高速率电刺激对豚鼠听神经兴奋性的影响

目的：评估高速率电刺激对豚鼠听神经兴奋性的影响。方法：切开受试豚鼠圆窗膜,标准电极插入鼓阶大约4mm,两个靠近蜗尖的电极作为刺激电极,切开圆窗用压碎的颞肌封住。随机选择一耳为刺激耳,另一耳则为对照耳,均安装上标准电极,刺激电极在整个测试过程中保持不动。在保持刺激强度处在临床正常水平[电刺激诱发听觉脑干电位(EABR)阈值上6dB]情况下,用200(n=14)、400(n=10)、1000(n=11)、2000(n=10)脉冲数／s(PPS)四种不同的电刺激速率急性刺激45只听力正常豚鼠鼓阶内电极2h,记录急性刺激前、后3h内EABR的阈值和Ⅰ波幅值,比较急性刺激前、后EABR的Ⅰ波幅值的变化。结果：急性刺激电流强度固定在临床正常水平(EABR阈上6dB),急性刺激后EABR的Ⅰ波幅值同急性刺激前相比,采用200PPS刺激速率平均约升高20％;400PPS约升高9％;1000PPS约升高7％;2000PPS约升高30％。结论：在刺激电流强度为阈上6dB的情况下,人工耳蜗言语编码策略如果应用1000、2000PPS高速率电刺激不会导致听神经的兴奋性下降。此实验为临床上研制新的人工耳蜗采取高刺激速率语音信号处理方案提供了依据。     

电刺激强度对豚鼠听神经兴奋性的影响

目的在不同电刺激下观察不同刺激强度的电诱发听性脑干反应(electricalevokedauditorybrainstemresponse,EABR)I波幅值的变化,评估刺激强度对豚鼠听神经兴奋性影响。方法选取健康短毛白色纯种红目豚鼠45只,标准电极分别插入豚鼠两侧鼓阶内大约4mm,靠近蜗尖两个电极作为刺激电极,随机选择一侧耳为刺激耳,另一侧耳为对照耳。选用二种不同的刺激强度[EABR阈上6dB(n=25)和阈上18dB(n=20)]电荷平衡双相脉冲电流,连续刺激2小时,刺激速率分别为200、1000PPS,观察刺激前和刺激后3小时内EABRI波幅值的变化。结果刺激强度为EABR阈上6dB、刺激速率为200PPS时I波幅值同刺激前相比升高20%,刺激速率为1000PPS时I波幅值较刺激者相比升高约7%;刺激强度为EABR阈上18dB时,应用200PPS刺激速率,I波的幅值在刺激后0分钟下降21%(P<0.05)、30分钟时下降11%(P<0.05)、60分钟时下降9%(P<0.05)、90分钟时下降7%(P>0.05)、120分钟时下降约2%(P>0.05)、150分钟时完全恢复正常;应用1000PPS的刺激速率,I波的幅值在刺激后0分钟下降56%(P<0.01)、30分钟时下降47%(P<0.01)、60分钟时下降58%(P<0.01)、90分钟时下降63%(P<0.01)、120分钟时下降约48%(P<0.01)、150分钟时下降43%(P<0.01)、180分钟时下降44%(P<0.01)。结论高速率和高强度连续电刺激导致EABRI波幅值持久下降反应了听神经的兴奋性变化不但受电刺激的速率影响,而且受电刺激的强度影响。     

人工耳蜗植入后EABR动态变化的初步研究

目的观察人工耳蜗植入儿童电诱发听性脑干反应(electrically auditory evoked response EABR)的动态变化,了解慢性电刺激后听觉通路反映特性的变化,为听觉系统的可塑性变化研究提供实验依据。方法本研究采用前瞻性设计,对19例平均年龄为3.2±1.0岁的语前聋儿童在接受人工耳蜗植入术中电极植入后进行EABR检测,患者在人工耳蜗植入后5.4±3.2月后再次进行EABR检测,观察EABR阈值、波III、波V潜伏期以及EABR输入输出曲线指标的变化。结果EABR平均阈值从人工耳蜗植入术中的196.9±11.1CL下降到术后5.4±3.2个月的189.2±13.2CL CL,配对t检验显示显著性差异（p=0.006）。平均阈上20CL V波潜伏期从人工耳蜗植入术中的4.72±0.21ms缩短到术后5.4±3.2月的4.60±0.18ms,配对t检验显示显著性差异（p=0.032）。结论耳聋儿童在接受人工耳蜗电刺激后EABR的反应阈值和潜伏期变化在最初的5.4±3.2月已经出现,提示3岁左右年龄段儿童听觉系统具有较好的可塑性,人工耳蜗植入后早期的听觉、言语训练尤为重要。     

