Research paper The time course of recovery from suppression and facilitation from single units in the mammalian cochlear nucleus Stefan Bleeck b , Mark Sayles a , Neil J. Ingham a , Ian M. Winter a, * a Centre for the Neural Basis of Hearing, The Physiological Laboratory, Downing Street, Cambridge CB2 3EG, United Kingdom b Institute of Sound and Vibration Research, University of Southampton University Road, Highfield, Southampton S017 1BJ, United Kingdom Received 28 September 2005; received in revised form 2 December 2005; accepted 5 December 2005 Available online 3 February 2006 Abstract The responses to two identical, consecutive pure tone stimuli with varying inter-stimulus intervals (Dts) were measured for 89 neurons in the cochlear nucleus of the anaesthetised guinea pig. We observed two main effects; either a decrease (suppression) or an increase (facil- itation) in response to the second tone followed by an exponential recovery. Response behaviour correlated with the unit type; primary- like, primary-like with notch and transient-chopper units showed a recovery from suppression that was very similar to that already reported in the auditory nerve. For chopper units the strength of the adaptation was correlated with the units regularity of spike dis- charge; sustained chopper (CS) units showed less suppression than transient choppers. Onset units showed complete suppression at short Dts. Pause/Build (PB) units responded with increased activity to the second tone. In contrast to previous studies in the cochlear nucleus the recovery from suppression or facilitation was well described by a single exponential function, enabling us to define a recovery time constant and a maximum suppression/facilitation. There appeared to be a hierarchy in the time constant of recovery with PB and CS units showing the longest recovery times and onset units showing the shortest. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Adaptation; Forward masking; Auditory brainstem; Onset unit; Primary-like; Chopper 1. Introduction Most neurons in the early auditory pathway change their response during the presentation of a tone. When the response is a decrease this change is referred to as ‘‘adaptation’’ (Adrian, 1928). In order to fully understand the representation of complex time varying stimuli a knowledge of the magnitude and time course of recovery from adaptation is essential (Delgutte, 1980; Mandava et al., 1995). This recovery can be measured by a forward masking procedure, in which the neuron’s response to a second tone, often called the ‘probe’, is compared to the response to a first, preceding, tone often called a ‘masker’ or ‘conditioning’ tone. In the auditory system adaptation has been most extensively studied at the level of the audi- tory nerve (for reviews see Smith et al., 1983; Eggermont, 1985). Of immediate relevance here is a study by Harris and Dallos (1979) who recorded from single auditory nerve fibres in anaesthetised chinchillas in response to a ‘‘paired- tone’’ stimulation paradigm. It was shown that the recov- ery from adaptation followed an exponential time course over approximately 200 ms. The reduced firing rate in response to the probe tone has often been hypothesised as a physiological correlate of the psychophysical phenom- enon of forward masking. Similar paired tone paradigms have also been used in the cochlear nucleus (CN), the termination site of all audi- 0378-5955/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.heares.2005.12.005 Abbreviations: PSTH, post-stimulus time histogram; CN, cochlear nucleus; CV, coefficient of variation; CAP, compound action potential; BF, best frequency; NPR, normalised probe rate; OCB, olivocochlear bundle; SR, spontaneous rate; CS, sustained chopper; CT, transient chopper; OC, conset chopper; OL, onset with later activity; PB, pause- build; PL, primary-like * Corresponding author. Tel.: +44 1223 333812; fax: +44 1223 333840. E-mail address: imw1001@cam.ac.uk (I.M. Winter). www.elsevier.com/locate/heares Hearing Research 212 (2006) 176–184