INDICATIONS FOR FEATURE DETECTION WITH THE LATERAL-LINE ORGAN IN FISH

1. Stimulated by a moving object or two stationary, oscillating sphere s at different positions to the fish, extracellular recordings were ob tained from 77 units in the torus semicircularis of a catfish (Ancistr us spp.). 2. Five mechanosensory units showed complex filter propertie s, e.g. they responded exclusively to complex hydrodynamic stimuli cau sed by the moving object, but not to oscillating spheres independent o f location, amplitude and phase. 3. Particular midbrain units reacted only to certain stimulus features and responded I) only to the visible vortices following the moving object with a latency of about 300, 900 or 2800 ms depending on the speed of the moving object (n = 8); II) o nly to a preferred direction of object motion (n = 12); III) only to a n exclusively contralateral sphere stimulus, not to a simultaneously b ilateral stimulation (n = 3). 4. Seven units have shown a long-lasting response to the moving object (2.3, 10 and 20 cm/sec) with one or two zones of inhibition, which reminds one of OFF or ON-center units in t he visual system. 5. The present findings let us assume the existence of filter properties of toral neurons in lateral line processing, whic h calls to mind ''feature detectors'' reported in other sensory system s. It can be regarded as evidence for complex perception of the hydrod ynamic environment in fish.