as an auditory rehabilitation device , the auditory brainstem implant ( abi ) has been used to treat deafness in individuals with neurofibromatosis type 2 since the first device was implanted in 1979 . in the following years , progressively more effective multichannel abis have been developed to palliate the deficits due to loss of integrity of the auditory nerves . furthermore , the indications for abi have also been expanded to other treatments , such as the treatments for nontumor patients , ossification or major cochlear malformations , aplasia or avulsion of the cochlear nerves , vestibular schwannoma on the only hearing ears , and bilateral temporal bone fractures . thus , an increasingly greater number of patients have acquired auditory perception again with the help of abi , and their quality of life has improved . despite the enormous potential of this increasingly applied treatment , the auditory performance of many implanted patients is limited , and the variability between cases hinders a complete understanding of the roles played by the multiple parameters related to the efficacy of the implant . although several researchers have committed to improve the efficacy of the implant , the predicament has not been resolved . to solve this problem , we attempted to establish a large animal model for abis in a rhesus macaque monkey to explore the array of the cochlear nucleus ( cn ) complex , which would thus lay the foundation for the investigation of the mechanisms of auditory recovery after implant surgery . the care and handling of animals were conducted in compliance with the chinese animal welfare act , the guidance for animal experimentation of capital medical university , and the beijing guidelines for the care and use of laboratory animals . this study was approved by the animal ethics committee of beijing neurosurgical institute , capital medical university ( no . 20131114 ) . the animals were in good medical condition at the time of testing and had no history of exposure to known ototoxic drugs . all monkeys were treated in the same way ( e.g. , diet and cage status ) . they were maintained in individual cages in a controlled environment of 120 cm ( h ) 80 cm ( w ) 75 cm ( d ) . the front , roof , and walls were composed of organic glass , whereas the bottom of the cage was composed of metal net . the monkeys were routinely released into a semi - closed environment that had environmental enrichment objects , such as toys , branches and climbing materials , mirrors , and tvs , as well as other objects . in the abi surgical procedure , general anesthesia was induced via intramuscular injections of ketamine ( 15 mg / kg , gutian pharmaceutical co. , ltd . , china ) , midazolam ( 0.5 mg / kg , enhua pharmaceutical co. , ltd . , china ) and atropine ( 0.02 mg / kg , jinyao amino acid co. , ltd . , orotracheal intubation was performed after an intravenous ( iv ) injection of fentanyl ( 0.001 mg / kg , astrazeneca ltd . , uk ) and rocuronium ( 0.3 mg / kg , n.v . organon , the netherlands ) . lumbar puncture and urethral catheterization were performed prior to surgery . during the surgery , the electrocardiogram , blood pressure , pulse rate , spo2 , end - tidal carbon dioxide partial pressure and temperature were monitored . for the magnetic resonance imaging ( mri ) , computed tomography ( ct ) scanning , auditory brainstem response ( abr ) , electrical abr ( eabr ) or lumbar puncture , the monkeys were initially anesthetized with intramuscular ketamine ( 15 mg / kg ) , midazolam ( 0.5 mg / kg ) and atropine ( 0.02 mg / kg ) . all of these experiments were performed under iv propofol ( propoflo , abbott laboratories , usa ) anesthesia with an initial bolus of 1.5 mg / kg and a continuous infusion of 0.5 mgkgmin . the electrode pad was comprised by medical silicone , slightly elliptical , and measured 12 mm  4 mm  2 mm . the 24 surface electrodes were attached to one side , and the other side was covered with a 1-cm - diameter polyethylene terephthalate mesh pad for stabilization purposes . we performed mri / ct scanning for all six monkeys prior to the surgery to evaluate the intracranial status and gasification degree of the mastoid , and to obtain a reference for surgical planning ; abr testing was conducted to assess auditory function . a lumbar puncture was performed , the cerebrospinal fluid ( csf ) was tested , and the results were subsequently reserved as a baseline . general anesthesia was performed via endotracheal intubation , and a left bench - park position was subsequently placed . a postauricular inverted l - shaped skin incision was performed under aseptic technique , and a right modified suboccipital retrosigmoid ( rs ) craniotomy was performed from the level of the superior nuchal line down to the foramen magnum . after the bone flap was removed , the