Steady state evoked potentials: a new tool for the accurate assessment of hearing in cochlear implant candidates.

Precise determination of residual hearing in prospective cochlear implant candidates is essential. As the minimum age of implantation for young children has reduced, the use of objective measures of hearing has become more important. At the University of Melbourne Cochlear Im­ plant Clinic, steady state evoked potential (SSEP) assessments are rou­ tinely carried out on all candidates under the age of 5 years using a micro­ computer and custom-designed hardware in the manner described by Cohen et al. [I]. Steady state evoked potentials are scalp potentials elicited in response to sinusoidally amplitude and/or frequency modulated tones. The resulting potential is periodic, and is phase locked to the modulation envelope of the stimulus, Good estimations of behavioural thresholds have been reported across a range of carrier frequencies (250-4,000 Hz), in both normal and hearing­ impaired awake adults using SSEP testing techniques at repetition rates of 40 Hz [2, 3]. In addition to these findings, Cohen et al. [I] demonstrated that the response can be elicited by stimuli at low sound pressure levels in sleeping adults if modulation rates above 70 Hz are used. We have also successfully employed these high modulation frequencies to test sleeping neonates, obtaining results consistent with those seen in awake and sleep­ ing adults, and suggesting that this technique is suitable for use as a mea­ sure of hearing acuity in sleeping children [4]. Furthermore, Rickards et al. [5] showed good agreement between SSEP thresholds and tone pip elicited 45 SSEPs in Cochlear Implant Candidates