Research paper Amplitude-modulation detection by gerbils in reverberant sound fields Andrea Lingner, Kathrin Kugler, Benedikt Grothe, Lutz Wiegrebe * Division of Neurobiology, Department Biology II, Ludwig-Maximilians-Universitaet Munich, Großhaderner Str. 2-4, D-82152 Martinsried-Planegg, Germany article info Article history: Received 19 November 2012 Received in revised form 5 April 2013 Accepted 9 April 2013 Available online 18 April 2013 abstract Reverberation can dramatically reduce the depth of amplitude modulations which are critical for speech intelligibility. Psychophysical experiments indicate that humans’ sensitivity to amplitude modulation in reverberation is better than predicted from the acoustic modulation depth at the receiver position. Electrophysiological studies on reverberation in rabbits highlight the contribution of neurons sensitive to interaural correlation. Here, we use a prepulse-inhibition paradigm to quantify the gerbils’ amplitude modulation threshold in both anechoic and reverberant virtual environments. Data show that prepulse inhibition provides a reliable method for determining the gerbils’ AM sensitivity. However, we find no evidence for perceptual restoration of amplitude modulation in reverberation. Instead, the deterioration of AM sensitivity in reverberant conditions can be quantitatively explained by the reduced modulation depth at the receiver position. We suggest that the lack of perceptual restoration is related to physical properties of the gerbil’s ear input signals and inner-ear processing as opposed to shortcomings of their binaural neural processing. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Natural sounds, such as speech, have pronounced temporal- envelope modulations. These modulations provide crucial infor- mation for speech intelligibility in quiet situations (Shannon et al., 1995, 1998). However, in our daily life, speech signals are nearly always accompanied by reverberation, resulting from reflections of the sound on its path from the source to the receiver. These re- flections create a ‘tail’ of acoustic excitation that is added to the sound, interfering with both temporal modulations and spatial cues of the sound. The detrimental effect of reverberation especially on amplitude modulation (AM) is known from the everyday challenge of understanding speech in highly reverberant environments like a church (Houtgast et al., 1980). Indeed, many studies have quantified the detrimental effects of reverberation on speech intelligibility (e.g. Knudsen, 1929; Lochner and Burger, 1961; Santon, 1976; Houtgast and Steeneken, 1973) and also pitch perception (Sayles and Winter, 2008). Earlier experiments have indicated perceptual compensation for reverberation for speech stimuli (Watkins, 1991 , 2005; Brandewie and Zahorik, 2010; Nielsen and Dau, 2010; Srinivasan and Zahorik, 2013). In line with these data, a report by Zahorik et al. (2011) suggests that humans’ sensitivity to AM in reverberation is higher than predicted from the modulation transfer function measured in an anechoic or diotic condition. Recent electrophysiological data obtained from rabbits provide a possible neural correlate of this compensation mechanism by binaural processes in the auditory midbrain (Delgutte et al., 2012). Specifically, neurons sensitive to changes in interaural correlation may contribute to the restoration of AM because the reverberant tail has a lower correlation than the direct sound. Thus, amplitude modulation may be partially recoded by the binaural system into a modulation of interaural correlation. These physiological data relate the perceptual compensation of reverberation to a body of literature on the perception and neural coding of periodic fluctu- ations in interaural correlation: Grantham and Wightman (1978), Grantham (1982) quantified the extent to which human listeners can detect periodic changes in interaural correlation of the Oscillating-correlation (Oscor) stimulus. Later electrophysiological work showed a neural correlate of human-perceptual sensitivity in the cat auditory midbrain (Joris et al., 2006). While in the classical Oscor stimulus the interaural correlation fluctuates sinusoidally between 1 and À1, Siveke et al. (2008) introduced an Oscor01 Abbreviations: AM, amplitude modulation; PPI, prepulse inhibition; IR, impulse response; MD, modulation depth. * Corresponding author. Tel.: þ49 89 2180 74 314; fax: þ49 89 2180 99 74314. E-mail addresses: lingner@zi.biologie.uni-muenchen.de (A. Lingner), kugler@ bio.lmu.de (K. Kugler), grothe@bio.lmu.de (B. Grothe), lutzw@lmu.de, wiegrebe@ zi.biologie.uni-muenchen.de (L. Wiegrebe). Contents lists available at SciVerse ScienceDirect Hearing Research journal homepage: www.elsevier.com/locate/heares 0378-5955/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.heares.2013.04.004 Hearing Research 302 (2013) 107e112