We studied the columnar and layer-specific response properties of neurons in the primary auditory cortex (A1) of six (four females, two males) anesthetized free-tailed bats, Tadarida brasiliensis, in response to pure tones and down and upward frequency modulated (FM; 50 kHz bandwidth) sweeps. In addition, we calculated current source density (CSD) to test whether lateral intracortical projections facilitate neuronal activation in response to FM echoes containing spectrally distant frequencies from the excitatory frequency response area (FRA). Auditory responses to a set of stimuli changing in frequency and level were recorded along 64 penetrations in the left A1 of six free-tailed bats. FRA shapes were consistent across the cortical depth within a column and there were no obvious differences in tuning properties. Generally, response latencies were shorter (<10 ms) for cortical depths between 500 and 600 μm, which might correspond to thalamocortical input layers IIIb-IV. Most units showed a stronger response to downward FM sweeps, and direction selectivity did not vary across cortical depth. CSD profiles calculated in response to the CF showed a current sink located at depths between 500 and 600 μm. Frequencies lower than the frequency range eliciting a spike response failed to evoke any visible current sink. Frequencies higher than the frequency range producing a spike response evoked layer IV sinks at longer latencies that increased with spectral distance. These data support the hypothesis that a progressive downward relay of spectral information spreads along the tonotopic axis of A1 via lateral connections, contributing to the neural processing of FM down sweeps used in biosonar.

中文翻译：

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自由尾蝙蝠初级听觉皮层中FM方向选择性的层状组织。

我们研究了六只（四只雌性，两只雄性）麻醉的无尾蝙蝠Tadarida brasiliensis在初级听觉皮层（A1）中神经元的柱状和层特异性反应特性，以响应纯音和上下调频（ FM； 50 kHz带宽）扫描。此外，我们计算了电流源密度（CSD），以测试侧向皮层内投影是否响应包含从兴奋性频率响应区域（FRA）到光谱距离远的频率的FM回波来促进神经元激活。在六只自由尾蝙蝠的左A1沿64次穿刺，记录了对一系列刺激的频率和水平变化的听觉反应。FRA形状在柱内的整个皮层深度上是一致的，并且调谐特性没有明显差异。通常，对于500至600μm的皮质深度，响应潜伏期较短（<10 ms），这可能对应于丘脑皮质输入层IIIb-IV。大多数单位对向下的FM扫描表现出更强的响应，并且方向选择性在整个皮层深度中均不变化。响应CF计算的CSD曲线显示电流沉位于500至600μm之间。低于引起尖峰响应的频率范围的频率无法引起任何可见电流吸收。高于产生尖峰响应诱发层IV的频率范围的频率以更长的等待时间下降，该等待时间随频谱距离而增加。这些数据支持这样的假设：光谱信息的逐步向下中继通过侧向连接沿着A1的Tonotopic轴传播，