Measor Kevin, Yarrow Stuart, Razak Khaleel A
Graduate Neuroscience Program, University of California, Riverside, CA, USA.
Department of Psychology, University of California, Riverside, CA, USA.
Hear Res. 2018 Sep;367:137-148. doi: 10.1016/j.heares.2018.05.017. Epub 2018 May 26.
Sound level processing is a fundamental function of the auditory system. To determine how the cortex represents sound level, it is important to quantify how changes in level alter the spatiotemporal structure of cortical ensemble activity. This is particularly true for echolocating bats that have control over, and often rapidly adjust, call level to actively change echo level. To understand how cortical activity may change with sound level, here we mapped response rate and latency changes with sound level in the auditory cortex of the pallid bat. The pallid bat uses a 60-30 kHz downward frequency modulated (FM) sweep for echolocation. Neurons tuned to frequencies between 30 and 70 kHz in the auditory cortex are selective for the properties of FM sweeps used in echolocation forming the FM sweep selective region (FMSR). The FMSR is strongly selective for sound level between 30 and 50 dB SPL. Here we mapped the topography of level selectivity in the FMSR using downward FM sweeps and show that neurons with more monotonic rate level functions are located in caudomedial regions of the FMSR overlapping with high frequency (50-60 kHz) neurons. Non-monotonic neurons dominate the FMSR, and are distributed across the entire region, but there is no evidence for amplitopy. We also examined how first spike latency of FMSR neurons change with sound level. The majority of FMSR neurons exhibit paradoxical latency shift wherein the latency increases with sound level. Moreover, neurons with paradoxical latency shifts are more strongly level selective and are tuned to lower sound level than neurons in which latencies decrease with level. These data indicate a clustered arrangement of neurons according to monotonicity, with no strong evidence for finer scale topography, in the FMSR. The latency analysis suggests mechanisms for strong level selectivity that is based on relative timing of excitatory and inhibitory inputs. Taken together, these data suggest how the spatiotemporal spread of cortical activity may represent sound level.
声级处理是听觉系统的一项基本功能。为了确定皮层如何表征声级，量化声级变化如何改变皮层整体活动的时空结构很重要。对于能够控制并经常快速调整叫声声级以主动改变回声声级的回声定位蝙蝠来说尤其如此。为了了解皮层活动如何随声级变化，我们在此绘制了苍白蝙蝠听觉皮层中响应率和潜伏期随声级的变化情况。苍白蝙蝠使用60 - 30千赫兹的向下调频（FM）扫描进行回声定位。听觉皮层中调谐到30至70千赫兹频率之间的神经元对用于回声定位的FM扫描特性具有选择性，形成了FM扫描选择区域（FMSR）。FMSR对30至50分贝声压级之间的声级具有很强的选择性。我们在此使用向下的FM扫描绘制了FMSR中声级选择性的地形图，并表明具有更单调速率声级函数的神经元位于FMSR的尾内侧区域，与高频（50 - 60千赫兹）神经元重叠。非单调神经元在FMSR中占主导地位，并分布在整个区域，但没有证据表明存在幅度拓扑结构。我们还研究了FMSR神经元的首次放电潜伏期如何随声级变化。大多数FMSR神经元表现出矛盾的潜伏期变化，即潜伏期随声级增加。此外与潜伏期随声级降低的神经元相比，具有矛盾潜伏期变化的神经元具有更强的声级选择性，并且调谐到更低的声级。这些数据表明在FMSR中，神经元根据单调性呈聚集排列，没有强有力的证据表明存在更精细尺度的地形图。潜伏期分析表明了基于兴奋性和抑制性输入的相对时间的强声级选择性机制。综上所述，这些数据表明皮层活动的时空扩展可能如何表征声级。
