1. Training experiments: a) When the catfish Ictalurus nebulosus (Amiurus nebulosus) is moving, it can perceive an inhomogeneous d. c. field (current density ≦0.0006 μA/mm2, current density gradient ≦0.0003 μA/mm2 per mm) (Fig. 1b, Notebook entry 1). b) The fish is also able to perceive the direction (polarity) of this field (Notebook 2, Fig. 4). 2. Recordings from afferent single fibers of the so-called small pit organs (Fig. 3a, 3b): a) The spontaneous frequency of nerve impulses is changed when the position of the fish is fixed and a similar but somewhat stronger field (0.006 μA/mm2 or 0.003 μA/mm2 per mm) is moved at speeds of 10-15 cm/sec around the fish (Fig. 2, 6). b) Impulse frequency is affected by both the polarity of the field and the latters direction of movement. Thus, with a single polarity, field movement in one direction elicits a frequency increase and in the opposite direction a frequency decrease (Figs. 2, 6). Likewise, movement in a. single direction with one polarity elicits an increase in frequency and with reversed polarity a decrease in frequency. c) The greater the current density and the faster the movement, the greater the change in frequency. d) The dependence of the reaction on the direction of current flow (see above) does not refer to any direction parallel to the plane of the fish's skin. 3. It follows that: a) The small pit organs respond to changes of current density per unit time, i.e., to current changes, to which they are exposed during relative motion between fish and field. b) The organ responds only to the current which flows perpendicular to the surface of the skin, i.e., along the longitudinal axis of the ampullary canal of the small pit organ (see Fig. 7). Positive ΔI/Δt (see p. 382) results in a frequency drop and negative ΔI/Δt in a frequency rise. The value of ΔI/Δt determines the amplitude of the change in frequency. c) Bodily movements of the fish suffice (at least at 0.006 μA/mm2 or 0.003 μA/mm2 per mm) to produce perceptible changes in field density. d) The stimulus, ±ΔI/Δt, can be produced in an inhomogeneous electric field through movement of the fish in a straight line (between points of different current density) or by turning. In a homogeneous field only turning movements can produce ±ΔI/Δt. With turning movement, the sign and amplitude of the stimulus and its response depend on the angle of the ampullary canal to the direction (polarity) of the field (see Fig. 8). Orientation with regard to field direction is possible by changing this angle and comparing the amplitude of the responses. 4. The following points are discussed: a) The reasons for the assumption that the fish perceives the field even at training threshold by means of its small pit organs despite the fact that the threshold of the individual organ is ten times higher. b) Whether the fish can perceive d. c. fields occuring in rivers (Fig. 9). c) The possibility of a functional analogy between the small pit organs and the ampullary organs of the mormyrids. © 1969 Springer-Verlag.
