Lateral-line responses to water surface waves in the clawed frog,Xenopus laevis

Abstract

1. The responses of free-standing lateral-line organs to water surface waves were examined in the clawed frog,Xenopus laevis, by recording from the organs' afferent fibers. 2. Responses to waves consisted of modulations of the fiber's spontaneous discharge frequency. Typically, the modulation followed the wave frequency, but occasionally transients or harmonic or subharmonic modulations were observed (Figs. 3, 10). 3. Stimulus intensity was encoded by the degree of spike synchronization with the excitatory phase of the wave cycle and by the organ's maximal firing rate (Figs. 3, 4). Synchronization reached saturation at about 29 dB above threshold, but maximal firing increased linearly with the logarithm of the stimulus amplitude up to 80 dB above threshold. 4. The sensitivity of the organs could fluctuate with time; such changes were correlated with shifts in the spontaneous firing rate of the organ (Fig. 5). 5. Stimulus intensity did not affect the phase of the response period, but within that period the mean phase of firing shifted slightly with stimulus intensity (Fig. 6). 6. The data demonstrate an asymmetry between the inhibitory and excitatory transduction processes. Inhibition saturated at about 29 dB above threshold, but excitation continued to increase with stimulus intensity through all amplitudes applied. On the other hand, inhibition was strong enough to completely suppress firing for up to 70% of the wave cycle (Fig. 4). 7. The responses of the organs were maximal at 5 Hz, the lowest frequency tested in our equipment. With higher wave frequencies, response threshold increased, and the upper frequency response limit was mostly below 20 Hz (Fig. 7). It is suggested that this shift of responsiveness toward lower frequencies is due to characteristics of the surface wave. 8. From the thresholds for surface waves, the minimal threshold amplitude for water movementsat the cupula was calculated to be 0.01 μm (Table 1). 9. Stimulus direction affected considerably the response latencies and thresholds of the organs (Figs. 8, 9). The main determining factor for the response threshold was the animal's wave shadow, which could be as high as 30–40 dB.