Kaunzinger I, Kramer B
J Exp Biol. 1995;198(Pt 11):2365-72. doi: 10.1242/jeb.198.11.2365.
The South American knifefish Eigenmannia sp. can detect the electric organ discharges (EODs; 250600 Hz) of conspecifics when they are superimposed over its own EOD. This study investigates the minimum frequency difference necessary for such signal perception, using the application of sine-wave stimuli. Electrosensory stimulus-intensity thresholds were determined for trained fish using stimuli associated with food rewards. These sine-wave stimuli were 'clamped' to the EOD frequency of the fish. Electrosensory thresholds were also determined for the spontaneous jamming avoidance response (JAR; a change in EOD frequency evoked by a stimulus of sufficiently similar frequency), in this case using unclamped stimuli. Over the wide frequency range investigated (0.33.01 times EOD frequency), the lowest stimulus-intensity thresholds of 0.6 µV cm-1 (peak-to-peak) (0 dB) at a water conductivity of 100 µS cm-1 were found close to (but not exactly at) the EOD fundamental frequency. At exact frequency identity between the EOD and the stimulus, the stimulus-intensity response threshold rose abruptly by more than 10 dB compared with slightly higher or lower stimulus frequencies. A similar 'needle-like' threshold increase was found at exactly two and three times the EOD frequency, but neither at harmonic ratios between stimulus and EOD frequency that represent fractions (e.g. at 5:4=1.25, 4:3=1.33, 3:2=1.5 or 5:3=1.67 times EOD frequency) nor at subharmonics such as half or two-thirds of the EOD frequency. The steepest increase of stimulus-intensity response threshold was in the range 0.9981.002 times EOD frequency, corresponding to a threshold change, or electrosensory filter slope, of 5000 dB per octave. For the spontaneous JAR, a similar stimulus-intensity threshold increase was observed when EOD frequency equalled stimulus frequency. Because of the longer rise time for the stimulus amplitude (400 ms rather than 35 ms) the stimulus intensity threshold was higher (up to 32 dB; mean, 20 dB) than in the other experiments (up to 15 dB; mean, 13 dB). A difference in frequency between the EOD and the applied stimulus as small as 1 Hz (that is, 0.2 % of the EOD frequency) was sufficient for good signal perception in Eigenmannia sp. The JAR appears to be useful in avoiding insensitivity at exact integer harmonics of the EOD frequency.
南美刀鱼 Eigenmannia sp. 能够在同种个体的电器官放电（EODs；250 - 600 赫兹）叠加在其自身 EOD 之上时检测到这些放电。本研究使用正弦波刺激来探究这种信号感知所需的最小频率差。通过与食物奖励相关的刺激，确定了经过训练的鱼的电感觉刺激强度阈值。这些正弦波刺激被“钳制”到鱼的 EOD 频率。还针对自发的干扰回避反应（JAR；由频率足够相似的刺激引起的 EOD 频率变化）确定了电感觉阈值，在这种情况下使用未钳制的刺激。在所研究的宽频率范围内（EOD 频率的 0.3 - 3.01 倍），在 100 μS cm⁻¹ 的水电导率下，最低刺激强度阈值为 0.6 μV cm⁻¹（峰 - 峰）（0 dB），该阈值出现在接近（但不完全是）EOD 基频处。当 EOD 与刺激频率完全相同时，与略高或略低的刺激频率相比，刺激强度反应阈值突然跃升超过 10 dB。在 EOD 频率的恰好两倍和三倍处也发现了类似的“针状”阈值增加，但在刺激与 EOD 频率的谐波比为分数的情况下（例如，在 5:4 = 1.25、4:3 = 1.33、3:2 = 1.5 或 5:3 = 1.67 倍 EOD 频率）以及在 EOD 频率的次谐波（如二分之一或三分之二）处均未发现。刺激强度反应阈值的最陡增加出现在 EOD 频率的 0.998 - 1.002 倍范围内，对应于每倍频程 5000 dB 的阈值变化或电感觉滤波器斜率。对于自发的 JAR，当 EOD 频率等于刺激频率时，观察到类似的刺激强度阈值增加。由于刺激幅度的上升时间较长（400 毫秒而非 35 毫秒），刺激强度阈值比其他实验中的更高（高达 32 dB；平均值为 20 dB）（其他实验中高达 15 dB；平均值为 13 dB）。EOD 与施加刺激之间小至 1 赫兹的频率差（即 EOD 频率的 0.2%）就足以使 Eigenmannia sp. 实现良好的信号感知。JAR 似乎有助于避免在 EOD 频率的精确整数谐波处出现不敏感情况。
