Kanwal Jagmeet S, Medvedev Andrei V, Micheyl Christophe
Department of Physiology and Biophysics, Georgetown University Medical Center, 3900 Reservoir Road, NW, Washington, DC 20057-1460, USA.
Network. 2003 Aug;14(3):413-35.
During navigation and the search phase of foraging, mustached bats emit approximately 25 ms long echolocation pulses (at 10-40 Hz) that contain multiple harmonics of a constant frequency (CF) component followed by a short (3 ms) downward frequency modulation. In the context of auditory stream segregation, therefore, bats may either perceive a coherent pulse-echo sequence (PEPE...), or segregated pulse and echo streams (P-P-P... and E-E-E...). To identify the neural mechanisms for stream segregation in bats, we developed a simple yet realistic neural network model with seven layers and 420 nodes. Our model required recurrent and lateral inhibition to enable output nodes in the network to 'latch-on' to a single tone (corresponding to a CF component in either the pulse or echo), i.e., exhibit differential suppression by the alternating two tones presented at a high rate (> 10 Hz). To test the applicability of our model to echolocation, we obtained neurophysiological data from the primary auditory cortex of awake mustached bats. Event-related potentials reliably reproduced the latching behaviour observed at output nodes in the network. Pulse as well as nontarget (clutter) echo CFs facilitated this latching. Individual single unit responses were erratic, but when summed over several recording sites, they also exhibited reliable latching behaviour even at 40 Hz. On the basis of these findings, we propose that a neural correlate of auditory stream segregation is present within localized synaptic activity in the mustached bat's auditory cortex and this mechanism may enhance the perception of echolocation sounds in the natural environment.
在觅食的导航和搜索阶段，髯蝠发出约25毫秒长的回声定位脉冲（频率为10 - 40赫兹），这些脉冲包含一个恒定频率（CF）成分的多个谐波，随后是一个短的（3毫秒）向下频率调制。因此，在听觉流分离的背景下，蝙蝠可能会感知到一个连贯的脉冲 - 回声序列（PEPE...），或者分离的脉冲流和回声流（P - P - P...和E - E - E...）。为了确定蝙蝠中流分离的神经机制，我们开发了一个简单而逼真的具有七层和420个节点的神经网络模型。我们的模型需要循环和侧向抑制，以使网络中的输出节点“锁定”到单个音调（对应于脉冲或回声中的CF成分），即对以高速率（>10赫兹）呈现的交替两个音调表现出差异抑制。为了测试我们的模型对回声定位的适用性，我们从清醒的髯蝠的初级听觉皮层获得了神经生理学数据。事件相关电位可靠地再现了在网络输出节点观察到的锁定行为。脉冲以及非目标（杂波）回声CF促进了这种锁定。单个单位反应不稳定，但当在几个记录位点求和时，即使在40赫兹时它们也表现出可靠的锁定行为。基于这些发现，我们提出听觉流分离的神经相关物存在于髯蝠听觉皮层的局部突触活动中，并且这种机制可能会增强在自然环境中对回声定位声音的感知。