外淋巴“枯竭“状态对听神经兴奋性影响

目的 通过记录电刺激诱发听觉脑干电位(EABR)阈值变化搞清楚内耳外淋巴“枯竭”状态对听神经兴奋性影响。方法 在手术显微镜帮助下,小心把标准刺激电极沿蜗轴插入鼓阶约4mm,用压碎的肌肉轻轻封住圆窗口。然后用波宽为50μs/phase、刺激速率为30次/秒电荷平衡双相脉冲电流刺激受试动物鼓阶内的靠近蜗尖的电极对(1/2),记录两次EABR阈值,取均值。然后把明胶海绵做成直径约为1mm,长约为4cm小的圆柱形,把一端放入前庭阶持续吸干外淋巴,直到显微镜下见鼓阶外淋巴消失,模拟外淋巴“枯竭”状,用以上电刺激参数再次记录电极对(1/2)的EABR阈值两次,取均值。比较前后两次EABR阈值的变化(t检验)。结果 耳蜗外淋巴“枯竭”状态时测试到的EABABR阈值0.63±0.11mA较充满外淋巴时0.27±0.08mA明显升高(P<0.001)。结论 EABR阈值升高说明耳蜗鼓阶外淋巴呈现“枯竭”状态时听神经兴奋性明显下降,临床上少数重聋或者全聋患者耳蜗外淋巴呈现“枯竭”状态,推测其电子耳蜗植入临床效果可能较耳蜗鼓阶内外淋巴正常患者差。     

电诱发听性脑干反应和电诱发镫骨肌反射阈值在内耳畸形儿童人工耳蜗植入术后调机中的运用

目的　探讨内耳畸形小儿人工耳蜗植入手术后,植入体电诱发听性脑干反应（electrically evoked auditory brainstem responses,EABR）、电诱发镫骨肌反射阈值（electrically evoked stapedius reflex threshold,ESRT）的变化特点及规律,以指导术后设备调试。方法　将88例澳大利亚Cochlear Nucleus24型人工耳蜗植入手术患儿分为耳蜗形态正常组与内耳畸形组,测试手术后1年内不同时期EABR和ESRT值,术后1年运用行为测试法检测主观阈值（T值）和最大舒适阈（C值）,分析特点及变化规律。结果  内耳畸形组患儿术后不同时期EABR和ESRT阈值较正常组高（P＜0.05）,两组EABR和ESRT阈值变化趋势相同,总的趋势是低频值较低,高频值较高,术后1年EABR和ESRT阈值逐渐增高;两组EABR与T值显著相关,ESRT与C值显著相关。结论　内耳畸形组人工耳蜗植入手术后EABR和ESRT阈值变化规律及特点与正常组患儿相同,阈值可用于指导内耳畸形人工耳蜗植入者手术后设备的调试。     

豚鼠噪声暴露后畸变产物耳声发射及神经元特异性烯醇化酶表达的观察

为观察豚鼠暴露于强噪声后畸变产物耳声发射(DPOAE)及神经元特异性烯醇化酶(NSE)的变化,选用13只Preyer's反射正常的健康豚鼠,分为二组,8只噪声暴露组,5只为NSE表达对照组。噪声强度115dB(A),连续暴露4小时,DPOAE幅值于噪声暴露前后进行测试,结果DPOAE幅值噪声暴露前后差异明显(P<0.001),豚鼠内耳内、外毛细胞及螺旋神经细胞胞浆、隧道贯穿纤维NSE免疫组化反应均呈阳性表达,暴震前后无明显变化。结果提示豚鼠接受短时间强噪声刺激后,DPOAE幅值的下降为暂时阈移,而内耳神经元及其末梢未受损伤。     

