Midbrain responses to electroreceptive input in catfish

Summary1.A limited region of the torus semicircularis in the midbrain of the catfish receives strong electroreceptive input. The electroreceptive region forms a 200–300 μm wide rostrocaudal band extending the length of the torus (Fig. 1). Tactile, auditory, and visual stimuli also influence unit activity in this region.2.The receptive fields of single units demonstrate a somatotopic organization of electroreceptive input, primarily from the contralateral body surface (Fig. 2): anterior body areas project to rostral torus, posterior body areas project to caudal torus.3.The electroreceptive evoked potential to a stimuluson oroff transient consists of a large negative wave, sometimes followed by a biphasic wave (Fig. 1c). The evoked potential amplitude increases logarithmically with stimulus intensity over a 65 db range (Fig. 4); EP latency decreases exponentially over this same range, its greatest rate of change being to stimuli of less than 100 μV/cm.4.The threshold stimulus intensity to elicit an evoked response, as extra-polated from amplitude versus intensity plots (Fig. 4) from the most sensitive region of the torus, is 0.6 μV/cm, which equals the behavioral detection threshold of catfish.5.The amplitude and latency of the evoked potential changes depending on the orientation of the electric field gradient ( $$\overrightarrow {\text{E}}$$ ) of the stimulus with respect to the fish (Fig. 5). For each torus region abest $$\overrightarrow E$$ can be found to which the response is maximal in amplitude and minimal in latency. At 90° to thisbest $$\overrightarrow E$$ orientation, the EP virtually disappears.6.Thebest $$\overrightarrow E$$ orientation changes parametrically with the rostrocaudal and mediolateral position of the recording site (Fig. 6). A possible mechanism responsible for $$\overrightarrow {\text{E}}$$ orientation preference is discussed.7.The occurrence of an electrical stimulus alters the excitability of the torus to subsequent electrical stimuli (Figs. 7 and 8). Twin pulse experiments show a period of response depression then facilitation following stimulus intensities greater than 500 μV/cm.