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  • Owl’s hearing acuity measured with IR pupil monitor

Oct 2007
Michael A. Greenwood

The barn owl is an almost perfect predator. It is powerfully built, fast and silent, and when it strikes, the result is usually lethal for whatever is on the receiving end.

But there may be a slight chink in the nocturnal raptor’s formidable armor.

Researchers at the University of Oregon in Eugene and at Monash University in Victoria, Australia, have found that the ability of the barn owl (Tyto alba) to track noises made by potential prey seems to be more acute when the prey is moving horizontally rather than vertically.


The auditory spatial acuity of a barn owl was tested by measuring pupil dilation (left) with an infrared sensor while subjecting the owl to a series of noises coming from speakers positioned at various locations vertically and horizontally in front of the owl (right). Reprinted from PLoS ONE with permission of the researchers.

The discovery was made with an infrared monitor that measured the owl’s pupil dilation in response to a series of noises emitted from speakers positioned on nearby vertical and horizontal axes, said researcher Terry T. Takahashi. The auditory map formed by the owl’s tracking of sound is reflected in the activity of neurons in the bird’s midbrain region, the researchers said. The technique provides insight into how response to external phenomena is converted into electrical activity in specific regions of the brain, which ultimately affects behavior. The monitoring technique eventually could be applied to humans.

Sensitivity changes

During experiments, the researchers secured the owl so that it was immobile and placed a pupillometer, consisting of an IR detector and emitter, in proximity to its cornea.

The emitter illuminated the retina with IR light, while the detector sensed the IR spectra reflected off the retina through the pupil. Earlier discoveries by the same group had shown that, when the owl detects a sound, its pupil dilates, and the detector picks up this increased reflectance. Presenting the same sound from the same place causes the pupil-dilation reflex to become insensitive, until the sound or the sound source’s position changes. It is this recovery of sensitivity that shows that the owl has detected a change in auditory stimulation.

The researchers positioned the owl a short distance from the horizontal-vertical array of speakers, the intersection of which was directly in front of the bird. Speakers along the vertical arm were used to assess discrimination in elevation, while those along the horizontal axis were used to assess discrimination in azimuth.

They found that the owl’s ability to detect change in the source of a noise was about two times finer in the azimuth than in the vertical. They also determined that the spatial tuning of neurons in the midbrain was better in the azimuth than in the vertical by a factor of about two.

The results suggest that the eyes, through the dilation of the pupils, can be used to gather information about the brain and to identify basic sensory deficits as well as damaged brain tissue.

PLoS ONE, online, Aug. 1, 2007.

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