Eaton-Peabody Laboratories, Massachusetts Eye and Ear, Boston, Massachusetts.
Department of Otolaryngology, Harvard Medical School, Boston, Massachusetts.
J Neurophysiol. 2019 Oct 1;122(4):1821-1842. doi: 10.1152/jn.00081.2019. Epub 2019 Aug 28.
While motion is important for parsing a complex auditory scene into perceptual objects, how it is encoded in the auditory system is unclear. Perceptual studies suggest that the ability to identify the direction of motion is limited by the duration of the moving sound, yet we can detect changes in interaural differences at even shorter durations. To understand the source of these distinct temporal limits, we recorded from single units in the inferior colliculus (IC) of unanesthetized rabbits in response to noise stimuli containing a brief segment with linearly time-varying interaural time difference ("ITD sweep") temporally embedded in interaurally uncorrelated noise. We also tested the ability of human listeners to either detect the ITD sweeps or identify the motion direction. Using a point-process model to separate the contributions of stimulus dependence and spiking history to single-neuron responses, we found that the neurons respond primarily by following the instantaneous ITD rather than exhibiting true direction selectivity. Furthermore, using an optimal classifier to decode the single-neuron responses, we found that neural threshold durations of ITD sweeps for both direction identification and detection overlapped with human threshold durations even though the average response of the neurons could track the instantaneous ITD beyond psychophysical limits. Our results suggest that the IC does not explicitly encode motion direction, but internal neural noise may limit the speed at which we can identify the direction of motion. Recognizing motion and identifying an object's trajectory are important for parsing a complex auditory scene, but how we do so is unclear. We show that neurons in the auditory midbrain do not exhibit direction selectivity as found in the visual system but instead follow the trajectory of the motion in their temporal firing patterns. Our results suggest that the inherent variability in neural firings may limit our ability to identify motion direction at short durations.
虽然运动对于将复杂的听觉场景解析为知觉对象很重要，但它在听觉系统中的编码方式尚不清楚。知觉研究表明，识别运动方向的能力受到运动声音持续时间的限制，但我们可以在更短的持续时间内检测到耳间差异的变化。为了了解这些不同的时间限制的来源，我们在未麻醉的兔子的下丘脑中记录了单个单位的反应，这些兔子对包含短暂的线性时变耳间时间差（“ITD 扫掠”）的噪声刺激做出反应，该刺激暂时嵌入在耳间不相关的噪声中。我们还测试了人类听众检测 ITD 扫掠或识别运动方向的能力。使用点过程模型将刺激依赖性和尖峰历史对单个神经元反应的贡献分开，我们发现神经元主要通过跟随瞬时 ITD 做出反应，而不是表现出真正的方向选择性。此外，使用最优分类器对单个神经元反应进行解码，我们发现方向识别和检测的 ITD 扫掠的神经阈值持续时间与人类阈值持续时间重叠，尽管神经元的平均反应可以在心理物理极限之外跟踪瞬时 ITD。我们的结果表明，下丘脑中并没有明确地编码运动方向，而是内部神经噪声可能限制了我们识别运动方向的速度。识别运动和识别物体的轨迹对于解析复杂的听觉场景很重要，但我们如何做到这一点尚不清楚。我们表明，听觉中脑中的神经元没有像在视觉系统中那样表现出方向选择性，而是在它们的时间发射模式中跟随运动的轨迹。我们的结果表明，神经发射的固有可变性可能限制了我们在短时间内识别运动方向的能力。
