Hopkins C D, Westby G W
Brain Behav Evol. 1986;29(1-2):77-104. doi: 10.1159/000118673.
We explored coastal streams, rivers, and swamps in the Guianas of South America and found eleven species of gymnotiform fishes with pulse discharges. Each species has a characteristic electric organ discharge (EOD) waveform of 0.5-5 ms duration; at least two species appear to have a natural sex difference in their EODs which is apparent when comparing large adult males and females. Three sensory coding mechanisms are proposed to explain how electric fish might be able to determine species and sex identity from such short electrical pulses. Spectral Coding: electroreceptors tuned to different frequencies encode the spectrum of the EOD as a cross-fiber stimulation pattern. Temporal Coding: EODs are encoded as a volley of nerve spikes patterned in the time domain. Scan Sampling: a receiver detects a signaler's EOD as an amplitude modulation or 'beat' set up by the combination of its own discharge with the signaler's. The receiver uses the modulation envelope to assess the signaler's EOD waveform. To distinguish between these three coding mechanisms, we tested the ability of one pulse gymnotiform, Hypopomus beebei, to discriminate one electric waveform from another by comparing the acceleration of the discharge rate to different stimuli. Stimuli are presented under two conditions: when the stimulus pulse train is free-running compared to the fish's pulse train, and when the stimulus train is phase-locked to the fish's discharge pulse train. Under the former condition scan sampling may be used; under the latter it will be impossible. Hypopomus discriminates the polarity of a single period sinusoidal stimulus under scanning conditions but does not discriminate under clamped conditions. Hypopomus gives the strongest response to single period sine waves of 670 Hz and weaker responses to sinusoids of lower and higher frequencies. Free-running and phase-locked stimuli evoke similar responses. Under free-running conditions, Hypopomus discriminates a series of EOD-like stimuli that have been phase-shifted by varying amounts, but under phase-locked conditions does not. Scan sampling is presented as a possible waveform recognition mechanism for pulse-discharging gymnotiform fishes.
我们对南美洲圭亚那地区的沿海溪流、河流和沼泽进行了探索，发现了11种具有脉冲放电的裸背电鳗目鱼类。每个物种都有一个持续时间为0.5至5毫秒的特征性电器官放电（EOD）波形；至少有两个物种在其EOD中似乎存在自然的性别差异，在比较成年大雄鱼和雌鱼时这种差异很明显。我们提出了三种感官编码机制来解释电鱼如何能够从如此短暂的电脉冲中确定物种和性别身份。频谱编码：调谐到不同频率的电感受器将EOD的频谱编码为一种跨纤维刺激模式。时间编码：EOD被编码为在时域中形成模式的一连串神经尖峰。扫描采样：接收器将自身放电与信号发送者的放电相结合所产生的幅度调制或“拍频”检测为信号发送者的EOD。接收器使用调制包络来评估信号发送者的EOD波形。为了区分这三种编码机制，我们测试了一种单脉冲裸背电鳗目鱼类——贝氏下口电鳗（Hypopomus beebei）通过比较放电率对不同刺激的加速度来区分一种电波形与另一种电波形的能力。刺激在两种条件下呈现：当刺激脉冲序列与鱼的脉冲序列自由运行时，以及当刺激序列与鱼的放电脉冲序列锁相时。在前一种条件下可能会使用扫描采样；在后一种条件下则不可能。贝氏下口电鳗在扫描条件下能够区分单个周期正弦刺激的极性，但在钳位条件下则不能。贝氏下口电鳗对670赫兹的单个周期正弦波反应最强，对较低和较高频率的正弦波反应较弱。自由运行和锁相刺激引发相似的反应。在自由运行条件下，贝氏下口电鳗能够区分一系列相位已发生不同程度偏移的类似EOD的刺激，但在锁相条件下则不能。扫描采样被认为是脉冲放电裸背电鳗目鱼类一种可能的波形识别机制。
