Spatial location is accurately tracked by human auditory sensory memory: evidence from the mismatch negativity Leon Y. Deouell, 1 Ariel Parnes, 1 Natasha Pickard 2 and Robert T. Knight 2 1 Department of Psychology, The Hebrew University of Jerusalem, Jerusalem, 91905, Israel 2 Department of Psychology and the Helen Wills Neuroscience Institute, University of California at Berkeley, Berkeley, California, USA Keywords: event-related potentials, mismatch negativity (MMN), planum temporale, sound localization, superior temporal gyrus Abstract The nature of spatial representation in human auditory cortex remains elusive. In particular, although humans can discriminate the locations of sounds as close as 1–10 degrees apart, such resolution has not been shown in auditory cortex of humans or animals. We used the mismatch negativity (MMN) event related brain potential to measure the neural response to spatial change in humans in narrow 10 degree spatial steps. Twelve participants were tested using a dense array EEG setup while watching a silent movie and ignoring the sounds. The MMN was reliably elicited by infrequent changes of spatial location of sounds in free field. The MMN amplitude was linearly related to the degree of spatial change with a resolution of at least 10 degrees. These electrophysiological responses occurred within a window of 100–200 milliseconds from stimulus onset, and were localized to the posterior superior temporal gyrus. We conclude that azimuthal spatial displacement is rapidly, accurately and automatically represented in auditory sensory memory in humans, at the level of the auditory cortex. Introduction A fundamental function of the sensory system is to alert the organism to environmental change. It is easy to envisage natural situations in which the ability to automatically and accurately detect unexpected changes in sound source locations may determine survival. However, animal studies suggest that auditory neurons have very broad receptive fields, frequently encompassing whole hemispaces (Ahissar et al., 1992; Brugge et al., 1996; Middlebrooks et al., 1998; Tian et al., 2001; Stecker et al., 2003, 2005a), and although auditory cortex lesions are reported to disrupt sound localization in humans (Zatorre & Penhune, 2001), nearly nothing is known about the resolution of acoustic space in human auditory cortex. In humans, automatic change detection is reflected by the mismatch negativity (MMN; Na ¨a ¨ta ¨nen et al., 2001) event related brain potential. MMN is elicited experimentally by an infrequent unattended sound (deviant) that differs along one or more features from previous unattended sounds (standards). The main generators of the MMN are in primary and secondary auditory cortex in the superior temporal gyrus (e.g. Alho, 1995; Halgren et al., 1995; Rosburg, 2003), with secondary generators in the frontal cortex (Giard et al., 1990; Deouell et al., 1998). The exact location of superior temporal gyrus generators is probably contingent upon the deviating acoustic feature (Giard et al., 1995; Rosburg, 2003). The MMN depends on the magnitude of deviance, increasing monotonically in amplitude as the difference between the standard and the deviant increases (e.g. along sound frequency, Tiitinen et al., 1994; Yago et al., 2001; duration, Amenedo & Escera, 2000; or stimulation rate, Sable et al., 2003). This MMN gain function provides an objective measure of the nature (e.g. linear or logarithmic) and accuracy of representation of the specific dimension in sensory memory (Tiitinen et al., 1994; Na ¨a ¨ta ¨nen & Alho, 1997). Conflicting results have been obtained regarding the MMN gain function for spatial change. The MMN amplitude increased with deviance magnitude when space was simulated with headphones, manipulating interaural time or level differences, or even using standard head-related transfer functions (Na ¨a ¨ta ¨nen et al., 1988; Paavilainen et al., 1989; Doeller et al., 2003; Nager et al., 2003; Sonnadara et al., 2006). Nevertheless, although humans may be able to discriminate deviations as small as 1–10° of azimuth in frontal space (Perrott & Saberi, 1990), and auditory attention can be tightly focused within a range of 6° or better (Teder-Sa ¨leja ¨rvi et al., 1999b), the spatial separation between deviants in these studies of nonatten- tional change detection was 30° at best. Moreover, studies that have employed a realistic ‘free field’ setup, using an array of loudspeakers, failed to find a clear relation between the degree of change and MMN amplitude. Rather, an ‘all or none’ phenomenon was found wherein the MMN simply indexed any change in spatial location without localizing information (Paavilainen et al., 1989; Colin et al., 2002). [The term ‘free field’ is used in the literature either to represent an anechoic environment or to distinguish presentation from environ- mental sources (including loudspeakers) as opposed to presentation via headphones. In this paper, we take the term to denote the latter meaning.] Thus, whether the location of sound is automatically tracked at the level of the auditory cortex in humans, with a fine resolution compatible with behaviour, is unknown. We used the MMN to assess the preattentive representation of sound location using a free-field setup and a finer scale of spatial resolution. Correspondence: Dr Leon Y. Deouell, as above. Email: msleon@huji.ac.il Received 11 May 2006, revised 19 June 2006, accepted 26 June 2006 European Journal of Neuroscience, Vol. 24, pp. 1488–1494, 2006 doi:10.1111/j.1460-9568.2006.05025.x ª The Authors (2006). Journal Compilation ª Federation of European Neuroscience Societies and Blackwell Publishing Ltd