Eggermont J J, Mossop J E
Department of Physiology, University of Calgary, Calgary, Alberta T2N 1N4, Canada.
J Neurophysiol. 1998 Oct;80(4):2133-50. doi: 10.1152/jn.1998.80.4.2133.
The neural representation of sound azimuth in auditory cortex most often is considered to be average firing rate, and azimuth tuning curves based thereupon appear to be rather broad. Coincident firings of simultaneously recorded neurons could provide an improved representation of sound azimuth compared with that contained in the firing rate in either of the units. In the present study, a comparison was made between local field potentials and several measures based on unit firing rate and coincident firing with respect to their azimuth-tuning curve bandwidth. Noise bursts, covering a 60-dB intensity range, were presented from nine speakers arranged in a semicircular array with a radius of 55 cm in the animal's frontal half field. At threshold intensities, all local field potential (LFP) recordings showed preferences for contralateral azimuths. Multiunit recordings showed in 74% a threshold for contralateral azimuths, in 16% for frontal azimuths, and in only 5% showed an ipsilateral threshold. The remaining 5% were not spatially tuned. Representations for directionally sensitive units based on coincident firings provided significantly sharper tuning (50-60 degrees bandwidth at 25 dB above the lowest threshold) than those based on firing rate (bandwidths of 80-90 degrees). The ability to predict sound azimuth from the directional information contained in the neural population activity was simulated by combining the responses of the 102 single units. Peak firing rates and coincident firings with LFPs at the preferred azimuth for each unit were used to construct a population vector. At stimulus levels of >/=40 dB SPL, the prediction function was sigmoidal with the predicted frontal azimuth coinciding with the frontal speaker position. Sound azimuths >45 degrees from the midline all resulted in predicted values of -90 or 90 degrees, respectively. No differences were observed in the performance of the prediction based on firing rate or coincident firings for these intensities. This suggests that although coincident firings produce narrower azimuth tuning curves, the information contained in the overall neural population does not increase compared with that contained in a firing rate representation. The relatively poor performance of the population vector further suggests that primary auditory cortex does not code sound azimuth by a globally distributed measure of peak firing rate or coincident firing.
听觉皮层中声音方位的神经表征通常被认为是平均放电率，基于此的方位调谐曲线似乎相当宽。与单个神经元放电率所包含的信息相比，同时记录的神经元的同步放电可能会提供更精确的声音方位表征。在本研究中，对局部场电位与基于单位放电率和同步放电的几种测量方法在方位调谐曲线带宽方面进行了比较。在动物前半场半径为55厘米的半圆形阵列中，从九个扬声器发出覆盖60分贝强度范围的噪声脉冲串。在阈值强度下，所有局部场电位（LFP）记录都显示出对侧方位偏好。多单元记录显示，74%对侧方位有阈值，16%对额方位有阈值，只有5%显示同侧阈值。其余5%没有空间调谐。基于同步放电的方向敏感单元的表征提供了比基于放电率的表征（带宽为80 - 90度）更尖锐的调谐（在比最低阈值高25分贝时带宽为50 - 60度）。通过组合102个单个单元的反应，模拟了从神经群体活动中包含的方向信息预测声音方位的能力。每个单元在偏好方位处的峰值放电率和与LFP的同步放电用于构建群体向量。在刺激水平≥40分贝声压级时，预测函数呈S形，预测的额方位与额扬声器位置一致。与中线夹角大于45度的声音方位分别导致预测值为-90度或90度。对于这些强度，基于放电率或同步放电的预测性能没有差异。这表明，尽管同步放电产生的方位调谐曲线更窄，但与放电率表征相比，整个神经群体中包含的信息并没有增加。群体向量相对较差的性能进一步表明，初级听觉皮层不是通过峰值放电率或同步放电的全局分布测量来编码声音方位的。
