Cheng Z, Wasserman G S
Department of Psychological Sciences, Purdue University, West Lafayette, IN 47907-1364, USA.
Biol Cybern. 1996 Aug;75(2):93-103. doi: 10.1007/s004220050277.
Nerve cell signals are different in form from the stimuli that evoke them and they exhibit complex spatio-temporal characteristics. This defines a neural coding problem which is addressed by two current theories: Multiple Meaning Theory holds that neural signals contain patterns that make statements about combinations of stimulus properties; the Task Dependence Hypothesis suggests that different features of identical neural signals mediate performance in different behavioral tasks. These coding issues were addressed by investigating the representation of sensory information in the distal nervous system after transduction of visual stimuli into bio-electric signals. The objects of study were light-evoked neural responses which had been intracellularly recorded from single retinula (photoreceptor) cells in Limulus lateral eyes. The efficacies with which sensory information was represented by various candidate neural codes were calculated using receiver operating characteristic (ROC) analyses to provide objective indices. The specific visual problem under investigation was discrimination between light flashes whose intensities differed by a very small amount. A wide range of light adaptation states and relative stimulus intensities were explored. Extremely stringent data quality standards were applied which restricted the investigation to cells whose potentials did not exhibit any statistically significant drift during the hours required for data collection. Seven cellular characterizations were simultaneously monitored to detect drift in a given cell's potentials; these characterizations included the value of the membrane potential and the values of six candidate codes. These codes were: the area under the light-evoked receptor potential (RP), the mean value of the RP, the peak height of the RP, the slope of the onset of the RP, the duration required for the RP to drop from its peak by a given amount, and the duration required for the RP to end. The results were: (1) Light adaptation increases efficacy. (2) Thus, light adaptation trades sensitivity for acuity (as characterized by ROC discriminations). (3) Increasing relative light flash intensity also increases efficacy. (4) The efficacies of the various codes are significantly different and fall in the following order: area > or = peak = mean > or = duration-end = slope = duration-drop. These findings further demonstrate that arbitrary characterizations of stimulus-response relationships are very likely to be incomplete. They particularly indicate that many commonly used and quite conventional neural analysis strategies may substantially under-estimate system performance.
神经细胞信号在形式上与引发它们的刺激不同，并且表现出复杂的时空特征。这就定义了一个神经编码问题，目前有两种理论来解决这个问题：多重意义理论认为神经信号包含对刺激属性组合进行陈述的模式；任务依赖假说表明相同神经信号的不同特征在不同行为任务中调节表现。通过研究视觉刺激转导为生物电信号后远端神经系统中感觉信息的表征，来解决这些编码问题。研究对象是从鲎侧眼中单个小眼（光感受器）细胞进行细胞内记录得到的光诱发神经反应。使用接受者操作特征（ROC）分析计算各种候选神经编码表征感觉信息的效率，以提供客观指标。所研究的具体视觉问题是辨别强度相差非常小的闪光。探索了广泛的光适应状态和相对刺激强度。应用了极其严格的数据质量标准，将研究限制在数据收集所需时间内其电位未表现出任何统计学显著漂移的细胞。同时监测七个细胞特征以检测给定细胞电位的漂移；这些特征包括膜电位值和六个候选编码的值。这些编码是：光诱发感受器电位（RP）的面积、RP的平均值、RP的峰值高度、RP起始的斜率、RP从峰值下降给定幅度所需的持续时间以及RP结束所需的持续时间。结果是：（1）光适应提高效率。（2）因此，光适应以敏锐度（由ROC辨别表征）换取灵敏度。（3）增加相对闪光强度也提高效率。（4）各种编码的效率显著不同，顺序如下：面积≥峰值 = 平均值≥持续时间结束 = 斜率 = 持续时间下降。这些发现进一步证明，刺激 - 反应关系的任意表征很可能是不完整的。它们特别表明，许多常用且相当传统的神经分析策略可能会大幅低估系统性能。
