Eric Knudsen - Auditory Sound Map of The Barn Owl

Auditory Sound Map of The Barn Owl

In 1978, Knudsen and Konishi presented the discovery of an auditory map of space in the midbrain of the barn owl. This discovery was groundbreaking because it unearthed the first non-somatotopic space map in the brain. The map was found in the owl’s midbrain, in the lateral and anterior mesencephalicus lateralis dorsalis (MLD), a structure now referred to as the inferior colliculus. Unlike most sound-localization maps, this map was found to be two-dimensional, with units arranged spatially to represent both the vertical and horizontal location of sound. Knudsen and Konishi discovered that units in this structure respond preferentially to sounds originating in a particular region in space.

In the 1978 paper, elevation and azimuth (location in the horizontal plane) were shown to be the two coordinates of the map. Using a speaker set on a rotatable hemispherical track, Knudsen and Konishi presented owls with auditory stimulus from various locations in space and recorded the resulting neuronal activity. They found that neurons in this part of the MLD were organized according to the location of their receptive field, with azimuth varying along the horizontal plane of the space map and elevation varying vertically.

Knudsen followed this discovery with research into specific sound localization mechanisms. Two main auditory cues used by the barn owl to localize sound are interaural time difference (ITD) and interaural level difference (ILD). The owl’s ears are asymmetric, with the right ear’s opening being directed higher than that of the left. This asymmetry allows the barn owl to determine the elevation of a sound by comparing sound levels between its two ears. Interaural time differences provide the owl with information regarding a sound’s azimuth; sound will reach the ear closer to the sound source before reaching the farther ear, and this time difference can be detected and interpreted as an azimuthal direction. At low frequencies, the wavelength of a sound is wider than the owl's facial ruff, and the ruff does not affect detection of azimuth. At high frequencies, the ruff plays a role in reflecting sound for heightened sensitivity to vertical elevation. Therefore, with wide-band noise, containing both high and low frequencies, the owl could use interaural spectrum difference to obtain information about both azimuth and elevation. In 1979, Knudsen and Konishi showed that the barn owl uses interaural spectrum information in sound localization. They presented owls with both wide-bandwidth noise and pure tones. The birds were able to successfully locate pure tones (since they could still gather information from IID and ITD), but their error rate was much lower when localizing wide-bandwidth noise. This indicates that the birds utilize interaural spectrum differences to improve their accuracy.

Together with John Olsen and Steven Esterly, Knudsen studied the pattern of response to IID and ITD in the space map. They presented owls with sound stimuli while recordings were made from the optic tectum. Consistent with the previous findings regarding the organization of the optic tectum in terms of elevation and azimuth, they found that ITD varied primarily along the horizontal axis and IID along the vertical axis. However, the map according to elevation and azimuth does not line up perfectly with the ITD/IID map. Azimuth and ITD do not have a strictly linear relationship. In addition, IID is used not only to determine elevation of the sound source, but also, to a lesser degree, azimuth. Finally, IID and ITD are not the only two cues the owl uses to determine sound location, as shown by Knudsen’s research into the effect of bandwidth on sound localization accuracy. Information from multiple types of cues is used to create the map of elevation and azimuth in the optic tectum.