电诱发听性脑干反应在人工耳蜗植入术后的临床应用分析[论著]

目的　探讨电诱发听性脑干反应（electrically evoked auditory brainstem responses,EABR）的特性和其在判断人工耳蜗植入术后听觉传导通路完整性中的作用,以及不同脉冲宽度的EABR阈值与人工耳蜗植入术后调试行为测试数值之间的相关性,为人工耳蜗植入术后首次开机不会配合行为测试的植入者科学设定刺激参数提供参考。方法　选取郑州市第三人民医院植入诺尔康晨星人工耳蜗（CS-10A植入体）、能配合行为测试的植入者20例,采用行为测试测得阈值（T值）和最大舒适阈（C值）,在标准屏蔽室内做ABR检查,采用3、10、20电极分别测脉冲宽度为25、50、75 μs/相的EABR平均阈值,分析引出率和波形分化特点,并对两种数据进行统计学分析。结果　EABR越容易引出波形越清晰,患儿的听觉反应越灵敏,也反映了术后植入体系听觉传导通路越完整（P<0.05）;EABR脉冲宽度为75 μs/相的平均阈值与人工耳蜗的C值有良好的相关性（P<0.05）。结论　EABR检测可客观的判断人工耳蜗植入术后植入体系完整的听觉传导功能,客观评价人工耳蜗植入效果。人工耳蜗调试中,可以通过EABR脉冲宽度为75 μs/相的平均阈值来指导不能配合主观行为测试的植入者C值的判定,为患儿早期开始听觉刺激,建立听觉重塑带来帮助。     

听觉剥夺及耳蜗内电刺激幼鼠听皮层和下丘核CREB 和 NMDAR1蛋白表达变化

目的：观察耳聋幼鼠及其耳聋后单耳植入电极电刺激后幼鼠听皮层和下丘核环磷酸腺苷反应元件结合蛋白（cyclic AMP response element － binding protein ,CREB）和N －甲基－D－天冬氨酸受体－1（N －methyl－D－aspartic acid receptor one ,NMDAR1）表达水平的变化。方法将66只12天龄SD幼鼠随机分为2大组,分别为耳聋造模后4周组（33只）及耳聋造模后6周组（33只）。将耳聋造模后4周组再分为对照1组、耳聋造模后4周组（4周组）及耳聋造模后3周耳蜗内电刺激组1（刺激时间为1周,简称“电刺激1组”）,每组11只大鼠;将耳聋造模后6周组再分为对照2组、耳聋造模后6周组（6周组）及耳聋造模后5周耳蜗内电刺激组2（刺激时间为1周,简称“电刺激2组”）,每组11只大鼠;对照1、2组均正常饲养。除对照1、2组外,在其余4组幼鼠颈背部、两侧下腹部皮下注射庆大霉素（总量为350 mg／kg ）,半小时后于相同部位注射呋塞米（总量为200 mg／kg ）,两周后行ABR检测,于耳聋造模成功后第3、5周分别对电刺激1、2组的幼鼠植入电极,在耳蜗内进行电刺激,每天3小时,持续7天。于耳聋造模后4、6周分别处死4、6周组大鼠取听皮层和下丘组织,通过免疫组织化学方法,观察CREB和NMDAR1表达水平的变化。结果耳聋造模成功后幼鼠ABR阈值均大于93 dB SPL ,4周组听皮层、下丘CREB和NMDAR1的表达较对照1组增加,电刺激1组CREB和NMDAR1的表达较4周组增加。6周组听皮层和下丘CREB和NMDAR1的表达较对照2组下降,电刺激2组CREB和NMDAR1的表达较6周组增加。结论听觉剥夺可导致幼鼠听皮层和下丘CREB和NMDAR1早期表达增加而晚期表达下降。耳蜗植入电极电刺激可导致幼鼠听皮层和下丘CREB和NMDAR1的表达增加,反映这两个部位神经元的可塑性变化。     

