Vogel A, Hennig R M, Ronacher B
Humboldt University Berlin, Institute of Behavioural Physiology, Dept. of Biology, Invalidenstr. 43, 10115 Berlin, Germany.
J Neurophysiol. 2005 Jun;93(6):3548-59. doi: 10.1152/jn.01288.2004. Epub 2005 Feb 16.
A key problem for neuronal information processing is the variability of spike trains, something that is likely to constrain the encoding of sensory signals. We measured interspike-interval variability (coefficient of variation) as well as spike-count variability (Fano factor) in the metathoracic auditory system of locusts. We performed simultaneous intracellular recordings at the first three processing levels to establish identical physiological conditions. This allows us to assess whether variability is generated anew or is reduced during synaptic transmission and processing. Both the interspike-interval variability as well as the spike-count variability revealed similar trends and showed an increase from the periphery to higher processing levels. This result was confirmed by single-cell recordings. A comparison of ascending interneurons coding for sound direction and those encoding sound patterns showed that the latter respond more reliably to repeated stimulus presentations. In general, the variability of spiking responses was much lower than expected from a Poisson process. Furthermore, we observed a strong dependence of variability on the spike rate, which differed at the three levels investigated. The differences in spike rates account for most of the differences in variability observed between processing levels. For auditory receptors, we found a good agreement between the Fano factor and the squared coefficient of variation, suggesting similarities to a renewal process of spike generation at the periphery. At the level of interneurons, the Fano factor was lower than the squared coefficient of variation; this indicates a higher reliability than expected from the interspike-interval distribution.
神经元信息处理的一个关键问题是脉冲序列的变异性，这很可能会限制感觉信号的编码。我们测量了蝗虫后胸听觉系统中的峰峰间隔变异性（变异系数）以及脉冲计数变异性（法诺因子）。我们在前三个处理水平上进行了同步细胞内记录，以建立相同的生理条件。这使我们能够评估变异性是在突触传递和处理过程中重新产生还是降低。峰峰间隔变异性和脉冲计数变异性都显示出相似的趋势，并且从外周到更高的处理水平都有所增加。单细胞记录证实了这一结果。对编码声音方向的上升中间神经元和编码声音模式的中间神经元的比较表明，后者对重复的刺激呈现反应更可靠。一般来说，脉冲发放反应的变异性远低于泊松过程的预期。此外，我们观察到变异性对脉冲率有很强的依赖性，在研究的三个水平上有所不同。脉冲率的差异解释了处理水平之间观察到的变异性差异的大部分。对于听觉感受器，我们发现法诺因子与变异系数的平方之间有很好的一致性，这表明与外周脉冲产生的更新过程相似。在中间神经元水平，法诺因子低于变异系数的平方；这表明其可靠性高于从峰峰间隔分布预期的可靠性。
