Effects of neonatal partial deafness and chronic intracochlear electrical stimulation on auditory and electrical response characteristics in primary auditory cortex

Abstract

The use of cochlear implants in patients with severe hearing losses but residual low-frequency hearing raises questions concerning the effects of chronic intracochlear electrical stimulation(ICES) on cortical responses to auditory and electrical stimuli. We investigated these questions by studying responses to tonal and electrical stimuli in primary auditory cortex (AI) of two groups of neonatallydeafened cats with residual high-threshold, low-frequency hearing. One group were implanted with a multi-channel intracochlear electrode at eight weeks of age, and received chronic ICES for up to nine months before cortical recording. Cats in the other group were implanted immediately prior to cortical recording as adults. In all cats in both groups, multi-neuron responses throughout the rostrocaudal extent of AI had low characteristic frequencies (CFs), in the frequency range of the residual hearing, and high-thresholds. Threshold and minimum latency at CF did not differ between the groups, but in the chronic ICES animals there was a higher proportion of electrically but not acoustically excited recording sites. Electrical response thresholds were higher and latencies shorter in the chronically stimulated animals. Thus, chronic implantation and ICES affected the extent of AI that could be activated by acoustic stimuli and resulted in changes in electrical response characteristics.

Second spatial derivative analysis of cortical surface potentials recorded in cat primary auditory cortex using thin film surface arrays: Comparisons with multi-unit data

“…This finding is consistent with our previous report using standard electrophysiology and penetrating microelectrodes in neonatally ototoxically partially deafened animals (Fallon et al, 2009b). The proportions in our previous study (82% and 58% for unstimulated and stimulated groups respectively) are much higher than those in the present study, reflecting the fact that recordings in that study were made with single microelectrodes.…”

“…Additionally, the responsiveness of the cortex in two groups of partially deafened cats was assessed, revealing a difference in the acoustic responsiveness of AI in cats that had received chronic intracochlear electrical stimulation compared to partially deafened, unstimulated controls. This difference, which was evident in the MU data and was also in accord with previously reported MU data (Fallon et al, 2009b), was not evident using the standard simpler analysis of the LFPs.…”

“…Surface LFPs were captured at a sample rate of 10 kHz using the Cerebus system (Cyberkinetics; Foxborough, Massachusetts) and processed off-line in IgorPro (Wavemetrics; Lake Oswego, Oregon). Following LFP recordings, MU responses were recorded using standard techniques (Fallon et al, 2009b). Briefly, a 7×7 planar silicon array (Cyberkinetics; Foxborough, Massachusetts) was inserted in the middle of the MEG, and a linear array (A1×32-6mm-100-413-A32, NeuroNexus Technologies; Ann Arbor, Michigan) inserted down the rostral bank of the posterior ectosylvian sulcus.…”

Behavioral frequency discrimination ability of partially deafened cats using cochlear implants

“…Each subject was chronically stimulated using a clinical stimulator and speech processor (Cochlear Limited) carried in a harness worn by the animals which did not limit the animal's ability to move (Fallon et al, 2009). A standard stimulation strategy (SPEAK; McDermott, 1989) with a clinical electrode-frequency allocation map was used.…”

A partial hearing animal model for chronic electro-acoustic stimulation

“…In particular, the hair cells are functional along the entire length of the basilar membrane instead of only at the (low frequency encoding) apex, as in the case of partially-deaf individuals (Von Ilberg, Baumann et al 2011). Although a few EAS studies using partially-deafened animals have been carried out (Coco, Epp et al 2007; Fallon, Shepherd et al 2009; Stronks, Versnel et al 2011), there is a need for further animal research which combines acoustic stimulation with chronic electrical stimulation to determine the nature of the interactions between these two modalities in a clinically relevant partial deafness model. In order for these studies to be clinically relevant, it is important that the animal model have a similar hearing loss to that seen in the clinical population targeted for cochlear implantation (i. e. steeply sloping audiogram).…”