The Intraparietal Sulcus and Perceptual Organization Rhodri Cusack Abstract & The structuring of the sensory scene (perceptual organi- zation) profoundly affects what we perceive, and is of increas- ing clinical interest. In both vision and audition, many cues have been identified that influence perceptual organization, but only a little is known about its neural basis. Previous studies have suggested that auditory cortex may play a role in auditory perceptual organization (also called auditory stream segrega- tion). However, these studies were limited in that they just examined auditory cortex and that the stimuli they used to generate different organizations had different physical charac- teristics, which per se may have led to the differences in neural response. In the current study, functional magnetic resonance imaging was used to test for an effect of perceptual organiza- tion across the whole brain. To avoid confounding physical changes to the stimuli with differences in perceptual organiza- tion, we exploited an ambiguous auditory figure that is some- times perceived as a single auditory stream and sometimes as two streams. We found that regions in the intraparietal sulcus (IPS) showed greater activity when 2 streams were perceived rather than 1. The specific involvement of this region in per- ceptual organization is exciting, as there is a growing literature that suggests a role for the IPS in binding in vision, touch, and cross-modally. This evidence is discussed, and a general role proposed for regions of the IPS in structuring sensory input. & INTRODUCTION Often, the sound arriving from the environment is a mixture from many sources, but we are only interested in a subset of these. The auditory system initially decomposes the sound in many ways, such as by frequency in the ear and by other features later in processing. The resulting vast array of incoming sensory information is then structured, so that parts originating from different sources are allocated to different streams (Figure 1). This process is often referred to as auditory stream segregation or perceptual organization, the latter term also being used for the analogous process in vision. Auditory stream segregation exploits common- alities across sounds in the environment. For example, sequential sounds that are similar in their frequency or the pitch they evoke are more likely to have come from the same source than sounds with very dissimilar frequency or pitch. Sound components that begin at the same time or change in amplitude in the same way are likely to come from a common source. These and many other heuristics have been found to be important (see Darwin & Carlyon, 1995; Bregman, 1990, for reviews). Perceptual organization has a strong effect on how we hear the world (Cusack & Carlyon, 2004). Once streams are formed, we can selectively attend to just one at a time, but can generally switch between them at will. 1 Listeners find it hard to make comparisons in the tem- poral relationship or order of sounds across different streams (Bregman & Campbell, 1971). Streaming has a strong effect on rhythm (e.g., van Noorden, 1975), timbre, and pitch (Bregman, 1990). For example, if two people’s footsteps are interleaved and we hear them as a single stream, we will hear an average tempo that is twice as fast as if we hear them as two separate streams. How we hear music is strongly affected by streaming, and composers explicitly manipulate cues for perceptual organization to achieve particular rhythms, harmonies, and melodic separation (Huron, 1991). Clinically, abnormal perceptual organization has been implicated in syndromes such as autism and dyslexia. Frith (1989) influentially proposed that a large compo- nent of autism could be described by a cognitive pro- cessing style of weak central coherence. An important element of this is a greater tendency to perceive the parts of the sensory scene in detail rather than more integrated percepts. This has been demonstrated for many different types of stimuli, both in vision and au- dition (e.g., Foxton et al., 2003; Happe, 1996, 1999). This suggests a bias in the default form of perceptual orga- nization in autism. It has been thought for some time that abnormal basic auditory processing may also play a role in dyslexia (e.g., Tallal, Miller, & Fitch, 1993; Tallal, 1980). Recently, Sutter, Petkov, Baynes, and O’Connor (2000) and Helenius, Uutela, and Hari (1999) presented simple tone sequences and found the perceptual orga- nization of dyslexic adults differs from that for normal MRC Cognition and Brain Sciences Unit D 2005 Massachusetts Institute of Technology Journal of Cognitive Neuroscience 17:4, pp. 641–651