The neurobiology of the hunt
A barn owl (Tyto alba) hunting in complete darkness operates a biological targeting computer of exceptional precision. The pioneering research on this system took place right here at the California Institute of Technology (Caltech), led for decades by neurobiologist Masakazu Konishi. His work demonstrated that these owls can locate prey with an accuracy of one to two degrees in both horizontal and vertical planes, an ability that surpasses most other animals tested. The basis of this skill is the owl's ability to process minute differences in the arrival time and intensity of sound between its two ears.
The owl determines a sound's horizontal position, or azimuth, by using interaural time difference (ITD). A sound coming from the owl's left will strike the left ear first. The owl's brain can detect timing differences as small as 10 microseconds (a millionth of a second), allowing it to calculate the sound's origin along the horizontal axis. The neural processing for this begins in auditory nerve fibers that can encode stimulus timing with a precision of about 30 microseconds.
For the vertical position, or elevation, the owl uses interaural level difference (ILD). This relies on a subtle physical asymmetry: a barn owl's left ear opening is slightly higher than its right. A sound from below will therefore be slightly louder in the right ear. The owl’s prominent facial ruff acts like a parabolic dish, collecting and amplifying sounds, particularly in the 4 to 8 kHz range where their hearing is most acute. This facial structure increases the directional sensitivity that makes ILD a reliable cue for elevation.
Building an auditory map
The way the owl's brain is how it combines these two data streams—ITD for azimuth and ILD for elevation—into a coherent map of auditory space. This computation was a central focus of Konishi's lab at Caltech. They discovered that specialized neurons in the owl's midbrain respond only to specific combinations of ITD and ILD.
The process starts in a brainstem structure called the nucleus laminaris, which is a coincidence detector for the timing differences. Axons from each ear deliver signals here, and specific neurons fire only when pulses from both ears arrive simultaneously, effectively mapping a particular ITD. This information travels to the inferior colliculus, where it is combined with intensity data. The final result is a two-dimensional neural map of auditory space in a midbrain structure called the optic tectum.
This map is so precise that individual neurons correspond to specific points in space around the owl. When a neuron at a particular location on the map fires, the owl "knows" exactly where the sound is coming from and can launch a silent and accurate attack. The discovery of this computational map in the 1970s was a moment in neuroscience, showing how a brain can synthesize a representation of space from raw sensory data.