Detection of small across-channel timing di¡erences by cochlear implantees Robert P. Carlyon a; *, Luc Geurts b , Jan Wouters b a MRC Cognition and Brain Sciences Unit, 15 Chaucer Rd., Cambridge, CB2 2EF, UK b Lab. Exp. ORL, KU Leuven, Kapucijnenvoer 33, 3000 Leuven, Belgium Received 12 March 1999; received in revised form 18 November 1999; accepted 26 November 1999 Abstract Five post-lingually deafened users of the LAURA cochlear implant were presented with two trains of biphasic pulses applied concurrently to two widely separated channels. They could all discriminate between stimuli where pulses on the two channels were nearly synchronous (inter-channel delay = 0.1 ms) and those where there was a longer delay applied to one channel. All showed an asymmetry, being more sensitive when the longer delay was on either the more basal or, depending on the listener, the more apical channel. For four out of the five listeners this asymmetry could be at least partly attributed to one stimulus, with a 0.1-ms delay in either the apical (three listeners) or basal (one listener) channel, sounding markedly different from all other stimuli used in the experiment. Both the overall sensitivity of listeners and the general pattern of results survived the presentation of maskers on intermediate channels, and did not vary markedly with changes in the polarity of the pulses applied to one channel. Although the results varied substantially across listeners, it is concluded that they demonstrate a genuine sensitivity to the relative timing of stimulation applied to discrete populations of auditory nerve fibers. ß 2000 Elsevier Science B.V. All rights reserved. Key words: Cochlear implant ; Across-channel timing ; Phase e¡ect 1. Introduction This article examines the ability of cochlear implant- ees to discriminate ¢ne timing di¡erences between pul- satile stimuli applied concurrently to two channels, which innervate widely separated regions of the cochlea. The ability, if it exists, can be thought of as an electrical analog of the sensitivity of the normal auditory system to timing di¡erences between the envelopes of acoustic stimuli occupying widely separated frequency regions (Strickland et al., 1989; Yost and Sheft, 1989; Carlyon, 1994). As discussed below, there are theoretical reasons to expect that the sensitivity of implantees to across- channel timing di¡erences might exceed that of normal listeners to across-frequency asynchronies. A demon- stration of such ¢ne sensitivity in conditions where the two channels excite discrete populations of auditory nerve (AN) ¢bers would lead to two important conclu- sions. First, it would show that acoustical stimulation underestimates the ability of the central auditory system to compare the pattern of activity between di¡erent populations of AN ¢bers. Second, it would indicate that, even for widely separated channels, the relative timing between electrical pulses can a¡ect the percepts experienced by cochlear implantees. This in turn might mean that the across-channel delays implemented in the `continuous interleaved sampling (CIS)' strategy (Wil- son et al., 1991) are perceptible, and could have impli- cations for the values of such delays used in future CIS- based algorithms. However, as also discussed below, existing evidence obtained with electrical stimulation does not support the existence of a mechanism sensitive to across-channel timing di¡erences. Psychoacoustical experiments have shown that nor- mally hearing listeners can detect phase di¡erences of about 30^60³ between the envelopes of sinusoidally am- plitude-modulated tones with di¡erent carrier frequen- cies (Strickland et al., 1989; Yost and Sheft, 1989). In 0378-5955 / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII:S0378-5955(99)00215-4 * Corresponding author. Tel.: +44 (1223) 355294, ext. 720; Fax: +44 (1223) 359062; E-mail: bob.carlyon@mrc-cbu.cam.ac.uk Hearing Research 141 (2000) 140^154 www.elsevier.com/locate/heares