Analysis of surface wave direction by the lateral line system of Xenopus: source localization before and after inactivation of different parts of the lateral line.

The turning responses of clawed toads (Xenopus laevis) to surface waves were examined in animals with an intact lateral line or with different combinations of lateral lines reversibly inactivated by CoCl2. The responses were characterized with respect to response frequency, turning accuracy, turning side, response time, and swim distance. After the inactivation most animals still responded to surface waves but the responses were different from those of animals with an intact lateral line. They also differed according to the combination of inactivated lines. In all experiments the responses for stimuli in some sectors of the surface did not differ from controls. The location of these sectors co-varied with the position of the intact lines, i.e., normal responses were found for frontal stimulus directions when head lines were intact and for caudolateral stimulus directions when trunk lines were intact. Their size was larger when lines on both sides of the body were intact and smaller when only lines on one side were intact. When the number of functional lines was reduced to one or two on one side of the body the turning angles shown within the sector of normal responses were maintained for stimulus directions outside these sectors. These results can be interpreted as indicating that head and trunk lines represent different "position values". When only a single line was functional the toads still turned towards the stimulus source more often than by chance. It is hypothesized that Xenopus uses two mechanisms to determine the direction of surface waves. One uses the position values of head and trunk lines; this mechanism is comparable to the "place value" postulated for individual head neuromasts of surface feeding fish. The other uses the information encoded in the activity pattern that is elicited in one line when the surface wave travels over the line. This second mechanism yields information about stimulus side but not about stimulus angle.