刺激电极在鼓阶内位置对听神经兴奋性的影响

目的　通过记录电诱发听性脑干反应 (electricallyevokedauditoryresponse ,EABR)阈值变化 ,探讨植入刺激电极在豚鼠耳蜗鼓阶内的位置对听神经兴奋性的影响。方法　在手术显微镜下 ,将标准刺激电极放在靠近蜗轴位置 ,用压碎的肌肉轻轻封住圆窗口 ,记录二次EABR阈值 ,取均值。然后取出刺激电极 ,再次植入电极靠近耳蜗鼓阶的外侧壁 ,再记录二次EABR阈值 ,取均值 ,然后比较前后二次记录到的EABR阈值变化。结果　植入电极在耳蜗鼓阶外侧壁的EABR阈值为 1.14± 0 .4 5mA ,较耳蜗内侧壁EABR阈值 (0 .2 8± 0 .0 9mA)明显升高 ,二者有显著性差异 (P <0 .0 1)。结论　电刺激听神经如果采用间距不宽的双极电极 ,其有效性同电极在耳蜗鼓阶内的位置密切相关 :植入电极越靠近被刺激的听神经成份 ,被检测到的EABR的阈值就越低。此为实验室设计蜗旁多导电极提供了依据     

EABR在人工耳蜗植入术前及术中应用研究

人工耳蜗通过在耳蜗鼓阶内植入电极,电刺激耳蜗螺旋神经节使双侧重度或极重度感音神经性聋患者获得或恢复听觉.自House植入第一例人工耳蜗以来,耳鼻喉科医生及听力师开始探索电刺激引发的神经电反应,促进了一系列电生理检查手段的临床应用.NRT以其快速、无需镇静的优势在人工耳蜗植入术后发挥了很大作用,但EABR技术在人工耳蜗植入术前选择、术中检测中仍有广泛的临床应用.近年来,对于特殊病例如听神经病、耳蜗畸形患者的EABR研究己成为热点.     

Reduction in excitability of the auditory nerve following electrical stimulation at high stimulus rates. IV. Effects of stimulus intensity.

High rate intracochlear electrical stimulation using stimulus intensities well above clinical limits can induce a significant reduction in the excitability of the auditory nerve as measured by a reduction in the amplitude of the electrically evoked auditory brainstem response (EABR). The purpose of the present study was to assess the effect of stimulus intensity on these stimulus induced changes by comparing the effects of acute stimulation using stimulus intensities within normal clinical levels (6 dB and 12 dB above EABR threshold) and significantly above normal clinical levels (> 20 dB above EABR threshold; 0.34 microC/phase). Stimulus rates of 200, 400, or 1000 pulses/s (pps) were delivered to bipolar scala tympani electrodes. EABRs were recorded before and periodically following 2 h of continuous stimulation. No reduction in EABR amplitude was observed following stimulation at 6 dB above EABR threshold for the three stimulus rates examined. However, EABRs were reduced when stimulated at 12 dB above EABR threshold at 400 pps, and significantly reduced when stimulated at a rate of 1000 pps. Immediate post-stimulus response amplitudes of wave III were 63% and 35% of the pre-stimulus amplitude at 400 and 1000 pps respectively. More significant reductions in EABR amplitude were observed following stimulation at levels more than 20 dB above EABR threshold for both 400 and 1000 pps stimuli. Our findings indicate that stimulus induced changes in EABR amplitude are related to both stimulus rate and stimulus intensity. Moreover, stimulation using intensities within the normal clinical range show little evidence of prolonged reductions in auditory nerve excitability at stimulus rates of up to 1000 pps.     

Reduction in excitability of the auditory nerve following electrical stimulation at high stimulus rates: V. Effects of electrode surface area