transverse sinus , sigmoid sinus , and their intersection were exposed , and the foramen magnum was also opened [ figure 1 ] . a line incision was then performed on the dura mater along the sigmoid sinus . under surgical microscopic guidance , the nerves in the cerebellopontine angle ( cpa ) ( xi , x , ix , viii , and vii ) were detected and distinguished from bottom to top [ figure 2 ] . the abr was used again to verify the normality of neural function . the right cochlear nerve ( viii ) was subsequently cut under intraoperative neuromonitoring . by the anatomic markers of the glossopharyngeal nerve ( ix ) , choroid plexus of the fourth ventricle and brainstem stub of cochlear nerve , the right luschka 's foramen of the fourth ventricle once the anatomical location was confirmed , the electrode assays were implanted in the lateral recess through luschka 's foramen . the electrode assays were subsequently fixed with autologous muscle , followed by dural closure , bone replacement and suturing of muscles and skin . at the end of the surgery , eabr and head ct were used to evaluate the validity of the electrophysiological function and anatomical placement . during the operation , the prophylactic use of iv ceftriaxone sodium when the animals awoke from the anesthesia , they were transported to the observation room for postoperative monitoring . ( a ) an intra - operative bone window ( marked with bold black line marked in figure b ) , the solid arrow points to the transverse sinus , and the sigmoid sinus is marked with hollow arrow . the stars point to viii cranial nerve , the solid arrow points to vii cranial nerve , and the hollow arrow points to xi , x and ix cranial nerves . the animals were housed in a separate cage and monitored for food intake , skin wound healing condition , behavioral and neurological changes , and postoperative complications . the modified tarlov classification and the modified canada disability scale were used for neural defects and general status evaluation , respectively . intramuscular ceftriaxone sodium 10 mg / kg was administered for the first 3 postoperative days . a lumbar puncture was again performed during the 1 week postoperation or as indicated by signs of intracranial infection . abr and eabr testing were performed every 2 weeks from the 1 month after the implant surgery as the brain edema subsided . neural function in the monkeys was assessed using abr before and 1-month after the abi surgery . the animal 's ear canals were inspected and cleaned of debris if necessary , and the condition of the tympanic membrane was determined . following sedation , the skin was wiped clean with alcohol wipes , and 0.22 gauge skin electrodes were placed subcutaneously behind each ear , on the forehead , and on the back of the neck the electrode impedance was checked after placement into the skin and was below 2 k ohm for all recordings . the abr recordings were obtained using a bio - logic auditory evoked potentials system ( version 6.2.0 , natus medical incorporated , usa ) controlled by a laptop computer . the evoked responses were amplified 100 k times , and our physiological filters were set to pass at 100500 hz . for the abr recordings , rarefaction clicks were delivered through insert earphones ( er-3a , etymotic research , inc . , il , usa ) at a rate of 13.3 stimuli / s and at a level of 80 db nhl ( peak sound pressure level ) . the abr waveforms were averaged over a minimum of 1500 repetitions . clicks and tone bursts were presented at 80 db nhl and decreased in 10 db nhl increments from 80 to 30 db nhl and 5 db nhl until the abr response was no longer detectable . the latencies and amplitudes of peaks i , ii , and iv were measured at 80 db nhl . the monaural responses were recorded for each monkey and averaged ( biologic traveler express ) in a 10 ms time window . the averages for 1000 sweeps were collected , and the responses were replicated to determine the waveform reliability . the first eabr test was conducted every 2 weeks since the 1 month after the abi procedure . the noninverting electrode was placed at the midline ( cz ) , and the inverting electrode was placed subcutaneously behind the ipsilateral ear . the recording electrodes were passed through an external low pass filter ( fc = 32 khz ) in an attempt to eliminate the frequency - modulated signal sent from the external to internal components of the abi . frequencies outside the 1003000 hz range were filtered out , and the response was sampled at 20,000 hz , which provided data points at 0.05 ms intervals and interpolation between the points . the waveforms were marked using the bio - logic auditory evoked potentials system , which provided the latency values to the 0.01 ms . signal averaging was performed between 5 ms and 80 ms relative to the stimulus onset ; however , a more limited