We take it for granted that we can pinpoint the direction that a noise comes from. Our brains rapidly calculate the difference between a sound arriving at both our ears to locate its source; but fish have a much bigger problem. With their ears close together deep in their heads and sound travelling 5 times faster in water than air, placing a sound must be much harder; yet fish manage. As toadfish are very vocal, and the females locate the positions of courting males by following their serenade, Peggy Edds-Walton and Richard Fay, from Loyola University Chicago, were curious to find out if the fish use information from both ears in the neural circuit that codes the direction of the sounds (p. 1483).
Knowing that most vertebrates, humans included, process auditory information from one ear before combining the information from both ears at a second site in the brain circuit, Edds-Walton and Fay decided to investigate the sites where auditory inputs from the left and right ears are combined by the toadfish's brain. But to find out, the team needed a way to alter the auditory input from one ear to cells in specific regions of the brain and measure the effect of the change on cells in the same brain region. For example, if the pair could change the `directional response' of a cell in the left descending octaval nucleus (DON: the first auditory region of the brain)by manipulating the right ear, this would suggest that sound information from both ears is brought together and processed in the DON.
Deciding to anaesthetise the auditory nerve from the saccule (one of the fish's hearing organs) in one side of a fish's head to alter its input to the brain, Edds-Walton tried injecting lidocaine into the saccule and was surprised when the electrical spike activity that she and Fay were recording in the DON disappeared. At first the pair thought that the saccule had been damaged, but as Edds-Walton removed the needle, the signal returned. Edds-Walton had inadvertently tipped the large calcareous otolith, to which the sound sensitive saccule is attached, and changed the strength of the saccule's directional response to sound. `This was one of those moments of serendipity in science,' says Edds-Walton. She and Fay had found a way of reversibly altering a saccule's input to the brain.
Having discovered this new method to test the fish's hearing, the duo played vibrations to toadfish over frequencies ranging from 50 to 300 Hz from different directions as they recorded the directional response patterns from cells in the midbrain and the DON while the otolith and saccule were tipped. The pair then repeated the experiments when the hearing structures were in their original orientations.
After 3 years of meticulous experimentation and analysis, it was clear that cells in the midbrain were processing sounds picked up by both ears, just like vertebrates. But so were cells in the DON. This means that toadfish begin combining auditory inputs from both saccules at an earlier stage in the auditory system than had been thought. The only other vertebrates that combine auditory inputs from both ears at the first auditory site in the brain are frogs and toads. `Maybe toadfish have more in common with toads than just being unattractive,' laughs Edds-Walton.
