Dear Tamas,
I'm not sure if you are familiar with cortical auditory evoked potentials (CAEPs), especially in the context of cochlear implants. For a good starting place in the literature, here is a study on 12 experienced adult users of cochlear implants, relating several types of evoked potentials and behavioural measures (such as speech perception scores) with age- and sex-controlled matches:
http://dx.doi.org/10.1016/j.clinph.2005.02.011In general, when looking at auditory processing objectively (such as with evoked potentials), there does seem to be a consistent latency increase for cochlear implant users, in not only the earlier Middle Latency Responses (see Gordan et al 2005 for a child specific study), but also P1, N1 and P2 of the obligatory cortical responses (aka CAEPs). Latency differs when compared to the expected age-controlled normal hearing latency, most commonly a delay of some length. I have seen in the ballpark of 50-80ms delays, but have also seen CI users who have latencies comparable to generalized hearing 'norms'. These are usually people who lost hearing either progressively, or lost hearing suddenly later in their life, but not always.
Usual explanations for the latency increases, are along the lines of auditory processing time is longer, dependant on the participants audiological history. In terms of objective evidence of that concept, I would suggest looking to the literature on Mis-Match Negativity (MMN), as this looks at attentional paradigms as well.
In terms of your specific question with unilateral loss and cochlear implants, I would be tempted to look at the engineering side of the device, or possibly the settings of the implant programming, but you mention you do not think the delay is a technical one. Another area to consider may be the idea of hemispheric connectivity. In your example of a unilateral loss with the CI in the deaf ear , it may be that the non-CI (and fully hearing ear) input is processing faster in the brain than the CI input is. This is an extension of the concept that auditory-deprivation impacts on plasticity . But I have nothing concrete to support this theory. Maybe someone else on this list might be of more help there.
As a CI user myself, I can say that fatigue is what makes me need to take more time to process. Often I can ask for a repetition, then halfway through the first word of the repeat, can realise what the whole sentence was from the first time it was said. That could be more scientifically explored/explained in terms of attention, I would imagine.
Kind Regards
Nathan Barlow
References:
Gordon KA, Papsin BC, and Harrison RV. (2005) Effects of cochlear implant use on the electrically evoked middle latency response in children.Hear Res. 204(1-2). 78-89.
Kelly A.S, Purdy, S.C, and Thorne, P.R. (2005). Electrophysiological and speech perception measures of auditory processing in experienced adult cochlear implant users, Clinical Neurophysiology, 116 (6); 1235-1246