time window of  210 ms was used for the analyses . sweeps that contained large signals between 2 ms and 80 ms latency were rejected from the average . at least 500 sweeps were typically averaged , and the responses were recorded at least twice . monopolar pulses ( mp 1 + 2 ) were delivered by the multichannel implants at the basal end of the array . the stimulation levels were increased in steps of 1030 clinical units until the maximum levels were reached ; in most cases , the eabrs recorded at least three increasing levels during this process . the measurement data were presented as the mean  standard deviation ( sd ) . for the analysis of the comparison of latencies and amplitudes of peaks i , ii and iv between the bilateral ears in the abr test , a dependent samples t - test was used . a paired samples t - test the p level was set to 0.05 for all analyses to determine the statistical significance . the care and handling of animals were conducted in compliance with the chinese animal welfare act , the guidance for animal experimentation of capital medical university , and the beijing guidelines for the care and use of laboratory animals . this study was approved by the animal ethics committee of beijing neurosurgical institute , capital medical university ( no . 20131114 ) . the animals were in good medical condition at the time of testing and had no history of exposure to known ototoxic drugs . all monkeys were treated in the same way ( e.g. , diet and cage status ) . they were maintained in individual cages in a controlled environment of 120 cm ( h ) 80 cm ( w ) 75 cm ( d ) . the front , roof , and walls were composed of organic glass , whereas the bottom of the cage was composed of metal net . the monkeys were routinely released into a semi - closed environment that had environmental enrichment objects , such as toys , branches and climbing materials , mirrors , and tvs , as well as other objects . in the abi surgical procedure , general anesthesia was induced via intramuscular injections of ketamine ( 15 mg / kg , gutian pharmaceutical co. , ltd . , china ) , midazolam ( 0.5 mg / kg , enhua pharmaceutical co. , ltd . , china ) and atropine ( 0.02 mg / kg , jinyao amino acid co. , ltd . , orotracheal intubation was performed after an intravenous ( iv ) injection of fentanyl ( 0.001 mg / kg , astrazeneca ltd . , uk ) and rocuronium ( 0.3 mg / kg , n.v . organon , the netherlands ) . lumbar puncture and urethral catheterization were performed prior to surgery . during the surgery , the electrocardiogram , blood pressure , pulse rate , spo2 , end - tidal carbon dioxide partial pressure and temperature were monitored . for the magnetic resonance imaging ( mri ) , computed tomography ( ct ) scanning , auditory brainstem response ( abr ) , electrical abr ( eabr ) or lumbar puncture , the monkeys were initially anesthetized with intramuscular ketamine ( 15 mg / kg ) , midazolam ( 0.5 mg / kg ) and atropine ( 0.02 mg / kg ) . all of these experiments were performed under iv propofol ( propoflo , abbott laboratories , usa ) anesthesia with an initial bolus of 1.5 mg / kg and a continuous infusion of 0.5 mgkgmin . the pulse rate , spo2 , respiratory rate and temperature were monitored . the electrode pad was comprised by medical silicone , slightly elliptical , and measured 12 mm  4 mm  2 mm . the 24 surface electrodes were attached to one side , and the other side was covered with a 1-cm - diameter polyethylene terephthalate mesh pad for stabilization purposes . we performed mri / ct scanning for all six monkeys prior to the surgery to evaluate the intracranial status and gasification degree of the mastoid , and to obtain a reference for surgical planning ; abr testing was conducted to assess auditory function . a lumbar puncture was performed , the cerebrospinal fluid ( csf ) was tested , and the results were subsequently reserved as a baseline . general anesthesia was performed via endotracheal intubation , and a left bench - park position was subsequently placed . a postauricular inverted l - shaped skin incision was performed under aseptic technique , and a right modified suboccipital retrosigmoid ( rs ) craniotomy was performed from the level of the superior nuchal line down to the foramen magnum . after the bone flap was removed , the transverse sinus , sigmoid sinus , and their intersection were exposed , and the foramen magnum was also opened [ figure 1 ] . a line incision was then performed on the dura mater along the sigmoid sinus . under surgical microscopic guidance , the nerves in the cerebellopontine angle ( cpa ) ( xi , x , ix , viii , and vii ) were detected and distinguished from bottom to top [ figure 2 ] . the abr was used again to verify the normality of neural function . the right cochlear nerve ( viii ) was subsequently cut under intraoperative neuromonitoring . by the anatomic markers of the glossopharyngeal nerve ( ix ) , choroid plexus of the fourth ventricle and brainstem stub of cochlear nerve , the right luschka 's foramen of the fourth ventricle once the anatomical location was confirmed , the electrode assays were implanted in the lateral recess through luschka 's foramen . the electrode assays were subsequently fixed with autologous muscle , followed by dural closure , bone replacement and suturing of muscles and skin . at the end of the surgery , eabr and head ct were used to evaluate the validity of the electrophysiological function and anatomical placement . during the operation , the prophylactic use of iv ceftriaxone sodium when the animals awoke from the anesthesia , they were transported to the observation room for postoperative monitoring . ( a ) an intra - operative bone window ( marked with bold black line marked in figure b ) , the solid arrow points to the transverse sinus , and the sigmoid sinus is marked with hollow arrow . the stars point to viii cranial nerve , the solid arrow points to vii cranial nerve , and the hollow arrow points to xi , x and ix cranial nerves . the animals were housed in a separate cage and monitored for food intake , skin wound healing condition , behavioral and neurological changes , and postoperative complications . the modified tarlov classification and the modified canada disability scale were used for neural defects and general status evaluation , respectively . intramuscular ceftriaxone sodium 10 mg / kg was administered for the first 3 postoperative days . a lumbar puncture was again performed during the 1 week postoperation or as indicated by signs of intracranial infection . abr and eabr testing were performed every 2 weeks from the 1 month after the implant surgery as the brain edema subsided . neural function in the monkeys was assessed using abr before and 1-month after the abi surgery . the animal 's ear canals were inspected and cleaned of debris if necessary , and the condition of the tympanic membrane was determined . following sedation , the skin was wiped clean with alcohol wipes , and 0.22 gauge skin electrodes were placed subcutaneously behind each ear , on the forehead , and on the back of the neck the electrode impedance was checked after placement into the skin and was below 2 k ohm for all recordings . the abr recordings were obtained using a bio - logic auditory evoked potentials system ( version 6.2.0 , natus medical incorporated , usa ) controlled by a laptop computer . the evoked responses were amplified 100 k times , and our physiological filters were set to pass at 100500 hz . for the abr recordings , rarefaction clicks were delivered through insert earphones ( er-3a , etymotic research , inc . , il , usa ) at a rate of 13.3 stimuli / s and at a level of 80 db nhl ( peak sound pressure level ) . clicks and tone bursts were presented at 80 db nhl and decreased in 10 db nhl increments from 80 to 30 db nhl and 5 db nhl until the abr response was no longer detectable . the latencies and amplitudes of peaks i , ii , and iv were measured at 80 db nhl . the monaural responses were recorded for each monkey and averaged ( biologic traveler express ) in a 10 ms time window . the averages for 1000 sweeps were collected , and the responses were replicated to determine the waveform reliability . the first eabr test was conducted every 2 weeks since the 1 month after the abi procedure . the noninverting electrode was placed at the midline ( cz ) , and the inverting electrode was placed subcutaneously behind the ipsilateral ear . the recording electrodes were passed through an external low pass filter ( fc = 32 khz ) in an attempt to eliminate the frequency - modulated signal sent from the external to internal components of the abi . frequencies outside the 1003000 hz range were filtered out , and the response was sampled at 20,000 hz , which provided data points at 0.05 ms intervals and interpolation between the points . the waveforms were marked using the bio - logic auditory evoked potentials system , which provided the latency values to the 0.01 ms . signal averaging was performed between 5 ms and 80 ms relative to the stimulus onset ; however , a more limited time window of  210 ms was used for the analyses . sweeps that contained large signals between 2 ms and 80 ms latency were rejected from the average . at least 500 sweeps were typically averaged , and the responses were recorded at least twice . monopolar pulses ( mp 1 + 2 ) were delivered by the multichannel implants at the basal end of the array . the stimulation levels were increased in steps of 1030 clinical units until the maximum levels were reached ; in most cases , the eabrs recorded at least three increasing levels during this process . the measurement data were presented as the mean  standard deviation ( sd ) . for the analysis of the comparison of latencies and amplitudes of peaks i , ii and iv between the bilateral ears in the abr test , a dependent samples t - test was used . a paired samples t - test was applied for the pre- and post - operative csf analysis . the p level was set to 0.05 for all analyses to determine the statistical significance . the average time for the abi procedure was 5.2 h ( range , 4.56 h ) , which included nearly half of the time for the eabr testing . the preoperative abrs indicated that all animals possessed adequate auditory function , and the thresholds were 515 db nhl [ figure 3 ] . the peaks i , ii and iv in the abr testing were distinguished at 80 db nhl , and their latencies and amplitudes are shown in table 1 . the latency and amplitude of peaks i , ii and iv in the abr testing for six monkeys ( mean  sd ) * the left ears were tested twice , and one for the right ; p : comparison between left and right ears . only one - peak eabr waves were induced during the abi operation [ figure 4 ] . however , both one - peak and two - peak eabr waves were observed during the postoperative testing [ figure 5 ] . the intra- and post - operative eabr waves confirmed that the devices were accurately placed where the stimulus could excite the complete auditory pathway . for both one - peak and two - peak eabr waves , the latencies did not appear to change with the stimulus intensity ; however , the amplitudes had a positive correlation with the intensity . the postoperative ct scans confirmed the anatomic site of the implant electrode arrays [ figure 6 ] . throughout the eabr testing , all monkeys remained stable , and no abnormal changes in the pulse rate , spo2 , respiratory rate , or abnormal limb movements were identified . the one intra - operative electrical auditory brainstem response wave ( ep i ) followed a stimulus artifact ( sa ) , and the stimulus intensity from top to bottom of the five lines were 200 , 150 , 100 , 50 and 0 current levels . ( a ) one - peak postoperative electrical auditory brainstem response ( eabr ) wave ( ep i ) , the stimulus intensities from top to bottom were 120 , 100 , 80 , 50 , 30 and 0 current levels ( cls ) ; ( b ) two distinguishable eabr waves ( ep i and ii ) , the stimulus intensities from top to bottom were 150 , 150 , 150 , 120 , 110 , 100 , 80 , 50 and 30 cls . a group of bone windows in postoperative computed tomography scans showed auditory brainstem implants ( arrows ) in the lateral recess of the fourth ventricle , which was the target anatomic location . all wounds healed well , and no csf leakage , subcutaneous hydrops , wound infection , intracranial infection , or death occurred . according to the tarlov classification the animals recovered to preoperative health status in 5.7 days ( range , 57 days ) , which was evaluated by the modified canada disability scale . by the time of the last test , no monkeys had died or were sacrificed ; they were all maintained for use in a subsequent study . one animal presented with transient facial nerve paralysis postoperation , which was evidenced by a reduced ipsilateral less blink , a shallow forehead , and facial wrinkles . the csf samples of the monkeys at different time points were colorless and clear , with a negative reaction in pandy 's test . the csf items were not significant differents between the pre- and post - operative tests , including intracranial pressure ( 80.0 mmh2o vs. 81.7 mmh2o , p = 0.742 ) , chloride ( 118.0 mmol / l vs. 120.0 mmol / l , p = 0.522 ) , glucose ( 2.3 mmol / l vs. 2.4 mmol / l , p = 0.771 ) , protein ( 17.2 mmol / l vs. 17.6 mmol / l , p = 0.946 ) , and cells ( 1.7 cells / visual field vs. 0.3 cells / visual field , p = 0.184 ) . the average time for the abi procedure was 5.2 h ( range , 4.56 h ) , which included nearly half of the time for the eabr testing . the preoperative abrs indicated that all animals possessed adequate auditory function , and the thresholds were 515 db nhl [ figure 3 ] . the peaks i , ii and iv in the abr testing were distinguished at 80 db nhl , and their latencies and amplitudes are shown in table 1 . the latency and amplitude of peaks i , ii and iv in the abr testing for six monkeys ( mean  sd ) * the left ears were tested twice , and one for the right ; p : comparison between left and right ears . only one - peak eabr waves were induced during the abi operation [ figure 4 ] . however , both one - peak and two - peak eabr waves were observed during the postoperative testing [ figure 5 ] . the intra- and post - operative eabr waves confirmed that the devices were accurately placed where the stimulus could excite the complete auditory pathway . for both one - peak and two - peak eabr waves , the latencies did not appear to change with the stimulus intensity ; however , the amplitudes had a positive