THE RESPONSES OF MIDBRAIN LATERAL-LINE UNITS OF THE GOLDFISH, CARASSIUS-AURATUS, TO OBJECTS MOVING IN THE WATER

Lateral line responses to hydrodynamic stimuli caused by small objects moving in the water were recorded extracellularly from single and ''f ew units'' in the midbrain torus semicircularis of the goldfish, Caras sius auratus. All units (n = 59) increased their discharge rate when a n object passed the fish laterally either from anterior to posterior o r in the opposite direction. Different units could show different temp oral response patterns. In 29 out of 49 units tested (59%) the number of spikes caused by an object moving from anterior to posterior along the side of the fish was significantly different from those caused by an object moving from posterior to anterior. Two units encoded motion direction by different temporal patterns of discharge. In general, dis charge rates decreased when object distance was increased or when obje ct speed was decreased. Changes in object size and shape did not affec t the strength of unit responses as much as changes in object speed an d distance. The data indicate that some midbrain lateral line units in the goldfish encode motion direction but are not tuned to other aspec ts of an object moving in the water. In this respect, the responses of units in the midbrain are similar to those reported for medullary uni ts of goldfish. However, comparing midbrain with peripheral and medull ary responses, it becomes clear that a substantial amount of neural co mputation has taken place and that there are distinct response types i n the midbrain of Carassius which differ from the uniform peripheral a nd from medullary response patterns.