Silently and suddenly, a barn owl (Tyto alba) sweeps from the sky to strike its prey with astonishing accuracy. How does it find its prey? Although in nature the barn owl’s keen night vision is important in locating prey, the sounds of a scurrying mouse are sufficient for the owl to strike with deadly precision. Laboratory tests have revealed that birds such as the barn owl are able to locate the source of sounds within 1° or 2° in both the horizontal and vertical planes (1° is approximately the width of your little finger held at arm’s length).
Because of its astounding ability to detect and locate the source of sound, this nocturnal predator can pinpoint its prey by the rustlings the prey makes, and it can precisely determine not only the prey’s location along the ground but also its own angle of elevation above the prey. How do we know that the hunting owl uses the prey’s sound?
For one thing, we know that barn owls can catch a mouse in a completely darkened room . In experiments, a barn owl was able to capture a skittering leaf pulled along the floor by a string in a dark room (indicating that sight and smell are not involved), and if unable to see, it will leap into the middle of an expensive loudspeaker from which mouse sounds emanate.
To locate its prey by using sound cues, the barn owl must place the source of the sound on a horizontal plane from left to right (i.e., its azimuth), as well as on a vertical plane (i.e., its elevation). We now know that a barn owl uses different cues for locating sound cues in horizontal and vertical planes. The owl uses time differences in the arrival of sound in each ear to place it on a horizontal plane and differences in intensity between the two ears to determine the elevation of the sound source learned this by playing sound in a barn owl’s ear through small earphones. An owl turns its entire head to face the direction from which it perceives the sound source, because its eyes are fixed in their sockets.
When the sound in one ear preceded that in the other, the owl turned its head in the direction of the leading ear. The longer the time difference, the further the owl turned its head. The intensity differences in the two ears vary with the elevation of the sound source largely because of the arrangement of the ear canals and facial feathers. The two ear canals that channel the sound toward the inner ears are, oddly enough, situated asymmetrically, with the right one higher than the left.
Because of this difference in ear placement, each ear responds differently to a sound at a given elevation. This helps the owl determine its own elevation above the sound source, information critical to an aerial predator. Also, the face of the barn owl is composed of rows of densely packed feathers, called the facial ruff, that act as a focusing apparatus for sound (Figure 6.12).
Troughs in the facial ruff, like a hand cupped behind the ear, both amplify the sound and make the ear more sensitive to sound from certain directions. The facial ruff assists the owl in localizing sounds by creating differences in intensity of the sound in both ears. Loudness is a cue to localizing the sound in both the horizontal and the vertical dimensions. Sound is generally louder in the ear closer to the source. Because of the structure of the facial ruff, the left ear collects low-frequency sounds primarily from the left side, and the right ear collects low-frequency sounds from the right side.
A comparison of the intensity of low-frequency sounds in each ear helps the owl determine from which side of the head the sound originates. However, the facial ruff channels high-frequency sound to each ear differently, depending on the elevation of the sound source. As a result, the right ear is more sensitive to high frequency sounds that originate above the head, and the left ear is more sensitive to high-frequency sounds from below the head. The owl compares the loudness of high frequency sounds in each ear to determine its position above or below the sound source.
As a sound source moves upward from below the bird to a position above the owl’s head, the high-frequency sounds would first be loudest in the left ear and then gradually become louder in the right ear. Information on the timing and loudness of sounds in each ear is then sent to the owl’s central nervous system over the auditory nerve in a pattern of nerve impulses.
The information is first sent to the cochlear nuclei. Each side of the brain (cerebral hemisphere) has two cochlear nuclei: the magnocellular nucleus and the angular nucleus. Every axon in the auditory nerve sends a branch to both of these nuclei. Whereas the branch of the auditory nerve that goes to the magnocellular nuclei conveys timing information, the branch to the angular nuclei transmits intensity information. Thus, the timing data that place the sound on a horizontal plane are processed separately from the intensity data that place the sound on a vertical plane. These different features of sound are processed in parallel along nearly independent pathways to higher processing stations, where a map of auditory space is eventually formed. The map of auditory space is formed in the external nucleus of the inferior colliculus of the midbrain.
Within the inferior colliculus are certain neurons that respond selectively to specific degrees of binaural differences in sound. For example, one neuron may respond maximally to differences that correspond to a sound originating 30° to the right of the owl. The sound would arrive a certain amount sooner and be a certain degree louder in the right ear than in the left.
Those exact differences in timing and loudness stimulate that particular neuron in the inferior colliculus. The degree of binaural difference varies with the location of the sound, and the binaural difference that stimulates cells of the inferior colliculus varies from neuron to neuron. The resulting auditory space map is then transmitted to the optic tectum (an area of the brain involved in localizing and orienting to visual information). Auditory maps can be formed without visual input, but the precision of the map is increased by visual experience.
Indeed, when the information from the auditory map conflicts with visual input, owls trust their vision over their hearing. Normally, the auditory space map in the inferior colliculus and the visual space map in the optic tectum are aligned. When the two maps are misaligned by blocking one ear, the owl initially mislocates sound in the direction of the open ear. After many weeks experience with an earplug, a young owl learns new associations between auditory and visual cues and orients itself correctly. The auditory and visual space maps also become misaligned when an owl wears goggles that shift the visual field by 10o. A young owl gradually adjusts the location of sound localization to match its distorted visual map.
0 Comments