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 wave s were examined in animals with an intact lateral line or with differe nt combinations of lateral lines reversibly inactivated by CoCl2. The responses were characterized with respect to response frequency, turni ng accuracy, turning side, response time, and swim distance. After the inactivation most animals still responded to surface waves but the re sponses were different from those of animals with an intact lateral li ne. They also differed according to the combination of inactivated lin es. In all experiments the responses for stimuli in some sectors of th e surface did not differ from controls. The location of these sectors co-varied with the position of the intact lines, i.e., normal response s were found for frontal stimulus directions when bead lines were inta ct and for caudolateral stimulus directions when trunk lines were inta ct. Their size was larger when lines on both sides of the body were in tact and smaller when only lines on one side were intact. When the num ber of functional lines was reduced to one or two on one side of the b ody the turning angles shown within the sector of normal responses wer e maintained for stimulus directions outside these sectors. These resu lts can be interpreted as indicating that head and trunk lines represe nt different ''position values''. When only a single line was function al the toads still turned towards the stimulus source more often than by chance. It is hypothesized that Xenopus uses two mechanisms to dete rmine the direction of surface waves. One uses the position values of head and trunk lines; this mechanism is comparable to the ''place valu e'' postulated for individual head neuromasts of surface feeding fish. The other uses the information encoded in the activity pattern that i s elicited in one line when the surface wave travels over the line. Th is second mechanism yields information about stimulus side but not abo ut stimulus angle.