Central auditory onset responses, and temporal asymmetries in auditory perception D.P. Phillips  , S.E. Hall, S.E. Boehnke Hearing Research Laboratory, Department of Psychology, Dalhousie University, Halifax, NS, Canada B3H 4J1 Received 22 January 2002; accepted 2 April 2002 Abstract Historically, central auditory responses have been studied for their sensitivity to various parameters of tone and noise burst stimulation, with response rate plotted as a function of the stimulus variable. The responses themselves are often quite brief, and locked in time to stimulus onset. In the stimulus amplitude domain, it has recently become clear that these responses are actually driven by properties of the stimulus’ onset transient, and this has had important implications for how we interpret responses to manipulations of tone (or noise) burst plateau level. That finding was important in its own right, but a more general scrutiny of the available neurophysiological and psychophysical evidence reveals that there is a significant asymmetry in the neurophysiological and perceptual processing of stimulus onsets and offsets: sound onsets have a more elaborate neurophysiological representation, and receive a greater perceptual weighting. Hypotheses about origins of the asymmetries, derived independently from psychophysics and from neurophysiology, have in common a response threshold mechanism which adaptively tracks the ongoing level of stimulation. ß 2002 Elsevier Science B.V. All rights reserved. Key words: Onset response; O¡set response; Auditory temporal asymmetry; Single unit; Perception 1. Introduction The responses of auditory neurons to tone and noise burst stimuli are increasingly dominated by an onset transient at successively more central stations of the auditory neuraxis. Historically, the sensitivity of central neurons to the intensity of a tone or noise burst stim- ulus has been described by plotting the response rate as a function of the plateau level of the stimulus. It has recently become clear that the transient onset response is not driven directly by the plateau amplitude of the stimulus, but by the dynamics of the stimulus’ onset ramp (rate of pressure change, in the case of linear rise-time pulses). Interestingly, however, there have been no comparable studies of response o¡sets. In prac- tice, this has been because o¡set responses are relatively uncommon in the central auditory system. This asym- metry in the neurophysiology raises the question of whether it supports perceptual asymmetries. The general purpose of this article is to draw together in one place two converging, but independent, lines of evidence on central auditory function. One thread of research concerns the evidence for asymmetries in the neurophysiological representation of sound onsets and o¡sets. The second line of research concerns psycho- physical evidence on auditory temporal asymmetries. We begin by describing the general properties of central auditory responses to tone and noise burst stimuli, em- phasizing studies of onset responses from the auditory forebrain. We then examine the properties of the sys- tem’s o¡set response. We then describe the recent re- interpretation of auditory onset responses, particularly as it pertains to sound amplitude coding. We then dis- cuss the adaptive nature of neural response thresholds. We discuss the notion that one way to conceptualize an aspect of central auditory function is temporal edge 0378-5955 / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII:S0378-5955(02)00393-3 * Corresponding author. Tel.: +1 (902) 494 23 83; Fax: +1 (902) 494 65 85. E-mail address: ears@is.dal.ca (D.P. Phillips). Abbreviations: ABR, auditory brainstem response; CF, characteristic frequency; SPL, sound pressure level Hearing Research 167 (2002) 192^205 www.elsevier.com/locate/heares