High rate intracochlear electrical stimulation at high intensities can induce significant reductions in the excitability of the auditory nerve as measured by a decrement in the amplitude of the electrically evoked auditory brainstem response (EABR). Such changes are primarily associated with stimulus induced neuronal activity, although direct current (DC) can also contribute. We examined the extent of stimulus induced change in auditory nerve excitability using large surface area platinum electrodes ('high-Q' electrodes). These electrodes have a surface area approximately 70 times greater than standard Pt electrodes of the same geometric area, resulting in lower DC and charge density (charge/electrode surface area) for a common stimulus. Guinea pigs were bilaterally implanted with either high-Q or standard Pt electrodes, and unilaterally stimulated for 2 h using stimulus intensities of 12 dB or 20-30 dB above EABR threshold (0.34 microC/phase) at stimulus rates of 200, 400, or 1000 pulses per second (pps). EABRs were recorded before and following the acute stimulation. While there were significant reductions in EABR amplitudes and elevated EABR thresholds following stimulation at 12 dB above threshold using 400 and 1000 pps delivered to standard Pt electrodes, there were fewer or no significant changes in the post-stimulus EABR amplitude and threshold using high-Q electrodes under equivalent stimulus conditions. At a higher stimulus intensity (20-30 dB above EABR threshold), no reduction in EABR amplitude was observed at 200 pps for both stimulating electrodes. However, EABRs were reduced significantly at 400 and 1000 pps. There was significantly greater EABR recovery following stimulation using high-Q electrodes compared with standard Pt electrodes at 400 (P<0.05) and 1000 pps (P<0.05). These data indicate that large surface area electrodes can significantly reduce stimulus induced changes in auditory nerve excitability, and may therefore have important clinical application.     

Effects on cochlear responses of activation of descending pathways from the inferior colliculus

The inferior colliculus (IC) has been shown anatomically to make direct descending connections with medial olivocochlear (MOC) neurones in the auditory brainstem. The MOC neurones project to the outer hair cells in the cochlea and inhibit cochlear neural output. This study investigated the effect of IC stimulation on cochlear output in both guinea pigs and rats in order to determine the functional significance of the IC-to-olivocochlear system projection. Stimulation of the central nucleus and the external cortex of the IC in paralysed guinea pigs, both contra- and ipsilaterally to the test cochlea, resulted in a small increase of the cochlear microphonic amplitude and a small decrease of the compound action potential (CAP) amplitude, the latter equivalent to a 3-6 dB change in acoustic input. Effects on the CAP were maximal in the frequency range 6-10 kHz. These effects were consistent with partial activation of the MOC system. In unparalysed rats, stimulation of the inferior colliculus evoked a large, prolonged suppression ranging from 5-12 dB in the amplitude of distortion product otoacoustic emissions (2f(1)-f(2); DPOAE), as reported previously by Scates et al. (1999). However, this suppression was decreased to only 0-3 dB when the animals were paralysed, suggesting that the larger suppression in the unparalysed state was the consequence of either a general masking effect caused by animal movement, or activation of middle ear muscles by the inferior colliculus stimulation. The results indicate a small but significant excitatory effect of the inferior colliculus on the medial olivocochlear system under conditions of anaesthesia and paralysis.     

Chronic intracochlear electrical stimulation in the neonatally deafened cat. I: Expansion of central representation

Intracochlear electrical stimulation via cochlear prostheses has been employed as a means of providing some hearing to deaf children. Since chronically restricted stimuli are known to have profound effects on central nervous system development, it is important to examine the effects of chronic intracochlear electrical stimulation in a neonatally deafened animal model. In this study neonatally deafened cats were implanted with a scala tympani electrode consisting of two pairs of electrodes. Chronic electrical stimulation was delivered using one electrode pair and consisted of charge-balanced biphasic pulses (200 μs/phase, 30 pps) at 2 dB above the electrically evoked auditory brain stem response (EABR) threshold for 4 h/day or at 6 dB 1 h/day, 5 days/week, for up to 3 months. The second electrode pair was unstimulated and served as an internal control. Following chronic stimulation, acute mapping experiments were performed in the central nucleus of the inferior colliculus (ICC) using single unit and multi-unit recording techniques and activating each electrode pair separately.In addition to these chronically stimulated animals, 2 other groups of experimental animals were studied: A normal group consisting of prior normal adult cats that were acutely implanted; and an unstimulated control group consisting of neonatally deafened adult cats that were either acutely implanted or implanted at 8–10 weeks of age but not chronically stimulated.

