Kick S A, Simmons J A
J Neurosci. 1984 Nov;4(11):2725-37. doi: 10.1523/JNEUROSCI.04-11-02725.1984.
The sensitivity of the echolocating bat, Eptesicus fuscus, to sonar echoes at different time delays after sonar emissions was measured in a two-choice echo detection experiment. Since echo delay is perceptually equivalent to target range, the experiment effectively measured sensitivity to targets at different ranges. The bat's threshold for detecting sonar echoes at a short delay of only 1.0 msec after emissions (corresponding to a range of 17 cm) was 36 dB SPL (peak to peak), but the threshold decreased to 8 dB SPL at a longer delay of 6.4 msec (a range of 1.1 m). Prior research has shown that, at even longer delays (corresponding to ranges of 3 to 5 m), the bat's threshold is in the region of 0 dB SPL. Contractions of the bat's middle ear muscles synchronized with the production of echolocation sounds cause a transient loss in hearing sensitivity which appears to account for the observed echo detection threshold shifts. The bat's echo detection thresholds increase by approximately 11 dB for each reduction in target range by a factor of 2 over the span from 17 cm to 1.1 m. As range shortens, the amplitude of echoes from small targets also increases, by 12 dB for each 2-fold reduction in range. Thus, when approaching a target, the bat compensates for changes in echo strength as target range shortens by changing its hearing threshold. Since this compensation appears to occur in the middle ear, the bat regulates echoes reaching the cochlea to a stable amplitude during its approach to a target such as a flying insect. In addition to this automatic gain control linked to target range, the bat aims its head to track a target's position during approach, thus stabilizing echo amplitude even if the target's direction changes. We hypothesize that the bat's directional emissions, directional hearing, middle ear muscle contractions, and head aim response collectively create a three-dimensional spatial tracking filter which the bat locks onto targets to stabilize echo amplitudes during interception of prey. We further hypothesize that this regulation, which cancels echo amplitude changes caused by the target's changing spatial position, leaves the bat free to observe echo amplitude changes caused by the target's own actions, such as insect wing beats. Elimination of spatially dependent echo amplitude changes removes the cause of potentially troublesome changes in neural response latency and keeps stimulation from echoes in the "tip" region of auditory nerve fiber tuning curves.(ABSTRACT TRUNCATED AT 400 WORDS)
在一项二选一的回声检测实验中，测量了棕蝠（Eptesicus fuscus）对声呐发射后不同时间延迟的声呐回声的敏感度。由于回声延迟在感知上等同于目标距离，该实验有效地测量了对不同距离目标的敏感度。棕蝠在声呐发射后仅1.0毫秒的短延迟（对应距离为17厘米）时检测声呐回声的阈值为36分贝声压级（峰峰值），但在6.4毫秒的较长延迟（距离为1.1米）时，阈值降至8分贝声压级。先前的研究表明，在更长的延迟（对应距离为3至5米）时，棕蝠的阈值在0分贝声压级左右。棕蝠中耳肌肉的收缩与回声定位声音的产生同步，会导致听力敏感度暂时丧失，这似乎可以解释观察到的回声检测阈值变化。在从17厘米到1.1米的范围内，目标距离每缩小2倍，棕蝠的回声检测阈值大约增加11分贝。随着距离缩短，小目标回声的幅度也会增加，距离每缩小2倍，幅度增加12分贝。因此，在接近目标时，棕蝠通过改变其听力阈值来补偿随着目标距离缩短而发生的回声强度变化。由于这种补偿似乎发生在中耳，棕蝠在接近诸如飞行昆虫等目标的过程中，会将到达耳蜗的回声调节到稳定的幅度。除了这种与目标距离相关的自动增益控制外，棕蝠在接近过程中会转动头部跟踪目标位置，从而即使目标方向改变也能稳定回声幅度。我们假设，棕蝠的定向发射、定向听觉、中耳肌肉收缩和头部转动反应共同创建了一个三维空间跟踪滤波器，棕蝠在拦截猎物时锁定目标以稳定回声幅度。我们进一步假设，这种调节消除了由目标空间位置变化引起的回声幅度变化，使棕蝠能够自由观察由目标自身动作（如昆虫翅膀振动）引起的回声幅度变化。消除与空间相关的回声幅度变化消除了神经反应潜伏期潜在麻烦变化的原因，并使来自回声的刺激保持在听觉神经纤维调谐曲线的“尖端”区域。（摘要截选至400字）
