benefits and opportunities associated with studies of “animal cocktail parties”. The fields of study we attempt to integrate are broad; therefore, the scope of our review is necessarily broad and we make no attempt to cover any single topic in great depth. For other treatments on these topics, readers are referred to Bregman (1990, 1993), Bronkhorst (2000), Carlyon (2004), Darwin and Carlyon (1995), and Darwin (1997) for work in humans, and Klump (1996, 2005), Feng and Ratnam (2000), Hulse (2002), Brumm and Slabbekoorn (2005), and Langemann and Klump (2005) for work in animals.