correlation with the intensity . the postoperative ct scans confirmed the anatomic site of the implant electrode arrays [ figure 6 ] . throughout the eabr testing , all monkeys remained stable , and no abnormal changes in the pulse rate , spo2 , respiratory rate , or abnormal limb movements were identified . the one intra - operative electrical auditory brainstem response wave ( ep i ) followed a stimulus artifact ( sa ) , and the stimulus intensity from top to bottom of the five lines were 200 , 150 , 100 , 50 and 0 current levels . ( a ) one - peak postoperative electrical auditory brainstem response ( eabr ) wave ( ep i ) , the stimulus intensities from top to bottom were 120 , 100 , 80 , 50 , 30 and 0 current levels ( cls ) ; ( b ) two distinguishable eabr waves ( ep i and ii ) , the stimulus intensities from top to bottom were 150 , 150 , 150 , 120 , 110 , 100 , 80 , 50 and 30 cls . a group of bone windows in postoperative computed tomography scans showed auditory brainstem implants ( arrows ) in the lateral recess of the fourth ventricle , which was the target anatomic location . all wounds healed well , and no csf leakage , subcutaneous hydrops , wound infection , intracranial infection , or death occurred . according to the tarlov classification the animals recovered to preoperative health status in 5.7 days ( range , 57 days ) , which was evaluated by the modified canada disability scale . by the time of the last test , no monkeys had died or were sacrificed ; they were all maintained for use in a subsequent study . one animal presented with transient facial nerve paralysis postoperation , which was evidenced by a reduced ipsilateral less blink , a shallow forehead , and facial wrinkles . the csf samples of the monkeys at different time points were colorless and clear , with a negative reaction in pandy 's test . the csf items were not significant differents between the pre- and post - operative tests , including intracranial pressure ( 80.0 mmh2o vs. 81.7 mmh2o , p = 0.742 ) , chloride ( 118.0 mmol / l vs. 120.0 mmol / l , p = 0.522 ) , glucose ( 2.3 mmol / l vs. 2.4 mmol / l , p = 0.771 ) , protein ( 17.2 mmol / l vs. 17.6 mmol / l , p = 0.946 ) , and cells ( 1.7 cells / visual field vs. 0.3 cells / visual field , p = 0.184 ) . to the best of our knowledge , this is the first report regarding the rhesus macaque monkey ( m. mulatta ) implanted with abi placement . the successful induction of an abi model in the rhesus macaque monkey paves the way for future studies . an appropriate surgical approach is vital for the exposure of the implant site and following abi implantation . two approaches have been used in clinical practice for abis : the translabyrinthine ( tl ) and rs approaches . the tl approach was originally advocated by edgerton et al . and is currently the only method approved by the us food and drug administration ( fda ) in abi clinical trials . however , an anatomical study by kuroki and moller on cadaver specimens has demonstrated that the tl approach to the cpa offers a limited view ; thus , it is necessary to medically retract the sigmoid sinus to obtain a medial view of the root entry zone of the cochlear nerve . in another anatomical investigation , friedland and wackym have demonstrated that the rs approach provides excellent visualization of the lateral recess of the fourth ventricle when a 30 endoscope is used , which provides a more direct appreciation of the implant site compared with the tl approach . previous studies have proven that the rs approach has various advantages , such as the adequate control of bleeding , dissection of the entire tumor under direct view , easy identification of the facial and cochlear nerves at their root entry zone and the distal end in the internal auditory canal , and adequate exposure of the foramen of luschka . in addition , the rs approach provides a unique opportunity for real - time intraoperative monitoring by direct recording of cochlear nerve action potentials , which could prove useful in attempts at hearing preservation . compared with humans , the rhesus macaque monkey possesses a more flat , smaller ( approximately 2 cm  1.5 cm on one side ) posterior fossa and relatively strong , thick head and neck muscles . the regular linear postauricular incision can not provide satisfactory posterior fossa bone exposure , which , therefore , makes it difficult to distinguish nerves during the abi procedure . after considering the advantages of the two approaches , the modified rs approach was adopted for this study . the inverted l - shaped flap and muscles , which had been fully freed , were turned outward , and the external occipital protuberance , middle line , superior nuchal line , part of superior border of the foramen magnum and mastoid process were completely exposed . during the nerve identification stage , the lower cranial nerves were recognized first , followed by the cochlear and facial nerves . the choroid plexus of the fourth ventricle and luschka 's foramen could then be located by anatomic landmarks . prior to electrode array implantation , the cerebellum and brainstem were separated from the bottom to top , which proved to be easier than the operation on the locale of luschka 's foramen . the experiences with the six monkeys suggested that the modified approach is feasible for multichannel abi implantation . several researchers have studied auditory evoked potentials in rhesus macaque monkeys . they have concluded that peak i in rhesus macaque monkeys ( which is analogous to peaks i and ii in humans ) was generated by eighth nerve fibers , whereas peak ii ( that is analogous to peak iii in humans ) was generated by the cn . the neural generator of peak iii ( which is analogous to peak iv in humans ) was not determined by a specific near - field recording , although the researchers assumption was that this peak had its origin in the superior olivary complex . finally , peak iv in monkeys ( which is analogous to peak v in humans ) was determined to originate from the contralateral lateral lemniscus . in our study , the latencies and amplitudes of peaks i , ii and iv were accurately calculated . the latencies of the waves were similar to a previously published study ; however , the amplitudes appeared lower than the data from the same research . intraoperative electrophysiology during abi implantation demonstrated that electrical stimulation from the implanted electrode elicits a response from the ascending auditory pathway , which is reflected in the recorded eabr . as the full surface of the cn is not visible , even an experienced neurosurgeon may have difficulty placing the electrode array into an optimal position every time . in our study , preoperative abr in the rhesus macaque monkey contains three explicated waves [ figure 3 ] ; however , only one - peak eabr waves can be observed during surgery [ figure 4 ] , and one - peak or two - peak eabr waves can be observed postoperatively [ figure 5 ] . in humans , the eabr elicited by the electrical stimulation of the cn is similar to the abr ; however , it typically does not contain components that correspond to peaks i or ii in the abr . the latencies of the peaks that are generated in the cn and the lateral lemniscus are shorter than the abr . referred the relevant information of humans and analyzed the waves in the abr and eabr of rhesus monkey , we concluded that the eabr waves in rhesus macaque monkeys do not contain components that correspond to peak i in abr , and ep i and ep ii [ figures 4 and 5 ] correspond to peaks ii and iv [ figure 3 ] , respectively . furthermore , peaks ii and iv were generated in the cn and contralateral lateral lemniscus . in addition , the latencies of ep i and ep ii were less than peaks ii and iv [ figures 4 and 5 , and table 1 ] . of particular note was that the waveform of the recorded eabrs differed considerably among individuals . during testing , we could observe that as the stimulus intensity increased , the amplitude of the eabr wave in the same electrode also increased in a certain scope . however , no similar correlation was identified between the latency and stimulus intensity . in the current study , we successfully established an abi animal model in the rhesus macaque monkey , and short - term data supported its feasibility and safety . however , the long - term survival state and electrophysiology of monkeys with implants requires further investigation . although eabr testing and postoperative ct could prove the effectiveness of this procedure in electrophysiology and anatomy , respectively , relevant behavioral studies should be considered in future implantations . any disorder that affects the secretion , absorption or transportation will cause changes in the components of the csf and/or neural activity . the short recovery time and stable csf components of all animals may demonstrate the safety of the abi . we concluded that the rhesus macaque monkey is a valid abi model , and the modified suboccipital rs approach can provide sufficient exposure for the implant surgery . during the intra- and post - operation of abis , the eabr test can guide us to the optimal implant sites , and the postoperative ct can confirm the implant location . ep i and ep ii of eabr wave were generated in the cn and contralateral lateral lemniscus . numerous studies have clearly established this species as an essential model for neurophysiologic investigations in conjunction with sophisticated behavior . we suggested that the demonstration of the feasibility of abi procedures in the macaque monkey opens a promising field of research regarding the mechanisms of auditory recovery after abi surgery .