He is best known for his discovery, along with Masakazu Konishi, of a brain map of sound location in two dimensions in the barn owl, tyto alba.
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.
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.
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.
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.
[6] Together with John Olsen and Steven Esterly, Knudsen studied the pattern of response to IID and ITD in the space map.
Knudsen altered owls’ auditory cues by plugging one ear or removing the ruff feathers and preaural flaps.
Earlier work had indicated that alteration of the sound map by experience was restricted to a period during development, and once this window of plasticity had passed, subsequent changes would not occur.
The adult auditory sound map is more readily altered if the bird was exposed to abnormal stimuli earlier in life, during a sensitive period.
In owls raised with displacing prisms, this persistent reliance on inaccurate information is particularly apparent: “Even though interaction with the environment from the beginning of life has proven to owls that their visual perception of stimulus source location is inaccurate, they nevertheless use vision to calibrate sound localization, which in this case leads to a gross error in sound localization”.
[11] While monaural occlusion and visual displacement both alter the associations between sensory cues and corresponding spatial locations, there are significant differences in the mechanisms at work: “The task under [the conditions of monaural occlusion] is to use vision[…]to assign abnormal combinations of cue values to appropriate locations in space.
In contrast, prisms cause a relatively coherent displacement of visual space while leaving auditory cues essentially unchanged.