Among the major findings of this study are: Electrical stimulation of the intracochlear bipolar electrode consistently produces activation of a reproducibly limited sector of the ICC. The location of this activated sector was found to be consistent with the known cochleotopic organization of the ICC and the intracochlear location of the stimulating electrodes. No major differences in the spatial representation of activated electrodes were found between prior normal cats and neonatally deafened unstimulated cats. The locations, shapes and widths of these spatial representations were virtually indistinguishable indicating that ICC cochleotopic organizations were equivalent in these two experimental groups. In contrast, the ICC representation of chronically stimulated electrode pairs were found to be significantly different. The average area activated by chronically stimulated electrode pairs at 6 dB above minimum threshold was approximately twice that of unstimulated deafened animals and prior normal animals; and it was larger, but not significantly so, than the average of the unstimulated electrode pair in the same experimental group.

These results indicate that: 1) One of the fundamental features of the central auditory organization, the orderly topographic representation of cochlear place, is unaltered (at least to the level of the midbrain) by the lack of normal acoustic input during development. 2) Electrical stimulation of very limited duration and intensity delivered to a restricted sector of the cochlear spiral can expand the central representation of that sector and alter cochleotopic representations in the auditory CNS.     

声强对豚鼠下丘给声响应神经元间隔探测的影响

目的 探讨声音信号强度对豚鼠下丘给声响应神经元间隔探测的影响。方法采用单细胞记录的方法,刺激信号由100ms和50ms白噪声相加组成,声音强度10～80dB SPL,中间间隔时间为0、1、2、4、8、16、48、96?ms,分别统计阈响应组和阈上10,20,30dB时,给声响应神经元的间隔阈值,并观察间隔阈值与放电率和潜伏期的关系。结果本实验共记录96个神经元,其中,给声响应神经元有35个,在阈响应强度时,给声响应神经元间隔阈值明显增大,四组强度的间隔阈值均值分别为(22.91±4.36)ms,(9.00±2.69)ms,(4.00±0.49)ms和(11.33±3.11)ms（P<0.01）,但阈上各组间的差异无统计学意义（P>0.05）。在间隔时间为48ms时,给声响应神经元间隔后放电恢复率的均值分别为0.44±0.15,0.83±0.12,0.88±0.07和0.61±0.10（P<0.01）。而首次放电潜伏期分别为(12.86±0.72)ms,(11.65±0.64)ms,(11.03±0.65)ms和(10.68±0.55)ms（P>0.05）。 结论 声音信号强度对豚鼠下丘给声响应神经元的间隔探测有一定影响。     

Monopolar Intracochlear Pulse Trains Selectively Activate the Inferior Colliculus

Previous cochlear implant studies using isolated electrical stimulus pulses in animal models have reported that intracochlear monopolar stimulus configurations elicit broad extents of neuronal activation within the central auditory system—much broader than the activation patterns produced by bipolar electrode pairs or acoustic tones. However, psychophysical and speech reception studies that use sustained pulse trains do not show clear performance differences for monopolar versus bipolar configurations. To test whether monopolar intracochlear stimulation can produce selective activation of the inferior colliculus, we measured activation widths along the tonotopic axis of the inferior colliculus for acoustic tones and 1,000-pulse/s electrical pulse trains in guinea pigs and cats. Electrical pulse trains were presented using an array of 6–12 stimulating electrodes distributed longitudinally on a space-filling silicone carrier positioned in the scala tympani of the cochlea. We found that for monopolar, bipolar, and acoustic stimuli, activation widths were significantly narrower for sustained responses than for the transient response to the stimulus onset. Furthermore, monopolar and bipolar stimuli elicited similar activation widths when compared at stimulus levels that produced similar peak spike rates. Surprisingly, we found that in guinea pigs, monopolar and bipolar stimuli produced narrower sustained activation than 60 dB sound pressure level acoustic tones when compared at stimulus levels that produced similar peak spike rates. Therefore, we conclude that intracochlear electrical stimulation using monopolar pulse trains can produce activation patterns that are at least as selective as bipolar or acoustic stimulation.     

