AN AUDITORY FOVEA IN THE BARN OWL COCHLEA

The distribution of frequencies along the basilar papilla of the barn owl (Tyto alba) was studied by labelling small groups of primary auditory neurones of defined frequency response and tracing them to their peripheral innervation sites. The exact location of marked neurones was determined in cochlear wholemounts with the aid of a special surface preparation technique. The average basilar papilla length (in fixed, embedded specimens) was 10.74 mm. The resulting frequency map shows the basic vertebrate pattern with the lowest frequencies represented apically and increasingly higher frequencies mapped at progressively more basal locations. However, the length of basilar papilla devoted to different frequency ranges, i.e. the space per octave, varies dramatically in the barn owl. The lower frequencies (up to 2 kHz) show values between about 0.35 and 1 mm/octave, which are roughly equivalent to values reported for other birds. Above that, the space increases enormously, the highest octave (5-10 kHz) covering about 6 mm, or more than half of the length of the basilar papilla. Such an overrepresentation of a narrow, behaviourally very important frequency band is also seen in some bats, where it has been termed an acoustic or auditory fovea.