Refs: Knudsen-E-I.  Knudsen-P-F. (1990) Sensitive and critical periods for visual calibration of sound localization by barn owls. J-Neurosci. 10(1).  Pp. 222-32.

Knudsen-E-I.  Brainard-M-S. (1991)Visual instruction of the neural map of auditory space in the developing optic tectum. Science. 253(5015).  P 85-7.

1. A number of previous studies by Knudsen and his colleagues have shown that baby owls use their visual system to calibrate the head turning behavior in response to sound stimuli.  Baby owls turn their heads towards the location interesting sounds while they are still in the nest.  This behavior is thought to be innate and not the result of learning.

2.  As a baby owl grows its head increases in size.  Thus changes the relative distance between the two ears which increases as the head grows.  Since the distance between the two ears determines the interaural time disparity, this functional parameter will be changing as the animal grows.

3. If baby owls are fitted with binocular prisms that displaces the visual image to the right or left, the animal mislocalizes visual stimuli.  A visual stimulus like a food pellet appears to be located at a location shifted away from where it actually is.  The owl with prisms consistently pecks at food to the left or right of where it is located.

4. When baby owls are old enough to sit on a perch, they can be trained to locate both visual and auditory stimuli.  An owl with prisms consistently makes an error in the location of both the visual stimulus and the auditory stimulus.

5. In a darkened room an auditory stimulus causes the owl to turn its head toward the sound, but it mislocalizes the sound by the amount of the visual prism displacement.  This suggests that the map of sound localization in the auditory system has been modified to adjust to the apparent location of the visual stimulus.

6.  When the prisms are removed from the owl and its localization behavior tested the visual localization is immediately correct, yet the owl still turns its head to the wrong location in response to the sound stimulus.

7. Gradually over time, the auditory localization behavior improves, until the animal regains it's ability to localize sounds.

8. The ability to affect the system by placing prisms on the eyes depends on how early in the owl's development the prisms are put on.  The "sensitive period" is the time during which prisms will affect the auditory localization behavior.  The longer the delay, the less of a shift in the auditory sound localization behavior.

9. The ability to recover from prism placement depends on the age at which the prisms are removed.  If an owl is older than 200 days before the prisms are removed, it will be unable to recover its auditory localization behavior.  This period of time is called the critical period.  It is not known what factors govern either the critical or sensitive periods.

Visual instruction of the neural map of auditory space in the barn owl: an electrophysiological study:

1. Neurons in the optic tectum of owls receive input from visual neurons as well as space-specific auditory neurons.  These tectal neurons have receptive fields for both visual and auditory stimuli.  In normal owls the visual and auditory receptive fields are aligned to receive input from the same location in space.

2. In prism reared owls, these receptive fields are in alignment when the the prisms are on the owl but are  out of alignment when the prisms are removed.  When the prisms are removed the tectal neurons' receptive field for visual stimuli is correct, where as it's receptive field for auditory stimuli is displaced by the amount of the prism displacment.

3. These results corroborate the behavioral data on developing owls. These experiments suggest that two maps exist in the optic tectum, a map of visual space and a map of auditory space.  Under normal conditions these two maps are in alignment.  Under prism rearing conditions, the auditory map shifts to conform to the visual error induced by the prisms.

4. These results suggest that sensory experience plays a major role in adjusting the neural circuits in the nervous system to adapt to the sensory environment.