Temporal integration of electrical stimulation of auditory nuclei in normal-hearing and hearing-impaired cat

Temporal integration functions were measured, before and after a sound-induced hearing loss, in 5 cats using trains of electrical pulses applied to auditory nuclei in the brainstem. The 8 stimuli ranged from 1 pulse (0.25 ms duration) to 16 pulses (0.25 ms pulses spaced over 240 ms). The stimuli were applied to inferior colliculus or cochlear nucleus via permanently implanted electrodes. One electrode was tested extensively in each animal to obtain 10 sets of behaviorally-measured electrical detection thresholds counterbalanced across stimuli. The animal was then exposed to a 110 dB SPL, 2 kHz tone for 48 h and pre- and post-exposure audiograms were measured. The mean permanent threshold shift for acoustic stimuli was 48.5 dB. Another 10 thresholds for each of the 8 electrical stimuli were then measured. In the normal hearing animals, the mean slope of the temporal integration function for electrical stimulation was -7.6 dB per factor of 10 pulses. Alternatively, the mean time constant was 139 ms. In the hearing impaired animals, the slope was reduced to -1.5 dB per factor of 10 pulses, which corresponded to a mean time constant of 17 ms. In addition, the hearing impaired animals showed a decreased threshold for the electrical stimuli (stimulation hypersensitivity) as well as reduced variability across electrical stimulation thresholds. The results suggest that a major contribution to temporal integration occurs in inferior colliculus or higher. In addition, the results suggest that the reduction in temporal integration that follows hearing impairment is a peripherally-induced, central effect.     

Stimulus properties influencing the responses of inferior colliculus neurons to amplitude-modulated sounds

The temporal pattern of the responses of neurons in the inferior colliculus of the anesthetized rat were studied using continuous tone or noise carrier signals, amplitude modulated by pseudorandom noise. Period histograms of the responses, cross-correlated with the pseudorandom noise, gave an estimate of the unit's impulse responses to modulation. The amplitude-modulation rate transfer function (MTF) was obtained by Fourier transforming the correlograms. At sound levels within approximately 15 dB of the unit threshold, the MTFs were near lowpass functions between 6 and 200 Hz but became more bandpass-like as the intensity was increased. There was a steep decline in the response to modulation at modulation frequencies above 200 Hz for all stimulus intensities. For the bandpass-type MTFs the greatest modulation of the discharge pattern occurred at modulation frequencies between 10 and 200 Hz with a maximum in the distribution of MTF peak values between 100 and 120 Hz. There was no consistent relationship with characteristic frequency of either the position of the MTF peak or the high-frequency cutoff of the MTF. The cross-correlograms obtained at high stimulus intensities (30-60 dB above threshold) often showed a negative peak, representing a decrease in the probability of firing in response to intensity increments in the stimulus, and denoting a nonmonotonic rate-intensity function. The MTFs for units responding to amplitude-modulated broadband noise were often flatter in the low frequency region than those generated with tone carriers at corresponding intensities. For some units addition of a broadband noise background to the modulated tone changed the response characteristic of the MTF from bandpass to lowpass and shifted the MTF peak to a lower modulation frequency. The results demonstrate that although neurons in the inferior colliculus are selectively sensitive to the modulation frequency of dynamic stimuli, the response characteristics are not invariant, but instead are closely dependent on the conditions under which the modulation is presented.     

Acoustic input to the lateral pontine nuclei

Axon and terminal degeneration were studied in the cat dorsolateral pontine nucleus (DLPN) after lesion of the inferior colliculus. In separate experiments the acoustic responses of 111 units of the lateral pontine nuclei were studied in cats anesthetized with chloralose-urethane.Lesions of all three nuclei of the inferior colliculus (central, pericentral and external) lead to a very similar pattern of terminal degeneration in a discrete region of ipsilateral DLPN. This is suggestive of a highly convergent projection in which topography may be blurred.Most units responded to binaural stimulation, and the most common binaural response consisted of xcitatory inputs from each ear which facilitatyed at some binaural intensity levels and occuluded at others. Discharge rates changed as a result of alterations in the number of spikes evoked at the onset of the stimulus, and sustained discharges were rarely encountered. Units broadly and irregularyly tuned; binaural inhibition was very uncommon.Unit response characteristics suggested that, while the projection from the inferior colliculus was highly convergent, only a subclass of inferior colliculus neurons may be involved. However, the acoustic properties of lateral pontine units were strikingly similar to those of the cerebellar vermis, a region to which DLPN is known to project.