Midbrain units in catfish: Response properties to electroreceptive input

Abstract

1. Single unit responses to uniform, rectangular and sine wave electric field stimulation were studied in the torus semicircularis of catfish (Ictalurus nebulosus) with the intention of characterizing functional unit types. Five unit types were discriminated on the basis of their responses to dc step stimuli (Table 1, Fig. 2): (1) phasic units with high resting discharges (Phrd) and (2) phasic units with low resting discharges (Plrd), both responded with a fast adapting increase in firing rate; (3) tonic units (T) responded with an impulse train that did not adapt appreciably for at least 250 ms; (4) phasic-tonic units (P-T) responded with a fast and a slow adapting component; and (5) weakly responding units (W) responded poorly or erratically. 2. Changing the orientation of the stimulus field drastically altered the response vigor of most units (Figs. 1, 3). Field orientation preference was measured for each unit based on its impulse response (Fig. 1). As a group,T units were most selective, followed byP-T andW units.Plrd andPhrd units were least selective responding to a wide range of stimulus orientations (Table 1). Frequency-response, intensity-response, and threshold measurements were determined with the stimulus field in the optimal orientation (best\(\vec E\)) for each unit. 3. Characteristic frequency thresholds fell between 0.8 and 15 μV/cm (8×10−13 and 15×10−12 A/mm2). Unit latencies ranged from 8 to 28 ms except for 3 unit latencies of 47, 93, and 130 ms (Fig. 4). 4. Increasing the intensity of rectangular wave stimulation usually resulted in a monotonic decrease in response latency (Fig. 11) while impulse response functions often saturated or declined at high intensities, especially amongT andP-T units (Fig. 10).W units showed no systematic effect of intensity on their responses. 5. Each unit was classified according to its frequency tuning properties into one of four groups (Fig. 5): broad passband, <0.1 Hz to 10–25 Hz; low passband, <0.1 Hz to 1–7 Hz; medium passband, 0.1–1.0 Hz to 7–25 Hz; and high passband, 1.0–5.0 Hz to 10–25 Hz. SomeT, P-T andW units displayed frequency specific inhibition, typically by frequencies above their excitatory frequencies (Fig. 6). 6. Phase locking to sinusoidal stimuli was a common attribute of torus units. Four distinct types of phase locking units were found (Fig. 7): monophase multispiking, monophase oligospiking, biphase multispiking, and biphase oligospiking. Firing phase could advance, recede or remain unchanged as a function of stimulus intensity (Fig. 8). After-discharges at the cessation of a high intensity sinusoidal stimulus was observed in 13% of the phase locking units.