Discrimination by the cod of sounds from opposing directions

Abstract

1. The ability of cod (Gadus morhua L.) to discriminate between sound waves coming from opposing directions was studied by means of cardiac conditioning in approximately free field conditions under a raft in a Scottish sea loch. The sound sources were positioned at 0° (frontal), 60° or 120° in the fish's median vertical plane, or at 60° or 90° in the transverse plane, other sources occupying diametrically opposed locations. Radial distances varied between 3.05 m and 4.20 m. 2. Switching a continually pulsed 120 Hz pure tone stimulus from a reference source to an opposing source was detected under all conditions. 3. Discrimination was absent when a standing wave was added to the opposing source, locally inverting the phase of sound pressure with respect to the acoustic particle motion without influencing the direction of propagation. 4. Switching to a completely synthesized standing wave field, simulating the phase relations of the reference source, was not detected. Switching to a simulation of the opposite source was detected. 5. It is concluded that the detection of sound propagation direction is based on the characteristic phase relationship between particle motion and sound pressure. Cues resulting from sound propagation itself are irrelevant, and do not appear to be perceived. 6. It appears that the 180° ambiguity in determining sound propagation direction by means of acoustic particle oscillation alone can be eliminated by the cod. This ability seems to hold for all stimulus directions, thus providing a basis for truly three-dimensional directional hearing.