Kittens deafened one day after birth, used in cochlear implant experiments, then killedFallon, J.B., Shepherd, R.K., Nayagam, D.A.X., Wise, A.K., Heffer, L.F., Landry, T.G., and Irvine, D.R.F. ‘Effects of deafness and cochlear implant use on temporal response characteristics in cat primary auditory cortex’. Hearing Research. 2014 Sep; 315:1-9. doi: 10.1016/j.heares.2014.06.001.
In an attempt to investigate whether deafness of moderate duration (less than 14 months) results in degradation of temporal resolution, and whether any such effects are offset by cochlear implant use, seventeen healthy young cats were used in an experiment by the University of Melbourne / Bionics Institute.
Fifteen kittens were administered with a daily injection of neomycin sulfate from one day after birth for seventeen days, or longer, until rendered profoundly deaf. The remaining two cats had normal hearing, and were deafened acutely just prior to cortical recording as young adults.
At eight weeks of age, ten of the deafened kittens underwent surgery in order to have a cochlear implant device implanted in the left cochlea (inner ear). Fourteen days after surgery, and every month thereafter during the chronic stimulation program, the cats were anesthetized, and an electrically evoked auditory brainstem response was recorded for each stimulated electrode.
At 7-15 months of age all but one cat underwent acute electrophysiological experiments under anaesthesia, to record the response properties of primary auditory cortex neurons. The cats were then placed in a stereotaxic frame (fixing them in place and rendering them immobile) in a Faraday room, and a craniotomy was performed to expose the auditory cortex and conduct recordings using microelectrodes.
At the end of the experiment the cats were killed with an overdose of sodium pentobarbital.
Relevance to Humans
Cats differ greatly from humans in their anatomy, genetics and metabolism, making them inappropriate for use in studying human deafness. In fact, within their publication the researchers themselves refer to the significant differences between even cats and rabbits, and note that the use of anaesthetised cats has profound effects on the relevance and utility of the experiment:
"Finally, it should be noted that our and Vollmer and Beitel's (2011) data on neural responses to ICES were obtained in acute experiments on barbiturate anesthetized animals. Barbiturate anaesthesia has profound effects on inhibitory synaptic conductances in AI (Tan et al., 2004; Wehr and Zador, 2005), and synaptic inhibition is an important determinant of suppression duration and BRR. The effects of anaesthesia are also highlighted by the recent report that temporal neural coding of ICES in the inferior colliculus of the acutely deafened, awake rabbits is better than that in anesthetized cats (Chung et al., 2013)."
"As a veterinary surgeon for 30 years I challenge the researchers responsible for this study to provide one shred of evidence that these cat experiments are predictive of what happens in people with hearing problems. The fact that these researchers make the claims they do in their published work indicates that they need to undergo a basic course in evolutionary biology."
~Dr Andre Menache.
The experiment received funding support from the National Institutes of Health, the National Health and Medical Research Council of Australia, and the Victorian State Government.
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