SOURCE LEVEL DISCRIMINATION BY THE LATERAL LINE SYSTEM OF THE MOTTLED SCULPIN, COTTUS-BAIRDI

A conditioned feeding response of the mottled sculpin was used to measure level discrimination by the lateral line system as a function of source level, frequency, and distance. Level discrimination limens (LDLs) were determined with a 6-mm-diam, sinusoidally vibrating sphere placed in front of the trunk lateral line system at varying distances from the trunk of the fish. LDLs were relatively independent of frequency over most of the detection bandwidth, with mean values ranging from 5-6 dB from 10 to 50 Hz at a fish-to-source distance of 15 mm. The mean LDL at 100 Hz, near the upper end of the frequency range of the lateral line system was 8-9 dB at the same source distance. Mean LDLs at 10 and 20 dB above detection threshold were not significantly different, nor were mean LDLs at distances ranging from 7.5 to 30 mm for a 50-Hz, 10-dB SL source. The level-encoding features of posterior lateral line fibers innervating the trunk were also determined from data previously collected under stimulus conditions nearly identical to those used for behavioral studies. Afferent fibers responded to level increments with increases in both spike rate and phase-locking over most of the conditions used in behavioral experiments. Although it was impossible to eliminate spike rate as a level-encoding mechanism at any stimulus condition, phase-locking appeared to be a strong candidate at low source levels and vibration frequencies. These results show that level discrimination abilities by the sculpin lateral line system are within the range of capabilities of vertebrate auditory systems and that phase-locking in peripheral fibers may be important for encoding stimulus level by the lateral line.