138 Hearing Research, 59 (1992) 138-156 © 1992 Elsevier Science Publishers B.V. All rights reserved 0378-5955/92/$05.00 HEARES 01722 Single-fibre and whole-nerve responses to clicks as a function of sound intensity in the guinea pig Huib Versnel, Ruurd Schoonhoven and Vera F. Prijs ENT Department, Unicersity Hospital, Leiden, The Netherlands (Received 11 February 1991; accepted 19 December 1991) This paper describes a study of the intensity dependence of click-evoked responses of auditory-nerve fibres in relation to the simultaneously recorded compound action potential (CAP). Condensation and rarefaction clicks were presented to normal hearing guinea pigs over an intensity range of 60 dB. The recorded poststimulus time histograms (PSTHs) were characterized by the latency (tp), amplitude (A e) and synchronization (Sp) of their dominant peak, parameters that are particularly important for the understanding of the CAP. For all fibres t i, decreased monotonically with increasing intensity, in a continuous way for fibres with high characteristic frequency (CF > 3 kHz), and in discrete steps of one CF-cycle for low-CF (CF < 3 kHz) fibres. An additional analysis of PSTH envelopes revealed that average latency shifts with intensity are similar for all CFs above 2 kHz. For all fibres Ap increased monotonically with intensity; the increase was stronger and maximum values were larger for low-CF than for high-CF fibres. A schematic model PSTH was then formulated on the basis of the experimental data. A sum of these model PSTHs from a hypothesized fibre population was convolved with an elemental unit response (Versnei et al., 1992) in order to simulate the compound action potential. Synthesized CAPs agreed with experimental CAPs in their main aspects. Poststimulus time histogram; Intensity; Click; Compound action potential; Convolution; Guinea Pig Introduction The compound action potential (CAP) as recorded by electrocochleography in human subjects or animals strongly depends on the intensity of the acoustic stimu- lus. Both for tonal and clic~ stimuli the amplitude of the CAP increases and its latency decreases with in- creasing intensity (e.g. Peake and Kiang, 1962; Egger- mont, 1976; Elberling, 1976; Salvi et al., 1979; Prijs and Eggermont, 1980; M¢ller, 1986). The CAP represents the summation of electric potential variations at the recording site (e.g. the round window) induced by discharges of the responding afferent fibres. If the unit response (UR) associated with a single discharge is identical across fibres of different CF and SR (cf. Kiang et al., 1976; Prijs, 1986; Versnel et al., 1992), if discharges of the individual fibres are mutually inde- pendent and if the single-fibre contributions add lin- early, then the relation between whole-nerve and sin- Correspondence to: H. Versnel, (Present address) Systems Research Center, University of Maryland, College Park, MD 20742, USA. Fax: (301)314-9920. Preliminary results were presented at the 2nd International Sym- posium on Cochlear Mechanics and Otoacoustic Emissions, Rome, March 1989. gle-fibre responses can be expressed by a convolution as (Goldstein and Kiang, 1958): -- S(t)U(t- 'r) dz (1) in which N is the number of fibres, C(t) is the CAP, U(t) is the unit response (UR), Pi(t) is the PSTH of the i th fibre as the experimental estimate of its discharge probability density function, and S(t) the compound or whole-nerve discharge latency distribution, i.e. the sum of Pi's. Several studies have been reported on the relation between the CAP and the underlying single-fibre dis- charge patterns using the convolution description (e.g. de Boer, 1975; Elberling, 1976; Wang, 1979; Bappert et al., 1980; Dolan et al., 1983; Versnel et al., 1990). De Boer (1975) modelled CAPs to tone bursts in cat applying a theoretical model in which the parameter choice for the Pi(t) constituting S(t) was based on reverse-correlation data for the cochlear filter com- bined with an empirical description of the inner hair